Comparing Evapotranspiration Rates Estimated from Atmospheric Flux and TDR Soil Moisture Measurements

Measurements of water vapor fluxes using eddy covariance (EC) and measurements of root zone soil moisture depletion using time domain reflectometry (TDR) represent two independent approaches to estimating evapotranspiration. This study investigated the possibility of using TDR to provide a lower limit estimate (disregarding dew evaporation) of evapotranspiration on dry days. During a period of 7 wk, the two independent measuring techniques were applied in a barley (Hordeum vulgare L.) field, and six dry periods were identified. Measurements of daily root zone soil moisture depletion were compared with daily estimates of water vapor loss. During the first dry periods, agreement between the two approaches was good, with average daily deviation between estimates below 1.0 mm d(-1). Toward the end of the measurement period, the estimates of the two techniques tended to deviate due to different source areas contributing to the flux estimates. With certain limitations, TDR-based evapotranspiration estimates are a promising approach for confining EC-based evapotranspiration.