Farmers can take some of the guesswork out of pre-irrigation decisions and possibly save some water and money if they determine appropriate pre-irrigation rates and application timing.
Recommendations depend on irrigation capacity and type of system.
Jim Bordovsky, Texas A&M research engineer at the Halfway Research Center, says growers hoe a thin row in determining how much water to apply before they plant. Adding too much results in runoff, excessive evaporation, or losing moisture out the bottom of the soil profile. Too little and cotton plants may suffer unnecessary moisture stress at critical growth stages.
“The High Plains irrigation period is typically divided into pre-plant and in-season irrigation periods,” Bordovsky says. “With pre-plant irrigation, farmers attempt to bank water in the soil profile prior to planting for later use in fields where in-season irrigation capacity cannot meet the evaporative demand of the crop.”
He says high wind speeds and low relative humidity can cause severe loss of pre-plant water applications.
“A recent study at Halfway documented water resource losses (both irrigation and rainfall) over three years at 67, 60, and 47 percent for low elevation spray, LEPA, and subsurface drip irrigation treatments, respectively. Water losses came from evaporation and movement below the root zone.”
Based on that study, Bordovsky developed the following pre-plant irrigation recommendations for LEPA, subsurface drip and spray irrigation systems.
Subsurface drip irrigation
“Growers should apply sufficient water to insure seed germination, wetting the effective root zone around the drip line and then begin seasonal irrigations early. Based on the study, attempting to fill the profile results in water loss below the root zone and generally failed to increase lint yield compared to treatments receiving less pre-plant irrigation.”
“Apply sufficient irrigation to fill the effective root zone just prior to planting. Use multiple pivot passes in a wedge of the field in quantities that do not cause runoff,” Bordovsky says.
He says the effective root zone and effective water holding capacity of LEPA irrigated soil profiles are small compared to spray systems. “Because water is typically applied in alternate furrows and irrigation water tends to move down instead of laterally, the effective root zone of a LEPA-irrigated field may be only half that of a spray-irrigated field. If the water holding capacity of a 4-foot deep soil is 2 inches per foot of depth, the effective root zone of a spray-irrigated field could hold 8 inches of water compared to a LEPA-irrigated field holding only 4 inches.”
Adding more than the effective water holding capacity results in loss below the root zone. But farmers have to add enough. “Failing to fill the effective root zone of LEPA treatments prior to planting resulted in significant yield losses over the three-year test period.”
“Yield results for spray irrigation depend on timing and amounts of rainfall,” Bordovsky says. He recommends farmers complete field operations (applying phosphorus, listing, and installing furrow dikes) before initial irrigations.
“In general, do not start pre-plant irrigation prior to April 1. In low irrigation capacity areas, irrigate one-fourth to one-third of the pivot area adequately (bring to 75 percent of field capacity) by ‘windshield wiping’ the area, applying the largest amount of water possible per pass without causing runoff.
“After the first section under the pivot is sufficiently wet, irrigate the next quadrant the same way. As soon as an area is reasonably dry, mulch with a rotary hoe or sand fighter.”
Bordovsky says farmers with inadequate irrigation capacity will not have sufficient time before planting to irrigate the entire pivot area; however, the probability of receiving 2 inches of rainfall in May is better than 60 percent.
“Experiments show significant yield reductions by eliminating pre-plant irrigation in areas with low irrigation capacity,” he says. “However, light pre-plant irrigations generally waste water. At irrigation capacities greater than 0.2 inch per day, heavy pre-plant irrigations failed to increase yield significantly over treatments where in-season irrigation began early in the crop year.”
The goal, Bordovsky says, is use water as efficiently as possibly without sacrificing yield potential. And this season, wise use of available water becomes even more important with the high cost of pumping it.