Evapotranspiration in Winter Wheat Under Different Grazing and Tillage Practices in the Southern Great Plains.

Precipitation in the Southern Great Plains (SGP) is highly variable both spatially and temporally with recurring periods of severe drought. Winter wheat (Triticum aestivum L.) – summer fallow system with conventional tillage is the principal dryland cropping system in this region for both grazing and grain. Not surprisingly, winter wheat is a drought avoidance species as it takes advantage of soil moisture that accumulates during summer fallow period and matures early enough to avoid hot and dry late summer conditions. Although summer fallow minimizes risk of crop failure, there are numerous sustainability issues such as poor precipitation use efficiency, increased soil erosion, and decreased soil organic carbon and nitrogen. No-till systems are known to alleviate these problems, but limited adoption is observed in this region. The main objective of this study is to document and compare daily and cumulative evapotranspiration (ET) over growing season and annual timescales in graze-out, graze-grain, and grain-only winter wheat in a 4-year winter wheat-canola (Brassica napus L.) crop rotation system with conventional (traditional) and no-till practices. This study is part of the larger SGP Long Term Agroecosystem Research (LTAR) and GRL-FLUXNET (a cluster of eddy flux towers) projects at the USDA-ARS Grazinglands Research Laboratory (GRL), El Reno, OK. Biometric measurements (e.g., biomass, leaf area index (LAI), percent cover, and canopy height) and ET were measured during the 2016-17 winter wheat growing season. Seasonal rainfall during the growing season was 740 mm. As expected, biomass, LAI, and ET rates were higher in the grain-only wheat than graze-grain and graze-out treatments. Daily maximum ET rates ranged from 4.18 (graze-grain) to 6.23 mm (grain-only). Similarly, growing season ET ranged from 396 mm (graze-grain) to 488 mm (grain-only). Long-term year-round measurements from our clustered and paired eddy flux towers can provide insights into the effects of tillage and grazing practices on water budgets of typical wheat cropping systems in the SGP.