12.2.2: Operational Chacteristics

Managing sprinkler irrigation systems involves adjusting several variables to meet crop water needs, avoid deep percolation and align with management goals and constraints. Moved-lateral systems are repositioned from set to set across the field. The time that the lateral is in one location is called the set time. These systems require significant effort and labor to move each set; therefore, the minimum set time acceptable to operators is about 8 hours with a maximum of 3 sets per day. The most common set time is 12 hours, but 24-hour sets may be used for high clay content soils or for salinity management—low application rates over long periods enhance leaching and minimize runoff. Laterals must be drained before moving which can take up to 1 hour. Therefore, the time that water is applied, the application time, will be less than the set time.

The depth of water applied is often large for periodically moved systems. The depth is determined by the average application rate (A_r) and the time of application as described from Equations 11.3 and 11.4. The application rate is determined by the flow rate from the sprinkler (q_s), the spacing of sprinklers along the lateral (S_L) and the spacing between lateral sets along the mainline (S_m):

\(d_g=A_r \times T_o=\dfrac{96.3 q_s}{S_L S_m} \times T_o \)

where: d_g = the gross depth of application (in),

A_r = the average application rate, inches/hour,

q_s = the sprinkler discharge rate (gpm),

S_L = the spacing of sprinklers along the lateral (ft), and

S_m = the spacing of lateral locations along the mainline (ft), and

T_o = the application time (hr).

The depth of water applied per hour (i.e., the application rate) for typical sprinkler and lateral spacings are listed in Table 12.3 for a range of sprinkler flows. For example, a typical system using a 40 ft x 60 ft spacing with a sprinkler discharge of 10 gpm applies 0.4 inches per hour. If four inches of water are needed, then water should be applied for 10 hours. This is the gross depth of application and must be multiplied by the application efficiency for the low quarter (E_LQ) to determine the net depth (d_n):

d_n = d_g x E_LQ

Table 12.3. Depth of water applied per hour (i.e., the application rate), (in/hr).

Lateral Spacing, SL (ft)	Spacing Along Mainline, Sm (ft)	Representative Area, SL x Sm (ft2 )	Sprinkler Discharge, qs (gpm) 4	Sprinkler Discharge, qs (gpm) 6	Sprinkler Discharge, qs (gpm) 8	Sprinkler Discharge, qs (gpm) 10	Sprinkler Discharge, qs (gpm) 12	Sprinkler Discharge, qs (gpm) 15	Sprinkler Discharge, qs (gpm) 20	Sprinkler Discharge, qs (gpm) 25
20	40	800	0.48	0.72	0.96	1.20	1.44	1.81
30	40	1200	0.32	0.48	0.64	0.80	0.96	1.20	1.61
40	40	1600	0.24	0.36	0.48	0.60	0.72	0.90	1.20	1.50
20	50	1000	0.39	0.58	0.77	0.96	1.16	1.44	1.93
30	50	1500	0.26	0.39	0.51	0.64	0.77	0.96	1.28	1.61
40	50	2000	0.19	0.29	0.39	0.48	0.58	0.72	0.96	1.20
50	50	2500	0.15	0.23	0.31	0.39	0.46	0.58	0.77	0.96
20	60	1200	0.32	0.48	0.64	0.80	0.96	1.20	1.61
30	60	1800	0.21	0.32	0.43	0.54	0.64	0.80	1.07	1.34
40	60	2400	0.16	0.24	0.32	0.40	0.48	0.60	0.80	1.00
50	60	3000	0.13	0.19	0.26	0.32	0.39	0.48	0.64	0.80
60	60	3600	0.11	0.16	0.21	0.27	0.32	0.40	0.54	0.67
30	70	2100	0.18	0.28	0.37	0.46	0.55	0.69	0.92	1.15
40	70	2800	0.14	0.21	0.28	0.34	0.41	0.52	0.69	0.86
50	70	3500	0.11	0.17	0.22	0.28	0.33	0.41	0.55	0.69
60	70	4200	0.09	0.14	0.18	0.23	0.28	0.34	0.46	0.57
70	70	4900		0.12	0.16	0.20	0.24	0.29	0.39	0.49
40	80	3200	0.12	0.18	0.24	0.30	0.36	0.45	0.60	0.75
50	80	4000	0.10	0.14	0.19	0.24	0.29	0.36	0.48	0.60
60	80	4800		0.12	0.16	0.20	0.24	0.30	0.40	0.50
70	80	5600		0.10	0.14	0.17	0.21	0.26	0.34	0.43
80	80	6400		0.09	0.12	0.15	0.18	0.23	0.30	0.38

The irrigation interval (I_i) is the amount of time required to irrigate the field. This can be thought of as the time between consecutive irrigations of the first set of the field. Periodically moved laterals are operated to utilize the longest possible irrigation interval to minimize labor input. The irrigation interval depends on the average crop water use rate during the interval and the amount of water that can be stored in the root zone without causing deep percolation. The net depth of water required for an irrigation is the product of the irrigation interval and the average net crop water use during the interval. The net water use rate equals the evapotranspiration minus the expected effective rainfall during the interval. This is analogous to the net system capacity (C_n , in/d) discussed in Chapter 4. Thus, the required net depth of application is given by:

d_r = I_i x C_n

The net application depth must be less than or equal to the allowable depletion (AD) determined from scheduling:

AD = R_d x AWC x f_dc

where: AD = allowable depletion before irrigating, in,

R_d = root depth for scheduling, ft

AWC = available water capacity, in/ft, and

f_dc = allowable depletion, fraction.

The relationship between the allowable depletion and the net crop water use is shown in Figure 12.5. The solid blue lines represent the cumulative net crop water use during an irrigation interval. The horizontal dashed lines represent the allowable depletion for six soils using a critical allowable depletion of 50%, a management root zone depth of 4 ft, and the available water capacities consistent with Table 2.3. For example, the allowable depletion for a silt loam soil for these conditions is 4.3 inches. Applying more water would cause deep percolation. If the average crop water use rate was 0.30 inches/day, then the longest acceptable irrigation interval would be (4.3 in ÷ 0.3 in/d = 14.3 days) or rounding down to 14 days. Sandy loam soils only hold about 2.9 inches for these conditions. Also, since the irrigation interval will be shorter than for the silt loam you would expect that the net water use rate would be higher for the shorter period. So, for a water use rate of 0.35 inches/day the maximum irrigation interval for sandy loam would be about eight days. Sandy soils have small water holding capacities which leads to short irrigation intervals, requiring short set times or more laterals in an equally sized field. The irrigation intervals shown in Figure 12.5 represent the maximum acceptable values. Shorter intervals could be used such as seven days for the sandy loam soil so that scheduling activities are more tractable. The irrigation interval is based on the more extreme water use periods during the season. The actual irrigation interval will depend on irrigation scheduling during the season.

Figure 12.5. Maximum allowable irrigation interval for 4-ft root zone depth and 50% allowed depletion.

Water quantities in Figure 12.5 represent net irrigation depths and must be increased to determine the gross depth to apply. The application efficiency for well managed systems could be about 75% (see Table 5.4). Therefore, the gross depth for the silt loam soil is 5.6 inches (i.e., 0.3 in/d x 14 d ÷ 0.75) and 3.7 inches (i.e., 0.35 in/d x 8 d ÷ 0.75) for the sandy loam soil. If using the seven-day interval for the sandy loam soil, the gross depth would be 3.3 inches.

The irrigation interval accounts for time that water is applied, time for draining and moving laterals, and time to relocate the lateral to the starting set. If relocating the lateral takes one day, then water would only be applied for 13 days for the silt loam and 7 days for the sandy loam soil—assuming an eight-day interval. If an irrigator selected a 12-hour set time, then two sets can occur per day and the total sets possible for one lateral for the silt loam would be 26 (i.e., 13 days x 2 sets/day). One lateral would only allow 14 sets per lateral for the sandy loam soil.

The number of laterals required for the field depends on the irrigation interval and the field dimensions. The number of laterals also depends on the number of sets in the field and the number of sets possible during the irrigation interval for one lateral. Since fields for periodically moved systems are usually rectangular, the amount of land irrigated per set is usually constant. An example of a moved-lateral system is shown in Figure 12.6. This is an eighty-acre field with laterals on the left and right halves with the mainline running down the middle of the field. The left side of the sketch shows the layout of the sets, and the right side shows the lateral position and field information.

Figure 12.6. Plan view of field layout for moved lateral examples. Note that both sides of the field are irrigated.

The amount of area per set (A_s) is shown in Figures 12.1 and 12.6 and is given by:

\(A_s=\dfrac{L \times S_m}{43560} \)

where: A_s = the area irrigated per set, acres

L = the length of the lateral, ft, and

S_m = the spacing of laterals along the mainline, ft.

Then the number of sets (N_s) in the field is the area of the field (A_f) divided by the area per set:

\(N_s=\dfrac{A_f}{A_s}\)

The number of sets must be an integer, so the value from Equation 12.6 should be rounded up to the nearest integer. For example, the area of the field in Figure 12.6 is 80 acres (2640 ft x 1320 ft ÷ 43560 ft² /acre) and the area per set is 1.82 acres; thus, 80 acres ÷ 1.82 acres/set gives 44 sets for the field. Equation 11.15 can be expanded for when the lateral length is less than the field length to also give the number of sets:

\(N_s=\dfrac{W_f \times L_f}{S_m \times L}\)

where the length and width of the field are illustrated in Figure 12.6. Equation 12.7 and the layout in Figure 12.6 show that 44 sets are required for this field. The spacing of 60 feet between laterals is common for moved lateral systems as that is the length of two pipe sections for portable mainline pipe. The number of laterals depends on the number of sets that can be irrigated with one lateral during the interval:

\(N_p=\dfrac{24 I_i-T_m}{T_s}\)

where: N_p = the number of sets per lateral

I_i = the irrigation interval, d

T_m = time required to reposition lateral to the first set, hr, and

T_s = set time, hr/set.

The number of sets per lateral must be an integer; so, round the number from Equation 12.8 down to the nearest whole number. Then the number of laterals (N_L) for the field is:

\(N_L=\dfrac{N_s}{N_p}\)

The number of laterals must be an integer, so you need to round up for the number of laterals. Rounding of results to integer values may require relaxation of some management criteria to provide reasonable configurations. For example, increasing f_dc to 55% on a sandy loam soil allows one more day for the irrigation interval which may provide a more acceptable number of laterals. Such compromises would not threaten crop yields in most cases.