13.5: Depth of Water Applied

The depth of water applied each irrigation greatly affects the amount of potential runoff. As indicated earlier, the maximum application rate does not change with the depth of water applied. However, the time required to apply the water is directly proportional to the depth applied. Since the infiltration rate of the soil decreases with time, the longer it takes to apply water the greater the chance of runoff. The example in Figure 13.8 shows that there would be little runoff for an application of 0.8 inches each irrigation. Contrast that to the potential runoff for an application of 2.4 inches. There would certainly be substantially more runoff for the larger irrigation. It is desirable to apply smaller depths of water each irrigation with center pivots than to apply one large irrigation. It is common to apply from 0.70 to 1.25 inches per irrigation with pivots. This will usually require irrigation every 3 or 4 days.

Two other factors affect the depth of water to apply each irrigation: the condition of the soil surface where the pivot must travel and the evaporation rate of the applied water. If large irrigations are applied the soil surface becomes much wetter, and in some conditions, the traction of the pivot suffers. Any water that runs off often accumulates in the tracks left by the pivot wheels. The water then either flows downhill in the track or ponds in the track and surrounding area. If the pivot still has to pass through the low spot for that irrigation, or if the water remains at the time of the next irrigation, the wheel tracks from the pivot can become very deep and the pivot may have difficulty moving through these areas. Applying smaller depths of water each irrigation can mitigate some of these factors leading to more dependable operation.

Figure 13.8. Effect of application depth on the potential for runoff (adapted from Martin et al., 2007).

The loss of water due to evaporation can be important for high-frequency irrigation. The amount of water that evaporates while the water droplets are in the air is much lower than many producers estimate. The maximum loss of evaporation while the drops are in the air is less than 5% for even the most severe wind and drying conditions. The major loss of water to evaporation comes after the water has reached the crop and soil. Research has shown that water on the canopy and bare soil will evaporate very quickly. In windy, arid conditions, such as the Great Plains of the U.S., corn canopies dry in about 1 hour after irrigation in the middle of the day. The water that evaporates from the canopy uses some energy that would have caused transpiration had the crop not been irrigated. Thus, not all of the canopy evaporation is truly a loss. However, high-frequency irrigations that wet the crop or soil will lead to increase evaporation and somewhat lower efficiency. Estimates are that under very windy and arid conditions in the southern High Plains of the U.S. the efficiency of pivots equipped with impact sprinklers is about 85%. The efficiency increases to about 90% for devices that apply water just above the crop canopy and to as high as 95% for LEPA systems that apply water near the soil surface without wetting the canopy. For application efficiencies to be this high, water must not runoff the field.

In any case, very high-frequency irrigation with small depths can lead to reduced efficiency if canopy evaporation becomes excessive. There have been reports that high- frequency irrigation maintains a wet soil surface that leads to reduced infiltration rates and increased runoff.

There are several conflicting conditions regarding the proper depth of application for pivots. It is critical that managers develop a routine of observing the performance of the pivot on the steepest areas of the field near the outer half of the pivot. Managers should experiment to determine the maximum depth that can be applied without runoff problems occurring. This depth may decrease during the season so monitoring during the season is important. Managers could then adjust the depth of application down from the maximum depth if desired. Irrigation intervals shorter than 2 days are probably impractical. If the system has to operate at that or shorter frequencies, the sprinkler package may be inappropriate or special tillage may be needed to prevent runoff.

The depth of application on pivots is adjusted by controlling the average speed of the end tower. On electric-drive pivots a timer can be set between 0 and 100%. At 100% the distal end tower is supplied power continuously and the tower moves at a constant speed. This setting produces the smallest depth of application possible. To apply larger irrigations the timer setting is reduced. The timer setting represents the percent of a 1-minute interval that the end tower receives power. For example, a 50% timer setting provides power to the end tower for 30 seconds and it moves at a constant speed. The end tower remains stationary for 30 seconds. Operation of hydraulically powered systems is slightly different. The end tower on these machines moves constantly. The control setting regulates the delivery of oil to the end tower and controls the speed. The control setting represents the relative speed of the end tower.

For electric drive systems the relationship of the control setting and the depth of water applied is given below:

\(C_s=0.0231\dfrac{R-l C_g}{v_m d_g} \)

where: C_s = control setting (%),

R_l = distance from pivot base to end tower (ft),

C_g = gross system capacity (gpm/ac),

v_m = maximum continuous speed for the end tower (ft/min), and

d_g = gross depth of irrigation water to apply (in).

For example, to apply 1.3 inches of water with a pivot that has a maximum speed of 8 feet per minute, a capacity of 7 gpm/ac and the last tower is 1,280 feet from the pivot base; a control setting of 20% would be required. Manufacturers supply a tabular solution of Equation 13.7 for specific pivot designs.

The maximum depth of application that can be applied with a center pivot depends upon the soil infiltration, surface storage available, and peak application intensity of the system. The Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service) has categorized soils into intake families. Examples are shown in Table 13.5. In general, a low intake family, such as 0.1, is characterized by its high clay content and low infiltration rate. A high intake family, such as 3.0, is characterized by its high sand content and high infiltration rate.

Table 13.5. NRCS soil intake families (adapted from https://efotg.sc.egov.usda.gov//refe...uide_index.pdf).

Intake Family	Surface Soil Texture and Subsoil Permeability	Representative Soil Series	Representative Soil Series
0.1	Clays, silty clays, clay loam, silty clay loams (with slowly & very slowly permeable soils)	Albaton c Luton sic Wabash sic Filmore sicl	Crete sicl Pawnee cl Wymore sicl
0.3	Silt loam, loam silty clay loam, loams (with slow or moderately slow permeability)	Butler sil Colo sicl Wood River sil Belfore sicl	Burchard cl Hastings sicl Moody sicl Sharpsburg sicl
0.5	Silt loam, loam (with moderately slow or moderate permeability)	Hall sil Holder sil Holdrege sil	Judson sil Keith l Richfield l
1.0	Fine sandy loam, sandy loam, silt loam, loam, very fine sandy loam (with moderately slow to moderately rapid permeability) Loam, silt loam, very fine sandy loam, clay loam, sandy clay loam (with moderate or moderately rapid permeability)	Hord fsl Keith fsl Mitchell fsl	Crofton sil Monona sil
1.5	Fine sandy loam, loam, very fine sandy loam, sandy loam, silt loam (with moderate or moderately rapid permeability)	Anselmo vfsl Bayard vfsl Cass vfsl Alda fsl Brocksburg fsl	O’Neill fsl Rosebud fsl
2.0	Loamy fine sand, loamy very fine sand, loamy sand (with moderately rapid permeability)	Alice lfs Anselmo lfs Libory lfs Ovina lfs	Hersh lfs Jayem lfs Sarben lfs Otero lvfs
3.0	Loamy fine sand, loamy sand, fine sand, fine sandy loam, loamy very fine sand (with rapid permeability)	Bankard ls Dunday lfs Inavale lfs	Thurman lfs Valent lfs Valentine lfs

Figure 13.9. Influence of field slope on depressional storage. Photograph courtesy of USDA-NRCS (adapted from Martin et al., 2017).

As stated earlier, the storage of water on the soil surface in depressions can help avoid runoff in cases where application intensity exceeds soil infiltration rate. Figure 13.9 illustrates the concept of the storage in depressions. The amount of storage that is available depends upon field slope. For “conventional” tillage practices, this storage can be estimated from Table 13.6.

Table 13.6. Allowable soil surface storage (without artificial storage) values for various slopes (from Dillon et al., 1972).

Slope (%)	Allowable Soil Surface Storage (in)
0–1	0.5
1–3	0.3
3–5	0.1
5	0.0

Figure 13.10. Effect of sprinkler packages on application rate.

Figure 13.11. Illustration of wetted diameter (adapted from Martin et al., 2017).

Peak application intensity is an important factor when considering the potential for runoff of water (maybe we’ve lost sight by now—we want to avoid runoff). Peak application intensity can be calculated from Equation 13.4. The results of Equation 13.4 are shown in Figure 13.10. Obviously, wetted diameter, illustrated in Figure 13.11, has a major influence on peak intensities as does system capacity and the distance from the pivot point (Figure 13.12).

Figure 13.12. Effect of distance from pivot point on application intensity.

Figure 13.13. Maximum irrigation application depth (d_g) for different soils and peak application rates for zero potential runoff. Applies to center pivot and lateral move systems of any length.

Figure 13.13 provides a “management guide” for avoiding runoff during water application (Gilley, 1984; Martin et al., 2007). The figure uses the important factors that we’ve just discussed to indicate how much water can be applied and yet avoid runoff. The use of Figure 13.13 is illustrated in the following Examples 13.1 and 13.2.

Example 13.1

A center pivot operates with the following design features and field conditions:

Given: Q = 800 gpm

A = 130 ac

Sprinkler device = above canopy spray heads with 40 ft wetted diameter

System length = 1,300 ft

Field conditions: Soil—Holder silt loam, field slope 2%

Find: The maximum water application depth without runoff. Is this depth acceptable?

Solution

\(A_p=\dfrac{0.0177C_g R}{D_c}\)

\(C_g=\text{ gross system capacity} = \dfrac{800\text{ gpm}}{130\text{ ac}}=6.15 \text{ gpm/ac}\)

With a slope of 2%, the allowable surface storage is 0.3 inches (Table 13.6). The Holder silt loam is in the 0.5 intake family (Table 13.5).

Referring to Figure 13.13, we find that the maximum depth of application without runoff is 0.9 inches.

Is this acceptable? The 0.9 inches falls within the acceptable range of 0.70 to 1.25 inches per application, thus this system can be operated efficiently.

Example 13.2

Repeat Example 13.1 for a linear move system with the same capacity, 800 gpm, and lateral length, 1,300 feet.

Solution

The peak application rate can be calculated with Equation 13.6:

\(A_p=\dfrac{122.5(800\text{ gpm})}{(40\text{ ft})(1300\text{ ft})}=1.88\text{ in/hr}\)

From Figure 13.13 we find that we can apply 1.6 inches before runoff would occur, slightly higher than for the center pivot.

What if runoff is a problem? There are several design, management, and cultural practices that can be used if runoff is a problem. These practices are summarized in Table 13.7.

Table 13.7. Methods for reducing runoff under center pivot and lateral move sprinkler systems and their potential disavantages

Reducing Runoff Methods	Potential Disadvantages
Reduce system capactiy	need to irrigate more hours per year increases chances of soil water stress
Reduce application depth	requires more revolutions per year increases frequency of leaf wetting
Change sprinkler package to increase wetted diameter	may require higher pressure changes to pump and power unit may be needed
Increase surface storage	special interrow tillage practices may be needed increased field operations
Increase soil surface cover with crop residues	may require significant change to farming operations

In Table 13.8 we present a method for estimating the required sprinkler wetted diameter to avoid runoff for various soil textures, surface storages, and desired application depths for the case of a 1300-ft center pivot lateral. The table is based on methods discussed in Martin et al. (2012), including the Green-Ampt approach for infiltration. The increase in soil surface storage using crop residues is also presented in Martin et al. (2017). One method for increasing wetted diameter is to use boom backs, illustrated in Figure 13.14. Boombacks have also been used to address problems with rutting in center pivot wheel tracks by keeping the wetting pattern from the sprinkler behend the wheels.

Table 13.8. Minimum allowable wetted diameter, ft. Applies tocenter pivots with a 1300-ft pivot lateral.[a][b]

Gross System Capacity, gpm/ac	Depth Applied, inch	Surface Storage, inch	Sand	Loamy Sand	Sandy Loam	Loam	Silt Loam	Sandy Clay Loam	Clay Loam	Silty Clay Loam	Sandy Clay	Silty Clay	Clay
4.0	0.8	0.1	<10	10	18	39	46	110	>150
		0.3			<10	19	22	53	88	88	>150
		0.5					<10	19	32	30	63	73	120
	1.0	0.1	<10	13	22	48	59	136	>150
		0.3		<10	13	28	33	79	132	135	>150
		0.5			<10	14	17	41	67	67	134	>150
	1.2	0.1	<10	15	26	56	71	>150
		0.3		<10	17	36	44	103	>150
		0.5		<10	10	22	26	63	104	107	>150
5.0	0.8	0.1	<10	13	22	48	58	137	>150
		0.3		<10	11	23	27	67	110	109	>150
		0.5					<10	24	40	38	79	91	>150
	1.0	0.1	<10	16	28	60	74	>150
		0.3		<10	16	35	42	99	>150
		0.5			<10	18	21	51	84	83	>150
	1.2	0.1	<10	18	33	70	89	>150
		0.3	<10	12	21	45	55	129	>150
		0.5		<10	13	28	33	78	130	133	>150
6.0	0.8	0.1	<10	16	27	58	70	>150
		0.3	<10	a	13	28	33	80	132	131	>150
		0.5			<10	10	11	29	48	45	95	109	>150
	1.0	0.1	<10	19	33	72	89	>150
		0.3	<10	11	19	42	50	119	>150
		0.5			<10	21	25	61	101	100	>150
	1.2	0.1	<10	22	39	84	107	>150
		0.3	<10	14	25	54	67	>150
		0.5		<10	15	33	39	94	>150
8.0	0.8	0.1	<10	21	35	77	93	>150
		0.3	<10	10	17	37	44	107	>150
		0.5			<10	14	15	39	64	60	126	146	>150
	1.0	0.1	<10	25	44	96	118	>150
		0.3	<10	15	25	55	67	>150
		0.5		<10	13	29	33	81	135	133	>150
	1.2	0.1	<10	29	52	112	142	>150
		0.3	<10	19	33	72	89	>150
		0.5	<10	12	20	44	52	126	>150
10	0.8	0.1	<10	26	44	96	116	>150
		0.3	<10	13	21	47	54	133	>150
		0.5			<10	17	19	49	79	75	>150
	1.0	0.1	10	32	55	120	148	>150
		0.3	<10	19	32	70	83	>150
		0.5		<10	16	36	41	102	>150
	1.2	0.1	12	36	65	140	>150
		0.3	<10	24	42	90	111	>150
		0.5	<10	15	25	55	66	>150