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1.0 INTRODUCTION TO IRRIGATION AND DRAINAGE
1.1 Irrigation defined
Irrigation is any process other than natural precipitat...
• It can ensure all year production hence food security.
• Helps in the control of flooding and hence erosion especially w...
1.5 Drainage of Agricultural Lands
1.5.1 Defintion
Agricultural drainage is the removal and disposal of excess water from ...
Thus the two fold function of drainage is;
(i) To lower the ground water levels below the maximum depth of root zone (this...
1.6 Irrigation extent, potential and development in Kenya.
1.6.1 Extent of Irrigation
From a global perspective, irrigated...
In most of the countries in developing world, the growth of agricultural production has not kept
pace with the population ...
Kenya has 582,000km2 of land mass out of which 16% (93,000km2
) is of medium to high
potential. Agriculture is the mainsta...
enterprises include; Horticulture i.e vegetables, fruits, cut flowers etc, Paddy rice, Coffee,
Food crops i.e maize, beans...
Small holder refers to schemes where the irrigation infrastructure is owned, operated maintained
and managed by the farmer...
Small holder refers to schemes where the irrigation infrastructure is owned, operated maintained
and managed by the farmer...
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Introduction to irrigation and drainage

Irrigation is any process other than natural precipitation, which supplies water artificially to the soil to make up the deficiency of moisture under natural conditions for the profitable growth of crops, which otherwise would not be assured.

The irrigation process involves investigation, planning, design, construction, maintenance and operation of structures and channels for the proper conveyance of water from the source to the point of application.

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Introduction to irrigation and drainage

  1. 1. 1.0 INTRODUCTION TO IRRIGATION AND DRAINAGE 1.1 Irrigation defined Irrigation is any process other than natural precipitation, which supplies water artificially to the soil to make up the deficiency of moisture under natural conditions for the profitable growth of crops, which otherwise would not be assured. The irrigation process involves investigation, planning, design, construction, maintenance and operation of structures and channels for the proper conveyance of water from the source to the point of application. 1.2 Why Irrigation? Factors that necessitate irrigation include; (i) Insufficient rainfall (ii) Non-uniform distribution of rainfall. Total rainfall during a year may be adequate to raise a crop, however it may not be evenly distributed over the crop growth period (iii) Control of water supply. Irrigation enables water to be applied to the crops as and when required thereby increasing the yield per unit area, (iv) Growing of cash crops. The growth of cash crops e.g. flowers, sugarcane, rice, etc. may be preferred as they bring better returns. Such crops may require abundant supply of water and frequent watering to maturity, which cannot be met from natural rainfall during the crop growth period. This makes irrigation essential. 1.3 Importance of Irrigation Includes among others the following: • It supplements inadequate and unreliable rainfall particularly in the ASAL areas,
  2. 2. • It can ensure all year production hence food security. • Helps in the control of flooding and hence erosion especially where dams exist. • Creates employment • Could be used to reclaim more potentially productive land and hence create more settlement opportunities. • Specialized production targeting high value crops is made possible • Could be used to control pests and diseases by disrupting their life cycles. • Could be used for strategic production as dictated by market forces. 1.4 Advantages and Disadvantages of Irrigation. Advantages The advantages of irrigated agriculture include; - It increases the area of land brought under cultivation. - It improves crop yield over rainfed agriculture three or four fold. - Allows for the greater cropping intensity. - It results in improved economic security for the farmer. - Reduces risks associated with drought hence farmers can easily use high yield seed varieties, increase the use of fertilizers, pesticides, pesticides and mechanization. They can also control the timing of delivery of their products to the market and also the labour demands. - It allows for the introduction of more valuable crops to farmers. Disadvantages The disadvantages of irrigated agriculture include - Over irrigation leads to water logging of the soil and this reduces yields. - Increased possibility of ground water pollution as a result of increased use of chemicals and fertilizers. - Due to the presence of water in open channels, there is an increased possibility of having several water borne diseases e.g malaria, bilharzias, etc. - It is complex and expensive undertaking; most of the time requiring donor funding yet most of them are not self sustaining in the long run.
  3. 3. 1.5 Drainage of Agricultural Lands 1.5.1 Defintion Agricultural drainage is the removal and disposal of excess water from agricultural lands. It is necessary to provide an adequate drainage system for every irrigated area. The crop requirements are that the root-zone does not remain in the saturated state. Poorly drained soils deprive crops of development by, among others, - lowering soil temperature as the water evaporates. - Stopping adequate air circulation and preventing bacterial activities. - Encouraging certain plant diseases - Limiting rooting penetration due to high water table 1.5.2 Sources of excess water The sources of excess water that necessitate drainage include; (a) precipitation (b) Irrigation water (c) Overland flow or underground seepage from adjacent areas (d) Artesian flow from deep aquifers (e) Flood water applied for such special purposes as leaching salts from the soil or for temperature control. 1.5.3 Importance of drainage It is necessary to provide an adequate drainage system for every irrigated area. The important functions of the drainage system are: 1. Removal of excess ground water and salts from the soil profile in the crops root zone: Excessive irrigation of an area over an extended period tends to raise the ground water level. When it rises to within the capillary lift/rise of the soil, soil moisture reaches the surface where it evaporates and in the process salts present either in the irrigation water or dissolved in the soil, get concentrated on the land surface, leading to salinity with harmful effects on plant growth.
  4. 4. Thus the two fold function of drainage is; (i) To lower the ground water levels below the maximum depth of root zone (this helps better aeration, plant nutrition and greater root development. (ii) To provide an outlet for the accumulated salts which are usually washed down from the upper soil layers and leached out thro' the drainage water. This will take place during rainy season if there is adequate precipitation. Otherwise deliberate excess application of irrigation water does leach the soil satisfactorily if the operation is not well managed. 2. The second function of the system is to remove excess surface water due to irrigation, likely to occur in more than one way: (a) In flat fields, water from either border or sprinkler irrigation collects in the surface depressions leading to ponding. This not only lowers uniformity of irrigation but also damages plants in the ponded areas. (b) Excess water accumulates in the fields under border or sprinkler irrigation, at the lower ends of the runs; (c) Water may accumulate and flow as runoff from sloping fields under sprinkler irrigation when the rate of application exceeds the rate of infiltration of the soil; leading to erosion on the slopes and harmful ponding on the lowlands. (d) Surplus irrigation water may be due to unforeseen factors as break in supply ditch or bursting of a pressure pipe. 3. The third function of the drainage system is to remove excess surface water resulting from precipitation. This is likely to occur in the regions with a distinct dryland wet/rainy season. Especially, a field laid out in level irrigation borders/ basin of such a region, excess precipitation causes logging of fields and hence the need for drainage.
  5. 5. 1.6 Irrigation extent, potential and development in Kenya. 1.6.1 Extent of Irrigation From a global perspective, irrigated agriculture accounts for 40% of the food production which comes mainly from approximately 260million ha of irrigated lands two thirds of which are under formal irrigation schemes. Out of the 11.6 billion hectares of land in 103 countries, only 8.67% is cultivated land. The irrigated lands account for only 20% of the cultivated land world over. The proportion of cultivated land under irrigation varies from country to country with some countries reporting 100% of their cultivated land under irrigation, while others have practically no irrigation. Table 1.1 shows the irrigated areas in some selected countries compared to areas under irrigation. Table 1.1: Proportions of arable land under irrigation in some selected developing countries. Country Total area (Million ha) Cultivated area (Million ha) Area irrigated (Million ha) Proportion of cultivated area to Total area. Proportion of irrigated area to total area Proportion of irrigated area to cultivated area. India 327.6 137.9 43.1 42.1% 13.2% 31.3% Indonesia 149.2 14.0 7.6. 9.4% 5.09% 54.3% Thailand 51.4 7.3 4.3 14.2% 8.4% 58.9% Japan 37.0 6.0 2.8 16.2% 7.6% 46.7% Philippines 30 8.3 1.6 27.7% 5.3% 19.3% Kenya 58.2 1.7 0.11 2.9% 0.19% 6.5%
  6. 6. In most of the countries in developing world, the growth of agricultural production has not kept pace with the population growth. Means must therefore be sought to increase world agricultural production. In the arid and semi arid zones of the world, one of the principal means to achieve this is through irrigation. The extent of arid and semi arid lands in the world is illustrated in Table 1.2 in which agricultural production limited by drought is shown. Table 1.2 Areas of land limited by drought. Region % land limited by drought North America 20 Central America 32 South America 17 Europe 8 Africa 44 South Asia 43 North East Asia 17 South East Asia 2 Australia 55 World 28 1.6.2 Overview of Kenya's agro-ecological potential
  7. 7. Kenya has 582,000km2 of land mass out of which 16% (93,000km2 ) is of medium to high potential. Agriculture is the mainstay of Kenya's economy. The sector contributes over 50% of the country's export earning's and employs about 80% of the population. The sector accounts for about 26% of the country's GDP. Note: The countries population is currently estimated to be increasing at 2.6% annually thereby exerting pressure on the available water and land resources causing migration of people from high and medium potential agricultural land to low potential arid and semi-arid lands putting pressure an already fragile ecosystem. Irrigation development is an option that will enable intensification of land use in high and medium potential areas and integrated use of ASAL's. This will promote future growth and development of the agricultural sector. 1.6.3 Irrigation potential and achievements in Kenya. The irrigation potential in Kenya is 539,000 ha (based on available surface water) and a drainage potential of 600,000 ha. Out of the total potential, 105,000 (19%) and 30,000 ha (5%) have been developed for irrigation and drainage respectively. The potential and development by basins is indicated below in Table 1.3. Table 1.3 Irrigation potential and development in Kenya based on the major basins Basin Potential ( ha ) Development (ha) Tana 205,000 68,700 Athi 40,000 11,000 Lake basin 200,000 10,700 Kerio Valley 64,000 5,400 Ewaso Ngiro 30,000 10,000 TOTAL 539,000 105,800 Irrigation is practiced over a wide range of agro-ecological zones from supplementary irrigation in the high and medium potential areas during the rainy seasons to near total irrigation in the ASALs and during the dry seasons in the high and medium potential areas. Major irrigated
  8. 8. enterprises include; Horticulture i.e vegetables, fruits, cut flowers etc, Paddy rice, Coffee, Food crops i.e maize, beans, sorghum etc. The rate of irrigation development has been very low despite rising need for food and high poverty level. Table 1.4 shows irrigated areas over the years. Table 1.4 Progress of irrigated areas over the years. Year 1985 1998 2005 Irrigated area 52,000 87,350 105,800 There is need for the country to increase investment to irrigation development to meet the food requirements and improve living standards. 1.6.4 Categories of irrigation schemes in Kenya and level of development. The categories include; Private schemes This category comprises schemes that are constructed, owned and managed by individual farmers or companies. They include large estates that are run as commercial enterprises with bias on high value crops mainly for the export market. The investment resources are privately sourced and the schemes have their own salaried technical expertise for operation, maintenance, and management. Main enterprises include pineapples, cut flowers, coffee etc. Public schemes These are irrigation schemes centrally managed by public agencies like the National Irrigation Board and Regional Development authorities (RDA's). In the case if NIB, the investment capital for the schemes is provided for by the government who gives the farmers tenancy rights for use of irrigation facilities in crop production activities. Smallholder group based schemes.
  9. 9. Small holder refers to schemes where the irrigation infrastructure is owned, operated maintained and managed by the farmers either directly or through contractual arrangements with the water undertakers. The small holder irrigation schemes are implemented through a variety of ways that involve partnerships between the communities, and other development agencies such as Government of Kenya, NGO's and international agencies. Table 1.5 gives the contribution to irrigated areas by the three categories. Table 1.5 Comparative Irrigation Development Status Sector Developed area ( ha ) 1985 1998 2005 Smallholder schemes 17,500 34,650 47,000 National Schemes 11,500 12,000 16,000 Private schemes 23,000 40,700 42,800 TOTAL 52,000 87,350 105,800
  10. 10. Small holder refers to schemes where the irrigation infrastructure is owned, operated maintained and managed by the farmers either directly or through contractual arrangements with the water undertakers. The small holder irrigation schemes are implemented through a variety of ways that involve partnerships between the communities, and other development agencies such as Government of Kenya, NGO's and international agencies. Table 1.5 gives the contribution to irrigated areas by the three categories. Table 1.5 Comparative Irrigation Development Status Sector Developed area ( ha ) 1985 1998 2005 Smallholder schemes 17,500 34,650 47,000 National Schemes 11,500 12,000 16,000 Private schemes 23,000 40,700 42,800 TOTAL 52,000 87,350 105,800

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