There are several ways water leaves a crop field – through evaporation, transpiration and runoff, to name a few. Typically, evapotranspiration, or ET, takes the lion's share of water, with transpiration accounting for about 70% to 80% of the water used through ET, notes Steve Melvin, Nebraska Extension irrigated cropping systems educator. However, losses to evaporation as well as losses to runoff can be mitigated by residue.

That leaves about 30% losses due to evaporation. For fully irrigated soybeans, which require about 24 inches of water throughout the growing season, that's 7.2 inches of evaporation, notes Melvin. "We're never going to get this to 0%," Melvin says. "But we can reduce this pretty significantly if we work at it. In spring, when plants are small, that's when we get the most evaporation."

Residue means more water
One way to reduce evaporative loss is to leave more crop residue on the field. Research at West Central Research and Extension Center near North Platte from 2007 to 2010 shows about 3.5 inches of irrigation water can be saved when corn and soybeans are planted into full residue as compared to when planted where the residue had been completely removed. "That amount of water is the value of that residue," says Steve Melvin. "Keep in mind that one of the costs your neighbors have that don't no-till is the cost of pumping this extra water." However, keep in mind that when you save water through residue on irrigated fields, using a good soil water monitoring irrigation scheduling system is key to not letting those savings go to waste.

"Residue is kind of like an insulation blanket out there," he explains. "The two functions residue performs is: 1. Keeping the sun’s energy from directly worming the soil surface, and if the air temperature is warmer than the soil, slows the energy movement into the soil to evaporate water and 2. Keeping the wind off the soil surface, which also lowers evaporation."

Runoff during heavy rain events is a loss that can be reduced with residue as well, Melvin adds. Research at UNL South Central Ag Lab near Clay Center shows no-till systems have higher infiltration rates than those with tillage. In a study on plots that had the same tillage practices for a 25-year period, the plots that were disked and then planted had a permeability rate of 0.4 inches per hour, taking 1.5 inches per hour of rainfall to create runoff. Meanwhile, a ridge-plant field saw a permeability rate of 1.5 inches per hour, taking 3.2 inches per hour to create runoff. The no-till field had a permeability rate of 4 inches per hour, able to take 6.4 inches per hour of rain before runoff occurred. Melvin notes this research was on a field with about 0.5 percent slope.

Sprinkler package matters
Runoff under a center pivot is unacceptable and something must be changed if it is occurring. While no-till is a great way to reduce runoff potential on irrigated fields, selecting the right sprinkler package is essential.

One of the big determining factors is the wetted diameter of the sprinkler package – that is, how far it throws water in front and behind. "That determines how fast the water is coming down in inches per hour. The bigger the diameter, the lower the intensity," says Melvin. "You want to be able to apply an inch of water and not have any runoff. If you're on a steeper slope, doing tillage, you may need nozzles with a big wetted diameter like an impact sprinkler. In a nice, level field with reasonably high infiltration rate, there's no reason to have sprinklers that have large wetted diameters. Let's use something with a smaller wetted diameter and requiring lower pressure." Of course, Melvin notes, residue is a big deciding factor in minimizing runoff.