Inter-Annual Carbon, Water and Energy Exchange of Irrigated and Dryland Cotton in the Texas High Plains

Cotton (Gossypium hirsutum L.) farming in the Texas High Plains largely depends on the Ogallala aquifer for irrigation. The declining water table in the Ogallala aquifer indicates a potential land use change from irrigated to dryland cotton systems that can modify the carbon and energy dynamics of the region. Therefore, the objectives of this study were to: 1) assess and compare carbon, water and energy (latent, soil, and sensible heat) exchanges of irrigated and dryland cotton and 2) investigate the environmental variables driving the carbon, water and energy exchanges. Eddy covariance systems were used for continuous measurements of carbon, water and energy fluxes from an irrigated and dryland cotton field in 2014 and 2015. In addition, meteorological variables were measured including air temperature, relative humidity, photosynthetically active radiation, solar irradiance, net radiation, and soil moisture. The irrigated cotton acted as source of carbon, releasing 104 and 134 g C m^-2 in 2014 and 2015, respectively. On the other hand, dryland cotton acted as weak sink of carbon in 2014 accumulating about 20 g C m^-2 in system and a weak source of carbon in 2015, releasing about 59 g C m^-2 into the atmosphere. Irrigated cotton (583 mm) recorded greater magnitude of evapotranspiration (ET) in 2014 than dryland cotton (470 mm). In 2015, ET was slightly higher in dryland cotton (697 mm) than irrigated cotton (656 mm) due to greater rainfall received in the area. Soil moisture and vapor pressure deficit were the major limiting factors of the carbon uptake and water exchanges of the system.