Identifying Contributing Factors of Evapotranspiration Drivers in US Mid-South Cotton Production

Cotton grown in the humid US mid-south can have lower evapotranspiration (ET) demand compared to more arid regions across the US Cotton belt due to higher humidity and temperature, and greater solar radiation variability. More regional measurements in the US mid-south are necessary to provide more accurate ET estimates for producers and atmospheric models. An eddy covariance system quantified H₂O flux (ET) by coupling high frequency (10-20 Hz) measurements of vertical wind speed and water vapor gas concentration to determine 30-minute fluxes. Measurements were made during the 2017 and 2018 production seasons (May-Oct) following typical production practices of the region. In addition to the eddy covariance system, four-component net radiometer, soil sensors and Phenocam were deployed to calculate surface albedo, soil moisture, and green chromatic coordinate (GCC) continuously. Leaf area index (LAI) in both years and normalized difference vegetation index (NDVI) in 2018 were measured weekly. Evapotranspiration was partitioned into modeled evaporation (E) and transpiration (T) components using LAI. Daily average surface albedo was noted to stay constant (0.19 ±0.0007) after canopy closure. ET was noted to increase 1-2 days after irrigation events. Early in the season, E from the soil dominated, ranging between 40-72% of ET, while later T dominated as nodes developed and canopy started to close, ranging between 60-72% of ET. Using single variable linear regression, ET was found to be influenced by solar radiation, air temperature, and relative humidity (R²=0.20-0.26). The low regression values suggest biotic processes such as boll loading, and node development have a greater contribution to ET more than meteorological conditions. These findings help address questions related to contributing factors to ET demand from agricultural fields and therefore can be used to guide irrigation decision-making.