Comparisons of lysimetric and Bowen ratio estimates of evapotranspiration

Abstract

Two sets of Bowen ratio and lysimeter measurements of evapotranspiration (ET) were compared for a field of winter wheat in Phoenix, Arizona. Daytime data for ten days of clear skies were examined. Daily lysimeter ET (LET) generally exceeded Bowen ratio ET (BRET). Advective cases were compared with lapse cases. For one set of lysimeter and Bowen ratio measurements, average LET was 10% more than BRET during advective conditions, while average LET and BRET were equal during lapse conditions. Results for the other pair of measurements were less conclusive due to unresolved lysimeter problems, with average LET 13% more than BRET during advection, and average LET 13% less than BRET during lapse conditions. These results suggest that the assumption of equal eddy diffusivities for heat and vapor caused BRET to underestimate evapotranspiration during advection. The Bowen ratio, wind speed, and wind direction were identified as possible variables for correcting BRET underestimation.