Introduction

Uncontrolled runoff in landscape irrigation systems has presented a problem for our industry for many years. Low head drainage is not only common throughout residential and commercial landscapes, it is ubiquitous.

When we consider that turfgrass consumes more irrigated acreage and water than the top five agricultural crops (Diep, 2011), it becomes incumbent upon us to use every drop of irrigated water wisely and efficiently.

Over the last number of years new technology has allowed manufacturers of irrigation equipment to develop new controllers. One of the tools we use to provide optimal irrigation water to landscapes that is widely supported by public agencies is the Weather-Based Irrigation Controller (WBIC). A core feature of all robust WBICs is a ‘run/soak’ cycle that is automatically programmed to reduce runoff.

WBICs take inputs of soil, slope, precipitation rate and crop coefficients to maximize efficiency overall and to mitigate against runoff, particularly in sloped conditions. For example, under extreme slope conditions, a WBIC may dictate a ‘run’ duration of two minutes and a ‘soak’ duration of ten minutes to be repeated five times to provide ten minutes of irrigation.

This study discusses the effectiveness of the ‘run/soak’ cycle and shows that it is greatly enhanced if drain checks are installed on the low-head sprinklers. This research assumes that there are two sources of wasted water when runoff is uncontrolled: the amount of water that drains from the system each time the zone is turned off, and the amount of water that is used to refill the irrigation line that is not applied to the crop. That is to say, when the WBIC determines the optimal amount of water required for each zone, the water that is used to fill the line does not reach the crop in the time allotted. These two water sources are termed ‘fill water’ and ‘runoff’.

Research methodology

This research study examines the consequences of uncontrolled runoff in landscape irrigation under conditions when a ‘run/ soak’ cycle has been determined by the WBIC as necessary to mitigate runoff.

This study simulates uncontrolled runoff during five ‘run/ soak’ cycles of two minutes of run time. The study examines two sources of wasted water: runoff from low head drainage, and the water that is used to fill up the irrigation lines after drainage of the line occurs. The research simulates the performance of: PVC lines 1'', 2'', 3'', 4'', 6'', 8'', 10'' and 12''; water velocities 5, 7 and 10 feet/second; and a fill-time of each line of 30 seconds. Rates of flow were calculated in gallons per minute (GPM) and water capacity was calculated in gallons per 100 feet (Diameter Velocity and Flow Rate).

Results

The following tables illustrate the data derived for the simulation:

Table 1: Flow rates at different velocities and Gallons per 100 feet

The flow rates for fill-time are calculated by dividing the gallons per minutes by one-half (30 seconds) and then multiplying the quotient by five to achieve fill-time for five cycles.

Table 2: Flow rate for fill-time and Flow rate for five cycles

The table below summarizes and illustrates the runoff water for five cycles, a 30-second fill, and a 30-second fill times five at different pipe sizes and different velocities:

Table 3: Runoff for five cycles, fill for one and fill for five cycles


The aggregate of water wasted and the percentage of water wasted when compared to the theoretical application of ten minutes of water is represented below.

Table 4: Total water wasted in quantity and percent


The following table is a summary illustration of the percentage of water lost, or wasted water, derived from the data in this study. The table indicates that for all velocities, at all pipe sizes, the percentage of water wasted is between 42 and 62 percent of total applied water when five two-minute cycles are programmed.

Table 5: Percent of total water loss

Percentage of total Water loss

Discussion

WBICs are estimated to save between 15 to 30 percent of water applied when compared to non-WBICs (U.S. EPA, 2011). This study provides evidence that there is a potential for more efficient use of applied water to control runoff. Controlling runoff, according to the evidence herein presented, saves between 42 and 63 percent of water in slope conditions when a run/soak cycle is applied. In order to control runoff, sprinklers that have built-in drain checks can help mitigate the amount of wasted water, but these sprinkler options are limited to the amount of head (pressure) that they can maintain. There are discrete drain checks on the market that can be integrated into irrigation systems to mitigate runoff under conditions of greater head (pressure).

Policy Implications

The implications for controlling runoff are greater than just water conservation. Frequently, liquid fertilizers, composed primarily of nitrogen, are applied through irrigation systems, and subsequently run off into groundwater and the ocean. In addition, when irrigation systems are pressurized by pumping systems, the efficiency of these pumps is reduced and the cost of pumping increased, when significant amounts of water are wasted. Finally subsidizing the costs of other conservation tools, while not recognizing the effectiveness of those tools to mitigate runoff, creates overall inefficiencies in rebate programs.

Conclusion

The stimulation study illustrates that run/soak cycles are only effective to the extent that runoff is mitigated by drain checks on the lowest heads. The effects of controlling runoff are positive and significant, and should be recognized as a critically important tool for water conservation supported by public policy agents and institutions.

References cited

Editor's Note: Mike Davidson is a partner in eagle spec sales group, a manufacturer's representative the western states. He has been involved in the irrigation industry for more than 30 years.