Eddy Covariance Measurements of Water and Carbon Dioxide Fluxes in Mid-South US Cotton

The land-atmosphere interaction on the field scale in cotton (Gossypium hirsutum) has been understudied. An eddy covariance system quantified this interaction by measuring water and carbon dioxide (CO₂) fluxes as crop evapotranspiration (ETc) and net ecosystem exchange (NEE) respectively. Measurements were made in a commercial field in Northeast Arkansas in 2016 and 2017 growing seasons (May-Oct) following typical production practices of the region. Over the past 22 years, the fields (Dundee silt loam soils) were center pivot irrigated and typically planted in cotton production, but also included corn, soybean, peanut, and wheat. Plant monitoring with COTMAN was used to assess changes in plant structure, maturity, and fruit retention. ETc increased after emergence likely due to higher transpiration demand and higher air temperatures, then decreased after physiological cutout during boll maturation, likely due to lower transpiration demand. Average ETc was 4.06 mm d^-1 (0.16 in d^-1). NEE decreased after emergence until first squares due to increasing gross primary productivity (GPP), stayed constant during squaring and flowering periods, then increased after physiological cutout during boll maturation due to decreasing GPP. Average (NEE) was -1.42 µmol CO₂ m^-2 s^-1 during the growing season. These findings begin to address questions related to greenhouse gas (GHG) emissions from agricultural fields.