There are good reasons to reduce the amount of water applied per irrigation. They include economics (becoming more efficient with irrigation water may reduce pumping costs or stretch irrigation district supplies) and regulations (overirrigation may result in the leaching of nitrates).

For instance, in California the Central Valley Regional Water Control Board regulations for dairies will limit the amount of nitrogen that can be applied to 140 percent of the nitrogen taken off in the crop.

(For example, if a crop removes 200 pounds of nitrogen, no more than 280 pounds of all forms of nitrogen should be applied).

If nitrate is leached by water draining through the root zone, it is not available for the crop, and you could end up with an underfertilized crop.

If a grower wants to adequately irrigate an entire field, the minimum amount of water that can be applied is the amount required to get water from the head to the tail of the field.

This may mean that the upper end of the field receives excess water, and it may mean that on the average, the field receives more water than is required to refill the crop’s root zone.

The excess water often results in deep percolation (drainage) of water and possible leaching of chemicals, such as nitrate, which the water can leach as it passes through the root zone.

One of the most effective ways of reducing the amount of water applied per irrigation is to use shorter field lengths.

There is a quick and easy way to determine how reducing the field length will impact the amount of irrigation water applied to a specific field. But first, a couple of important facts:

1. Irrigation water does not “advance” down the field at a constant rate.

Those who have watched an irrigation have noticed that at first water moves quickly down the field but, later in the irrigation, water advances more slowly.

Often near the end of an irrigation set, the water seems to be just slowly creeping down the field. This “slowdown” occurs because as more and more of the field is covered with water, there is increasingly more water soaking into the soil (infiltrating).

More infiltration means there is less water available to “push” water down the field, so the rate of advance slows.

The extreme case is that if there is not enough water flowing on to the field, all the water may infiltrate in the area already wetted and then the water stops advancing and never reaches the end of the field.

2. For most soils, the infiltration rate starts out high and then decreases the longer water is in contact with the soil.

Often, after a number of hours, the infiltration rate reaches a lower, relatively constant rate. This is actually a good thing for the efficiency of irrigation.

If the infiltration rate remained constantly high (some sandy soils are like this), the portions of the field that had water sitting on them the longest could be significantly overirrigated. A decreasing rate of infiltration mitigates this effect somewhat.

Determining the impact of reducing field length is an easy thing to do in furrow-irrigated or border strip-irrigated fields. All that is needed is a measuring tape, a marker and a watch. Here is a step-by-step procedure:

• Step 1: Measure the length of the field and place a marker at half the distance down the field.

• Step 2: Keep track of the times when the irrigation starts, when the advancing water reaches the half-way marker, and when the water reaches the end of the field.

What is the impact of reducing a 1,250-foot field into two 625-foot fields? We placed a marker at 625 feet down the field. The time for water to reach the 625-foot marker was three hours.

The time for water to reach the 1,250-foot marker at the end of the field was 10.5 hours.

If the field length was shortened to 625 feet, two irrigation sets, each three hours long, would be needed to cover what would have been the long field.

The first set would irrigate the top 625 feet of the field and then the second set would irrigate the lower 625 feet. The total irrigation time for the two 625-foot fields would be six hours (2 x 3).

How much water could be saved? The flow rate to the field is not even needed to determine this.

10.5 hours = 1,250-foot field
6 hours = two 625-foot fields

Subtract the six hours of irrigation for the two 625-foot-long fields from the 10.5 hours of irrigation for the 1,250-foot field. The water savings would be 4.5 hours of irrigation – over a 40 percent savings.

This would be a 40 percent reduction in pumping costs or a savings of 40 percent of district water to be used on another field, and it would likely mean that the amount of drainage water and leaching of nitrate from the root zone would be reduced.

In a field study on a 1,250-foot field where flow rates were measured, 10.5 hours of irrigation applied 12.9 inches of water, while the six hours (three hours per each of the two 625-foot fields) would have applied 7.1 inches.

The 12.9 inches was significantly more water than the root zone could hold, so drainage and possibly nitrate leaching occurred. Applying 7.1 inches of water significantly reduced this.

So why doesn’t everyone use short fields? Shorter fields mean more pipelines to supply those fields, a significant cost.

It can also mean additional roads and additional head and tail ditches, which take land out of production. It means more irrigation sets. If a tailwater return system is used, that is an added complication.

Finally, shorter fields are harder to farm. Moving equipment through shorter fields is more expensive and time-consuming.

Some growers have reduced the cost of shortening fields by running gated pipe across the center of the field, but this has its own complications such as gated pipe cost and labor to take pipe in and out of the field.

It comes down to balancing the cost and inconvenience of farming shorter fields versus the water savings and drainage and nitrate loss reductions which shorter fields offer.

From strictly a water savings standpoint, shorter fields probably are not justified unless water becomes extremely expensive or in very short supply, but when the nitrogen application limits for dairies are added in, shorter fields look better.

The choice may be an underfertilized and poorly yielding “long” field in which drainage occurred and leached the nitrate from the root zone versus adequately fertilized, higher-yielding “short” fields that have significantly less drainage with less nitrate lost to leaching.

It’s easy and costs nothing to determine how much water could be saved by cutting field length in half. Take the measurements and make the calculation. It’s good information to have when making management decisions.  FG

Larry Schwankl is an irrigation specialist and Carol Frate is a farm adviser, both with the University of California Cooperative Extension.

—Excerpts from California Dairy Newsletter, Vol. 2, No. 5

PHOTO
Photo taken at Bown Dairy, Fayette, UT.
Courtesy of Kristi Spence, Dairy Farmers of Utah. Click here to visit their blog.

Larry Schwankl
Irrigation Specialist
University of California Cooperative Extension