Cotton yields are strongly influenced by the availability of water in the soil. The demand for irrigation is projected to rise as results of climate change, bringing increased consume of water resources and making the practice of irrigation more expensive. Therefore, there is a need to estimate evapotranspiration (ET) over cotton for water management. ET is a combination of evaporation from the soil surface and transpiration from the plant and is controlled by the plant (e.g., size, morphology, and canopy closure) and environmental factors (e.g., the soil water, soil surface conditions, and the atmospheric conditions). In this study, we will analyze how changes in environmental factors affect ET over an irrigated cotton field using the data measured in the Energy Balance Experiment (EBEX) in the summer of 2000.

EBEX was conducted in San Joaquin Valley, California over a flood-irrigated flat cotton field with an area of 1.6 km by 0.8 km. Eddy fluxes of sensible (H) and latent (LE) heat were measured in each level by an eddy covariance system that consisted of a three-dimensional sonic anemometer (CSAT3, Campbell Scientific), a fine-wire fast-response thermocouple (Campbell Scientific), and an open-path hygrometer (KH20, Campbell Scientific). Sonic anemometers measured fluctuations of three components of wind velocity and fluctuations of sonic temperature of the atmosphere. Hygrometers measured fluctuations of densities of water vapor. Along with the turbulent fluxes, a variety of micrometeorological variables were also measured as 30-min averages of 1 s readings. Net radiation was measured with net radiometer (Model Q-7.1, REBS). Additionally, thermocouples and soil moisture probes (CS615, Campbell Scientific) were buried at several depths to measure the profiles of soil temperature and soil moisture. The soil heat flux (G) at a depth of 10 cm was measured with two soil heat flux plates at each site (Model HFT3, REBS).

We will present the surface energy budget, radiative flux, sensible heat flux, and latent heat flux and their diurnal variations under different atmospheric conditions, and quantify the contribution of nighttime ET to daily ET. The results will be examined using a Penman–Monteith model to separates control of ET into the radiation (equilibrium) and atmospheric demand (imposed) components.