An eddy covariance quantification of soybean evapotranspiration in the Mississippi Delta

Authors: Anapalli, Saseendran; Wagle, Pradeep; Gowda, Prasanna; Reddy, Krishna; Fisher, Daniel; Sui, Ruixiu

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2018
Publication Date: N/A
Citation: N/A

Interpretive Summary: The Mississippi Delta relies mostly on groundwater from the Mississippi River Valley Alluvial Aquifer for irrigating its crops. Pumping water from this shallow aquifer beyond its natural recharge levels has resulted in significant groundwater-level declines, declining well production, and threatening future water-availability for the region. Of crops grown in the region, soybean represents the most irrigated acreage (53%) and the remaining is shared between rice, corn, cotton, and aquaculture. Quantifying water requirements and providing the crops with the right amount of irrigations at the right time not only helps optimize productivity but can help in containing further depletion of the groundwater resources in the region as well. For the first time in the MS Delta, rsearchers with the USDA ARS Crop Production Systems Research Unit at Stoneville, MS, and the Forage and Livestock Production Unit, Grazinglands Research Laboratory, USDA ARS, El Reno, OK, undertook a study to measure water requirements of soybean in the region. Average daily water requirements of soybean in the region was 0.2 inch. In a typical year, a 120 day (emergence to harvest) soybean crop uses about 25 inches of water for optimum growth and yield. The data generated will be useful for scheduling soybean irrigations using routine weather data collated at climatological observatories in the region.

Technical Abstract: Accurate quantification of evapotranspiration (ETc) from crops is critical in management decisions to enhance water-productivity in agriculture. We quantified ETc from soybean (Glycine max L.) using the eddy covariance (EC) approach (ETe). Recognizing the unresolved problems in balancing energy fluxes in the EC approach, we also monitored ETc by computing latent heat energy flux (LE) from the system following a residual energy balance (EB) approach (ETb) using added instrumentation and compared the fluxes. The unclosed energy fluxes in the EC was post-analysis corrected using the Bowen ratio (BR) and LE methods. The measurements were conducted in a 35 ha clay soil field planted to irrigated soybean in the lower Mississippi Delta, USA, in 2016. The crop was planted on April 24, emergence occurred in nine days, and physiological maturity occurred in 126 days after emergence (DAE). Maximum LAI was 5.7 m2 m-2 and average grain yield harvested was 4900 kg ha-1. The EC and EB measurements were started on DAE 38 and continued until physiological maturity. The EC measured fluxes showed an energy balance closure of about 81%. The ETe was 17.9, 18.1, and 10.1 % lower than ETb, and ETe corrected using BR (ETebr) and using LE (ETele) approaches, respectively. Average soybean seasonal ETe, ETb, ETebr, and ETele were 412, 486, 487, and 453 mm, respectively. For scheduling irrigations in soybean, based on grass and alfalfa reference crop ET calculated from weather data, averages of the ETb, ETebr, and ETele daily estimates were used in deriving crop coefficients (Kc). The Kc for grass reference crop varied between 0.56 and 1.29 and for alfalfa reference crop varied between 0.56 and 1.02. The experiments are being continued for collecting data for deriving long-term average Kc in soybean for irrigation scheduling applications across different soil and climate types in the region.