With water levels in the southern part of the Ogallala Aquafer declining, farmers in western Texas are working to squeeze the most production out of every drop of irrigation water they apply to their fields. With that goal in mind, the Texas Alliance for Water Conservation (TAWC), a partnership between producers, industry and Texas Tech University, has engaged in a series of on-farm trials and demonstrations since 2004.
Glen Schur, a farmer from the Texas Panhandle and Chair of the TAWC producer board, recently discussed the results of several comparisons of irrigation technology. In an initial three-year trial in 2011, 2012 and 2013, researchers compared results of using low energy spray application (LESA) with low energy precision application (LEPA) on 80-inch centers, on cotton crops under the same pivot.
In this trial, with both designs delivering the same water volume, the LEPA system provided better water penetration, higher yields and net returns more than double those from the broadcast LESA application over each of the three years. The cotton crop under the LEPA application produced net returns of $177, $184 and $184 per acre over the three years. Net returns under the LESA system were $60, $37 and $72.
Researchers then followed up with a comparison of LEPA (on 40-inch and 80-inch centers), low drift nozzles (LDN), precision mobile drip irrigation (PMDI) and LESA (broadcast spray from 80-inch centers). The researchers installed each of the systems to a single pivot on a flat field with uniform soil types, and calibrated them for identical water output.
This test tracked production of a sorghum crop and cotton crop under the same pivot during 2016. Using drones and sophisticated “Aqua Spy” probes, the researchers evaluated coverage, water penetration and dispersal through the root zones for each system. At each probe location, the team used one AquaSpy probe on the top of the furrow, one on the bottom and one midway between, to track water movement within the soil profile. Each 48-inc probe uses 12 sensors to measure soil moisture at four-inch depth intervals.
AquaSpy Agronomist David Sloane, PhD, says data from the probes illustrates considerable differences between irrigation systems in terms of water dispersal within the plant root zones. “You need to know where the roots are and where your water is going,” he says.
Based on probe data, Sloane says the LEPA system with 40-inch centers provides deep, uniform wetting in each row. LEPA with 80-inch centers provides lower initial cost, deep wetting with twice as much water from each nozzle, but coverage can be less uniform and result in dry rows. PMDI provides uniform wetting and typically deep infiltration, as do the LDN systems.
In this trial, the LEPA system on 80-inch centers provided deeper infiltration than LEPA on 40-inch centers, and also good lateral movement through the soil. Sloane notes that the field had been managed under a strip-till system, leaving old root channels to facilitate water movement between rows. Generally speaking, he says, no-till or minimum-tillage systems benefit water infiltration and irrigation efficiency.
For the 2016 sorghum crop, the rows under the PMDI system produced the highest yield (7,066 pounds per acre), followed by LEPA on 80-inch centers (7,033 pounds), then LEPA on 40-inch centers (6,612 pounds), while the LESA application produced 6,267 pounds per acre.
For the cotton crop, the results were somewhat different, with the LESA application produced the highest yield at 1,500 pounds per acre, with PDMI next at 1,420 pounds followed by LEPA on 40-inch centers, LEPA on 80-inch centers and LDN on 80 inch centers respectively.
Normally, Sloane says, the LEPA and PDMI systems would outperform the LESA applications. The difference here could be due to the LESA rows being on the inside span of the pivot, resulting in a longer soak time, although the total volume of irrigation water was the same. He also notes that the 2016 growing season in the Texas Panhandle area included unusually heavy and timely rains during August and September, and the broadcast spray might have helped infiltration of rainfall. Finally, he says, the rows under the LESA system were harvested just prior to a rainfall, with the other rows harvested after the rain, which might have negatively affected yields from those rows.
Sloane notes that in this trial, probe data showed water was able to move across plant lines under all treatments. Periodic rainfall probably helped in that water moves through wet soil more easily than through dry soil. In all the treatments, the crops used all the available moisture, with the probes showing dry soil to 48 inches at the end of the season. PMDI provides the best and most uniform distribution in the root zone, but cost is a factor, Sloane says. Likewise, LEPA on 40-inch centers provides good performance and flexibility but at a higher cost compared with LEPA on 80-inch centers. LDN on 80-inch centers outperformed LEPA on 80-inch centers in this test.
Generally, Sloane says, of the systems in this test, he would expect PMDI to provide the highest efficiency, followed by LDN, LEPA and LESA respectively.
If we need to reduce the volume of water applied, Schur says, we need to get away from broadcast spraying. Broadcast irrigation provides about 65% efficiency compared with 90 to 95% for precision applications.
The Texas Alliance for Water Conservation offers several video presentations about this project, and other water-management resources on its website.