Weighing Lysimeters for Evapotranspiration Research on Clay Soil

Local crop evapotranspiration (ET) measurements are needed for northeastern Louisiana. Objectives were to install, calibrate, and assess soil water characteristics of paired weighing lysimeters at Saint Joseph, LA. Installed in 2005, the lysimeters have inner tanks 1.5 m long, 1.5 m deep, and 1.0 m wide that rest on load cells within outer tanks. The inner tanks contain refilled Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquerts) and allow removal of freely draining water. Available irrigated fetch is >= 50 m in all directions with two discontinuities. Calibration yielded, with little hysteresis, a linear relationship (R-2 > 0.99) between applied mass and summed load cell output for each lysimeter. Measurement errors for mass changes during calibration were consistently equivalent to < 0.2 mm water. In 2007, at a 1.2-m depth, lysimeter and surrounding field soil water potentials were similar. The lysimeters appear suitable in mechanical operation and drainage to measure daily well-watered crop ET.