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Chapter 5 methods of irrigation Dr. Thomas Abraham_19-3-14
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Chapter 5 methods of irrigation Dr. Thomas Abraham_19-3-14



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  • 1. Irrigation Methods are mainly classified into : 1. Surface Irrigation or Gravity Irrigation 2. Subsurface Irrigation or Sub-irrigation 3. Sprinkler or overhead irrigation 4. Drip or Trickle irrigation ◦
  • 2. 1) SURFACE IRRIGATION  Irrigation water flows across the field to the point of infiltration  Primarily used for field crops and orchards  Water is applied to the soil surface and the water flows by gravity either through furrows, strips or basins.  Water is applied from a channel located at the upper reach of the field.  Loss of water by conveyance and deep percolation is high and the efficiency of irrigation is only 40-50% at field level in surface method of irrigation.  Properly constructed water distribution systems to give sufficient control of water to the fields  And effective land preparation to permit uniform distribution of water over the field are very important.
  • 3.  Water is applied to the field in either the controlled or uncontrolled manner.  Controlled: Water is applied from the head ditch and guided by corrugations, furrows, borders, or ridges.  Uncontrolled: Wild flooding.  Surface irrigation is entirely practised where water is abundant.  Low initial cost of development is later offset by high labour cost of applying water.  Deep percolation, runoff and drainage problems
  • 4.  1. Furrow irrigation  2. Border irrigation  3. Basin irrigation
  • 5.  Furrow irrigation - in which the water poured on the field is directed to flow through narrow channels dug between the rows of crops, instead of distributing the water throughout the whole field evenly.  The furrows must all have equal dimensions, in order to guarantee that the water is distributed evenly.  Like flood irrigation, furrow irrigation is rather cheap in areas where water is inexpensive.
  • 6.  In furrow irrigation, only a part of the land surface (the furrow) is wetted thus minimizing evaporation loss.  Irrigation can be by corrugation using small irrigation streams.  Furrow irrigation is adapted for irrigating on various slopes except on steep ones because of erosion and bank overflow.
  • 7.  There are different ways of applying water to the furrow.  As shown in Fig 3.1, siphons are used to divert water from the head ditch to the furrows.  There can also be direct gravity flow whereby water is delivered from the head ditch to the furrows by cutting the ridge or levee separating the head ditch and the furrows.  Gated pipes can also be used. Large portable pipe(up to 450 mm) with gate openings spaced to deliver water to the furrows are used.  Water is pumped from the water source in closed conduits.  The openings of the gated pipe can be regulated to control the discharge rate into the furrows.
  • 8.  The Major Design Considerations in Surface Irrigation Include:  Storing the Readily Available Moisture in the Root Zone, if Possible;  Obtaining As Uniform Water Application As Possible;  Minimizing Soil Erosion by Applying Non-erosive Streams;  Minimizing Runoff at the End of the Furrow by Using a Re-use System or a Cut -Back Stream;  Minimizing Labour Requirements by Having Good Land Preparation,  Facilitating Use of Machinery for Land Preparation, Cultivation, Furrowing, Harvesting Etc.
  • 9.  The Specific Design Parameters of Furrow Irrigation Are Aimed at Achieving the Above Objectives and Include:  a) Shape and Spacing of Furrows: Heights of ridges vary between 15 cm and 40 cm and the distance between the ridges should be based on the optimum crop spacing modified, if necessary to obtain adequate lateral wetting, and to accommodate the track of mechanical equipment.  The range of spacing commonly used is from 0.3 to 1.8 m with 1.0 m as the average.
  • 10.  d) Field Slope: To reduce costs of land grading, longitudinal and cross slopes should be adapted to the natural topography.  Small cross slopes can be tolerated.  To reduce erosion problems during rainfall, furrows (which channel the runoff) should have a limited slope (see Table 3.1).
  • 11. Soil Type Maximum slopes* Sand 0.25 Sandy loam 0.40 Fine sandy loam 0.50 Clay 2.50 Loam 6.25 Source: Withers & Vipond (1974)  *A minimum slope of about 0.05 % is required to ensure surface drainage.
  • 12.  In this, Parallel ridges are made to guide a sheet of flowing water when the water moves down the slope.  The field is divided into several long parallel strips called borders that are separated by low ridges.  Field should be even surface over which the water can flow down the slope with a nearly uniform depth.  Every strip is independently irrigated by turning a stream of water at the upper end.  Then water spreads and flows down the strip in a thin sheet.  Water moves towards the lower end without erosion covering the entire width of the border.  Sufficient moisture is provided to the soil to entire length of the border.  Border method is suitable for most of the soils, while it is best suited for soils having moderately low to high infiltration rates.  However, it is not suitable for course sandy and clay textured soils.
  • 13. Border Irrigation System  In a border irrigation, controlled surface flooding is practised whereby the field is divided up into strips by parallel ridges or dykes and each strip is irrigated separately by introducing water upstream and it progressively covers the entire strip.  Border irrigation is suited for crops that can withstand flooding for a short time e.g. wheat.  It can be used for all crops provided that the system is designated to provide the needed water control for irrigation of crops.  It is suited to soils between extremely high and very low infiltration rates.
  • 14. Border Irrigation System
  • 15. Border Irrigation
  • 16. Border Irrigation Contd.  In border irrigation, water is applied slowly.  The root zone is applied water gradually down the field.  At a time, the application flow is cut-off to reduce water loses.  Ideally, there is no runoff and deep percolation.  The problem is that the time to cut off the inflow is difficult to determine.
  • 17.   Basin method of irrigation is adopted mainly in orchards.  Usually round basins are made for small trees and square basin for large trees.  These basins allow more water to be impounded as the root zones of orchard plants are usually very deep.  Each basin is flooded and water is allowed to infiltrate into the soil.  Based on type of crop and soil, nearly 5-10 cm depth of water may be needed for every irrigation.  The advantage of basin method is that unskilled labour can be used as there is no risk of erosion.  Disadvantages : there is difficulty in using modern machinery and it is also labour intensive.
  • 18.  Basin irrigation is suitable for many field crops.  Rice grows best when its roots are submerged in water and so basin irrigation is the best method to use for this crop.  Other crops which are suited to basin irrigation include:  Pastures, e.g. alfalfa, clover;  Citrus, banana;  Crops which are broadcast, such as cereals, and  To some extent row crops such as tobacco.
  • 19. Basin Irrigation Diagram I rrigation time.
  • 20. Size of Basins  The size of basin is related to stream size and soil type(See Table below).  Table : Suggested basin areas for different soil types and rates of water flow  Flow rate Soil Type  Sand Sandy loam Clay loam Clay  l/s m3 /hr .................Hectares................................  30 108 0.02 0.06 0.12 0.20  60 216 0.04 0.12 0.24 0.40  90 324 0.06 0.18 0.36 0.60  120 432 0.08 0.24 0.48 0.80  150 540 0.10 0.30 0.60 1.00  180 648 0.12 0.36 0.72 1.20  210 756 0.14 0.42 0.84 1.40  240 864 0.16 0.48 0.96 1.60  300 1080 0.20 0.60 1.20 2.00  Note: The size of basin for clays is 10 times that of sand as the infiltration rate for clay is low leading to higher irrigation time. The size of basin also increases as the flow rate increases. The table is only a guide and practical values from an area should be relied upon. There is the need for field evaluation.
  • 21.  Most common among surface irrigation  Suitable for close growing crops like groundnut, wheat, finger millet, pearl millet, paragrass etc.  In this method field is divided into small plots surrounded by bunds on all four sides.  Water from head channel is supplied into the field channel one after the other.  Each field channel supplies water to two rows of check basins and water is applied to one basin after other.
  • 22.  In this, field is laid out into long, narrow, strips, bordering with small bunds.  Most common size of strips are 30-50 m length and 3-5 m width.  Borders are laid out along the general slope.  Water from the channel is allowed into each strip at a time.  This method is suitable for close growing crops and medium to heavy textured soils.  Not suitable for sandy soils.
  • 23.  It should be applied only to flat lands that do not concave or slope downhill so that the water can evenly flow to all parts of the field.  Yet even so, about 50% of the water is wasted and does not get used by the crops.  Some of this wasted water accumulates at the edges of a field and is called run-off.  In order to conserve some of this water, growers can trap the run-off in ponds and reuse it during the next round of flood irrigation.
  • 24.  In flood irrigation, a large amount of water is brought to the field and flows on the ground among the crops.  In regions where water is abundant, flood irrigation is the cheapest method  This low tech irrigation method is commonly used by societies in developing countries.
  • 25.  However a large part of the wasted water can not be reused due to massive loss via evaporation and transpiration.  One of the advantages of flood irrigation is its ability to flush salts out of the soil, which is important for many saline intolerant crops.  However, the flooding causes an anaerobic environment around the crop which can increase microbial conversion of nitrogen from the soil to atmospheric nitrogen, or denitrification, thus creating low nitrogen soil.  Surge flooding is an attempt at a more efficient version of conventional flood irrigation in which water is released onto a field at scheduled times, thus reducing excess run-off.
  • 26.  - Irrigation to crops by applying water from beneath the soil surface either by constructing trenches or installing underground perforated pipe lines.  In this system, water is discharged into trenches.  And allowed to stand during the whole period of irrigation for lateral and upward movement of water by capillarity to wet the soil between the trenches.
  • 27.  Conditions that favor subsurface irrigation  An impervious subsoil at a depth of 2 m or more.  A very permeable subsoil of reasonably uniform texture permitting good lateral and upward movement of water.  Permeable loam or sandy loam surface soil.  Uniform topographic conditions and moderate slope.  Existence of high water table.  Irrigation water is scarce and costly.  Soils should be free of any salinity problem.  It must be ensured that no water is lost by deep percolation.  Subsurface irrigation is made by constructing a series of ditches or trenches 60 to 100 cm deep.  Width of the trenches is about 30 cm and vertical.  Spacing between the trenches varies between 15 to 30 m depending on soil types and lateral movement of water in soils.
  • 28.  Various types of crops, particularly with shallow root systems are well adapted to subsurface irrigation system.  Wheat, potato, beet, peas, fodder crops etc.  Advantages  Maintenance of soil water at favorable tension  Loss of water by evaporation is held at minimum  Can be used for soils with low water holding capacity and high infiltration rate where surface irrigation methods cannot be adopted and sprinkler irrigation is expensive.
  • 29.  Presence of high water table.  Poor quality irrigation water cannot be used-good quality water must be available.  Chances of saline and alkali conditions being developed by upward movement of salts with water.  Soils should have a good hydraulic conductivity for upward movement of water.
  • 30.  Sprinkler irrigation is a method of applying irrigation water which is similar to natural rainfall.  Water is distributed through a system of pipes usually by pumping.  Water under pressure is carried and sprayed into the air above the crop through a system of:  Overhead perforated pipes, nozzle lines, or through nozzles fitted to riser pipes attached to a system of pipes laid on the ground.  Nozzles of fixed type or rotating under the pressure of water are set at suitable intervals in the distribution pipes.  Sprayed water wets both the crop and the soil and, hence, has a refreshing effect.  Water is applied at a rate less than the intake rate of soil so that there is no runoff.  Measured quantity of water is applied to meet the soil water depletion.
  • 31.  Sprinkler irrigation is suited for most row, field and tree crops and water can be sprayed over or under the crop canopy.  Large sprinklers are not recommended for irrigation of delicate crops such as lettuce because the large water drops may damage the crop.  Suitable slopes  Sprinkler irrigation is adaptable to any farmable slope, whether uniform or undulating.  Lateral pipes supplying water to the sprinklers should always be laid out along land contour.  This will minimize the pressure changes at the sprinklers and provide a uniform irrigation.
  • 32.  Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils.  Application rate from the sprinklers (in mm/hour) is always chosen to be less than the basic infiltration rate of the soil - so that surface ponding and runoff can be avoided.  Sprinklers are not suitable for soils which easily form a crust.
  • 33.  A typical sprinkler irrigation system consists of the following components:  Pump unit  Mainline  Laterals  Sprinklers   Suitable irrigation water  A good clean supply of water, free of suspended sediments, to avoid problems of sprinkler nozzle blockage and spoiling the crop by coating it with sediment.
  • 34. Components of Sprinkler Irrigation
  • 35. 52 Sprinkler irrigation (cont..) • Uniform application by “artificial rain” • Good application efficiencies (0.7 – 0.8) – dependent on wind, temperature, humidity • Fairly terrain independent (but design must take terrain into account) • Can have a low labour content However, • High(ish) investment cost • High maintenance cost due to pumping • Can be complex to run
  • 36. 53 Sprinkler irrigation: Criteria • Must permit cost recovery within one to two years (and double investment in a short time) • Must be suitable for use on small and irregular shaped plots • Must require only simple maintenance and tools • Have a low risk of component failure • Be simple to operate • Be durable and reliable – able to withstand rough and frequent handling without serious damage
  • 37. 54 Sprinkler irrigation: System layout
  • 38. 55 Sprinkler irrigation
  • 39. 56 Sprinkler irrigation: Drag hose system
  • 40. 57 Sprinkler
  • 41. 58 Sprinkler irrigation: Spray pattern
  • 42. 59 Sprinkler irrigation: Spray pattern
  • 43. 60 Sprinkler Spray pattern: Variation in pressure
  • 44. 61 Sprinkler irrigation: Variation in pressure
  • 45. 62 Sprinkler irrigation: Hand move laterals
  • 46. 63 Sprinkler irrigation: Drag hose system
  • 47. 64 Sprinkler irrigation: Drag hose system
  • 48. 65 Sprinkler irrigation: Centre pivot system
  • 49. 66 Sprinkler irrigation: Centre pivot system
  • 50. 67 Sprinkler irrigation: Centre pivot system
  • 51. 68 Sprinkler irrigation: Centre pivot system
  • 52. 69 Sprinkler irrigation: Linear move system
  • 53. 70 Sprinkler irrigation: Linear move system
  • 54. 71 Sprinkler irrigation: Linear move system
  • 55. 72 Sprinkler irrigation: Mobile Raingun
  • 56. 73 Sprinkler irrigation: Mobile Raingun
  • 57. 74 Sprinkler irrigation: Appropriateness Type Score Crops Piped distribution 16 All “Low tech” 16 All Drag hose 15 All Solid set 14 Orchards Hand move laterals 12 All Perforated pipe 11 Soft fruit and veg Static gun 10 Cereals, Row crops Side roll 7 Short cereals, row crops Traveling gun 7 Cereals, Row crops Boom 6 Cereals, Row crops Centre pivot 5 Cereals, Row crops Linear move 5 Cereals, Row crops Side move 4 Cereals, Row crops
  • 58. Raingun Irrigation System
  • 59. Linear Move
  • 60. Centre Pivot
  • 61. Pivot of a Centre Pivot System
  • 62. System Layout  Layout is determined by the Physical Features of the Site e.g. Field Shape and Size, Obstacles, and topography and the type of Equipment chosen.  Where there are several possibilities of preparing the layout, a cost criteria can be applied to the alternatives.  Laterals should be as long as site dimensions, pressure and pipe diameter restrictions will allow.  Laterals of 75 mm to 100 mm diameter can easily be moved.  Etc. - See text for other considerations
  • 63. Design of Laterals  Laterals supply water to the Sprinklers  Pipe Sizes are chosen to minimize the pressure variations along the Lateral, due to Friction and Elevation Changes.  Select a Pipe Size which limits the total pressure change to 20% of the design operating pressure of the Sprinkler.  This limits overall variations in Sprinkler Discharge to 10%.
  • 64. Lateral Discharge  The Discharge (QL) in a Lateral is defined as the flow at the head of the lateral where water is taken from the mainline or submain.  Thus: QL = N. qL Where N is the number of sprinklers on the lateral and qL is the Sprinkler discharge (m3/h)
  • 65. Selecting Lateral Pipe Sizes  Friction Loss in a Lateral is less than that in a Pipeline where all the flow passes through the entire pipe Length because flow changes at every sprinkler along the Line.  First Compute the Friction Loss in the Pipe assuming no Sprinklers using a Friction Formula or Charts and then:  Apply a Factor, F based on the number of Sprinklers on the Lateral.
  • 66. Pressure at Head of Lateral  The Pressure requirements (PL)where the Lateral joins the Mainline or Submain are determined as follows:  PL = Pa + 0.75 Pf + Pr For laterals laid on Flat land  PL = Pa + 0.75 (Pf Pe) + Pr For Laterals on gradient.  The factor 0.75 is to provide for average operating pressure (Pa) at the centre of the Lateral rather than at the distal end. Pr is the height of the riser.
  • 67. Pumping Requirements  Maximum Discharge (Qp) = qs N Where:  qs is the Sprinkler Discharge and  N is the total number of Sprinklers operating at one time during irrigation cycle.  The Maximum Pressure to operate the system (Total Dynamic Head, Pp) is given as shown in Example.
  • 68. DRIP OR TRICKLE IRRIGATION  3.4.1 Introduction: In this irrigation system:  i) Water is applied directly to the crop ie. entire field is not wetted.  ii) Water is conserved  (iii) Weeds are controlled because only the places getting water can grow weeds.  (iv) There is a low pressure system.  (v) There is a slow rate of water application somewhat matching the consumptive use. Application rate can be as low as 1 - 12 l/hr.  (vi) There is reduced evaporation, only potential transpiration is considered.  vii) There is no need for a drainage system.
  • 69.  Drip irrigation / trickle irrigation - involves dripping water onto the soil at very low rates (2-20 litres/hour)  -from a system of small diameter plastic pipes fitted with outlets called emitters or drippers.  Water is applied close to plants so that only part of the soil in which the roots grow is wetted (Figure 60 in Notes).  With drip irrigation water, applications are more frequent (usually every 1-3 days).  This provides a very favourable high moisture level in the soil in which plants can flourish.
  • 70. 87 Drip irrigation: Layout
  • 71. 88 Drip Irrigation System • The Major Components of a Drip Irrigation System include: • a) Head unit which contains filters to remove debris that may block emitters; fertilizer tank; water meter; and pressure regulator. • b) Mainline, Laterals, and Emitters which can be easily blocked.
  • 72. 89 Water Use for Trickle Irrigation System • The design of drip system is similar to that of the sprinkler system except that the spacing of emitters is much less than that of sprinklers and that water must be filtered and treated to prevent blockage of emitters. • Another major difference is that not all areas are irrigated. • In design, the water use rate or the area irrigated may be decreased to account for this reduced area.
  • 73. 90 Micro irrigation: Drip irrigation
  • 74. 91 Micro irrigation: Root zone
  • 75. 92 Micro irrigation: Infiltration Sandy soil Clay soil
  • 76. 93 Micro irrigation: Emitters
  • 77. 94 Micro irrigation: Thick walled drip hose
  • 78. 95 Micro irrigation: Thin walled drip hose
  • 79.  While drip irrigation may be the most expensive method of irrigation, it is also the most advanced and efficient method in respect to effective water use.  Usually used to irrigate fruits and vegetables  System consists of perforated pipes that are placed by rows of crops or buried along their root lines and emit water directly onto the crops that need it.  As a result, evaporation is drastically reduced and 25% irrigation water is conserved in comparison to flood irrigation.  Drip irrigation also allows the grower to customize an irrigation program most beneficial to each crop.  Fertigation is possible.  Caution : Water high in salts / sediments should be filtered - otherwise they may clog the emitters and create a local buildup of high salinity soil around the plants if the irrigation water contains soluble salts.
  • 80.  Drip irrigation is most suitable for row crops (vegetables, soft fruit), tree and vine crops where one or more emitters can be provided for each plant.  Generally only high value crops are considered because of the high capital costs of installing a drip system.
  • 81.  Drip irrigation is adaptable to any farmable slope.  Normally the crop would be planted along contour lines and the water supply pipes (laterals) would be laid along the contour also.  This is done to minimize changes in emitter discharge as a result of land elevation changes.
  • 82.  Drip irrigation is suitable for most soils.  On clay soils water must be applied slowly to avoid surface water ponding and runoff.  On sandy soils higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.
  • 83.  One of the main problems with drip irrigation is blockage of the emitters.  All emitters have very small waterways ranging from 0.2-2.0 mm in diameter and these can become blocked if the water is not clean.  Thus it is essential for irrigation water to be free of sediments.  ]If this is not so then filtration of the irrigation water will be needed.  Blockage may also occur if the water contains algae, fertilizer deposits and dissolved chemicals which precipitate such as Ca and Fe.  Filtration may remove some of the materials but the problem may be complex to solve and requires an experienced professional.
  • 85.  A typical drip irrigation system is shown in Figure 61 and consists of the following components:  Pump unit  Control head  Main line  Laterals  Emitters or drippers.
  • 86.  Pump unit takes water from the source and provides the right pressure for delivery into the pipe system.  The control head consists of valves to control the discharge and pressure in the entire system.  It may also have filters to clear the water.  Common types of filter include screen filters and graded sand filters which remove fine material suspended in the water.  Some control head units contain a fertilizer or nutrient tank.  These slowly add a measured dose of fertilizer into the water during irrigation.  This is one of the major advantages of drip irrigation over other methods.
  • 87.  Supply water from the control head into the fields.  They are usually made from PVC or polyethylene hose and should be buried below ground because they easily degrade when exposed to direct solar radiation.  Lateral pipes are usually 13-32 mm diameter.  Emitters or drippers are devices used to control the discharge of water from the lateral to the plants.  They are usually spaced more than 1 metre apart with one or more emitters used for a single plant such as a tree.  For row crops more closely spaced emitters may be used to wet a strip of soil.  Many different emitter designs have been produced in recent years.  The basis of design is to produce an emitter which will provide a specified constant discharge which does not vary much with pressure changes, and does not block easily.
  • 88.  The water savings that can be made using drip irrigation are the reductions in deep percolation, in surface runoff and in evaporation from the soil.  These savings, it must be remembered, depend as much on the user of the equipment as on the equipment itself.  Drip irrigation is not a substitute for other proven methods of irrigation.  It is just another way of applying water.  It is best suited to areas where water quality is marginal, land is steeply sloping or undulating and of poor quality, where water or labour are expensive, or where high value crops require frequent water applications.
  • 89. Water Use for Trickle Irrigation System Contd.  Karmeli and Keller (1975) suggested the  following water use rate for trickle irrigation design  ETt = ET x P/85   Where: ETt is average evapotranspiration rate for crops under trickle irrigation;  P is the percentage of the total area shaded by crops;  ET is the conventional evapotranspiration rate for the crop. E.g. If a mature orchard shades 70% of the area and the conventional ET is 7 mm/day, the trickle irrigation design rate is:  7/1 x 70/85 = 5.8 mm/day  OR use potential transpiration, Tp = 0.7 Epan where Epan is the evaporation from the United States Class A pan.
  • 90. Emitters  Consist of fixed type and variable size types. The fixed size emitters do not have a mechanism to compensate for the friction induced pressure drop along the lateral while the variable size types have it.  Emitter discharge may be described by:  q = K h x  Where: q is the emitter discharge; K is constant for each emitter ; h is pressure head at which the emitter operates and x is the exponent characterized by the flow regime.
  • 91. Water Distribution from Emitters  Emitter discharge variability is greater than that of sprinkler nozzles because of smaller openings(lower flow) and lower design pressures.  Eu = 1 - (0.8 Cv/ n 0.5 )  Where Eu is emitter uniformity; Cv is manufacturer's coefficient of variation(s/x ); n is the number of emitters per plant.  Application efficiency for trickle irrigation is defined as:  Eea = Eu x Ea x 100  Where Eea is the trickle irrigation efficiency; Ea is the application efficiency as defined earlier.
  • 92. Pressure Head at Manifold Inlet  Like Sprinklers, the pressure head at inlet to the manifold:  = Average Operating Head = 8.9 m  + 75% of Lateral and Manifold head Loss = 0.75 (0.51 + 0.68)  + Riser Height = Zero for Trickle since no risers exist.  + Elevation difference = Zero , since the field is Level  = 9.79 m
  • 93. Solution Concluded  Total Head for Pump  = Manifold Pressure = 9.79 m  + Pressure loss at Sub-main = 6.59 m  + Pressure loss at Main = 2.90 m  + Suction Lift = 20 m  + Net Positive Suction head for pump = 4 m (assumed)  = 43.28 m  i.e. The Pump must deliver 3.23 L/s at a head of about 43 m.
  • 94. SUB-SURFACE IRRIGATION  Applied in places where natural soil and topographic condition favour water application to the soil under the surface, a practice called sub-surface irrigation. These conditions include:  a) Impervious layer at 15 cm depth or more  b) Pervious soil underlying the restricting layer.  c) Uniform topographic condition  d) Moderate slopes.
  • 95. SUB-SURFACE IRRIGATION (Contd…)  The operation of the system involves a huge reservoir of water and level is controlled by inflow and outflow.  The inflow is water application and rainfall while the outflow is evapotranspiration and deep percolation.  It does not disturb normal farm operations. Excess water can be removed by pumping.