Irrigation Chapter 1 Why manage water for irrigation?

The goal of irrigation management is to use water in the most profitable way at sustainable production levels. For production agriculture this generally means supplementing precipitation with irrigation. In recent years we have seen declines in groundwater levels in Nebraska, almost statewide. Much of the State of Nebraska is considered fully or over-appropriated. This means that in those over-appropriated areas, there will be no new development of irrigated acres. Some Natural Resources Districts have established pumping restrictions for irrigation water. Increases in fuel prices means that pumping “extra” irrigation water increases irrigation expenses without increasing yield or income. In addition, high nitrate levels have been found in many areas of the state. Nitrates in drinking water can be attributed, in part, to over-application of nitrogen fertilizer and/or over-irrigation. All of these factors indicate that irrigators should be scheduling their irrigation applications to make maximum use of precipitation and reduce excess use of irrigation water to protect and conserve our water resources. Author: Chuck Burr, University of Nebraska Lincoln Extension Educator in Phelps and Gosper counties.

Perhaps the best way to achieve the goal of using water in the most profitable way at sustainable levels is to schedule irrigations with the appropriate amounts of water applied with the appropriate frequency. This is accomplished by routinely measuring the soil water status, rainfall, irrigation water applied and estimating crop water use. Local information on evapotranspiration (crop water use) is available via the internet:  active root zone and the needs of the crop. Irrigation scheduling is discussed in depth in Chapter 10.

Irrigation water must be applied uniformly and with little runoff. Center pivot systems and sprinklers must be matched to the soil intake rates and terrain to limit runoff and irrigations must be properly scheduled. After irrigation the soil should be probed or monitored to determine the penetration of applied water and uniformity of application. The irrigation system and tillage system should be modified if necessary to improve the application uniformity and efficiency.

With furrow irrigation, adjustments can be made to the stream size and set time to achieve the most uniform water penetration. Other techniques are similar to those used with a sprinkler system. Schedule irrigations properly, probe the soil or monitor the soil moisture, and modify the system for the best use of irrigation water.

If irrigation and rainfall are insufficient to meet crop demand, yield reduction is likely. Irrigating too much causes percolation of excess water below the root zone and/or unacceptable runoff.

There are immediate short range costs associated with either excess irrigation or less than full irrigation. For each acre-inch of irrigation less than what the crop demands, corn yields can be reduced by 6-10 bushels per acre.

The long range costs of over-pumping the aquifer — excessive percolation which moves nutrients below the root zone or runoff of water and chemicals — are more difficult to assess. Over irrigation can result in reduced yields, additional costs and contribute to ground water contamination. One way to minimize both long-term and short-term costs is to match irrigation to crop needs.

Basically, five items are needed to insure proper irrigation water management:

1. The irrigation system must be properly designed and maintained for efficient and uniform water delivery.
2. An irrigation water measuring device (flow meter) should be installed on each well. Irrigators must know whether they’re applying the needed amount of water.
3. A rain gauge is a simple, but necessary tool in irrigation water management. It can be used to determine how much rainfall to credit to the field’s soil water balance.
4. Several soil moisture sensors will help you check the soil water content and irrigation penetration.
5. A reliable source of crop water use information is critical when determining when to irrigate. It can be used to help determine the irrigation amount and timing. Crop water use information can be obtained from several sources, including television, radio, the internet and telephone hotlines. Average crop water use estimates will often over estimate crop water use needs and will often under estimate the crop water use needs, thus timely, accurate information must be used.

This has been a brief discussion of the importance of irrigation water management. The Irrigation Home Study Course will provide the information necessary to help an irrigator meet the challenges of maintaining groundwater quality and profitably produce crops in the face of a less abundant and increasingly costly water supply. This course will detail the finer management points needed to achieve these and other irrigation goals described in this chapter.