CALIBRATION AND SCALE PERFORMANCE OF BUSHLAND WEIGHING LYSIMETERS

Weighing lysimetry is the primary method to directly measure evapotran spiration, and the scale performance of weighing lysimeters is often a ffected by wind loading. This study was conducted to calibrate the wei ghing lysimeters at Bushland, Texas, and to determine the effects of w ind on the measurement accuracy of the lysimeter scales. Applied mass amounts equivalent to 150% of the lysimeter range were applied, and ly simeter scale calibrations were determined. Wind influences were measu red by covering the lysimeters with a rubber sheet to minimize evapora tion 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% t otal error over the 250-mm range (2.25 Mg), insensitive to load distri bution on the lysimeter surface, and sensitive to surface pressures cr eated by wind loading. The effects of wind can be minimized with data smoothing but not eliminated. The USDA-ARS weighing lysimeters at Bush land Texas, have evapotranspiration measurement accuracy necessary to determine evapotranspiration rates as small as 0.05 to 0.1 mm/h over t ime periods of 30-min or greater.