Radiometric Sensing As a Turfgrass Evapotranspiration Measurement Tool.

The current array of turfgrass evapotranspiration (ET) measurement tools being used have a variety of weaknesses, which makes small scale research and comparison between turfgrass species and cultivars difficult. The objective of this research was to evaluate the utility of local radiometric sensing as a tool to measure turfgrass ET, in attempt to have a centralized ET measurement method. The experimental plot measures 31 x 31 m and consists of Kentucky bluegrass (variety unknown) with 1% bentgrass contamination, surrounded by several hectares of assorted turf species. Evapotranspiration will be measured and compared by lysimeters, the FAO-56 Penman-Monteith equation, the Bowen ratio, and the radiometric method. Lysimeters are constructed of polyvinyl chloride (PVC) with inside diameter of 15 cm and height of 30.5 cm with a plexiglass sealed bottom with one 1.3 cm drainage hole for drainage outside of measurement intervals. Lysimeters were hand packed with native soil to existing bulk density and a 15 cm native sod core was inserted in the top of the lysimeter. Four lysimeters were randomly placed in the 31 x 31 m plot area with a 10 month preconditioning period. Solar radiation, wind speed, humidity and temperature needed for the FAO-56 Penman-Monteith equation were measured via a local weather station. Fine wire thermocouples measured air temperature and chilled mirror hygrometers measured dew point needed to calculate ET via the Bowen ratio. The dual-temperature difference method (DTD) was used to determine sensible heat flux needed in the energy balance equation to determine ET. Net radiation was measured with a net radiometer and soil heat flux with a heat flux plate for the FAO-56 Penman-Monteith equation, Bowen ratio and radiometric method. Daily ET was compared between all four methods for three seasonal periods in spring, summer and fall.