Evaluation and validation of the ASCE standardized reference evapotranspiration equations for a subhumid site in northeastern Austria

Employing evapotranspiration models is a widely used method to estimate reference evapotranspiration (ET_REF) based on weather data. Evaluating such models considering site-specific boundary conditions is recommended to interpret ET_REF-calculations in a realistic and substantiated manner. Therefore, we evaluated the ASCE standardized ET_REF-equations at a subhumid site in northeastern Austria. We calculated ETREF-values for hourly and daily time steps, whereof the former were processed to sum-of-hourly values. The obtained data were compared to each other and to ET-values measured by a weighing lysimeter under reference conditions. The resulting datasets covered daily data of the years 2004 to 2011.

Sum-of-hourly values correlated well (r² = 0.978) with daily values, but an RMSE of 0.27 mm specified the differences between the calculation procedures. Comparing the calculations to lysimeter measurements revealed overestimation of small ET_REF-values and underestimation of large values. The sum-of-hourly values outperformed the daily values, as r² of the former was slightly larger and RMSE was slightly smaller. Hence, sum-of-hourly computations delivered the best estimation of ET_REF for a single day. Seasonal effects were obvious, with computations and measurements being closest to each other in the summer months.