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Irrigation water management


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Irrigation water management

  2. 2. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY CHAPTER 3 IRRIGATION EFFICIENCY,& SCHEDULING 3.1 IRRIGATION EFFICIENCY  Irrigation efficiency Ei is the ratio, usually expressed as per cent, of the amount of water used to meet the consumptive use requirement of the crop plus that necessary to maintain a favourable salt balance in the crop root zone to the total volume of water diverted, stored or pumped for irrigation.  Irrigation systems are designed and operated to supply the irrigation requirement to each fie'd of the farm.  The performance of an irrigation system determined by the efficiency with which water is stored in the surface reservoir at the head works, diverted and conveyed to the irrigated area through the conveyance system and applied to the fields and by the adequacy and uniformity of water application in each field.  The overall efficiency of an irrigation system is defined as the per cent of water supplied to the farm that is beneficially used for irrigation on the farm.  Overall system efficiency, often known as irrigation efficiency, is expressed as follows: Et = 100 (S-P-R-O)/S in which, Ei = Irrigation efficiency % 1= Irrigation requirement L = Leaching requirement S = Amount of water supplied to the farm P = Total deep percolation on the farm R - Total runoff from the farm O = Operational losses in channels and pipelines Water losses in an irrigation system can be considered in two groups, namely, 1. the losses in the storage reservoir and the conveyance system, and 2. the losses in the field.  Losses in the storage reservoir are due to evaporation from the water surface in the reservoir, seepage losses from the sides and bottom of the reservoir and spillage from the reservoir.  Conveyance losses include seepage losses in the canal system, spillage due to overtopping of the canals and breaches and rat holes in the canal bunds, runoff to drains from the canals and evaporations from the water surface in the canal network Field losses are mainly due to deep percolation below the crop root zone and runoff from irrigated fields in case of land served by surface irrigation methods and evaporation during water application in case of land served by sprinkler irrigation. IRRIGATION WATER MANAGEMENT Page 2
  3. 3. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  In evaluating the performance of irrigation system it is useful to examine the efficiency of each component of the system. This allows identification of the components which are not performing well and taking suitable measures to improve them.  The overall system efficiency may be expressed as follows – in which, Eo = Irrigation efficiency overall (%) Er = Reservoir storage efficiency (%) Ec = Water conveyance efficiency (%) Ea - Irrigation application efficiency Reservoir storage efficiency – The term defines the efficiency with which water is stored in a reservoir. Reservoir storage efficiency is expressed as in which, Ve = Evaporation volume from the reservoir Vs = Seepage volume from the reservoir V, = Inflow to the reservoir during a time interval V0 = Volume of outflow from the reservoir during time interval S = Change in reservoir storage during the time interval. The S term is often neglected when long time periods are considered. Example 3.1. A Persian wheel discharges at the rate of 11,000 litres per hour and works for eight hours each day. Estimate the area commanded by the water lift if the average depth of irrigation is 8 cm and irrigation interval is 15 days. Solution: The discharge of Persian wheel in 15 days Water required to cover one hectare to a depth of 8 cm Area commanded by Persian wheel Example 3.2. The following data were obtained in determining the soil moisture content at successive depths in the root zone prior to applying irrigation water: IRRIGATION WATER MANAGEMENT Page 3
  4. 4. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Depth of sampling Wt. of moist soil sample Oven dry wt. of soil sample Cm gm gm 0-25 134.6 126.82 25-50 136.28 127.95 50-75 122.95 115.32 75-100 110.92 102.64 The bulk density of the soil in the root zone was 1.50 gm/cc. The available moisture holding capacity of the soil was 17.8 cm/m depth. Determine (i) the moisture content at the different depths in the root zone, (ii) moisture content in the root zone at the time of irrigation, (iii) net depth of water to be applied to bring the mositure content to field capacity, and (iv) gross irrigation requirement at an estimated field irrigation efficiency of 70 per cent. Solution: (i) Soil moisture content at different depths Depth of root Moisture content before Depth of water required at irrigation root zone (omc *ϒ*d) Cm In % In cm 0-25 6.13 x 1.5 x 0.25 = 2.30 25-50 6.53 x 1.5 x 0.25 = 2.44 50-75 6.60 x 1.5 x 0.25 = 2.47 75-100 8.06 x 1.5 x 0.25 = 3.02 (ii) Moisture content in the root zone at the time of irrigation = 2.30 + 2.44 + 2.47 + 3.02 = 10.23 cm (iii)Net irrigation requirement = 17.80 - 10.25 = 7.55 cm (ic)Gross irrigation requirement =7.55/70 x 100 = 10.78 cm Example 3.3. Compute the gross depth of water application, the total number of irrigations and the irrigation interval of tomatoes of 5 months growing period in loamy soil. The irrigation water need of tomatoes for the whole growing period, estimated month wise is 720 mm. solution: Net depth of irrigation for tometoes in loamy soil is 40 mm. IRRIGATION WATER MANAGEMENT Page 4
  5. 5. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Therefore , the gross irrigation depth = 720/40 =18 mm The growing season = 5 x 30 = 150 days. Irrigation interval = 150/18 = 8.33 days. rounded of to 8 days. Example 3.4. Compute the efficiency of an irrigation system based on the following data: 1800 litres / min of water is diverted to the farm each day of 24 hrs. Each day 0.8 ha of maize and 1 ha of wheat are irrigated. The irrigation requirement of maize is 8 cm and that of wheat 12 cm. Solution: Assuming that the leaching requirement is nil Efficiency of an irrigation = volume of water diverted to field/ volume of water required Volume of water diverted = 1800 litres/min = (1800 x 60 x 24)/1000 = 2592 cubic metres Volume of water required to irrigate maize = area x depth = 0.8 x 0.08 x 1000 = 640 cubic metres Volume of water required to irrigate wheat =0.12 x l x 1000 = 1200 cubic metres Total water use by maize and wheat = 640 + 1200 = 1840 cubic metres Efficiency of an irrigation = 1840 / 2592 = 70.99%, say 71%. Example 3.5. Compute the reservoir storage efficiency for a 24-hr period when 3100 liters/minute of water are being diverted from the reservoir based on the following data: The rate of inflow into the reservoir of 4420 litres/minute. S = 410 cubic metres (the quantity of to be removed to restore the initial water level in the reservoir. Solution- VI = 4420 litres/minute: = 6364.8 cubic metres per day VO = 3790 litres/minute = 5457.6 cubic metres per day S= 410 cubic metres (given) 92.2 % 3.2 PROJECT IRRIGATION EFFICIENCY  Irrigation efficiency is usually expressed as the percentage ratio of the amount of water stored in crop root zone for crop use in the project command area to the amount of water diverted from the project source. It is expressed as, where, Ep = project irrigation efficiency in per cent Ws = amount of water stored in crop root zone soil Wd = amount of water diverted or pumped from the source  COMPONENTS OF PROJECT IRRIGATION EFFICIENCY Irrigation efficiency may be considered in stages from the point of diversion of water from a source to its actual use by crops. The components are: (i) water conveyance efficiency EC and (ii) water application efficiency Ea. IRRIGATION WATER MANAGEMENT Page 5
  6. 6. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY where, Ep = project irrigation efficiency in per cent Ec = water conveyance efficiency in per cent Ea = water application efficiency in per cent (i) Water Conveyance Efficiency(IF QUESTION CONTAIN 3 TO 4 MARKS) Water is conveyed through canal network, watercourses and channels from sources such as reservoirs, rivers and dams to fields or farms for crop use. Conveyance efficiency is used to evaluate the efficiency of the system conveying water. It is also used to measure the efficiency of channels conveying water from wells and ponds to fields. Water conveyance efficiency may be defined as the percentage ratio of the amount of water delivered to fields or farms to the amount of water diverted from sources. It is expressed as, where, Ec = water conveyance efficiency in per cent Wf = amount of water delivered to fields or farms (at the head of field supply channel or farm distribution system) Wd = amount of water diverted from sources (NOTE - IF THE QUESTION IS EXPLAIN IT FOR SEVEN MARKS THEN ALSO WRITE FOLLOWING three steps shown in capital letters) 1. STAGES OF CONVEYANCE EFFICIENCY –  The conveyance system for Water supplied to farms from sources through irrigation agencies.  The conveyance system within a farm for irrigating fields from outside sources such as canal. Thus the farmer himself to evaluate the water conveyance system in his farm. 2. FACTORS AFFECTING CONVEYANCE EFFICIENCY/effect of losses on conveyance efficiency  It is affected by water losses through evaporation, seepage, and transpiration by undesired vegetation and leakage through water control structures in the conveyance system.  The losses may vary from 25 to 60 per cent of the water diverted for irrigation.  In unlined canals, waterways and channels, water loss is heavy mainly due to seepage. IRRIGATION WATER MANAGEMENT Page 6
  7. 7. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Soil permeability, wetting perimeter and depth of water in conveyance structures, depth of water table below the canal bed, time for which water is run, temperature of water and types of materials used in construction of the conveyance system are the principal factors influencing the seepage loss.  There is often a growth of undesirable vegetation along and in canals and in channel beds and sides. The loss through transpiration by this vegetation may sometimes be considerable.  Occasionally, animals like rats and rabbits make burrows in channel bunds. There may be cracks in channel beds and bunds leading to water losses. 3. IMPROVEMENT MEASURES FOR CONVEYANCE EFFICIENCY  The efficiency can be improved considerably by lining canals, waterways and channels with impervious materials like bricks or stone masonry, bitumen clay mixture, concrete slabs, asphalt membrane and so on.  High-density polyethylene sheets may be used in irrigation channels as a cheap lining material.  Repairs of cracks, holes, burrows, erosion damages, leaks in water control structures should be done as a part of continuous maintenance.  Weed growth should not be allowed in unlined canals, waterways and field channels.  Pipes may be laid for water conveyance in farms or wherever feasible to cut the water conveyance losses.  Lining of conveyance system or laying pipes to convey water is a costly affair that becomes prohibitive in many occasions.  However, the extent of water losses, water scarcity and its high cost demand particularly that the conveyance system should have lining. (ii) Water Application Efficiency  Water application efficiency refers to the efficiency of water application to fields. Water is applied to fields by various methods. Those may be surface, subsurface, sprinkler or drip irrigation methods.  The efficiency of those methods individually or for single irrigation or irrigation of farm fields may be estimated.  The water application efficiency may be defined as the percentage ratio of the amount of water stored in the crop root zone to the amount of water delivered to fields. It is expressed as, where, Ea = water application efficiency in per cent Ws = amount of water stored in the crop root zone soil Wf = amount of water delivered to fields. IRRIGATION WATER MANAGEMENT Page 7
  8. 8. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY (NOTE - IF THE QUESTION IS EXPLAIN IT FOR SEVEN MARKS THEN ALSO WRITE FOLLOWING two steps shown in capital letters) 1. EFFECT OF LOSSES ON APPLICATION EFFICIENCY  This efficiency is determined to evaluate the irrigation practices in a farm. It accounts for loss of water by seepage in the supply channel, deep percolation and occasionally run-off occurring in fields.  Application losses of water in crops other than rice, on an average, account for about 17 per cent of the water supply reaching the fields.  The deep percolation loss in wet rice is exceptionally high and ranges from 38.3 per cent to as much as 80 per cent of the water applied in various soils.  The efficiency may be very low in a badly managed farm and 75 per cent in a wellmanaged farm. It can however be increased to approach 100 per cent if crops are under-irrigated by applying lower amount water than needed.  It is sometimes done because of water scarcity or high-priced water. Underirrigation may completely prevent deep percolation and run-off of water, but it is undesirable as crops suffer from water stress and give low yields.  Improper land levelling and grading, faulty choice of irrigation methods, application of excess water, frequent irrigation, very small or very large stream sizes, improper attentions during irrigation by the irrigator and faulty design of fields are the principal factors that cause low efficiencies. 2. IMPROVEMENT MEASURES FOR APPLICATION EFFICIENCY  Proper land levelling and grading is a prerequisite for efficient water application. This is needed to avoid accumulation of excess water in lower spots leading to deep percolation loss and under-irrigation of higher spots, and to achieve uniform run and distribution of water in the field.  Proper selection of irrigation methods according to crops, soil types, topography, climate and stream sizes are important to secure high efficiency through uniform application of water and preventing the deep percolation and run-off losses.  There is a tendency of farmers to frequently irrigate and give excess water to the crops when the supply of water is abundant.  Besides, absence of water measuring devices and ignorance of farmers in deciding the time of irrigation and quantity of water required for irrigation often pose problems.  A smaller size stream needs a longer time to irrigate a long and big strip of land and causes greater infiltration and deep percolation of water at the upper reach of the field.  On the other hand, a bigger size stream makes effective control of flow difficult and may cause surface run-off. The size of a stream requires to be decided based on soil type, method of irrigation, length of water run and depth of water to be applied.  The size of field and length of water run are determined according to stream sizes available, intake rates of soils, methods of irrigation adopted, prevailing land grade IRRIGATION WATER MANAGEMENT Page 8
  9. 9. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY and topography. Longer water run causes a greater intake of water and consequently deep percolation near the entry point of the stream. 3.3EFFICIENCY OF IRRIGATION PRACTICES, OPERATION OF IRRIGATION SYSTEM WATER USE AND 3.3.1 Water Distribution Efficiency  Water distribution efficiency measures the extent to which water is uniformly distributed and stored in the effective root zone soil along the irrigation run.  Not only the application of the right amount of water to the held but also its uniform distribution over the field is important.  Permissible lengths of irrigation runs are controlled to a large extent by the uniformity of water distribution which is possible for a given soil and irrigation management practice.  Water distribution efficiency indicates the extent to which water is uniformly distributed in the field. It is described as. where, Ed = water distribution efficiency in per cent y = average numerical deviation in depth of water stored in root zone soil along the irrigation run from the average depth of water stored during irrigation d = average depth of water stored during irrigation along the water run  Water distribution efficiency dictates the permissible length of irrigation run. It provides a measure of efficiency of an irrigation system or method over the other. 3.3.2 Water Use Efficiency  Water use efficiency is determined to evaluate the benefit of applied water through economic crop production. It is very important in crop production and irrigation water management. It is described in the following two ways: (i) Field water use efficiency. This may be defined as the ratio of the amount of economic crop yield to the amount of water required for crop growing. It is obtained as follows, where, Eu = field water use efficiency expressed in kilogram of economic yield per hectarecm or hectare-mm of water Y = economic crop yield in kilogram per hectare WR = water requirement of the crop in hectare-cm or hectare-mm (ii) Crop water use efficiency. IRRIGATION WATER MANAGEMENT Page 9
  10. 10. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY This may be defined as the ratio of the amount of economic yield of a crop to the amount of water consumptively used by the crop. It is found out as follows,      where, Ecu = c r o P water use efficiency in kilogram of economic yield per hectare cm or hectare-mm of water WUE = water use efficiency of crop in kilogram of economic yield per hectare-cm or hectare-mm of water Y = economic yield of crop in kilogram per hectare CU = consumptive use of water in hectare-cm or hectare-mm ET = evapotranspiration in hectare-cm or hectare-mm Water use efficiency is influenced by changes in either numerator or denominator or both. The numerator is the crop yield that depends on various factors of crop production and losses due to pests, diseases, weather hazards and other environmental conditions. The denominator is influenced by various plant, soil and climatic conditions and by soil and crop management practices. An increase in crop yields or decrease in consumptive use and water requirement of crops improves the water use efficiency. Factors of crop production influencing crop yields are many. Climatic conditions, amount, distribution and intensity of rainfall, occurrences of drought, soil characteristics, drainage, irrigation, fertilizers use, crop varieties, crop management practices and prevalence of weeds, pests and diseases are some very important factors. The consumptive use of crops is similarly influenced by climatic conditions, soil characteristics, crop types and varieties, water supply and irrigation practices, tillage practices, weed control, fertilizer use and use of mulch, antitranspirants and growth retardants. 3.3.4 Increasing Water Use Efficiency by crop and Soil Management Practices  Water use efficiency (crop yield/evapotranspiration of crop area) is influenced by crop and soil management practices.  The numerator of this formula, namely, crop yield can be changed appreciably by management practices.  The evapotranspiration or denominator of the formula is more difficult for man to control because it is dependent on climate and the availability of water for the crop.  Water use efficiency is not closely dependent on the water available if the supply is within the evapotranspiration limit.  The crop yields depend on the adequacy of the water supply. Most crops are sensitive to water stress at the critical phases of their growth. plant species adaptation. One of the primary ways of increasing crop yield and water use efficiency in a particular environment is to select plant species adapted to the total amount and distribution of IRRIGATION WATER MANAGEMENT Page 10
  11. 11. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY water in an area. Plant species vary greatly in water requirement. Species selection, plant selection and plant breeding have contributed greatly to increasing the efficiency of the water available in an area. Planting pattern. Planting patterns have a direct effect on yield, solar energy capture, and evaporation and thus an indirect effect on water use efficiency. Two important planting pattern practices are plant density and row spacing. Planting date. The main reason for choosing the optimum dates for sowing is to ensure good germination, by placing the seed in the optimum moisture zone, neither too deep nor very shallow. After the seeds germinate, the moisture should be optimum for root growth and root penetration and to envelope maximum soil volume for nutrient uptake. Planting dates also indicate the right type of climate for the shoot growth and optimum utilisation of moisture by the roots under normal rainfall conditions. Short season and early maturing crop species are often grown to escape the draught part of the season. Weed control. One of the main management means of obtaining more efficient water use is the elimination of weeds in crops. Weeds compete with crops for soil nutrients, water and light. Technology for efficient weed control, both mechanical and chemical, have been developed which should be availed in increasing water use efficiency of crop plants. Disease and insect pests. An often overlooked means of increasing water use efficiency lies in the area of disease and pest management of crops. It is possible to select crop varieties which are resistant to pests and diseases common to an area. Efficient crop and water management also includes the judicious use of chemical pesticides and selecting appropriate cropping patterns and cropping sequences. Growing the same crop year after year on a land tend to increase disease incidences in crops. Planting dates could influence pest incidence. For example, in North India the recommended sowing date for mustard crop is in the early part of the last week of September. Delayed sowing will result in delayed flowering which usually coincides with the incidence of aphids. . 3.3.5 Economic irrigation efficiency – Economic efficiency is ratio of total production attained with the operating irrigation system, compared to the total production expected under ideal condition. This parameter is a measure of overall efficiency, because it relates the final output to input. EXAMPLES – FROM DILIPKUMAR MAJUMDAR - IRRIGATION WATER MANAGEMENT Page 11
  19. 19. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Example 10.17. A stream of 135 litres per second was diverted from a canal and 100 litres per second was delivered to the field. An area of 1.6 hectare was irrigated in eight hours. The effective depth of root was 1.8 m. The runoff loss in the field was 432 cu. m. the depth of water penetration varied linearly from 1.8 m at the head end of the field to 1.2 m at the tail end. Available moisture holding capacity of the soil is 20 cm per meter depth of soil. Determine the water conveyance efficiency water application efficiency, water storage efficiency and water distribution efficiency. Irrigation was started at a moisture extraction level of 50 per cent of the available moisture. 3.3.6 CROP - WATER RELATIONSHIP 1. Water utilization efficiency and harvest index. Water requirement and yield of crops vary between crops and their varieties. The total dry matter and yield produced per unit quantity of water (kg per cubic metre) vary with the crop. This is expressed as the water utilization efficiency IRRIGATION WATER MANAGEMENT Page 19
  20. 20. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY in kg per cubic metre. Usually only a part of the dry matter like grain, oil content or sugar content is useful. The ratio between the total dry matter and the harvested yield is known as the harvest index. The harvest index varies with the crop and its variety. 2. Yield response factor of a crop. The response of crop yield to water supply is quantified using the yield response factor ky, which relates relative yield decrease (1 - Ya/Ym) to relative evapotranspiration deficit (1 – ETa/ETm). Results of research on actual crop yield Ya in relation to maximum potential yield Ym under different water supply regimes have resulted in the following empirical relationship (1 - Ya / Ym) = ky (l- ETa/ ETm ...(10.65) in which, Ky= Yield response factor of a crop Ya= Actual harvested yield of the crop Ym= Maximum (potential) harvested crop yield ETa= Actual evapotranspiration ETm= Maximum potential evaporation In addition to the availability of water in terms of the amount, time, and crop growth stage, crop yields are influenced by its variety, density of plants, nutrient availability, soil salinity, and control of crop pests and diseases. 3.3.7 Crop response to Water at Different stages of Growth  It has been observed that water requirement of crops varies with the stage of its growth. When water supply is limited, it is necessary to take into account the critical stages of crop growth with respect to moisture.  The term critical stage is commonly used to define the stage of growth when plants are most sensitive to shortage of water.  Each crop has certain critical stages at which if there is shortage of moisture, yield is reduced.  When there is shortage of water, it is better take care of the critical stages first to obtain increased water use efficiency.  The critical stages of crops growth are required to be studied for different crops. Growth stages of cereals in relation to irrigation. The following terms are used to describe the growth and developmental stages of grain crop in relation to irrigation. Stages Details Germination the appearance of the radicle, Tillering the formation of tillers, i.e., branches produced from the base of the stem. Jointing the stage when 2 nods can be seen, i.e., the beginning of shooting. (During this stage the internodes of the stem are elongating). Shooting the stage of elongation of internodes. Booting end of shooting stage and just prior to the emergence of ears. Heading the emergence of the ear from the tube formed by the leaf sheaths, Flowering the opening of the flowers. IRRIGATION WATER MANAGEMENT Page 20
  21. 21. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Grain formation the period of grain development from fertilization until maturity, which can be further subdivided into: 'milk stage'-grain contents have a milky consistency, 'dough stage'-grain contents have a doughy consistency, 'waxy-ripe'-grain contents have a waxy appearance, 'full-ripe'-grain contents are hard, 'dead-ripe'-grain ripe for harvesting. 3.4 IRRIGATION SCHEDULING (IF QUESTION OF 7 MARKS THEN ALSO WRITE 3.4.1,3.4.2,3.4.3 and list out irrigation indicies and explain each shortly in one to two line) Irrigation scheduling is “the process of determining the time to irrigate and how much water is to be applied (irrigation depth) in each irrigation”. Proper scheduling is essential for the efficient use of water and other inputs in crop production. Irrigation schedules are planned to either fully or partially provide the estimated water requirement of the crop. 3.4.1 Amount of water to apply per irrigation. (water requirement of irrigation) With ‘full irrigation’ the amount of water required per irrigation is computed as follows: 1 = Dr(fc-fm)/irrigation efficiency in which, Dr = Depth of root zone fc = Soil moisture content at field capacit fm = Soil moisture content prior to irrigat 1. Full irrigation provides adequate water to meet the entire irrigation requirement and is aimed at achieving the maximum production potential of the crop. Excess irrigation, however, reduces crop yields by adversely influencing soil physical properties like soil aeration soil temperature and the microbial activities in the soil. Full irrigation is justified when there is no scarcity of water and the cost of irrigation is low. 2. Deficit irrigation meets the water requirement of crops only partially. It is economically justified when reducing the amount of irrigation supply below the full level causes production costs to decrease faster than the decline in the value of crop harvest. Deficit irrigation is practiced when there is water scarcity or when the irrigation system capacity is limited. With ‘deficit’ irrigation the crop root zone is not always filled to the field capacity moisture content level. In locations with appreciable amounts of precipitation during the irrigation season, it is possible to fill the root zone only partially so that some precipitation can be stored in the crop root zone. Many research studies indicate that with increasing water shortage globally it may be more economical to plan irrigation system to obtain economical crop production per unit of land. However, care should be taken to ensure that there is no water shortage during the 'critical stages' in the growth of the crop. IRRIGATION WATER MANAGEMENT Page 21
  22. 22. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY 3.4.2 Irrigation schedules based on prediction of ET. Irrigations can be suitably scheduled on a farm if the allowable maximum water depletion in the crop root zone and the evapotranspiration (ET) for short periods during the growing period are known. 3.4.3 Irrigation scheduling based on pan evaporation. Prihar et al. (1974) suggested a 'practical approach' relating irrigation requirement to the cumulative evaporation during a time period as measured in a standard open pan. The relationship is expressed as IW/Epan, in which IW is the depth of irrigation water and Epan is the cumulative open pan evaporation. The rainfall during the period, if any, is subtracted from the value of Epan. The validity of the procedure will depend on the proper installation of the evaporation pan and the rain gauge and the precision in measurements of pan evaporation and rainfall. Further, the suitability of the method is situation-specific and limited to the particular variety of a crops. 3.5 Irrigation Indices Scientific irrigation must be based on an understanding of the soil-water-plantatmospheric relationship. Irrigation needs of crops depend on the evaporation demand of the ambient atmosphere, soil-water regime in the crop root zone and the plant foliage, 1. plant symptoms. Visual symptoms in plants to decide on the time to irrigate are changes in the colour of plants curling of the leaves and tendencies of wilting, these are observed by looking at the crop as a whole and not individual plants. When the crop comes under water stress, appearance changes from vigorous growth to retarded growth and often there is marked change in the colour of the leaf which gets darker and sometimes grey. However, symptoms may sometimes be misleading due to changes in crop varieties and nutritional disorders as well as insect and pest incidence. 2. water content and water potential. The status of water in the plant is generally indexed from the measurement of leafwater content and leaf-water potential. Their values at any time of the day will depend on the lag between the evaporative demand of the atmosphere and the moisture uptake rate by the crop. When their values fall below certain critical limits specific to the plant species and their growth stages, important physiological and growth factors are adversely affected. Hence, these values can serve as reliable indicators for irrigating crops, 3. plant temperature. Almost the entire radiation energy received on the leaf surface during the daylight hours is utilized for evapotranspiration (ET). The remaining energy, if any, is used to heat the leaf tissues and the ambient air. When ET reduces due to water deficit in the plant, the energy saved in the process is partly used to raise the leaf IRRIGATION WATER MANAGEMENT Page 22
  23. 23. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY temperature. Many investigations have shown that the leaf canopy temperature is a sensitive index in crops. 4. Soil water regime. A commonly recommended method to decide on irrigation schedules is soil moisture measurements in the field. When the soil moisture content has dropped to a certain critical level, say about 50% of field capacity level in the crop root zone, irrigation is applied. Normally irrigation is not delayed beyond 60% field capacity moisture content. 3.6 General Guidelines on Planning Irrigation Schedules The characteristics of different soil groups and soil depth influencing irrigation scheduling are as follows: (i) Shallow and I or sandy soil. In sandy soils in general and soils with limited depth (due to the presence of hard pan close to the ground surface) the soil moisture holding capacity is low. These soils require frequent irrigations and the amount of water applied in each irrigation is kept low. iii) Loamy soils. Loamy soils can store more water than sandy soils or shallow soils. Hence, irrigation is applied less frequently with larger depth of water in an irrigation. (iii) Clayey soils. The moisture holding capacity of clayey soils is more than loamy soils. Hence, irrigations are less frequent and the quantity of water in an irrigation is higher. Adjusting irrigation depth for non-peak periods. The most irrigation schedule is for peak period moisture use situations. During the early growth stages of crops it is desirable to keep the irrigation interval at shorter intervals and reduce the quantity of water used in irrigation. If the irrigation interval is kept far apart due to the low value of ET0, the young plants may suffer from water shortage as their roots are notable to take up water from the lower layers of the root zone. On the other hand crops which are harvested dry like maize need much less water at the late season stage. Hence frequency of irrigation during the late seson is reduced. Adjusting irrigation schedules to rainfall occurring during growing season – the data presented mostly is on the assumption that no rainfall ocurrs during the crop growing season. If contribution from rainfall is significant during the period of schedule is to be suitably adjusted. This is usually done by making the irrigation interval longer or reducing the net irrigation depth. procedure to arrive at approximate solutions to irrigation schedule. A simple procedure to make estimates leading to approximate values for scheduling irrigation comprise of the following steps: 1. Estimate the net and gross irrigation depths (mm), 2. Compute the irrigation water need over the total growing season of the crop (mm), 3. Compute the total number of irrigations over the growing season, 4. Calculate irrigation interval (days). The number of irrigation applications over the growing season of a crop is obtained by dividing the irrigation water need during the growing season by the net irrigation depth per application. The interval between two irrigations is obtained by dividing the number of IRRIGATION WATER MANAGEMENT Page 23
  24. 24. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY days of the growing season by the number of irrigations. To be on the safe side the irrigation interval is rounded of to the lower whole figure. Exapie 10.13. Compute the gross depth of water application, the total number of irrigations and the irrigation interval of tomatoes of 5 months growing period in loamy soil. The irrigation water need of tomatoes for the whole growing period, estimated monthwise is 720 mm. 3.7 TIME OF IRRIGATION – Time of irrigation is usually governed by two major conditions namely, (1) water need of crops and (2) availability of irrigation water. Water need of crops is, however the prime consideration to decide the time of irrigation. (1) Water Needs of Crop  Crop plants require water to meet the transpiration loss, build up body tissues and to carry on biochemical and physiological activities within the body. Transpiration, which is considered as a vital physiological activity of plants, occurs continuously as long as the water supply is maintained.  Also, a continuous evaporation occurs from the moist soil surface in crop field. After irrigation, the evapotranspiration begins at a peak rate drawing water from the moist soil below and continues till there is available water in soil. This causes a continuous decline in soil water content.  Rate of evapotranspiration decreases continuously sometime after completion of irrigation with reduction in avalable soil water below the field capacity.  A stage is reached within a few days after irrigation when the rate at which soil water is available for extraction by crop plants becomes equal to the normal consumptive use rate. This stage of soil water is considered as the lowest point of the optimum soil water regime.  The optimum soil water regime means the" range of available soil water in which plants do not suffer from water stress and all the plant activities occur at an optimal rate. Field capacity is the uppermost limit of optimum soil water regime for crops other than rice.  A soil water deficit below optimum soil water regime causes water stress in plants causing decline in growth and yield, as the rate of availability of soil water falls short of the normal consumptive use rate.  Irrigation is, therefore, needed when this lowest limit optimum water regime is reached and it is considered as the most opportune time for irrigation.  The stage of available soil water below which water stress begins to cause a serious fall in crop growth and subsequently the yield is termed as the critical level of soil water for crop plants. This level of available soil water coincides with the lowest level of optimum available soil water regime. The optimum available soil water regime and the critical level of soil water may be diagrammatically represented as in Fig. 10.1.  Every crop has a characteristic optimum water regime. The regime varies with the crop abilities to extract water from different soil layers. (2) Availability of water IRRIGATION WATER MANAGEMENT Page 24
  25. 25. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Irrigation water is often in short supply in most locations and therefore demands a careful and economic use. Economy of water helps to bring more areas under protective irrigation and leads to a greater crop production in areas of limited water supply.  In areas where water is scarce, farmers are not able to apply normal irrigation to crops and are forced to skip some irrigations. It is therefore necessary that one decides a priority of stages of crops when irrigations are to be applied and the stages when one can afford to miss irrigation. The critical stages of water need of crops receive the foremost attention. It is necessary to simultaneously consider and weigh the relative the importance of the various stages for irrigation and the availability of water.  A preferential status of crop stages according to their relative importance to yield should be considered for irrigation in areas of water scarcity. 3.8 CRITICAL STAGES OF WATER NEED OF CROPS  During the life cycle of a crop plant, there are some crucial stages in the life cycle of a crop plant when the plant is badly in need of water. Denial of water during these stages causes a definite set back to growth processes and the yield is adversely affected. These stages are referred to as the critical stages of water requirement. IRRIGATION WATER MANAGEMENT Page 25
  26. 26. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  These stages do not usually coincide with the periods of peak consumptive use by crops. It will not be correct to consider that crops at these critical stages require more water as their water needs are utmost.  Critical stages of water requirement are usually the turning points in plant life cycle. This can be represented by a sigmoid or S-shaped growth curve (Fig. 10.2).  The curve shows two most important points of change in the growth rate, viz. the point of inflection and the point of deflection.  The point of inflection indicates a sudden increase in vegetative growth and the point of deflection represents the slowing down of vegetative growth and initiation of the reproductive phase.  Crops demand for adequate water at these stages and cannot afford to stand water stress without serious reduction in growth and yield. These two stages of crop life are, therefore, considered as the most critical stages of water requirement.  When crop plants are young and delicate, and start of grand growth period, biochemical activities in plants occur at a higher rate. This leads to tremendous increase in water need of the crop and the supply of water should be adequate to maintain the normal rate of active growth and evapotranspiration.  Water stress at the sensitive stages causes a serious retardation in growth process that ultimately decresses the yield. The sensitive stages differ from one crop to the others.  Water stress at these stages causes lower tillering, branching, pegging, tuber bulking, inadequate flowering and in extreme case, flower drops, poor setting of IRRIGATION WATER MANAGEMENT Page 26
  27. 27. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY     grains or fruits, bad filling of grains or serious fruit drops depending on the type of crops. In dwarf wheats crown root initiation stage is the most important critical stage of water need as crown roots fail to develop in dry soils. Lack of adequate water at this stage reduces tillering and affects the yield adversely. It is true that crops require adequate water supply throughout their life cycle for best growth and yield. Only in the later stages of crop maturity, water supply is reduced or cut-off to obtain uniform and quicker crop maturity. Crops may be allowed to stand water stress to some extent during certain periods of life excepting at the critical stages to save some water under situations of water scarcity. The critical stages of water need of crops that do not have distinguishable stages are decided experimentally. Determination of Critical Periods of Water Need To make a judicious use of irrigation water particularly when the water supply is limited, it is essential to determine the critical periods of water need of crops. For this purpose, a crop is subjected to predetermined water stress at different stages of growth and then the corresponding yield reductions are considered. It is then related to the yield of crop that has not been subjected to any water stress and irrigated according to the normal schedule. Another way to decide the critical periods is to miss irrigations at different stages of the crop and then relating the corresponding yield reductions with the yield from control plot which is irrigated normally. Periods at which yield reductions are significant are considered as the critical periods of water need in the life of the crop. 3.9 CRITERIA/APPROACHES FOR SCHEDULING IRRIGATION (ANSWER THE QUESTION AS PER REQUIREMENTS.) The criteria for scheduling irrigation as attempted from time to time may be grouped into three categories, namely, (1) plant criteria, (2) criteria based on soil water status and (3) meteorological criteria. (1) Plant Criteria Plants show up certain characteristic changes. These changes in plants are often valuable pointers to the time of irrigation. Different plant criteria considered to schedule irrigation are the general appearance of crop plants, plant water potential and water content of plant tissues, growth, critical periods of water need, indicator plant, stomatal opening, leaf diffusion resistance and plant temperature. A. Plant appearance  With water stress, some characteristic changes usually occur in the general appearance of plants.  There may be changes in the normal colour of plant or distortions of plants such as wilting or drooping of plants and curling or rolling of leaves. Some crops are very sensitive to soil-water changes and develop scarcity symptoms easily, while others do not.  Some plants develop symptoms easily, while others do not. IRRIGATION WATER MANAGEMENT Page 27
  28. 28. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Changes in colour appear first in the lower leaves. On the other hand, deep green and light green colours in alfalfa are indicative of water stress and adequate supply of water respectively.  Water stress is also shown by rolling of leaves during hottest part of day. While some plants not easily shows water stress until decrease in growth takes place.  This technique is however quite simple and rapid, but suffers from many deficiencies. Changes in colour may be misleading since nutritional disorder, insect damage, disease attack and varietal character cause variable changes in foliage colour.  However, distinct changes in colour have been used to schedule irrigation to beans. B. Plant water potential and water content  The relative leaf water content (RLWC) and leaf water potential change with variations in soil water availability or make lag between water absorption by plants and evaporative demand of the atmosphere.  It has been noted adverse physiological and growth phenomena specific to plant species with fall in the RLWC and water potential below certain critical limits.  However, sophisticated equipment, intricate measuring devices, high cost and lack of proper standardization of instruments deter the use of this technique on a large scale. C. Plant growth  Cell elongation, increase in height and radial changes in stem is considered as the growth process that suffers first with water stress in plant.  Timing of irrigation can be set as and when the normal growth rate is observed to decline. This is, however, possible in places where a continuous measurement of plant growth is maintained.  This technique offers difficulties owing to unavailability and high costs of equipment, inadequate standardization of the method, difficulties in selection of proper growth parameter and precise growth measurement and so on.  The serious objection to this approach of scheduling irrigation is that the plants suffer before they show any retardation in growth processes. D. Critical crop stages of water need  Irrigation scheduling may be decided based on stages of growth more conveniently in crops in which the physiological stages are distinct to locate the critical periods of water need.  However, it may be a little difficult in crops where stages are not so well defined. E. Indicator plant IRRIGATION WATER MANAGEMENT Page 28
  29. 29. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  There are some plants sensitive to soil-water variations. They may be used for detecting the water stress in crops that do not show symptoms of water stress easily or exhibit the same when they have already suffered seriously.  Sunflower plants are often used as in indicator plants in onion crop. An indicator plant for irrigation should be such that it shows the water stress before the crop has suffered from it.  When an indicator plant is grown in a crop field, care should be taken not to shade the plant by crop plants. F. Stomatal aperture  Opening of stomata in plants is regulated by soil-water availability. Stomata remain fully open when the supply of water is adequate, whereas they start closing with scarcity of water in soils to restrict the transpiration.  Water deficit in plants is directly related to availability of soil water and that may be used for scheduling irrigation in crops. G. Leaf diffusion resistance  Leaf resistance to vapour diffusion is regulated mainly by the leaf water deficit.  Several scientists reported a close relationship between leaf diffusion resistance (LDR) and plant water stress. It is observed that on cloudy days LDR is a sensitive index of internal water balance in the mild to moderate stress range and holds a promise for scheduling irrigation.  LDR, may be a useful tool in irrigation scheduling. LDR is a sensitive index of plant water stress in sugarcane and can be used as a basis for scheduling irrigation. H. Plant temperature  Solar radiation received on earth heats up leaf tissues besides causing evapotranspiration and heating up the ambient air. With water deficit in plant the temperature of leaf tissues rises. Many investigations have shown that leaf or canopy temperature is a sensitive index of plant water status in crops.  The difference between the stressed and unstressed canopy temperatures was a better index of water deficit than the difference between plant canopy and air temperatures. (2)Criteria Based on Soil Water Status Scheduling irrigation based on soil water content is the most accurate and dependable method. Determination of available soil water is rather more important than estimating the total water content of soils. For the purpose, an information on the optimum water regime of crops and the available water holding capacity of soils is essential. A. Soil water content  Early attempts were made to schedule irrigation when the soil water content reached a certain value. The idea did not succeed since there existed a wide variation in the water content retained by the different classes of soils.  Later advanced a new concept of scheduling irrigation based on the lower limit of soil water content for potential evapotranspiration of a crop. IRRIGATION WATER MANAGEMENT Page 29
  30. 30. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY B. C. D. E.     They assumed that the growth of crop was likely to suffer below this level of soil water. The threshold limit could be decided for various crops, soil types and atmospheric evaporativity. Depth-interval of irrigation  Attempts to schedule irrigation based on depth of water applied or interval between irrigations was not found very effective because of the water retentive capacity of soils varies widely with soil types and soil physical conditions,  Root zones of crops vary with types of crops and their rooting characteristics at different growth stages, the depth and interval of irrigation require modifications in different soils and at various crop growing periods.  An arbitrarily fixed depth or interval of irrigation has misleading effects on crop growth and yield. Critical level of soil water  This level once established experimentally for various crops in different soil types and soil conditions can be profitably used for scheduling irrigation. This approach has been widely suggested for adoption.  A periodical determination of soil water content is made to know the time when the soil water is likely to reach the critical level.  This criterion is synonymous with the concept of available soil water depletion for deciding the time of irrigation. The depth of irrigation however needs revision upwards every time with increasing vegetative growth and rooting depth of an actively growing crop. Soil water tension  Many scientific workers have used this criterion for scheduling irrigation to crops in various parts of the world. In many countries, the tensiometers has been considered as a useful device for scheduling irrigation.  The use of tensiometers for controlling irrigation did not find much favour with common farmers since the device presents certain difficulties in its use. The tensiometers can be used only in the lower tensions up to 0.85 bars.  It does not show the actual soil water content for direct calculation of the depth of irrigation to be applied. The water content is calibrated from the soil-tension curve.  Sometimes the tensiometer showing the energy status of soil water earlier to the existence of the actual soil water content. Electrical resistance The concept of electrical resistance that varies inversely with the water content in soils was also tried to schedule irrigation. For this purpose, resistance blocks made of gypsum, nylon, nylon-resin etc. were used. Crops were irrigated when the electrical resistance reached a certain value. The value could be decided experimentally for various crops by using the resistance blocks. This method has however many limitations and did not become popular. The limitations are: (i) resistance blocks cannot be used at low tension at which most of the available water is held by soils, (ii) difficulty of deciding the depth of irrigation as resistance blocks do not directly show the prevailing soil water content and (iii) IRRIGATION WATER MANAGEMENT Page 30
  31. 31. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY the existence of a time-lag in tension-equilibrium between the porous block and the surrounding soil which causes showing up the energy status of soil water earlier. (3) Climatological Approach A. Empirical formulae  For this purpose, empirical formulae using different meteorological parameters have been developed. Penman (1948) and Thornthwaite (1948), Blaney-Criddle (1950) and Christiansen (1968) developed formulae for estimating potential evapotranspiration.  This used the estimated evapotranspiration for scheduling irrigation by water budget method.  The adoption of empirical formulae for irrigation control demands the knowledge of water holding capacity of soil and a continuous record of rainfall and other meteorological parameters. This approach of scheduling irrigation to crops is complicated for an ordinary farmer. B. Evaporimeter  Irrigation is applied when crops consume the available soil water to a certain limit, calculated on the basis of consumptive use rate as determined by evaporimeters. Sunken screen evaporimeter value can be used from the period of 25 per cent ground coverage by crops till their maturity.  The irrigation is applied when a certain amount of water gets evaporated from the pan. The values of pan evaporation for this purpose are found for various crops at their different growth stages under different soil and climatic conditions. C. Irrigation water/Cumulative pan evaporation ratio (IW/CPE ratio)  The ratio of the amount of irrigation applied to cumulative pan evaporation values has been used for scheduling irrigation. The pan evaporation values are added up every day till it is equal to a certain ratio of the amount of water applied as irrigation. The ratio for various crops is determined experimentally by estimating the evapotranspiration by lysimeter studies. 3.10 NECESSITY OF IRRIGATION SCHEDULING  Since irrigation water is of limited supply in most of the places, an emphasis should be laid on making the most efficient and economic use of water for crop production.  In a situation where adequate water is available on demand, farmers often irrigate their crops earlier to the time of actual need in their eagerness to obtain good growth and high yield of crops.  Their attention is to produce more yields per unit area of land without much consideration of the amount of water used. This leads to waste of valuable water and it may sometimes cause damage to crops and lands owing to over-irrigation.  On the other hand, a delay in irrigation for lack of proper knowledge may force water stress to crop causing decline in yield.  The optimum scheduling of irrigation under this situation should be based on crop needs to avoid both over- and under-irrigation and to ensure a high water use efficiency. IRRIGATION WATER MANAGEMENT Page 31
  32. 32. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Since adequate water is not available in most places, attentions must be given to produce the maximum yield per unit of water used by rational distribution of water among crops over the growing seasons.  A thorough understanding of the soil-water-plant-atmosphere relationships is essential for proper scheduling of irrigation since irrigation needs of crops are decided by the evaporative demand of the ambient atmosphere, soil water status and plant characteristics. 3.11 FREQUENCY AND INTERVAL OF IRRIGATION 3.11.1 FREQUENCY OF IRRIGATION  Irrigation frequency refers to the number of days between irrigations during periods without rainfall. It depends on the consumptive use rate of a crop and on the amount of available moisture in the crop root zone.  It is a function of crop, soil and climate; sandy soils must be irrigated more often than fine textured deep soils.  Moisture-use rate varies with the kind of crop and climatic conditions and increases as the crop grows larger and the days become longer and hotter.  With higher frequency of irrigation, surface soils remain moist for longer periods leading to higher evapotranspiration losses. Thus, frequency of irrigations should be as low as possible to avoid waste of water. Of course, frequent irrigations with smaller depths of water each time are often more conducive to higher yields than heavier irrigations at long intervals, the delta of water remaining the same within a certain limit. This has been represented in Fig. IRRIGATION WATER MANAGEMENT Page 32
  33. 33. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  In general, irrigation should start when about 50 per cent and not over 60 per cent of the available moisture has been used from the zone in which most of the roots are concentrated.  The stage of growth of the crop with reference to the critical periods of growth is also kept in view while designing irrigation frequency.  In designing irrigation systems, the irrigation frequency to be used is the time (in days) between two irrigations in the period of highest consumptive use of the crops grown.  Irrigation frequency depends on how fast soil moisture is extracted when a crop is transpiring at its maximum rate. The average moisture-use rate during this period is used to plan irrigation systems. For an irrigation system to be adequate, it must have sufficient capacity to supply the water required during this period. The design irrigation frequency may be computed as folows: 3.11.2 IRRIGATION INTERVAL  The term, interval of irrigation indicates the time gap, usually expressed in days, between two subsequent irrigations.  The total amount of water required by a crop for producing an optimum yield is termed as delta of water and it is synonymous with water requirement of crop.  Fewer irrigations at longer intervals encourage saving of water. Losses of water occur in the irrigation practice and the losses may take place in conveyance channels and in fields every time irrigation is applied.  The loss may get increase due to inadequate knowledge and experience of the farmers in water application.  Immediately after irrigation when the soil is wet, evapotranspiration occurs at a potential rate. It starts declining some days after irrigation as the surface soil dries up. Dry and loose soil surface helps to reduce evaporation.  Since soil water declines progressively owing to continuous evapotranspiration, the rate of evapotranspiration also declines progressively with the advance of time after irrigation. Therefore, the longer is the interval between irrigations, the greater is the saving of water.  Besides, a longer interval between two irrigations cuts down the number of irrigations during the growing season. Care should, however, be taken not to cause any water stress beyond a certain limit by making the irrigation interval unduly long unless compelled to do so for reasons of water scarcity.  Irrigation is usually advised at the lowest limit of the optimum water regime, as already stated earlier. The interval between two irrigations should normally be the time taken by crops to reduce the soil water from field capacity to the lowest level of optimum soil water regime. IRRIGATION WATER MANAGEMENT Page 33
  34. 34. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY 3.11.3 Design Irrigation Frequency  It is same as the irrigation period and refers to the time, usually expressed in days, between two irrigations that is necessary to irrigate the design crop area during the period of peak consumptive use of the crop to be irrigated.  The design irrigation frequency is used to decide the capapcity of the irrigation system to be able to supply the required water to crops in the area.  Irrigation frequency depends on how fast soil water is extracted when crop is transpiring at its maximum rate. The average consumptive use rate during this period is used for planning the system.  The design irrigation frequency may be decided by the following relationship, where, Fc = field capacity, per cent Mb = soil water content just before irrigation, per cent As = apparent specific gravity of soil or bulk density of soil (dimensionless) D = depth of crop root zone, cm 3.12 Factors Affecting Frequency of Irrigation  The two main considerations namely, water need of crops and the availability of irrigation decide the irrigation frequency. Once these two are known, the frequency of irrigation is influenced mainly by: (a) Climate and season (b) Soil characteristics (c) Crop characteristics and (d) Crop and water management practices. A. Climate and season  Climate is responsible for causing variations in consumptive use rate and frequency of irrigation. High temperature, low humidity, high wind velocity, greater solar radiation in a place emphasis the need to irrigate crops more frequently.  This is particularly evident in arid regions and during summer season. Since air remains dry, temperature is high during summer months and the rainfall is low in the arid region, there is a necessity to irrigate crops frequently.  With greater evapotranspiration, frequent replenishment of soil water becomes necessary to maintain the optimum growth of crops.  On the other hand, higher rainfall and greater relative humidity during the rainy season reduce the irrigation requirement of crops and irrigations may be applied at longer interval, if it at all becomes necessary.  In humid areas where rainfall is higher, the irrigation requirements of crops are lower and irrigations are applied less frequently. IRRIGATION WATER MANAGEMENT Page 34
  35. 35. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  During winter months the crop demand for water is much less due to the lower temperature and evaporative demand of the atmosphere. The irrigation requirement during winter months is, therefore, lower and irrigations may be applied at longer interval. B. Soil characteristics  A soil with greater water retentive capacity serves as a bigger water reservoir for crops and can supply water for longer duration. Consequently, frequency of irrigation is lower and interval of irrigation is longer in heavier soils and in soils with crumb structure, good organic matter content and low content of soluble salts.  On the other hand, the frequency is higher in porous sandy soils with coarse texture, poor structure and low organic matter content. Retention of greater amount of available water is considered more important than total quantity of water retained by a soil.  Depth of soil is another factor that influences the frequency of irrigation. A shallow soil cannot hold enough water to meet the crop demand for a longer period. Necessarily, frequent irrigations are required with smaller depth of water each time. Irrigations at longer interval is applied to deep soil that has a greater water storage capacity.  Such a soil can supply water for longer duration particularly when the root system is quite deep and extensive. C. Crop characteristics  Crops vary in their consumptive use of water, sensitivity to water stress, water extraction capacity and optimum water regime. Frequency of irrigation thus varies with crops. A crop having higher consumptive use rate consumes the soil water quickly and requires more frequent replenishment of soil water.  Many crops have varieties that are either sensitive or tolerant to drought conditions. Varieties sensitive to drought conditions require frequent irrigations compared to tolerant varieties.  Rooting characteristics of crops such, When the root system is shallow and fibrous, crops are not able to utilize water from deeper soil layers and are frequently irrigated with smaller depth of water to wet only the upper soil layers.  Crops with deeper and extensive root system command a greater depth of soil and water reserve and require irrigations at longer interval.  Maximum quantity of water is extracted from the upper 25 per cent of the effective rooting depth and the extraction is least from the last 25 per cent of the rooting depth which is about one-fourth of that from the upper section. The water extraction pattern shows that a higher frequency with smaller depth of irrigation each time is preferable for crops with shallow root system that extract most of their water need from the upper soil layers. IRRIGATION WATER MANAGEMENT Page 35
  36. 36. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Irrigation frequency varies with stages of crop growth. A crop when young and delicate needs frequent irrigations. Plants at this stage are very susceptible to water stress, because their root system is not deep and extensive enough to draw water from deeper soil layers.  Subsequently, the consumptive use rate gradually increases and at the same time the root system also develops. Irrigations can then be applied at longer interval, as roots are able to draw water from greater volume of soils.  When a crop approaches maturity, the demand for water greatly declines because of steep fall in consumptive use rate. Besides, the well-developed root system can also draw water from deeper layers. The irrigation requirement, therefore, declines with approach of maturity and crops are irrigated at longer intervals. D. Crop and water management practices  Soil water conservation practices such as artificial or soil mulching and crop cultural practices like weeding and hoeing help to reduce the evaporation loss and conserve more soil water for crop use. Thus, there is a reduction in irrigation requirement of crops.  Method of irrigation, depth of water applied each time and the water distribution efficiency influence the frequency of irrigation. Sprinkler irrigation adopted in porous and lighter soils demands frequent irrigations which cause more evaporation loss as the surface soil remains moist for a longer period. A smaller depth of irrigation is, therefore, applied each time.  On the other hand, irrigations are applied at longer interval when the surface irrigation is adopted in soils that have higher water retentive capacity and are not too porous. IRRIGATION WATER MANAGEMENT Page 36
  37. 37. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY 3.13 Irrigation Period  Irrigation period is the time, usually expressed in days, that can be allowed for applying one irrigation to a given design crop area during the peak consumptive use period of the crop.  It is a function of the peak-period consumptive use rate. It is considered for designing the irrigation system capacity and equipment.  The irrigation system must be so designed that the irrigation period is not greater than the irrigation interval. It is estimated as follows, 3.14 DEPTH OF IRRIGATION  Depth of irrigation is a function of the water retentive capacity of the rootzone soil and the extent of soil water depletion at the time of irrigation. It refers to the depth to which the applied water would cover an area. As for example, a 10-centimetre depth of irrigation to a hectare of land represents the volume of water which when allowed to stand without any loss and infiltration into the soil would stand over one hectare area to a depth of 10 cm.  The net depth of irrigation is decided by the amount of water required to bring the soil water content just before an irrigation to field capacity in the root zone soil. The water content of soil just before irrigation must be known to calculate the net depth of water required to be applied. It is calculated by the following formula,  Therefore, irrigation should consist of the quantity of water required to replenish the soil water depleted in the crop root zone just before irrigation applied. It should neither be lighter nor heavier than actual depth of water needed.  A lighter irrigation leads to under-irrigation keeping the lower soil dry, while a heavier one results io over-irrigation causing loss of water through deep IRRIGATION WATER MANAGEMENT Page 37
  38. 38. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY percolation. Roots do not grow in dry soil. Therefore, the root development gets restricted and shallow rooting takes place when the lower soils remain dry in case of under-irrigation.  Roots of an actively growing plant move deeper and deeper provided the soil below the growing point of roots is moist. Therefore, the depth of irrigation is increased with growth of crop to wet the extending root zone.  Shallow irrigation causes the roots already grown in deeper layers to gradually die for lack of water. On the other hand, when a heavy irrigation is made, some amount of water percolates down beyond the root zone and gets wasted. 3.15 Factors Affecting Depth of Irrigation Factors that govern the frequency of irrigation, influence also the depth of irrigation. Factors mainly concerned in modifying the depth are as follows: (a) Depth of effective root zone soil, (b) Water retentive capacity of soil, (c) Degree of soil water depletion, and (d) Apparent specific gravity of soil.  Depth of soil to be made wet regulates the depth of irrigation. When roots extend deeper into soil, more water is necessary to wet the soil layers up to which roots have developed since roots function as the principal water absorbing organ of plants. A smaller irrigation keeps the lower soils dry and it would lead either to shallower development of roots or the deeper roots that have grown earlier will die down gradually, if the amount of water applied continued to be smaller. Therefore, a greater depth of water is applied each time to an actively growing crop so that roots grow deeper.  A smaller depth of water is applied to crops that have shallower root system. The depth of water is also smaller when the crop is young and it roots are shallower. If a higher amount of water is the applied, some water will percolate down beyond the root zone and get wasted. With growth of the crop and its root system, the depth of irrigation is increased.  Depth of irrigation is a function of the water retentive capacity of root zone soil. Soils of heavier texture with a greater amount of capillary pores can retain more water in their pore spaces than lighter soils with more of non capillary pores. Again, soils with crumb structure, higher organic matter content and water-stable aggregates retain more water. The depth of irrigation is necessarily more in soils with a greater water retentive capacity compared to soils with a lower water retentive capacity.  The consumptive use of crop decides the depth of irrigation. Soil water gets depleted continuously after an irrigation is applied till the next irrigation. The amount of soil water depleted from the field capacity level in the effective root zone is to be replenished to bring back the soil water content to the field capacity to continue the normal crop growth. IRRIGATION WATER MANAGEMENT Page 38
  39. 39. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  The apparent specific gravity of soil is determined to estimate the net depth of water to be applied to the crop. The value of bulk density is important since it gives the mass of soil solids involved in water retention. The depth of irrigation increases with increase in bulk density of soil. The apparent specific gravity is the bulk density without considering the units of the bulk density.  Tillage operation, soil texture, structure, organic matter content influence the bulk density of soil. 3.16 IRRIGATION WATER QUALITY CRITERIA The several factors influencing water quality, the generally accepted criteria for judging the quality are: 1. Total salt concentration, as measured by electrical conductivity, 2. Relative proportion of cations as expressed by sodium adsorption ratio (SAR), and 3. Bicarbonate and boron contents. The suitability of irrigation water (SIW) can be expressed as SIW= f (QSPCD) 'where, Q = quality of irrigation water S = soil type P = salt tolerance characteristics of the plant C = climate D = drainage characteristics of the soil  Some other factors like the depth of water table, presence of a hard pan of lime or clay, calcium carbonate content in the soil and potassium and nitrate ions in irrigation water also indirectly affect the suitability of irrigation water.  This is probably the main reason for the several classifications, varying in limits of salinity and other chemical indices. The soil type, major crops of the area, climate and drainage characteristics profoundly influence the suitability of particular water for irrigation,  Highly saline water may be suitable in a well-drained, high textured, fertile soil while a much less saline water may be more harmful for the same crop grown on a heavy textured soil with impeded drainage.  It is the actual salt concentration near the root zone, which determines the suitability of irrigation water rather than the chemical properties of irrigation water alone.  The quality of irrigation water is generally judged by its total salt concentration, relative proportion of cations or sodium adsorption ratio and the contents of bicarbonate and boron, 1. salinity.  Irrespective of the ionic composition, the harmful effects of an irrigation water increase with its total salt concentration as it increases the soil salinity signifiicantly. IRRIGATION WATER MANAGEMENT Page 39
  40. 40. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY 2. 3. 4. 5. 6.  Waters of low salinity (EC < 3 mmhos/cm) are generally composed of higher proportions of calcium, magnesium and bicarbonate ions. Highly saline waters (EC >10 mmhos/cm) consist mostly of sodium and chloride ions.  Moderately saline (EC = 3 to 9 mmhos/cm) waters have Varying ionic compositions. Waters containing high concentrations of sodium, bicarbonate carbonate ions have high pH. Sodium adsorption ratio (SAR).  Any increase in the SAR of irrigation water increases the SAR of the soil soltion. This ultimately increases the exchangeable sodium of the soil. Generally, there is a linear relationship between SAR and exchangeable sodium percentage ESP) of the soil up to moderate ESP levels, and at high ESP levels, the relationship tends to be curvilinear.  In judjing the suitability of irrigation water, both salinity and SAR should be kept in view along with the salinity and sodicity developed during the cropping period.  Salinity increases the osmotic stress while adsorption of sodium is increased both by salinity and SAR Megnecium Calcium ratio.  At the same level of salinity and SAR, but with varying proportions of calcium and magnesium, adsorption of sodium by soils and clay minerals is more at higher Mg : Ca ratios. This is because the bonding energy of magnesium is generally less than that of calcium, allowing more sodium adsorption. It is more important if the Mg : Ca ratio in irrigation waters happens to be more than 4. Bicarbonate.  Irrigation water rich in bicarbonate content tend to precipitate soluble calcium and magnesium in the soil as insoluble carbonates:  This leaves a higher proportion of sodium to divalent cations in the soil solution and increases the SAR. This bicarbonate-induced increase in the SAR of the soil solution ultimately results in higher adsorption of sodium on the soil exchange complex. Boron.  Though boron is an essential nutrient for plant growth, it becomes toxic beyond 2 ppm in irrigation water for most of the field crops. It does not affect the physical and chemical properties of the soil, but at high concentrations it affects the metabolic activities of the plant. Potassium and nitrate.  Potassium and nitrate ions are often present in significant amounts in irrigation waters. Being essential nutrients, they act favourably in reducing the harmful effect of saline water on crop growth by way of providing these nutrients regularly, rather than by reducing soil salinity.  Among these, the effect of nitrate ion has been found more spectacular than potassium because irrigated soils are themselves deficient in nitrogen status and are generally well supplied with potassium. IRRIGATION WATER MANAGEMENT Page 40
  41. 41. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY  Regular supply of nitrate form helps in mitigating the salt-induced nitrogen deficiency and in increasing crop productivity. 3.17 Quality of Water Sources The quality of irrigation water is influenced primarily by its source. 1. River waters. The water quality of Indian a few exceptions, and those too in the hot generally good. An analysis of 13 important river water taken at several places during 1955 to 1966 at monthly intervals has revealed that the water quality of Gandak, Kosi, Brahmaputra, Kaveri, Beas nad Narmada is good. As their EC values fall below 350 micromhos/cm while salinity of those of Chambal, Yamuna, Tapti, Godavari and Krisha ranged from from 450 to 1400 micromhos/cm. These waters are mostly alkaline. The rivers having low EC values show less monthly and yearly variations as compare to those of moderate salinity. The waters of low EC are suitable for irrigation purposes throughout the while while those of moderate salinity can be used cautiously. In South India, waters of Hagari and Tungbhadra rivers are of moderate salinity. In some arid and salt affected areas of Gujarat, waters of some seasonal rivers show a salt concentration up to 7000 ppm. 2. Canal waters. The quality of canal water depends mainly on the river from which it originates Consequently, the quality of all the canal waters in India is good. The development of salt affected lands in the canal irrigated regions mainly due to the rise of water table. Extensive salt affected areas in the canal irrigated regions of Punjab, Rajasthan and UP have been developed because of the rise in the depth of water table and accumulation of salts on the soil surface through capillary conductivity and evaporation. 3. Quality of other surface water sources. Under the minor irrigation projects, some areas are irrigated by the waters of lakes, tanks, reservoirs and drains. In most of the cases, their quality is fairly good, except in case of the drains or the reservoirs, which are fed by small streams flowing over salt-affected areas. Drainage waters of Godavari and drainage channels in UP are of moderate salinity. 4. Quality of ground waters Well water forms an important source of Irrigation. Its quality is highly variable due to climatological and hydro-geological conditions. Paliwal (1972) classified the quality of groundwater resources of India into three main groups: (i) water quality of arid and semi-arid regions having rainfall below 450 mm per annum. (ii) water quality as influenced by hydrological conditions as high water table, consisting of some of the areas (iii)water quality of wells in some areas of the coastal regions of the country as influenced by backward flow of sea water and inundation. IRRIGATION WATER MANAGEMENT Page 41
  42. 42. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Of all the factors, aridity is the most important single factor responsible for a very degree off salinity of well waters. The problem of boron hazard is not serious in the irrigation water sources of India, except in a few cases in the arid areas where its concentration ranges from 2 to 5 ppm. The toxic effect of boron in irrigation water is primarily dependent on its concentration, soil texture, calcium carbonate content, pH and organic matter. It is also influenced by the type and amount of clay in the soil, its adsorption and release characteristics; depth to water table and the drainage condition of the soil. The equilibrium between different forms of boron and their interactions with the above factors and the boron tolerance limit of crops determine the toxic effects of boron on a particular crop species 5. Seasonal variation in water quality. Salinity of ground water is considerably influenced by the climatic conditions. It increases during the summer and is significantly reduced during the monsoon due to dilution by rain water. The degree of variation in salinity and its ionic composition depends upon the depth of water table, infiltration capacity of the soil and the rainfall characteristics of the area concerned. Besides these factors, some recharge of water takes place in low-lying areas due to flooding by rain water. 6. Variation in water quality with depth. The quality of ground-water in many regions shows wide variations with the depth of the aquifers. The depth at which good quality water occurs varies from place to place and even within the same area, at a distance of a few metres. At several locations in the Delhi territory, for instance, the water quality has been found to deteriorate with depth. In such areas, the water quality of dug wells is superior to tubewells tapping deeper aquifers. 3.18 irrigation with saline water 3.19 improving quality of water 3.19.1 IRRIGATION WITH POOR QUALITY WATER  Where there is no alternate source of good quality irrigation water in an area, it is inevitable to use the available water of poor quality. However yield potential of such areas can be increased by adopting proper management practices such as:  Improvement of sodium and bicarbonate rich waters by gypsum application,  choice of salt tolerant crop and their varieties,  optimum fertilization and manuring.  proper irrigation management, and  breaking of any impervious layers by deep ploughing. 3.19.2 improvement of Water Quality The harmful effect of irrigation water can be minimized to some extent by modifying its ionic composition by adding such chemicals, which tend to precipitate the harmful IRRIGATION WATER MANAGEMENT Page 42
  43. 43. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY constituents such as bicarbonate and carbonates in the form of less soluble salts or tend to create a favourable cationic (Ca : Mg : Na) ratio. 1. For all practical purposes, gypsum should be powdered up to 0.5 mm size or passed through a 30 mesh sieve. The gypsum requirement of water should be calculated, depending upon the relative concentration of sodium, magnesium and calcium ions in irrigation water and the solubility of gypsum. Mixing of gypsum directly in irrigation water poses some problems. If it is placed in bags or containers between the entry and outlet of water, it does not dissolve uniformly with increase of time, and most of it is deposited at the place of entry of water in the field and forms bigger granules from powdered gypsum on coming in contact with water, however, some of the above limitations can be overcome by applying its saturated solution by a mechanical device in the flowing water at a constant rate. 2. Choice of salt-tolerant crops. Some crop and their varieties are more salt-tolerant than others. Hence salt-tolerant crops are to be grown in salt affected areas till the soils are improved by vegetation or adopting the reclamation procedures. 3. Use of fertilizers. Generally, saline and alkali soils or soils irrigated with poor quality waters are low in their fertility status, especially with reference to nitrogen or sometimes to phosphorus. Better crops can be grown by raising their fertility status. Nitrogen response to crops is better when it is applied to soil along with manures. It has been observed that for wheat, barley, bajra and maize, the usual does of fertilizers, as applied on normal soils can be applied up to an EC value of 6.5 millimhos/cm and an ESP of about 30. However, excessive fertilization on a highly saline alkali soil is of no value. 4. Soil management practices. In order to adopt irrigation with poor quality water on a long-term basis, it is important to have a detailed analysis of representative soil profiles for their physical, chemical and morphological characteristics Soil analysis should include its structure, texture, pK, lime content, location and amount of gypsum (if present in the profile), total soluble salts, their ionic composition and exchangeable cations. Information on water transmission properties of the soil and depth of water table are to be obtained. Data on the climatic parameters as amount of rainfall, its intensity, distribution, and evaporation are to be obtained, as they control the periodic as well as annual salt build-up in an irrigated area. Accumulation of excessive amount of salt and development of high sodicity are the main limitations in the safe utilization of poor quality irrigation water. These depend upon the nature and amount of clay minerals in the soil and the quality of irrigation water. Success of irrigation with saline water lies in the degree of accuracy in the predicted values of soil salinity and sodicity expected to be developed during the cropping period. Saline waters can more safely be used in coarse than in fine textured soils. 5. Lime requirement for the reclamation of acid soils.  The beneficial effect lime application in soils when soil acidity is a constraint to agricultural production has been discussed earlier. As gypsum requirement for the IRRIGATION WATER MANAGEMENT Page 43
  44. 44. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY   6. 7. reclamation of alkali soil is determined on the basis of desired replacement of sodium from the soil complex, the lime requirement is determined based on the requirement of raising soil pH from less than 6 to 8 desirable higher value. A field infested with the problem of salinity should be levelled carefully. This will help in uniform spread of water and facilitate its downward movement, otherwise at high spots (if salts are not leached) it will lead to salt accumulation and will cause heterogeneity in salt concentration. The success of gypsum or flushing of salts lies in the drainability of the soil. Hence, if the improvement of internal drainage by tile drains is not economically feasible. Irrigation management. While irrigating with poor quality waters, accumulation of salts increases with fineness of soil texture. it is essential to adopt irrigation practices such that the salinity at the root zone is kept to the minimum. For this purpose the quantity of water and the frequency of irrigation are planned to meet the leaching requirement of soil and the consumptive use of the crop grown. In case of shallow salt concentration, A heavy pre-sowing irrigation to leach this surface salts will improve germination and early growth and is sometimes an essential practice. It is made far enough in advance of the desired planting date to allow for cultivation to remove weeds and prepare the seedbed. Seed placement. Obtaining a satisfactory stand of furrow-irrigated crops on saline soils when using poor quality water is important. If salinity is a problem, planting seeds in the centre of a single-row raised bed will place the seed exactly in the area where salts concentrate. A double-row raised planting bed by comparison offers an advantage. The two rows are placed so that each is near a shoulder of the raised bed, thus placing the seed away from the area of greatest salt accumulation. By this method higher soil and water salinities can be tolerated than with the single-row plantings because the water moves the salts through the seed area to the centre of the ridge. Alternate furrow irrigation is often advantageous. If the beds are wetted from both sides, the salts accumulate in the top and the centre of the bed, but if alternate furrows are irrigated, the salt can be moved beyond the single seed row , thus reducing the extent of salt accumulation. Off-centre, single-row planting on the shoulder of the bed closest to the watered furrow aids germination under salty soil conditions. 3.20 Leaching requirements The depth of irrigation water per unit depth of soil, required to produce any specified increase in soil salinity for any given electrical conductivity of irrigation water can be calculated from the following equation: = depth irrigation water = depth oi soil IRRIGATION WATER MANAGEMENT Page 44
  45. 45. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY = density of soil (bulk density) = density of irrigation water SP= saturation percentage of the soil = increase in electrical conductivity of saturation extract of the soil ECW = electrical conductivity of irrigation water. Under high water table conditions, the increase in soil salinity by the evaporation of ground water can be determined by the following formula: The fraction of irrigation water that must be leached through the root zone to keep the salinity of the soil below a specific limit is termed as leaching requirement (LR). Mathematically, it can be expressed as in which, LR = leaching requirement, expressed as a ratio or as per cent ECt = electrical conductivity of irrigation water, mmhos/cm ECd = electrical conductivity of drainage water, mmhos/cm Information on the consumptive use of water by the crop is necessary if the leaching requirement concept is to be used for determining either the depth of irrigation water that must be applied or the minimum depth of water to be drained, in order to keep the soil salinity from exceeding a pecified value. The depth of irrigation water DI, is related to consumptive use Dc and the equivalent depth of drainage water Dd by the equation Dl = Dc + Dd . Using above equation eliminate Dd from LR equation Expressing the leaching requirement (LR) in above equation in terms of the EC ratio of irrigation and drainage waters, as given in LR equation, provides the following relationship: In above Equation gives the depth of irrigation water, which is required to satisfy both the evapotranspiration and leaching requirements of the soil. Of the above factors, ECj is known from the chemical analysis of the irrigation water and ECd is taken on the basis of permissible salt tolerance limits of the crop. Generally, the salt tolerance limits are expressed in terms of EC of the saturation IRRIGATION WATER MANAGEMENT Page 45
  46. 46. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY extract of the soil solution (ECe). In most cases, ECd is assumed to be approximately twice as high as the corresponding ECe. IRRIGATION WATER MANAGEMENT Page 46
  48. 48. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY 3.21 Plant response to Saline and Alkali Soils(this is less important)  Excessive salinity usually causes poor stands of crop, stunted growth and reduced yield. It may also cause leaf burns in some crops and deep blue-green colour in other.  In some cases, symptoms of excessive salts are similar to those of drought or low fertility status. Soil salinity causes a decrease in the size and vigour of the plant. The germination of seeds is delayed and retarded but the cropping period is slightly reduced with the increase in salinity.  However, its effects on the yield and quality of the crop are variable. In general, grain yield is affected more than the height of the plant. 1. Effect of Salts on Plant Growth It is effected by:  increasing the osmotic pressure in the soil solution,  accumulating certain ions in toxic concentration in the plant tissue, and  by altering the plants mineral nutritional characteristics. a. Osmotic pressure and plant growth. It has been observed that there is an inverse relationship between the osmotic pressure of the soil solution and plant growth. The reduced crop growth is due to reduced water availability to plants. Thus, as the osmotic pressure increases with the concentration of salts, it is presumed that salinity inhibits plant growth primarily by reducing the availability of water. IRRIGATION WATER MANAGEMENT Page 48
  49. 49. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY On the other hand, recent evidence indicates that the water absorption capacity is relatively unaffected by salinity and plants develop their own internal pressure against salt. Hence, this theory suggests that salt tolerance of the plant may be defined as the degree to which osmotic adjustment can be made without any sacrifice in plant growth. In addition to the osmotic effect, the force by which water is held by soil particles also affects plant growth. This force, called soil moisture tension, increases as the moisture content of the soil decreases. The osmotic pressure of the soil solution and soil moisture tension are additive in their effect on plant growth. Their combined effect is called total soil moisture stress. It has been observed that plant growth is closely related to the soil moisture stress, regardless whether this stress arises primarily from salinity or soil moisture tension or both. Through controlled leaching, the osmotic pressure of the soil solution should be maintained at the lowest feasible level. By careful management of the irrigation system, the soil moisture tension in the root zone is maintained in a range that will provide the greatest net return of the crop grown. b. Specific ion effect. Certain ions exert specific effects which depress crop growth and yield independent of the salt concentration. These specific ion effects may be toxic or may cause nutritional disorder in the plant, influencing its physiological and metabolic activities. The toxic effect may be due to the presence of an ion in excessive amount in the soil solution, the higher uptake of which damages the plant growth. Nutritional disorders in plants occur when the accumulation of one or more nutrients in excessive amounts inhibits the uptake of other essential nutrients. When the amount of these essential nutrients is below their critical limit which is essential for normal physiological functions of the plant, the growth of the plant is inhibited. Plants vary widely in their nutrient requirement and their ability to absorb specific nutrients. Hence, the effects of salts on nutrition differ markedly from species to species and even between varieties of the same crop. c. Salt-tolerance of crops. Salt-tolerance means the ability of a plant to tolerate salt concentrations in the root zone. Knowledge of salt tolerance is important in selecting a particular crop and determining leaching requirement. d. Boron tolerance of crops. Boron, though essential plant growth, becomes toxic beyond a concentration m few ppm. It does not seem to affect the germination seeds up to 20 ppm or even more, but the growth rate height of seedlings are significantly reduced beyond 1 to 2 ppm. IRRIGATION WATER MANAGEMENT Page 49
  50. 50. IRRIGATION WATER MANAGEMENT DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY Relatively, legumes are salt-sensitive but tolerate more boron than cereals. Amongst field wheat and barley can tolerate from 1 to 2 ppm of boron in irrigation water. Boron is translocated in the leaves and accumulates in the tip and margin. Toxicity symptoms such as yellowing and tip burn is usually visible. If boron toxicity is indicated in the plant, its source should be found. If irrigation water has excessive boron, it should be mixed with good quality water and if soil is the cause, the boron should be reduced by leaching, though its leaching is more difficult than of salts. 3.22 Leaching methods.  Leaching can be done by ponding the required depth of water on the soil surface by means dikes or ridges, thus establishing a downward water movement through the soil. Leaching can be done at any time of the year, depending on the availability of water.  It is advantageous to practice leaching during the monsoon season. The quality of water applied in leaching should be in conformity with the tolerance limit of the crops to be grown. The cost of leaching will be high if salts are leached beyond a threshold value, as no additional benefits will accrue, because yields stabilize at the threshold value.  Information on crop tolerance of salt will help in selecting crops. From the initial salinity and the crop tolerance level, fractions of salts to be leached could be derived by the relation:  In case any crop is known to be sensitive at the germination stage, the threshold value of this particular stage should be used in deciding the fraction of the salts to be leached.  A thumb rule commonly used is that 1 cm of water should be applied for each cm of the soil to be reclaimed. However, the quantity of water required may vary with soil texture and fraction of the salts to be leached.  For medium textured soils (sandy loam), 0.5 cm of water for each cm of soil is usually sufficient to remove 80 per cent of the salts present in the soil profile. continuous and intermediate leaching.  In continuous leaching, water is ponded continuously over the soil surfc; e to allow water to pass through the root zone. In the case of intermittent leaching, water is ponded at intervals.  A net saving of 30-35 per cent can be made through. intermittent, compared to continuous leaching, in the amount of water needed to leach the salts to the same degree. However, intermittent leaching may not be advantageous for highly degraded alkali soils with restricted infiltration rates.  Rectangular check basins and level borders are used when the land is flat. Contour checks can be used when the land slope is considerable. IRRIGATION WATER MANAGEMENT Page 50