FO-03875     Reviewed 1991

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Irrigation Water Management Considerations for Sandy Soils in Minnesota

Irrigation Water Scheduling

Irrigation water management or scheduling involves more than just turning on the machine because it has not rained for a few days or the neighbor is irrigating. Irrigation scheduling is a decision-making process to determine when and how much water to apply to a growing crop to meet specific management objectives (Rogers, 1989). To be successful requires the blending of the latest scientific information, technologies, and personal irrigation experiences into an effective and sound water management program.

A sound irrigation scheduling program can help an operator:

• prevent economic yield losses due to moisture stress.
• maximize efficiency of production inputs.
• minimize leaching potential of nitrates and other agrichemicals below the rooting zone.
• conserve the water resource and maximize its beneficial use.

Leaching of chemicals cannot be totally eliminated by proper irrigation scheduling, according to some specialists (Kranz, 1989; Fishbach et al., 1988; Ritter et al., 1988; Hergert, 1986; and Ritter, 1986). For example, if a significant rainfall occurs shortly after an irrigation, the excess water will percolate deep in the soil and may carry some agrichemicals below the root zone. Likewise, large rainfall during the off-season may leach some agrichemicals that remain in the root zone. It is estimated that 70-80 percent of the annual recharge to surficial ground water in central Minnesota occurs after harvest and before planting (personal communication with Nieber, 1989). Figure 2 shows the normal monthly corn crop water use rates and respective precipitation for west central Minnesota. Months where rainfall is significantly larger than crop use indicate highest potential for ground water recharge and possible leaching of agrichemicals.

Figure 2. Normal Monthly Corn Crop Water Use and Precipitation Rates for West Central Minnesota.

Effective irrigation is possible only with regular monitoring of soil-water-plant conditions in the field, predicting future crop water needs, and following the best recommended water management strategies. This also requires a basic understanding of soil-water-plant relationships and soil moisture monitoring techniques.

To set up and operate an effective irrigation scheduling program these sequenced procedures need to be followed for each field:

1. Determine the crop's active rooting depth and the corresponding available water-holding capacity for each soil type in the field.
   
   
2. Select the predominant soil type(s) that should be used for irrigation water management purposes.
   
   
3. Define the allowable soil water depletion limits for the selected soil types and the crop(s) to be grown.
   
   
4. Establish a soil moisture monitoring system and regularly (at least twice a week) keep track of the soil water deficit.
   
   
5. Initiate an irrigation when the soil water deficit is expected to approach the selected allowable soil water depletion limit by the time the irrigation cycle is completed.

A brief discussion of each of these steps is presented later in this publication. This procedure typically takes 5 to 20 minutes of the operator's time daily to keep updated after determining the initial soil water characteristics. If operator time is not available to regularly monitor the soil moisture, consider finding a crop consultant to assist in achieving the management objectives.

This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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