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Center pivot irrigation systems typically wet the soil surface 15 to 20 times during the irrigation season. Each wetting cycle produces evaporation directly back into the atmosphere, depending on the amount of energy reaching the soil surface. Even with developed crop canopies, energy-limited evaporation can be a substantial portion of evapotranspiration (ET), about 30% of the ET during the irrigation season for fully irrigated corn grown in sandy soils.
Crop residues on the surface have a role in reducing evaporation from soils. UNL research at North Platte demonstrated that the equivalent of 6000 lb/ac of wheat stubble laying flat could reduce bare soil evaporation by half under a fully irrigated corn crop. This crop received only nine irrigation events during the growing season each year. The projected full season evaporation (120 days) is shown in Table 1.
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Evaporation in the fully irrigated trial was reduced by 6.8 inches due to the crop canopy and another 3.8 inches due to the straw mulch. This savings of almost 4 inches of water in evaporation due to the crop residue is important, but represents only part of the story. This savings was measured during 120 days of the growing season. Additional benefits of crop residue extend throughout the year and include runoff control and soil moisture capture and snow capture. These other benefits could easily add 2 inches or more soil moisture to irrigation programs, as has been shown in dryland systems with residue management.
More recent KSU irrigation research at Garden City, Kansas, showed that corn and wheat residues reduced evaporation by 50%, or about 0.03 to 0.05 inches per day (Table 2). This quantity is impressive on a daily basis because extrapolated over 100 days of a growing season, the water savings totals 3 to 5 inches. Reducing soil water evaporation with the adoption of crop residue management can lead to reduced pumping and energy costs for irrigators with adequate water and increased crop production for irrigators with limited water supplies.
Norman Klocke
Former UNL Extension Irrigation Specialist
Professor in Water Resources Engineering
Kansas State University
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