Error analysis of evapotranspiration measurements

Abstract

Model, site, sampling and instrumentation requirements for the Bowen ratio energy budget method were evaluated to provide guidelines for proper application. An error analysis procedure was developed to evaluate system performance and identify potential areas of improvement. The procedures were applied to two sets of field data to identify the measurement errors. Bowen ratio energy budget measurements were made over an extensive stand of mesquite (Prosopis pubescens) in the San Pedro River valley near Mammoth, Arizona in May, 1978 and over a kochea (Kochea scoparia) pasture adjacent to the Pecos River near Roswell, New Mexico in July, 1979. Mean daily evapotranspiration over the mesquite stand was about 5 mm. The error analysis indicated mean half-hour 95 percent confidence intervals for latent and sensible energy of 0.48 ± 0.09 and 0.21 ± 0.09 cal/cm²/min respectively, with a majority of the error originating in the Bowen ratio measurement. Diode circuitry and calibration procedures were improved for the July 1979 study. The mean daily evapotranspiration over the kochea pasture was about 3 mm. The 95 percent confidence intervals on the mean half-hour estimates of latent and sensible energy were 0.27 ± 0.01 and 0.23 ± 0.01 cal/cm²/min respectively. The increased measurement precision was attributed to the improvements made in the system.