Surface Irrigation

Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman

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Pages 185-207 (doi: 10.13031/ISM.2021.10) in Irrigation Systems Management. ,

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Abstract.  See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.

Keywords. Advance, Recession, Infiltration, Water Balance, Efficiency, Management of Sloping Furrow Irrigation Systems, Basin and Border Irrigation, Runoff Recovery, Surge Flow Irrigation, Textbook

10.1 Introduction

Surface irrigation is the oldest irrigation application method in the world. In fact, according to Price and Purcell (2011), the practice was used as early as 6000 years ago in Mesopotamia. Approximately 84% of the world’s irrigation (FAO, 2021) and 35–45% of the U.S. irrigation uses the surface method (FAO, 2021 and USDA, 2019). Surface irrigation includes border, furrow, and basin irrigation (Figure 10.1). Surface irrigation requires less pressure than does sprinkler or microsystems. In addition, worldwide, water is commonly furnished to the surface irrigated field using only gravity to deliver and distribute the water; pumping is not required. In the U.S. Midwest, much of the water for surface irrigation is pumped from groundwater and the primary energy cost using surface irrigation is due to lifting the water to the soil surface. If the topography of the land is such that surface irrigation is possible with only moderate leveling, surface irrigation may be less expensive than other methods.

(a) (b) (c) (d)

Figure 10.1. (a) Gated pipe furrow irrigation, (b) large-scale basin irrigation, (c) small-scale basin irrigation, and (d) border irrigation (photo d courtesy of Jan Feyen, KU Leuven, Belgium).

In all surface irrigation systems, water is applied at the inlet end and the water then flows to the downstream end. A portion of the water infiltrates as it advances across the field. Water is usually applied through gated pipes, siphons, or gates as shown in Figure 10.2.

(a) (b)

Figure 10.2. Water application in surface irrigation: (a) gated pipe, (b) siphons.

(a) (b)

Figure 10.3. (a) Land grading and (b) planing in preparation for surface irrigation.

Figure 10.4. Forming furrows for furrow irrigation.

Surface irrigation can be an efficient application method if the soils and fields are well suited to this method. But, it can be very inefficient if the soils and other factors are not properly considered when developing and managing the system. The soil infiltration rate is especially critical in the efficient operation of surface irrigation systems. If the infiltration rate of the soil is too high, the depth of water that infiltrates near the inlet will be much larger than that at the last point to receive water, the downstream end. The land slope and its uniformity also have a major impact on surface irrigation. Slopes that are too steep cause excess runoff and erosion. Acceptable slopes are usually less than 2%. The uniformity of the slope is also critical so that water does not accumulate in depressions on the surface.

Surface irrigation requires land preparation such as grading and leveling (Figure 10.3). With furrow irrigation, furrow forming or bedding is also required (Figure 10.4).

If a surface-irrigated field is too long, or the inlet flow is too small, a long period of time may be required for water to reach the downstream end of the field. This usually results in nonuniform distribution of water and excessive deep percolation (Figure 10.5). Runoff of water from the downstream end of a field can be one of the largest losses of water for surface systems. Relatively uniform distribution of water in furrow irrigation may require that 20 to 30% of the applied water runs off the field. If this water is not captured in a runoff recovery or reuse system, the application efficiency (E_LQ) is usually less than 60 to 70%.

In this chapter, the fundamentals of surface irrigation will be presented and discussed to illustrate the importance of proper application and management. Guidelines for good management will be presented for surface irrigation.

10.2 Advance, Recession, and Infiltration

Figure 10.5. Illustration of surface irrigation showing deep percolation, runoff, and evaporation.

To the casual observer, surface irrigation looks like a very simple concept. The water is applied at the inlet end and the irrigator allows gravity to move the water across the field. As the water moves across the field, part of it infiltrates and part of it is stored on the soil surface. After the water reaches the end of the field, runoff occurs unless the flow is blocked by an earthen dike. Water is usually not applied to the entire field simultaneously but rather is applied to only a portion of the field at one time. These portions of the field are referred to as sets. A set may be an individual border strip, a single basin, or a group of furrows. The water is applied for a fixed time period called set time.

Even though the concept of surface irrigation appears simple, the science of surface irrigation can be very complicated. This is largely because of the many interactions that occur between the rate of inflow, land slope, roughness of the land slope, uniformity of the land slope, and most importantly, the infiltration rate of the soil during irrigation.

In surface irrigation the soil infiltration rate has a large impact on the ultimate distribution of water and the ultimate amount of water that runs off the edge of the field. This is in contrast to sprinkler and microirrigation where the hardware of the system has more control on how the water is distributed and whether or not the water infiltrates at the desired location. The hardware can be designed so that the application rate is less than the infiltration capacity of the soil allowing all of the water to infiltrate at the point of application. This is not true with surface irrigation. Once the water leaves the inlet end of the field, the manager no longer has control of the water; the soil now has control. Infiltration during surface irrigation can vary significantly on land that is cultivated annually. It depends upon whether it is the first irrigation of the season or whether it is a subsequent irrigation, and where tractor tires have traveled and compacted the soil. Some of the variations in infiltration are illustrated in Figure 10.6. There can be many other sources of infiltration variability within the field.

In practice, many surface irrigators have developed an art of irrigating, rather than applying science to irrigation management. What we hope to do in this chapter is to balance the two: the art and the science. It is unlikely that we will ever get to the point where we can completely manage based on theory alone because there are so many variables that are out of the manager’s control.

Figure 10.6. Trends in cumulative infiltration as influenced by irrigation sequence and wheel traffic.

Figure 10.8. Students measuring stream size, advance, recession, and runoff in a furrow irrigation system. (Photo courtesy of Laszlo Hayde, IHE Delft Institute for Water Education.)

Let us take a look at the fundamentals that apply to surface irrigation. The first concept is advance and recession of water. In Figure 10.7, two curves are shown: the advance curve and the recession curve. The advance curve is a graphical picture of how rapidly water moves from the inlet end to the downstream end of the field, which can be measured directly in the field (Figure 10.8). The curve is not linear. As water moves further and further from the inlet end, the rate at which the wetting front moves decreases. It is typical that it takes about one-third as much time to get halfway across the field as it does to get from the starting point to the downstream end of the field. For example, if it took 3 hours to get to the midpoint of the field, we would estimate approximately 9 hours total to reach the downstream end.

Figure 10.7. Advance and recession curves for surface irrigation.

The recession curve is a plot of how the furrow drains after irrigation has been stopped, and can also be measured directly. Usually, the surface begins to drain from the upstream end. For the example illustrated in Figure 10.7, drainage occurs in approximately 1 hour. This is in contrast to the advance time, which was 9 hours before water reached the downstream end.

Why are advance and recession important? The amount of water that infiltrates at any point in the field depends upon how long water was at that point. In our example in Figure 10.7, water was at the inlet end for 12 hours because irrigation was continued for 3 hours after water reached the downstream end of the field. At the downstream end, water arrived after 9 hours of application. Further, the recession took approximately 1 hour at the downstream end. That is, recession stopped at hour 13. So, how long was water present at the downstream end? In this case, 4 hours (13 - 9). At the upstream end water infiltrated for 12 hours, while at the downstream end, water had the opportunity to infiltrate for only 4 hours. You can now see why the amount of infiltrated water would not be uniformly distributed.

Figure 10.9. Opportunity time for surface irrigation.

Figure 10.10. Example infiltration vs. opportunity time.

The time difference between the recession curve and advance curve is called opportunity time. The opportunity time curve shown in Figure 10.9 is the time difference between the advance and recession curves in Figure 10.7. In this example, opportunity time decreased as you move from the inlet end to the downstream end of the field. If the infiltration characteristics of the soil are uniform throughout the field, we would expect more infiltration at the inlet end compared to the downstream end.

Table 10.1. Data for Figures 10.7, 10.9 and 10.10.
Distance (ft)	Advance Time (h)	Recession Time (h)	Opportunity Time (h)	Infiltration (in)
0	0.0	12.0	12.0	4.2
300	0.8	12.2	11.4	4.0
600	2.7	12.5	9.8	3.6
900	5.5	12.8	7.3	3.2
1200	9.0	13.0	4.0	2.4

What is necessary to achieve good uniformity? For perfect uniformity the opportunity time curve would have to be horizontal, i.e., equal at all locations within the field. This can only happen if the advance curve and recession curve are parallel to one another. In other words, advance time would have to equal recession time at all points in the field. Even though we commonly picture more opportunity time at the inlet end than at the downstream end, it sometimes happens that recession is slower than advance. In this case, opportunity time would increase with distance from the inlet end.

Now, let us look at the development of the infiltration distribution profile. In Figure 10.10, we illustrate an example relationship between the cumulative infiltration and opportunity time. The data are listed in graphical as well as tabular form.

In Table 10.1, the advance time, the recession time, and the opportunity time have been tabulated. By combining the opportunity time information with the infiltration characteristics of the soil you can determine the infiltration at any position. Use 600 feet as an example distance. Here the advance time was 2.7 hours and recession occurred at 12.5 hours. Thus, the opportunity time was 9.8 hours. From Figure 10.10 we find that the infiltration would be approximately 3.6 inches. A similar procedure can be followed to obtain infiltration at any point along the furrow. The infiltration distribution curve shown in Figure 10.11 is based on the data from Table 10.1. Since the opportunity time decreased with distance from the inlet end, the infiltration also decreased with distance. We will return to this example later as we develop the relationships between water applied, infiltration, runoff, and the effective amount of water stored in the soil.

Figure 10.11. Infiltration profile.

10.3 Water Balance

In surface irrigation, just as in basic hydraulics, there must be conservation of mass. The primary components of the mass balance for surface irrigation may be represented as volumes. The volume balance is written as:

V_g = V_z + V_s + V_r (10.1)

where: V_g = gross application volume,

V_z = infiltration volume,

V_s = storage volume on the soil surface, and

V_r = runoff volume.

We assume that evaporation of water during application is negligible. While water is being applied, some water exists as storage on the surface until the inflow is stopped and recession is complete. Thus, V_s is transient; it only occurs while water is on the surface.

The water balance may also be described using the depth of water:

d_g= d_z+ d_s + d_r (10.2)

where: d_g= average gross application depth,

d_z= average infiltration depth,

d_s = surface storage depth, and

d_r = runoff depth.

As usual, depths represent the volumes divided by the irrigated area.

The gross application depth in furrow irrigation is calculated as:

(10.3)

where: d_g= average gross application depth (in),

q_s = furrow stream size (gpm/furrow),

t_co = cutoff time, i.e. set time (hr),

W = spacing of watered furrows (in), and

L = length of furrow (ft).

or for an entire set:

(10.4)

where: N = number of furrows watered per set, and

Q_t = total inflow rate to the field.

The total inflow rate is equal to the sum of the inflow from the water supply and, when a closed runoff recovery system is used, water reused on the same field. Thus, Q_t = Q_w + Q_p where Q_w = flow rate of the original supply and Q_p = flow rate of the recovery system. The W equals the spacing of the furrows if every furrow is irrigated. If every other furrow is irrigated, then W equals twice the furrow spacing. Often the furrow stream size (q_s) is constant for the duration of the irrigation. When the labor supply is available, efficiency can be improved by reducing furrow stream size after water advance across the field is complete. This is called cutback irrigation.

The gross application depth for basins and border irrigation is:

(10.5)

where: W_b = the width of the border or basin (ft), and

L_b= length of border or basin (ft).

The average infiltration depth (d_z) can be determined from the infiltration profile such as Figure 10.11. It occurs at about 60% of the field’s length from the inlet for open-ended systems. In our example, it occurs at 720 feet and equals 3.5 inches. After the irrigation and recession has stopped, the water stored on the surface (d_s) has either infiltrated or has run off; therefore, the depth stored is zero. In Equation 10.2, the only remaining variable is the runoff depth (d_r). The depth of runoff is the total volume of runoff water divided by the area of the irrigation set, basin, or border, or the area irrigated by an individual furrow. By rearranging terms, Equation 10.2 can be used to determine the amount of runoff from a surface irrigated field.

10.4 Efficiency

10.4.1 Calculation of Irrigation Efficiency

The concepts of in-field efficiency and water distribution uniformity as presented in Chapter 5 can be applied to surface irrigation by using the mass balance equations and water distribution graph. This is illustrated in Example 10.1.

The efficiency calculated in Example 10.1 was for a system where runoff is not reused. Later in this chapter we will discuss the use of runoff recovery systems as one method for improving irrigation efficiency. The effects of runoff recovery on efficiency can be determined when two things are known: the amount of runoff and the effectiveness of the runoff recovery system itself, that is, how much of the runoff water is actually captured and applied. The efficiency depends upon whether the recovery system is a closed system in which the runoff water is captured and returned to the field of origin, or whether it is an open system where the runoff is captured from one field and applied to another field with runoff being allowed to leave the second field. These systems are illustrated in Figure 10.12. The equations that apply for calculating efficiency are shown below.

Closed system: (10.6)

Open system: (10.7)

where: d_e= effective depth stored,

d_e= d_LQ if d_LQ = SWD,

d_e= SWD if d_LQ > SWD;

d_g = gross application;

R_r = runoff ratio; and

R_t = return ratio (efficiency of recovery system) = volume applied from the recovery system divided by volume of runoff.

The runoff ratio is:

(10.8)

10.4.2 Improvement of Surface Irrigation Systems

The application efficiencies of field-scale systems are often reported to be quite low, in the range of 40-50%. Much work has been done to develop methods for improving the efficiencies including the following:

• Converting earthen field ditches to lined-ditches or gated pipe delivery systems to reduce seepage and/or evaporation.
• Recovery or reuse of tailwater to reduce runoff losses.
• Improved land forming methods especially with the use of laser and GPS controlled land grading equipment (Dedrick et al., 2007) for improved application uniformity.
• Cutback irrigation and blocked-end systems to reduce runoff losses (USDA, 2012).
• Surge flow irrigation (Walker and Skogerboe, 1987) to control infiltration and runoff, and to improve uniformity of infiltration.
• Automation of water delivery systems (Humphreys, 1986; Koech et al., 2010; Koech et al., 2014) and semi-automation to better match set times to optimal set times.
• Development of computer-based models for improvement in design and selection of more efficient management options such as set-time and stream size (Bautista et al., 2009).

Figure 10.12. Closed and open runoff recovery systems.

In this book we concentrate mainly on the set-time and stream size management options as well as tailwater recovery and the management of surge flow systems.

10.5 Management of Sloping Furrow Irrigation Systems

Good management of surface irrigation systems is extremely important. The manager must respond to the effect of infiltration variability on the performance of the system during each irrigation. In addition to satisfying the water needs of the crop, the goals of management might include low runoff, low deep percolation, or that the sum of these two losses be minimized. We’ll discuss management practices to minimize the sum of runoff and deep percolation. In management of surface irrigation, the irrigator has control of three things: set time (i.e. cutoff time), stream size, and the soil water deficit before water is applied. All three can be changed without changing the system characteristics.

Many, if not all, textbooks, management guides, and computer software establish set time and stream size recommendations so that a required or preplanned desirable irrigation depth infiltrates in a large proportion of the field area (such as 90%). Unfortunately, it is only possible to compute the optimum set time-stream size combination if the infiltration vs. time relationship, such as the one illustrated Figure 10.10, is known with reasonable accuracy on the planned day of irrigation. This requirement is seldom, if ever, satisfied. However, even when the infiltration characteristics are known with confidence, simulation results from models can result in set times or stream sizes that are simply too unreasonable to put into practice. Labor constraints are often a problem.

Given these two problems, the infiltration uncertainty and the possible constraints of labor availability, we have chosen to take a reactive or adaptive approach to surface irrigation management. As explained in Chapter 6, we do not have to refill the crop root zone during irrigation to meet the ET requirements. In fact, that is seldom done with pressurized systems. We simply adjust the irrigation schedule according to how much effective water was applied during each irrigation. Here we follow that same philosophy with surface irrigation.

To overcome the labor-set time dilemma we attempt to adjust set times that fall within the constraints of the irrigator’s labor supply. In many areas this may mean that the shortest set time possible is 12 hours or even longer. With methods of semi-automation, such as using surge irrigation valves for example, set times can easily be reduced by 50%. In the Great Plains of the U.S., we commonly refer to set times in intervals of 6 hours, that is, 6, 12, 18, and 24 hours. For example, using a surge irrigation system which irrigates 2 sets simultaneously, 6-hour set times require that the irrigator return to the field only once every 12 hours.

The irrigator can also change stream size. If a water supply rate to a field is constant, then furrow stream size can be changed by changing the number of furrows per set. This is illustrated in the following equation:

(10.9)

where: q_s= furrow stream size,

Q_t = total inflow rate to the field, and

N = number of furrows irrigated per set.

Maximum furrow stream size must be kept below the flow that will cause erosion and be low enough so that the furrow has adequate capacity to prevent overflowing. The maximum nonerosive stream size is approximated by:

(10.10)

where: q_max= maximum nonerosive stream size (gpm) and

S = field slope (%).

For example, if the field slope is 0.4%, then the maximum nonerosive stream size would be 25 gpm. As stated, Equation 10.10 is an approximation. The NRCS (USDA, 2012) provides more specific guidelines for permissible maximum water velocities to prevent soil erosion in furrows. The important point is that stream size can also be a constraint to the management of furrow irrigation systems.

Another factor that the irrigator can change is the soil water deficit by controlling the frequency of irrigation. The maximum soil water deficit allowable is equal to the management allowed deficit (AD). If an irrigator is having difficulty attaining a high efficiency because of excessive irrigation, the soil water deficit can be increased, up to AD, by irrigating less frequently.

How do stream size, set time, and AD interact? In Section 10.2 we indicated that to obtain a perfectly uniform distribution of water, the advance curve and recession curve have to be parallel. Unfortunately, the tradeoff for uniform distribution is excessive runoff. On the other extreme, if the irrigator’s goal is to reduce runoff, then it might be desirable just to get the water to the end of the field and then shut it off or even shut it off before advance is complete. Obviously, this will result in low runoff, but will also result in poor distribution of infiltration and high deep percolation. So, what is the optimum compromise between runoff and deep percolation that results in the highest system efficiency? A distance-based management parameter that is useful for this determination is called “cutoff ratio.” It is defined as:

(10.11)

Figure 10.13. Conceptual graph illustrating ELQ vs. cutoff ratio for sloping furrows.

Figure 10.14. ELQ vs. cutoff ratio for sloping furrows with fine textured soils (clays, silty clays, silty clay loams, and clay loams). Assumes advance time exceeds recession time.

where: CR = cutoff ratio,

t_L = advance time to the end of the field, and

t_co = cutoff time (set time).

A rapid water advance (low t_L) results in a low cutoff ratio. Conversely, a slow water advance (high t_L) will yield a high cutoff ratio. Low cutoff ratios result in large amounts of runoff and good uniformity. High cutoff ratios result in poor distribution of water, high deep percolation, and low runoff. This concept is illustrated in Figure 10.13.

The cutoff ratio that provides maximum efficiency, where the sum of runoff losses and deep percolation are minimized, is dependent upon the soil characteristics and whether or not the system has runoff recovery. In Figures 10.14 to 10.16 you see that efficiency varies with cutoff ratio and by soil texture for sloping furrow irrigation systems. Here the fine textured soils include clays, silty clays, silty clay loams, and clay loams. Silts, silt loams, loams and sandy clays are considered medium textured soils, and sandy clay loams, sandy loams, loamy fine sand and fine sand are course textured soils. The efficiency term in the graph is the application efficiency of the low quarter (E_LQ). Figures 10.14 to 10.16 are based on the assumption that water advance time to the downstream end of the field exceeds water recession time following cutoff. This condition will be met in most cases for long, sloping furrows. This condition might not be met on fields with inadequate slope or small fields with short furrow length (such as smallholder farms). In that case, the principles for border or basin irrigation systems (Section 10.6) may be applicable.

Based on Figure 10.15, if the soil is of medium texture and a closed runoff recovery system is used, the maximum efficiency occurs at a cutoff ratio of about 0.40. Without runoff recovery, the maximum efficiency occurs at about 0.70. How can a manager use these curves? Suppose, because of time constraints, the irrigator can only change sets every 12 hours. If the medium textured soil is considered and the system has runoff recovery, the peak efficiency would occur with a cutoff ratio of 0.40. The desired advance time is then 0.40 × 12 hours or 4.8 hours. Hence, the irrigator would adjust the furrow stream to achieve the 4.8 hours advance time. Of course, the stream size that has been determined may not be feasible if it exceeds the maximum nonerosive stream size for that slope condition.

Figure 10.15. ELQ vs. cutoff ratio for sloping furrows with medium textured soils (silts, silt loams, loams and sandy clays). Assumes advance time exceeds recession time.

Figure 10.16. ELQ vs. cutoff ratio for sloping furrows with coarse textured soils (sandy clay loams, sandy loams, loamy fine sand and fine sand). Assumes advance time exceeds recession time.

The expected maximum efficiency for the system described above would be about 85% (Figure 10.15). For these efficiencies to be attainable, the depth infiltrated at the low quarter, d_LQ, must be less than the soil water deficit, SWD. If this is not true, the figures are not applicable and the manager should consider allowing a higher SWD before irrigation without exceeding AD. If SWD already equals AD, then other practices that reduce infiltration depths, such as every other furrow irrigation or shorter set times, must be considered.

As discussed above, usually the irrigator does not know the soil’s infiltration characteristics prior to irrigation. The irrigator learns these characteristics by irrigating a portion of the field. Once the advance time is known for a given furrow flow rate, or stream size, then the irrigator can make the appropriate adjustments to maximize efficiency.

In the example used so far in this chapter, the furrow stream size was 11 gpm (760 ÷ 70) and the advance time was 9 hours. According to Figure 10.15, the cutoff ratio that would result in maximum efficiency is about 0.70 with no runoff recovery. Thus, the desired advance time is 0.70 × 12 hours or 8.4 hours. This is close to the measured 9-hour advance time.

What if in the above example a runoff recovery system is used? Now, the desired cutoff ratio is about 0.40 (Figure 10.15). The desired advance time is 0.40 × 12 hours or 4.8 hours. The ratio of the desired time to the original time is equal to 0.53 (4.8 hours ÷ 9 hours). What would the stream size have to be for this to occur? Or, another way of looking at it, how many furrows would have to operate to achieve this goal? Table 10.2 contains correction factors for the number of furrows to irrigate for a fixed Q_t. The ratio of the new advance time to the old one is 0.53. Interpolating from Table 10.2, the number of furrows that should be watered is 63% of the number of furrows that were originally watered (find this under medium textured soil, N₂/N₁ = 0.63). Thus, the irrigator should irrigate 44 furrows (0.63 × 70) instead of the original 70. The furrow stream size would now be 760 gpm ÷ 44 = 17 gpm per furrow. If the furrow slope in this example is 0.3%, the maximum nonerosive stream size is 33 gpm. Thus, the 17 gpm flow rate is acceptable.

Table 10.3. Infiltration factors (ratio of dLQ to dg) for sloping furrows. (Assumes advance time exceeds recession time.)
	Infiltration Factors
Cutoff Ratio	Soil Texture
	Fine	Medium	Coarse
0.1	0.19	0.32	0.50
0.2	0.32	0.45	0.61
0.3	0.42	0.55	0.68
0.4	0.51	0.62	0.71
0.5	0.59	0.66	0.72
0.6	0.65	0.69	0.71
0.7	0.70	0.70	0.69
0.8	0.73	0.69	0.66
0.9	0.74	0.66	0.61

What is the depth infiltrated at the low quarter for the new condition? Table 10.3 relates the depth of low quarter to the gross depth applied and the cutoff ratio. The infiltration factor given in Table 10.3 is defined as:

Infiltration factor = (10.12)

Table 10.2. Correction factor (N2/N1) for predicting how many furrows to irrigate per set with a constant water supply. (Table based on equation in Cahoon et al., 1995.)
TL2/TL1	Fine	Medium	Coarse		TL2/TL1	Fine	Medium		Coarse		TL2/TL1	Fine	Medium	Coarse
0.1	0.1	0.2	0.4		1.3	1.3	1.2		1.1		2.5	2.6	2.0	1.5
0.2	0.2	0.3	0.5		1.4	1.4	1.3		1.2		2.6	2.7	2.0	1.5
0.3	0.3	0.4	0.6		1.5	1.5	1.4		1.2		2.7	2.8	2.1	1.6
0.4	0.4	0.5	0.7		1.6	1.6	1.4		1.2		2.8	2.9	2.2	1.6
0.5	0.5	0.6	0.7		1.7	1.7	1.5		1.3		2.9	3.1	2.2	1.6
0.6	0.6	0.7	0.8		1.8	1.9	1.6		1.3		3.0	3.2	2.3	1.6
0.7	0.7	0.8	0.9		1.9	2.0	1.6		1.3		4.0	4.3	2.8	1.9
0.8	0.8	0.8	0.9		2.0	2.1	1.7		1.4		5.0	5.4	3.3	2.1
0.9	0.9	0.9	1.0		2.1	2.2	1.7		1.4		6.0	6.6	3.8	2.2
1.0	1.0	1.0	1.0		2.2	2.3	1.8		1.4		7.0	7.7	4.3	2.4
1.1	1.1	1.1	1.0		2.3	2.4	1.9		1.5		8.0	8.9	4.8	2.5
1.2	1.2	1.1	1.1		2.4	2.5	1.9		1.5		9.0	10.0	5.2	2.7
											10.0	11.2	5.6	2.8
N2 = Correct number of furrows to water per set. N1 = Original number of furrows watered per set.								TL2 = Desired advance time. TL1 = Original advance time.

Table 10.4. Runoff ratios (Rr) for sloping furrows. (Assumes advance time exceeds recession time.)
Cutoff Ratio	Runoff Ratios
	Soil Texture
	Fine	Medium	Coarse
0.1	0.81	0.68	0.50
0.2	0.68	0.53	0.36
0.3	0.56	0.42	0.26
0.4	0.46	0.32	0.19
0.5	0.37	0.24	0.14
0.6	0.28	0.18	0.09
0.7	0.21	0.12	0.06
0.8	0.14	0.08	0.03
0.9	0.08	0.04	0.02

In Example 10.1, the cutoff ratio was 0.75 and the gross depth applied was 4.2 inches. The depth of low quarter would be equal to 4.2 inches times the factor from Table 10.3 (0.70) or 2.9 inches. This closely agrees with our original graphical analysis (Figure 10.11), 2.8 inches. This number can now be compared with the SWD before irrigation. If it exceeds the SWD, then the irrigator has two choices. The first, and easiest to implement, is to change the irrigation frequency so that SWD is higher during irrigation. Again, the constraint is that AD is the upper limit. The second approach is to change the irrigation cutoff time so that less water infiltrates during the irrigation and thus the depth of low quarter might be maintained less than the soil water deficit.

The irrigation frequency now will be dependent upon the effective water applied. As we discussed in Chapter 5, the effective water applied will be equal to the d_LQ if it is less than or equal to SWD. The effective water applied is equal to SWD if d_LQ is greater than SWD. In our example, where the effective water applied was 2.9 inches, the irrigation interval should be about 10 days if ET is equal to 0.3 inches per day.

The runoff ratio (R_r, Equation 10.8) must be known to calculate E_LQ when a runoff recovery system is used. Table 10.4 gives runoff ratios for various conditions.

Notice that the efficiency is lower in Example 10.4 than in Examples 10.2 and 10.3, even though the lower cutoff ratio was supposed to increase efficiency. What went wrong? In Example 10.4, the d_LQ > SWD. To improve efficiency to its potential, the d_LQ must either be reduced or SWD must be increased. Suppose AD = 3.4. The SWD cannot be increased without yield reduction, so a reduction of d_LQ must be attempted. Let us try a 6-hour set time.

An alternative to free flow at the furrow outlet is to block the downstream ends with a dike to prevent runoff. This is usually practical when field slopes are low. While blocking the ends prevents runoff, poor distribution of water can occur because of the ponded water behind the dike (Figure 10.17). Cutoff ratio guidelines that result in maximum efficiency have been established for all the cases discussed so far in this chapter. They are presented in Table 10.5. In general, when the ends are blocked, recommended cutoff ratios are higher than for the nonblocked case. This minimizes the size of the pond and the quantity of deep percolation beneath the pond.

In Examples 10.1 to 10.5, we reacted to what occurred in the field, i.e., we reacted to how fast the water advanced across the field. We refer to this as reactive management of surface irrigation. For someone irrigating a field for the first time, the data shown in Table 10.6 can help keep the flow rates, advance times, and field lengths within reasonable range.

Table 10.5. Recommended cutoff ratios to achieve maximum efficiency for sloping furrows.
Type of System	Soil Texture
	Fine	Medium	Coarse
No reuse	0.90	0.70	0.50
Closed reuse system	0.50	0.40	0.20
Open reuse system	0.70	0.50	0.35
Blocked ends (low slope, 0.1%)[a]	0.95	0.85	0.70
Blocked ends (moderate slope, 0.5%)[a]	0.90	0.80	0.65
[a] Based on data from Cahoon et al., 1995.

Are there alternatives to changing set time, stream size, and soil water deficit? As we have illustrated, one option for improving irrigation efficiency is to recover and reuse runoff water. The facilities necessary for recovering runoff are discussed in Section 10.7. Another option is to consider the furrow spacing. Alternate furrow or irrigating every other furrow should be considered if application depths are too large. In general, this practice will reduce infiltration by about 25 to 30%. For the same stream size, changing to every other furrow will increase advance time by about 30 to 40%, because of the longer time it takes for the wetting fronts between the irrigated furrows to meet. Watering every other furrow is usually a practice that can be used to reduce infiltration because even though the advance time is longer, the set size is twice as large as for every furrow irrigation.

Another field factor that can be changed, although not often desirable, is to reduce the furrow length. If the maximum nonerosive stream size is the limiting factor in achieving high efficiency, then furrow length should be reduced so that optimum advance times can be achieved.

Figure 10.17. Infiltration profile for blocked or diked end sloping furrows.

Surface irrigation efficiency can sometimes be improved by land smoothing. Land smoothing and laser grading will remove low and high spots and pot holes and provide uniform surface slopes. This will increase the advance rate of the water and uniformity of application.

Table 10.6. Furrow irrigation management recommendations for various soil types.
Soil Texture	Basic Infiltration Rate (in/hr)	Basic Infiltration Rate (gpm/100 ft)	Maximum Furrow Length (ft)	Recommended Stream Size[a] (gpm/100 ft)
Loamy sand	2.0–5.0	2.4	600	4.8
Sandy loam	0.5–4.0	1.9	800	3.8
Fine sandy loam	0.2–2.0	1.7	1000	3.4
Silt loam	0.2–1.5	1.1	1100	2.2
Silty clay loam	0.05–0.25	0.1	1300	1.4
[a] Actual stream size must be less than maximum nonerosive stream size.

Other options that can be used to overcome some of the constraints in surface irrigation are automation and semi-automation. This would eliminate the constraint of set time. Semi-automation of surface irrigation can be easily accomplished using surge flow irrigation valves, which will be discussed later. Another option is to use timers to terminate the inflow at the desired time in the absence of the irrigator. For example, if the irrigator can only return to the field every 12 hours but an 8-hour set time is desired, the timer could be set to shut off the water at 8 hours. The limitation of this procedure is that the system capacity, as discussed in Chapter 5, must be large enough to allow for the off time or down time that occurs between the time the system shuts off and the time that the irrigator returns to restart it.

If infiltration rates are too high to achieve the desired efficiency, then furrow packing and smoothing using special tillage tools might be helpful.

10.6 Basin and Border Irrigation

The management guidelines given so far in this chapter have focused on sloping furrow irrigation systems. For closely-spaced crops like alfalfa and orchard crops, basin and border systems are often more appropriate. Also, furrows sometimes are used in level basins which contain row crops. For basin irrigation systems, since the bottoms of basins are level and all of the water is retained within the basins by dikes, no runoff will occur. Thus, deep percolation is the only loss of water (again ignoring evaporation). In general, high stream sizes and low set times are appropriate since runoff is not an issue with these closed-level systems. Kay (1986) provides management guidelines for these systems. Readers can use computer simulation models such as WinSRFR (Bautista et al., 2012) to develop optimal management strategies for their individual site.

Border irrigation has many similarities to furrow irrigation in that the borders or bays have slope in the direction of flow and that they can be open-ended at the downstream end. Therefore, in some cases the management guidelines given for sloping furrow irrigation can apply to borders as well. However, sometimes the flow resistance of the closely spaced vegetation in borders can result in a significant amount of water stored on the soil surface before cutoff which can lead to long recession times and even to recession time exceeding advance time. When this occurs, the assumptions that we made in Section 10.5 would be invalid. In fact in some cases, optimal management is to stop inflow before advance is complete (Kay, 1986).

10.7 Runoff Recovery

10.7.1 Options for Managing Runoff

One of the challenges with surface irrigation is to achieve uniformity of infiltration while minimizing runoff from the field. Water must be present at the downstream end of the field long for uniform infiltration. This creates a potential for runoff. Runoff is an inherent problem with border and furrow irrigation systems.

As discussed above, blocking the downstream end of the field is one method for retaining runoff. When the slope is low enough, the retained water will spread back over a relatively large portion of the field. However, if the slope is too large, the ponded water infiltrates into only a small area. The result is poor water distribution. Blocking can also reduce the yield of crops that are sensitive to prolonged submergence.

Another option for minimizing runoff water is cutback irrigation. The concept here is to use a large inflow rate during the advance phase. Following advance, the inflow is reduced to a rate that approximately equals the steady-state infiltration rate of the wetted area. Without automation, this practice is labor intensive and requires good management. The correct cutback flow rate is difficult to estimate without considerable experience.

Recovering or reusing runoff water is another option. With a runoff recovery system, the runoff water is captured and returned to the field of origin or is delivered to another field. With runoff recovery, either less water from the original source is required to irrigate the same land area or more land can be irrigated with an equal volume. In either case, irrigation efficiency is increased. Runoff recovery has many other advantages including reduced nuisance problems associated with runoff, reduced energy requirements for irrigation, reduced labor, increased crop yields, and easier compliance with local regulations.

Often runoff causes nuisance problems downstream of the irrigated field. This can cause conflicts between neighboring farmers because surface drainage problems may occur on the downstream land. Capturing runoff reduces these problems.

If the original supply water is pumped, runoff recovery saves energy when the total dynamic head required to pump the runoff water is less than that required for the original supply. Usually less labor is required for irrigating if runoff recovery is employed. With less worry about the fate of runoff, irrigators do not monitor the water as closely or change sets as often. Crop yields sometimes are improved with runoff recovery if it results in more completely irrigating the downstream end of the field.

An important advantage of runoff recovery can be the ability to comply with water laws and regulations. In some regions, especially where groundwater is being depleted by irrigation, regulations limit the total volume of water that can be pumped from the aquifer. Sometimes the regulations specifically state that runoff cannot leave the irrigated farm. Runoff recovery systems facilitate compliance with these types of regulations.

10.7.2 Description of Runoff Recovery Systems

(a)(b)

Figure 10.18. (a) Runoff recovery reservoir, and (b) sump and pump for runoff recovery.

A runoff recovery system (Figure 10.18) has the following components:

• Drainage ditches for collecting and conveying runoff from the downstream edge of the field to the storage facility.
• A sump or reservoir for storing the runoff water.
• Inlet facilities to the sump or reservoir. These include a desilting basin for settling sediment from the runoff water, screens for removing trash from the water, and a chute, drop, or pipe inlet to deliver the water to the sump without causing serious erosion.
• A pump and power unit for withdrawing the water from the sump and, if necessary, pressurizing it for conveyance.
• A conveyance system, pipelines or open channels, for transporting the water from the storage facility to the field of use. Runoff water can either be returned to the field of origin or be delivered to another field for application. Often, using the water on a different field reduces initial costs because the runoff water is conveyed a shorter distance and normally down slope. If runoff is the only source of water for the receiving field, a very accurate estimate of the volume of runoff from the field or origin is necessary.

10.7.3 Design of Runoff Recovery Systems

The design of only the reservoir and pumping facilities will be discussed here. Two alternative designs, a continuous pump and an intermittent pump, will be considered.

For the continuous-pump system, the reservoir is designed to store the runoff from one irrigation set (plus allow for any necessary freeboard and unusable or dead storage). The capacity of the pump should equal the time averaged rate of runoff or, stated another way, equal to the volume divided by the time of cutoff (set time). The volume of storage that is required depends on the field and management conditions, but typically is from 20 to 55% of the volume applied to one irrigation set. If the runoff ratio, the ratio of the volume of runoff to the total volume applied, is known and the runoff recovery reservoir is full at the start of the irrigation, the following equations apply:

(10.13)

(10.14)

where: V_r = runoff volume from one set (active volume of return reservoir),

Q_r = capacity of the runoff recovery pump,

R_r = runoff ratio,

t_co = cutoff time,

Q_w= inflow rate to the field from the original source,

F = design factor,

F = 1, if the runoff is returned to the field of origin, and

F = 0, if the runoff is delivered to another field.

Since some of the runoff water will not be recovered, due to seepage and evaporation, these design equations contain a margin of safety. However, it still is important to include an additional margin of safety by using a high estimate of R_r. Suggested values for R_r are from 0.30 to 0.40.

As the name implies, the continuous-pump system operates continuously, or nearly so. There is very little flexibility in the management of these systems. The intermittent system allows for more flexibility. In this case, the reservoir is designed to store the runoff from two or more irrigation sets and the pump only operates on an intermittent basis. This makes management easier. The return pump must have more capacity than that for the continuously-operating system. Usually, the recovery pump will have a capacity in direct proportion to the reservoir volume. That is, if the reservoir can store the runoff from two sets, the pump would have twice the capacity as the pump for a continuously-operating system. The irrigator can then operate this system when adequate water is present in the reservoir. This system is particularly useful where the water is used to irrigate another field.

Rainfall runoff should be diverted away from the storage reservoir to minimize the accumulation of sediment in the reservoir. A gate on the reservoir inlet can be used to prevent the undesired inflow. If the runoff water is being returned to the field of origin, and if the original supply is groundwater, a check valve should be installed on the water supply pump to prevent the backflow of contaminated water to the groundwater reservoir in the event that the supply pump fails. If the recovered water is used to irrigate a different field, be aware of the potential of unwanted pesticides that may accumulate in the runoff from the field of origin.

10.8 Surge Flow Irrigation

10.8.1 The Surge Flow Process

Surge irrigation or surge flow is the process of intermittently applying water in surface irrigation (Yonts et al., 1996) as compared to continuous flow where water is applied for the entire irrigation set time. Surge irrigation was first studied as a method of reducing the amount of runoff that occurred during irrigation (Stringham and Keller, 1979). It was discovered that the time required for water to move to the end of the field was reduced by applying water intermittently rather than continuously.

Figure 10.19. Tee-type surge irrigation valve.

Water can be applied intermittently by cycling irrigation water between two irrigation sets. In years past, irrigation water was cycled when it was not getting to the end of a field. The irrigator would move on to subsequent sets and return in 1 or 2 days to finish irrigating the partially watered sets. The second time, the irrigation water could be moved all the way to the end of the field because the soil surface had sealed where previously wetted by irrigation and thus more water was available at the point where flow had stopped. This same process is used with surge irrigation, except 3 to 6 cycles are used and the cycling is done automatically for short durations of 20 minutes to 2 hours.

When water first contacts the soil in an irrigation furrow, the infiltration rate is high. As the water continues to run, the infiltration rate at that point in the furrow is reduced to a near constant rate. If water is shut off and the furrow is briefly allowed to dry, the surface soil particles consolidate and form a surface seal in the furrow. When water is reintroduced to the furrow, the infiltration rate is low due to this sealing action. The result is more water moving down the furrow rather than infiltrating into the soil in the initial reach of the furrow. Surge flow can increase irrigation performance by providing a more uniform application.

Figure 10.20. Field installation of a surge valve (Yonts et al., 1991).

Rather than turning the water on and off to achieve an on-off cycle, an irrigation surge valve (Figure 10.19) is used to alternate flow between two irrigation sets. Figure 10.20 shows one method of using a surge valve. Cycle times used with surge irrigation vary with soil texture and slope. Fine-textured soils respond less to surge irrigation than do coarse- textured soils that have higher initial infiltration rates. If field slope is so steep that it causes a rapid rate of advance, the effects of surge irrigation will be reduced. If the infiltration rate of a soil is low due to soil texture or compacted layers, surge irrigation is likely to be ineffective in reducing the irrigation advance times below those for continuous flow.

Surge flow has been used to reduce irrigation runoff in some cases by using short duration cycles after the water has reached the end of a field. This helps maintain high uniformity of water application and improve overall irrigation performance. Another application of a surge valve is to use it for semi-automatic operation. The surge controller provides a 2-set semi-automated furrow irrigation system. For example, if an irrigator is limited to returning to the field to every 12 hours, two 6-hour sets can be accomplished in that time frame.

Surge flow may not always reduce the advance time of water down the furrow. If it does not, there may still be benefits of labor savings and runoff reduction.

10.8.2 Management of Surge Flow Irrigation

Normally intermittent application is accomplished by using surge valves to alternate the water between a left and right irrigation set (Figure 10.20). The irrigation on times, during which water is applied to one side of the surge valve, are normally between 20 minutes and 2 hours. For each irrigation, an equal amount of off time occurs during each cycle. This will not be the case when different cycles times are used to compensate for an irregular shaped field. A cycle time—the time it takes to complete a full on time and off time cycle—is based on furrow length, soil texture, and field slope. The number of surge cycles used should be based on field length and field condition. Long fields and fields with high intake soils will require more cycles (5 to 6); shorter fields with low intake soil will need fewer cycles (3 to 4).

It is common to advance water during each surge cycle a distance that is equal to that fraction of the number of surge cycles used. For example, if using 4 surge cycles during advance, divide the field into 4 parts and advance the water one-fourth of the field distance during the first surge cycle. The time required to move the water that distance is the Cycle 1 on time. For the second and subsequent on times, multiply the factors given in Table 10.7 by the Cycle 1 on time. Table 10.7 provides the on-time factors for four and six cycles during advance.

Table 10.7. Surge irrigation on time factors for four and six surge cycles during advance. (Table based on Yonts et al., 1996, and Fekersillassie and Eisenhauer, 2000.)
Cycle No.	Four Cycles			Six Cycles
	Fraction of Field	On Time Factor		Fraction of Field	On Time Factor
1	0.25	1.0		0.17	1.0
2	0.50	1.9		0.34	1.9
3	0.75	2.4		0.51	2.4
4	1.00	2.9		0.68	2.9
5	-	-		0.85	3.3
6	-	-		1.00	3.7
Post advance or cutback	-	0.8–1.6		-	1.5–3.0

Following the final advance cycle, set the valve for the cutback or post-advance phase. During cutback, the valve cycles the water at a shorter frequency between the two irrigation sets until irrigation is complete. Table 10.7 gives the on time factors for the post advance or cutback cycles.

If water does not reach the end of the field by the last surge cycle, adjustments are necessary. Options include increasing the number of surge cycles or decreasing the number of furrows in the set to increase furrow flow rate. If water reaches the end of the field sooner than desired, increase the number of furrows or decrease the number of surge cycles.

Cycle times and the number of cycles can be adjusted for each set of conditions. Many commercially sold valves will have preprogrammed cycle times based on furrow length or expected advance time. In addition, cycle times can be developed based on individual conditions. The valve will automatically change at those times selected.

10.9 Summary

Surface irrigation includes the methods of furrow, border, and basin irrigation. It is the oldest form and most commonly used method of irrigation in the world. Water is usually delivered to surface irrigation sets through gated pipe, siphons, or gated inlets. Water flows over the land by gravitational force. The land must be graded to a uniform surface with slopes from 0 to 2 %.

With surface irrigation it is important to properly proportion any water losses so that the total of runoff and deep percolation is minimized. This is accomplished by choosing the appropriate irrigation frequency, inflow rate or stream size, and set time.

To control or reduce runoff losses, the irrigator can choose to either block the downstream end of the field, use cutback irrigation, or use runoff recovery. All methods have advantages and disadvantages.

Surge flow irrigation can be used to help reduce set times and reduce infiltration rates of the soil so that infiltration is more uniform within the field. Surge flow is accomplished with special valves that are equipped with a programmable controller to cycle the water as desired.

Questions

1. What is the major factor that determines the effective depth of water applied in a surface irrigation system without runoff recovery? How can this factor be modified?

2. Graph an advance-recession curve that characterizes uniform water distribution. Explain each component and the required conditions.

3. Without making considerable alterations to a furrow irrigation system, what changes can a manager make to minimize runoff and deep percolation? How do these adjustments minimize water losses?

4. Graph and compare advance-recession curves for surface irrigation on a fine-textured soil and a coarse-textured soil. Explain the difference expected.

5. Given a furrow irrigated field with a medium textured soil and data below, determine: d_g, d_z, d_LQ, d_r, percent runoff, DU, and E_LQ. Assume that the advance and recession curves given in Figure 10.7 and the data in Table 10.1 apply to this problem.

Q = 900 gpm L = 1,200 ft Tco = 12 h

N = 40 watered furrows, every other furrow watered Furrow spacing = 30 in SWD = 4.0 in

You’ll need to use Tables 10.3 and 10.4 for this problem.

6. If the slope in Question 5 is 0.3%, is maximum non-erosive stream-size being exceeded?

7. What would the E_LQ be for the conditions of Question 5 if a closed reuse system were installed and R_t = 0.85?

8. a. For the medium textured soil used in Question 5, determine the desired cutoff ratio to achieve maximum efficiency with a closed runoff recovery system (use Figure 10.15).

b. How many gates should be opened to achieve the cutoff ratio given in 8.a.?

c. Is the maximum non-erosive stream-size being exceeded?

d. What is the theoretical maximum efficiency for the advance time in Question 8.a. without a 1-hour recession? Assume R_t = 0.85 and loam soil (use Figure 10.15).

9. Discuss the differences in efficiency between Questions 7 & 8.

10. Does the d_LQ exceed SWD in any cases given in Questions 7 or 8?

11. A farmer is making a conversion from continuous flow furrow irrigation to surge flow. Your job is to determine how to set the controller and estimate the savings in water due to surging. The following conditions apply:

Q = 800 gpm Row length = 1,200 ft Row spacing = 30 in Every other furrow is watered	ET = 0.25 in/d Irrigation frequency = 6 d (assume SWD = ET × time interval between irrigations) Net (effective) irrigation required per year = 10 in Field slope = 0.3%
Continuous flow:		Surge flow:
Furrows per set = 45 Set time (cutoff time) = 12 hr Average depth infiltrated (dz) = 2.7 in DU = 70% Advance time = 9 hr		Furrows watered per side of valve = 30 Set time = 6 hr on each side of valve (12 hr total for both sides) Average depth infiltrated = 2.0 in DU = 80% Inflow time for advance = 4.5 hr

a. Determine the on-times for each surge cycle using 4 advance cycles and 2 cycles after advance is complete (post-advance).

b. Determine the gross depth applied and effective depth applied for each irrigation and for the year for both surge flow and continuous flow.

c. How much less water was applied by surging? Express your answer in inches/year and percent savings.

d. Determine the E_LQ for the continuous flow and surge systems.

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