Irrigation efficiency is the ratio of the average depth of irrigation water beneficially used to the average depth applied, expressed as a percentage.
Irrigation efficiency represents the percentage of applied water that is potentially accessible to crop
cooling, crop quality control, and leaching of salts from the soil profile. The irrigation efficiency is af- fected by the uniformity of distribution and losses. If either the uniformity of distribution decreases or losses increase, the overall irrigation efficiency gener- ally decreases. Irrigation efficiency is directly related to the percentage of irrigated area being under irri- gated or over irrigated. Therefore, irrigation system designs that maximize uniform application as well as minimize water losses caused by improper manage- ment (often poor irrigation scheduling), evaporation, wind drift, runoff, or deep percolation produce the greatest irrigation efficiencies.
Irrigation efficiency is a function of: the irrigation method used, physical condition of the irrigation system, physical condition of the soil, plant or crop type, spacing and population, timing and amount of irrigation water applied, water management level and skill, and environmental condition at the time water is applied. The way in which these functions interact with respect to uniformity and losses determines the irrigation efficiency.
(1) Water losses during application
Water loss varies with the type of irrigation method and system, the environmental conditions under which the system is operated, and the type and condition of conveyance system. For a well designed and installed irrigation system that fits the crop, water, and site conditions, water management is the principal means available by the irrigator to ensure that losses are held to a minimum. In the absence of reliable irrigation scheduling practices, the general tendency is to over water, resulting in excess runoff and deep percolation below the root zone. However, if the dryness appear- ance of plants is used as an indicator for scheduling, the tendency is to under irrigate. During hot, dry days a plant whose root development depth was restricted by over irrigation early in the growing season, can show stress even though adequate moisture exists in the normal root zone. This condition usually results in over irrigation.
(2) Sprinkle irrigation method
For sprinkle irrigation, major sources of water loss include:
• Improper water management (applying water when it is not needed or in excessive amounts) • Evaporation from droplets, the wetted canopy,
• Wind drift
• Runoff and deep percolation
• Leakage from conveyance system, worn nozzles, gaskets, or other equipment
Direct evaporation from droplets during irrigation is typically small except under very high evaporative conditions. Such losses are normally less than 5 per- cent of the total water that is applied, but increases as droplet size decreases and vapor pressure deficit increases. Losses as high as 45 percent have been measured under very low relative humidity and high temperature and wind conditions. Likewise, the longer droplets are airborne, the greater will be the water loss. Therefore, designs are preferred that minimize the height of sprinklers above the soil or canopy while still maintaining adequate uniformity and appropriate application rates.
For a given set of environmental conditions, evapora- tion from the wetted canopy is dependent on the type of crop, stage of growth, total leaf area, wind speed, wind direction, ambient air temperature, humidity, and duration of irrigation. Evaporation from the leaf sur- face of a crop canopy that covers the entire soil sur- face is the principal source of evaporative loss during irrigation. It amounts to as much as 25 percent of the total water loss for the day (Thompson, et al. 1988). Depending on the environmental conditions, the canopy may remain wet for 30 minutes or more after irrigation has ended. Therefore, the longer the irriga- tion set, the smaller the percentage of loss will be. Low Pressure In Canopy (LPIC) continuous-move laterals apply water a few inches above the soil surface low within the crop canopy, thereby eliminating the evapo- ration losses from the canopy. Where continuous- move LPIC sprinkler laterals apply water to less than half of the surface area and are used with appropriate soil, plant, and water management that controls water translocation, the system can be a Low Energy Preci- sion Application (LEPA) system.
Evaporation from the soil surface varies with wind speed, temperature, and canopy development. A canopy that provides full shading for the soil surface reduces soil evaporation losses. Likewise, wind in- creases turbulence at the soil surface, increasing soil evaporation. The total water loss attributed to soil evaporation is typically less than evaporation from the leaf surfaces, but may become relatively significant immediately after sprinkle irrigation. Where soil loss is
high, evaporation from canopy is generally low (low canopy cover). Where canopy loss is high, typically soil loss is low (shading affect from high canopy cover).
Wind drift is primarily considered a uniformity prob- lem although it can also contribute significantly to water losses if water droplets are small or if the drift is carried outside the field. Losses are typically less than 5 percent of the applied water, but vary depending on system type, operating pressure, and orientation to the wind. Drift losses are greater where the wind direction is parallel to the lateral or line of sprinklers and the wind blows toward the outer edge of the field. In comparison, drift losses are minimized for center pivots because the angle between the wind and lateral is continually changing.
Drift is a function of droplet size, droplet shape, and wind speed. It increases rapidly for droplets that are smaller than 0.04 inch. Because wind speed increases with height above the soil surface or canopy, the greater the trajectory angle or height of the nozzle, the more drift affects uniformity of application and the potential for loss. Therefore, designs are preferred that use low sprinkler trajectories or that place the nozzle closer to the crop canopy or in the crop canopy. Properly designed and managed sprinkle irrigation systems should not produce runoff or deep percola- tion. Therefore, the key to minimizing such losses is proper management. For solid-set systems, differences in application uniformity because of wind drift may result in some areas of the field receiving more than the design depth of application and other areas receiv- ing less. Fields irrigated by center pivots are subject to runoff near the outer edge where application rates are greatest. As surface roughness and residue decrease during the growing season because of tillage, rainfall, and irrigation, soil infiltration and surface storage capacity decrease. Application rates that are accept- able early in the season can result in runoff later. In addition as water droplets impact the soil surface, infiltration rates may decrease because of surface seal formation.
For well maintained sprinkle irrigation systems, pipe leakage and drainage losses should be held to 1 per- cent or less. Sprinkler drainage losses can be elimi- nated by incorporating antidrain valves at each sprin- kler. As with all irrigation systems, proper water
management and a routine maintenance program should be established and adhered to for preventing water loss and ensuring proper operation of the system.
(3) Micro irrigation method
For micro irrigation, major sources of water loss in- clude:
• Improper water management (applying water where it is not needed or in excessive amounts) • Evaporation from the wetted soil surface • Runoff and deep percolation
• Leakage from conveyance system
Water is normally not discharged into the atmosphere above the crop; therefore, micro systems are not sub- ject to drift nor to droplet and canopy evaporation except with micro sprinklers and spray nozzles. Be- cause application rates are typically quite low, the potential for runoff is reduced compared to sprinkle irrigation. Water infiltration rates are also more con- stant during the growing season since surface sealing caused by puddling from droplet impact is reduced.
(4) Surface irrigation method
Major sources of water loss for surface irrigation in- clude:
• Improper water management (applying water where it is not needed or in excessive amounts) • Evaporation from the wetted soil and water
surfaces
• Runoff and deep percolation • Leakage from conveyance system
Surface systems are not subject to wind drift losses nor evaporation from the wetted canopy. However, runoff and deep percolation generally are greater for graded surface systems than for well managed sprinkle irriga- tion systems. Typically, the combined losses of deep percolation and runoff dominate to the point that evaporation loss from the soil surface is relatively minor in comparison. However, with the appropriate match of soils, crops, field gradients, and water volume, a properly designed, installed, and managed surface system can have a higher efficiency than that for sprinkle irrigation.
Because the soil is used to transport water across the field, the infiltration opportunity time varies as water is moved from the inflow end to the outflow end of the field or is stored on the soil surface generally in lower
surface irrigation system for a wide variety of crops and adequately irrigating all parts of the field without over-watering another part is more difficult unless a tail water reuse system is included. With a very low gradient system, runoff can be minimized or elimi- nated by blocking off the end of the field or furrow, by decreasing grade or having level sections at the lower end, or by reusing the runoff water on the same or adjacent fields. Improper decreasing of tail water runoff without a reuse system can result in increased deep percolation losses and a lower distribution uniformity. Level basin, level furrow, and contour levee surface irrigation systems are relatively easy to design, install, and manage if soils are uniform and large volumes of water can be diverted or pumped onto the irrigated area.
(5) Subsurface irrigation method
(subirrigation and water table control)
For subsurface irrigation systems, major sources of water loss include:
• Improper water management (primarily irriga- tion scheduling resulting from poor timing of water introduction, the water table being kept too high or too low)
• Evaporation from soil and water surfaces • Deep percolation and lateral seepage
Subirrigation systems are not subject to wind drift or evaporation from wetted plant surfaces. Deep percola- tion losses can become significant depending on the permeability of the restricting barrier that supports the water table. Lateral seepage losses can vary greatly depending on the depth of water table in adjacent land, location of deep channels, and permeability and depth of soil strata.
Because the water table should be maintained at a nearly constant elevation, provisions should be made for irrigation water inflow or drainage and rainfall outflow. Water management involves maintaining a nearly constant water table elevation within desirable levels during periods of rainfall, no rainfall, and crop water use. Unless excess chemicals are applied to plant and soil surfaces where they can be subjected to runoff, good ground water quality can be maintained with a subsurface irrigation system. Percolating water containing soluble chemicals tend to concentrate the chemicals in the upper few inches of a free water table. As plants use water, the water table drops,