Calibration and Scale Performance of Bushland Weighing Lysimeters

Weighing lysimetry is the primary method to directly measure evapotranspiration, and the scale performance of weighing lysimeters is often affected by wind loading. This study was conducted to calibrate the weighing lysimeters at Bushland, Texas, and to determine the effects of wind on the measurement accuracy of the lysimeter scales. Applied mass amounts equivalent to 150% of the lysimeter range were applied, and lysimeter scale calibrations were determined. Wind influences were measured by covering the lysimeters with a rubber sheet to minimize evaporation during an extended period. The lysimeters were sensitive to mass changes as small as 0.05 mm (450 g), highly linear with less than 1% total error over the 250-mm range (2.25 Mg), insensitive to load distribution on the lysimeter surface, and sensitive to surface pressures created by wind loading. The effects of wind can be minimized with data smoothing but not eliminated. The USDA-ARS weighing lysimeters at Bushland, Texas, have evapotranspiration measurement accuracy necessary to determine evapotranspiration rates as small as 0.05 to 0.1 mm/h over time periods of 30-min or greater.