Application efficiency is the ratio of the average depth of irrigation water infiltrated and stored in the root zone to the average depth of irriga- tion water applied, expressed as a percentage. Application Efficiency Low Quarter (AELQ) is the ratio of the average of the lowest one-fourth of measurements of irrigation water infiltrated to the average depth of irrigation water infil- trated, expressed as a percentage. This term is most often used in defining management effectiveness. The greatest irrigation water loss generally results from applying too much water too soon (improper irrigation scheduling). Deficit irrigation of a significant part, or all, of the irrigated area is an exception. Other water losses include evaporation from soil and leaf surfaces, runoff, percolation below the plant root zone, and wind drift. In all cases irrigation water management has a large influence on the net amount of water available for beneficial use. Application efficiency is primarily affected by operator irri- gation water management decisions. An ad- equately designed and installed system can be badly mismanaged.
Evaporation losses vary with the irrigation method, system used, and system operation. It can occur di- rectly from the wetted soil or water surface, wetted plant canopy, and droplets discharged from sprinkler nozzles. Evaporation from the soil surface relative to other losses decreases as the depth of application increases. Surface, subsurface, and micro systems (except micro sprinklers and sprays) are subject only to evaporation at the soil surface since the canopy is not wetted during irrigation. As the crop canopy develops and the soil is shaded, soil evaporation losses are further reduced. Evaporation from sprinkle irriga- tion tends to be greater than that from surface and micro systems because of the increased surface area wetted as well as that water may be discharged di- rectly into the atmosphere above the crop canopy. As wind speed and vapor pressure deficit increase and droplet size decreases, droplet evaporation increases. Runoff is a function of soil surface slope and storage, the infiltration rate of the soil, and the application rate of the irrigation system. Properly designed micro and
sprinkler systems should have no runoff if correctly designed, installed, and managed. Water management is important with all irrigation systems. This is espe- cially true for sprinkler systems because the impact of droplets on the soil surface can reduce surface storage and can produce a surface seal that reduces infiltra- tion during subsequent irrigations.
To be adequately and fully irrigated, all graded surface irrigation systems must have some runoff unless the end of the field is severely underwatered, level field sections are provided at the outflow end, or the ends are blocked on low gradient fields. With nearly level surface irrigation systems, small dikes across the end can be used to increase irrigation uniformity. Blocked ends are most effective when opportunity time is increased on the lower third to fourth of the field. Runoff loss from the field can also be reduced if tail water is collected for reuse on the same or adjacent fields.
Deep percolation occurs where the infiltrated volume of water exceeds the volume needed to bring the soil- water content in the plant root zone to field capacity. Properly designed and managed irrigation systems that are installed on suitable sites can have very little or no water lost to deep percolation. Unless the upper fourth of the field is chosen for the design application depth, some deep percolation always occurs where graded
surface irrigation is used. This is necessary to ensure sufficient stream size and infiltration opportunity time at the outflow end of the field for filling the root zone to field capacity or to some planned lesser level. Cutback, tail water reuse, surge, or cablegation tech- niques can be used to minimize deep percolation losses. Often irrigation stream size is decreased to reduce or eliminate tail water runoff at the expense of increasing deep percolation and irrigation nonunifor- mity. Runoff and deep percolation should be managed because they largely affect efficiency and are the primary transport mechanisms for off-site surface and ground water pollution.
The term most often used to define management effectiveness is application efficiency (Ea). However, because application efficiency is a function of water losses, a high value does not necessarily mean an effective and uniform irrigation. For example, runoff and deep percolation can be eliminated by severely underwatering, but an Ea near 100 percent can result. (Ea cannot exceed 100 percent.) If insufficient water is stored in the root zone in most of the irrigated area to meet the crop water requirements, crop performance (yield or biomass) will be reduced. Therefore, a more complete definition of an effective irrigation should include the concepts of adequacy and uniformity of application. (See equations below.)
E AELQ a = = 100 100
Average depth of irrigation water stored in the root Average depth of irrigation water applied
Avg. depth of irrig. water stored in the low quarter root zone Avg. depth of irrigation water applied
(e) Irrigation adequacy
(1) Adequacy of irrigation
Adequacy of irrigation is the percentage of the field that receives the desired amount of water. In arid and semi-arid regions where the probability of sufficient rainfall is low, each irrigation typically fills the soil profile to field capacity or to some planned lesser level. In sub-humid and humid regions, this may be less than field capacity to provide storage for rainfall that may occur between irrigations. The choice of how much water to apply may also be a function of the sensitivity of the crop to water stress, its market value, and water supply.
Adequacy of irrigation can most easily be evaluated by plotting a depth of application distribution as shown in figure 2–46. The curve is developed by grouping field measurements of application depth in descending order, accounting for the field area that each measure- ment represents. The point where the curve intersects the line for desired application depth indicates the percentage of the field that is being adequately irri- gated. Note that the distribution gives the amount of water applied (received by each part of the field and can be used to calculate DU, the area under irrigated, and the area over irrigated). Deep percolation moves chemicals below the root zone and can contribute to ground water pollution. Both under and over irrigation can result in crop yield and quality reduction.
Figure 2–46 Distribution of field application depth indicating adequacy of irrigation
10
0 20 30 40 50 60 70 80 90 100
Percent of field area
Application depth
Adequacy of irrigation Desired application depth
Actual depth applied
Stress Deep percolation
The relationship DU and Ea is demonstrated in figure 2–47. Here, two irrigation systems (A and B) having the same adequacy, but different uniformity’s and Ea are shown for the same field. The application depth for each system is equal to the area under the curve for full irrigation (i.e., field capacity). Therefore, if unifor- mity of application (DU or CU) was 100 percent, both curves would fall exactly along the line for full irriga- tion. Note that since curve A is flatter, it has the better uniformity. The amount of over and under irrigation
for system A is represented by the area a1 and a2, respectively, and for system B as a1+b1 and a2+b2, respectively. Because over irrigation (potential for runoff and deep percolation) is greater for system B, that system has a lower Ea than system A. Therefore, for irrigation systems designed to apply water to field capacity, improving application uniformity also im- proves the Ea. However, this is not be true for systems that under or over water the entire field because the total amount of water loss remains unchanged.
Figure 2–47 Distribution for two irrigation systems having equal adequacy but different uniformity and application efficiency
, , , , ,, ,, ,, ,, ,, ,,
Average depth applied
Full irrigation requirement
, , , A B a1 b1 a 2 b 2 A B 0 10 20 30 40 50 60 70 80 90 100
Percent of field area
Figure 2–48 Distribution for two irrigation systems having equal uniformity but different adequacy and application efficiency
0 10 20 30 40 50 60 70 80 90 100
Percent of field area
Application depth ,,, ,,, ,,, ,,, ,,, ,,, ,,, ,
Full irrigation requirement c1 c2 ,,, ,,,, C A a 2 a1
The relationship of adequacy and Ea is shown in figure 2–48. Here, a third system (C) is used that has the same uniformity as system A. System C has a lower adequacy and therefore is not applying sufficient water for the root zone to be filled to field capacity. The amount of over and under irrigation for system A is represented by a1+c1 and a2, respectively. For sys- tem C, this is c1 and a2+c2, respectively. Because system A has the greater percentage of over irrigation (potential runoff and deep percolation), system C now has the greater Ea. However, improving Ea by decreas- ing application depth below full irrigation does not necessarily result in a more effective irrigation. De- pending on the market value, water-stress sensitivity of the crop, and price of energy and water, this may or may not improve net income.
(2) Sprinkler systems
A concept that combines a measure of uniformity and Ea and provides for adequacy considerations is the Application Efficiency of the Low Quarter (AELQ) or the Application Efficiency of the Low Half (AELH).
AELQ is the ratio of the average of the lowest one-fourth of measurements of irrigation water infiltrated to the average depth of irrigation water infiltrated, expressed as a percentage. AELH is the ratio of the average of the low one- half of measurements of irrigation water infil- trated to the average depth of irrigation water infiltrated, expressed as a percentage. AELQ and AELH can be measured by conducting field tests of existing systems.
Application efficiencies are termed to be potential when the amount of water applied equals the design amount needed in all areas. This condition seldom exists because of the many variables the irrigation decisionmaker must consider. These variables include under or over estimating soil water needed to refill the plant root zone to field capacity, nonuniform irrigation system application, nonuniform soil characteristics, and nonuniform plant water use.
For sprinkle irrigation systems, potential AELQ can be estimated for design and planning purposes by:
potential AELQ=DU×Re (2–103) where:
AELQ = application efficiency of the low-quarter (%) DU = distribution uniformity (%)
Re = effective part of the applied water that reaches the soil surface
Re is a function of wind drift and evaporation loss and normally varies between 0.8 and 1.0.
To include the consideration of adequacy for medium to high value crops, the gross depth of irrigation water to be applied can be determined by dividing the Soil Moisture Deficit (SMD) by AELQ for the system. This will result in about 10 percent of the total field area receiving less water than needed to reach field capac- ity with the rest of the field reaching or exceeding field capacity. This is acceptable for medium to high-valued crops, but may be impractical for lower valued crops or irrigation in a water-quality sensitive area. With lower value crops, an application efficiency based on the average low-half of applied depth may be more practical.
For design purposes, the ratio of the average low-half of irrigation water available to the crops to the average depth of water applied to the field (AELH) can be estimated by:
potential AELH = CU x Re [2–104] where:
AELH = application of efficiency of the low-half (%) CU = Christiansen coefficient of uniformity To include the consideration of adequacy for low to medium value crops, the gross depth of irrigation water to be applied can be determined by dividing the
SMD by AELH. This will result in about 20 percent of the total field area not reaching field capacity after irrigation with the rest at or above field capacity. A typical range of AELQ and AELH values for various types of sprinkle irrigation systems is shown in table 2–48. These values are based on the assumptions of a fully developed crop canopy and a properly designed and managed sprinkler system that is well maintained. Values will be lower where proper water and system management are not followed.
For sprinkler systems having a CU of more than 60 percent, sprinkle water application generally is distrib- uted normally. Using this fact, Walker (1979) has shown that system application efficiencies can be determined based on the fractional area of the field that is under irrigated (Au) and the coefficient of uniformity (CU) of water distribution.
The relationship between application efficiency, Ea, and CU is shown in figure 2–49. Ea can be solved explicitly using the following relationship:
[2-105]
where:
Ea = application efficiency (%)
Au = fraction of the field that is deficitly irrigated CU = coefficient of uniformity
This equation assumes that runoff and in-air losses are negligible.
Table 2–48 Probable application efficiencies of the low- quarter (AELQ) and the low-half (AELH) for various types of sprinkler systems (adapted from the USDA-SCS National Engineering Handbook, Sprinkler irrigation)
System type AELQ (%) AELH (%)
Periodic move lateral 60 – 75 70 – 85 Gun or boom sprinklers 50 – 60 60 – 75
Fixed lateral 60 – 85 70 – 88 Traveling sprinklers 55 – 67 65 – 77 Center pivot 75 – 85 80 – 88 Lateral-move 80 – 87 85 – 90 Ea = −
(
− CU)
− Au + Au 100 1 1 25 0 0125. . 3 634 1 123. . 0 3. 0 003. 1 233.Figure 2–49 Application efficiency as related to the coefficient of uniformity and the percent of the area that is deficitly irrigated 60 70 80 90 100 50 60 70 80 90 100 Application efficiency Coefficient of uniformity Percent of field deficitly irrigated 50 40 30 20 10 5
Table 2–49 Example water application efficiencies (%) for furrow irrigation by slope and intake family assuming no reuse of runoff1/
Furrow length = 900 ft Furrow spacing = 2.5 ft Manning’s n = 0.04
- - - Furrow Intake family - - - -
0.3 0.5 0.7 1.0 1.5
Uniform slope (So) - - - Fn 2/ (in) - - - -
(ft/ft) 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 level 3/ 80 85 85 70 80 80 65 75 80 60 70 75 50 65 70 0.0010 50 50 0.0020 50 50 55 55 60 55 60 0.0030 50 55 50 60 60 50 65 70 65 70 0.0040 50 55 60 55 65 55 70 75 70 75 0.0050 55 60 65 60 70 60 75 80 0.0075 60 70 70 80 80 85 0.0100 70 75 75 85 0.0150 80 85 90 90 0.0200 85 90 0.0250 90 0.0300 90
1/ Design efficiencies below 70 percent generally are not recommended. 2/ Fn is the desired net depth of application.
3/ Results for level fields assume no runoff (i.e., diked ends).
(3) Micro systems
The relationship shown in figure 2–49 can be applied equally well to micro systems (Howell, et al. 1986). Additional information is available from the USDA- SCS National Engineering Handbook, Trickle Irriga- tion.
(4) Surface systems, graded furrow
Typical values of water application efficiencies for furrow irrigation systems are shown in tables 2–49 and 2–50. These values are for no runoff reuse and for 75 percent runoff reuse respectively. Efficiency values represent the maximum or partial application effi- ciency that could be typically attained, based on the SCS method of furrow irrigation design and a net depth of application for the end of the furrow. For example, a furrow length was assumed to be 900 feet and furrow spacing 2.5 feet, with a roughness coeffi- cient of 0.04 and constant stream inflow. Maximum set time was 12 hours, and maximum flow rate was based on the maximum nonerosive stream size (i.e., Qmax, gpm = 10/slope in percent) for low erosion resistant soils.
Blanks in tables 2–49 and 2–50 represent situations where it was not possible to achieve these conditions. These were mostly soils in SCS furrow intake families of 0.5 or less. Excessive set time is the primary cause. These conditions could not be met for soils in the 0.1 intake family that have slope of more than 0.1 percent at net application, Fn, depth values greater than 2 inches. Therefore, graded furrow irrigation is not recommended on these soils. For intake families greater than 0.5, as slope increases, the stream size required to provide sufficient flow at the end of the furrow typically exceeds the maximum nonerosive stream size. For these conditions, either a shorter furrow length should be used or other irrigation sys- tems considered.
The data in tables 2–49 and 2–50 provide initial esti- mates of application efficiencies for furrow systems and were derived using standard USDA-SCS methods (NEH, Furrow Irrigation, 2nd ed.). Many conditions are not represented by these tables. They include more or less erosive soils with associated maximum stream sizes, different set times, different furrow lengths or spacing, cracking soils, nearly level fields, and blocked end furrows. More advanced surface irrigation simula- tion methods, such as kinematic wave zero-inertia, should be considered. Obviously, consideration of all these factors is beyond the scope of this chapter.
Values in tables 2–49 and 2–50 represent a range of values that are appropriate for initial design and plan- ning for the selected site condition. The final design requires use of standard USDA-SCS methods for furrow irrigation.
Example 2–25 illustrates the use of tables 2–49 and 2– 50. A more detailed analysis, including equations and recommended flow rates, is in the USDA-SCS National Engineering Handbook chapter on Furrow Irrigation.
Table 2–50 Example water application efficiencies (%) for furrow irrigation by slope and intake family assuming a runoff reuse efficiency of 75 percent1/
Furrow length = 900 ft Furrow spacing = 2.5 ft Manning’s n = 0.04
- - - Furrow Intake family - - - -
0.3 0.5 0.7 1.0 1.5
Uniform slope (So) - - - Fn 2/ (in) - - - -
(ft/ft) 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 level 3/ 80 85 85 70 80 80 65 75 80 60 70 75 50 65 70 0.0010 55 60 55 65 65 55 65 65 50 60 65 60 65 0.0020 65 60 60 70 70 60 70 70 55 65 70 50 65 70 0.0030 65 60 65 70 70 65 70 75 60 70 75 65 75 0.0040 70 55 70 75 70 65 75 80 60 70 75 70 75 0.0050 70 55 70 75 70 70 75 80 65 75 80 0.0075 75 75 80 70 80 85 80 85 0.0100 75 75 85 75 85 90 0.0150 80 85 90 90 0.0200 85 90 0.0250 90 0.0300 90
1/ Design efficiencies below 70 percent generally are not recommended. 2/ Fn is the desired net depth of application.
Given: Intake family (If) 0.5 Net depth of application (Fn) 4 in Furrow slope (So) 0.0040 ft/ft Roughness coefficient (n) 0.04
Furrow length 900 ft
Determine: Gross application depth required.
Solution: Using table 2–49, find the column heading for the soil intake family of 0.5 and locate the column for Fn = 4 inches. Move downward until you intersect the row having a value of So = 0.0040 ft/ft in the left most column and read an Ea = 60 percent. The gross application depth required is:
F F E g n a = = = 100 100 4 60 6 7 % % . in in
Therefore to ensure that the design net application depth of 4 inches was applied at all locations in the furrow, a gross depth of 6.7 inches must be applied.
If runoff water was reused with an efficiency of 75 percent (i.e., 75% of all runoff was applied back to the same or an adjacent field), then using table 2–50 and the same procedure as above, Ea would equal 75 percent.
F F E g n a = = = 100 100 4 75 5 3 % % . in in
Therefore to ensure that the design net application depth of 4 inches was applied at all locations