10.7: Runoff Recovery

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.

Description of Runoff Recover Systems

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.

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

(a) (b)

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:

\(V_r=\dfrac{R_r t_{co}Q_w}{1-R_r F} \) (10.13)

\(Q_r=\dfrac{V_r}{t_{co}}=\dfrac{R_r Q_w}{1-R_r F} \) (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.

Example 10.6

A continuous pump recovery system that returns the runoff to the field of origin is to be designed.

Given: Q_w = 500 gpm

t_co = 360 min

R_r = 0.30

F = 1

Find: V_r

Q_r

Solution

\(V_r=\dfrac{R_r t_{co}Q_w}{1-R_r F} \) (Eq. 10.13)

\(V_r=\dfrac{(0.30)(360\text{ min})(500\text{ gpm})}{\left[1-(0.30)(1.0)\right]} \)

\(Q_r=\dfrac{V_r}{t_{co}}=\dfrac{R_r Q_w}{1-R_r F} \) (Eq. 10.14)

\(Q_r=\dfrac{(0.30)(500\text{ gpm})}{\left[1-(0.30)(1.0)\right]}=214\text{ gpm} \)