Development of Bahiagrass Crop Coefficients Using Weighing Lysimeters and Eddy Correlation Methods

Bahiagrass (Paspalum notatum) crop coefficients (K_c) were determined in central Florida in 2004 and 2005. Crop evapotranspiration (ET_c) rates were measured simultaneously using weighing lysimeters and an eddy correlation system. Reference evapotranspiration (ET_o) values were calculated by the standardized ET_o equation by the Environmental and Water Resources Institute of American Society of Civil Engineering using weather data from a nearby reference ET station. The K_c values were developed from the measured ET_c and calculated ET_o values. Relevant soil moisture results indicated that the soil at the experimental sites was not water stressed and had a full canopy cover. A 12 cm grass height was closely maintained during the two year study period. Lysimeters were considered as the standard to directly measure ET_c. In this research, three relative large weighing lysimeters, each equipped with four commercial load cells, provided an accuracy of 0.29 mm. The lysimeters were double steel tanks with an inner surface area of 2.32 m² and a soil depth of 1.37 m. An eddy correlation system was used to directly measure the turbulent fluxes of water vapor (LE) and sensible heat (H) above the crop canopy. The distance between the lysimeters and the eddy stations was 80 m. Daily crop coefficient values were estimated using ET_c measured by the lysimeters and the eddy correlation system. The K_c values determined by the eddy correlation method were similar to that by the lysimeters during the summer season. The largest K_c difference was found in winter time when the grass growth was minimal and irrigation requirement was negligible.