Evapotranspiration in high alpine catchments – an important part of the water balance!

In this paper the third water balance component, the actual evapotranspiration (ETA), is analysed. Although evapotranspiration rates decrease with increasing altitude, it can be concluded that substantial quantitative differences are found among temperature and energy balance-based techniques for estimating the ETA. The objective of this study is to apply a distributed and continuous hydrological model and to utilise standard meteorological datasets with a high spatio-temporal resolution (1 km² and 60 min) to estimate the evapotranspiration in high Alpine Austrian catchments. Compared with the Hargreaves and Thornthwaite methods the ASCE-Penman-Monteith approach yields substantially higher potential evapotranspiration (ETP) rates, with basin-values up to 24% higher compared with the temperature-based methods. The decrease of ETP with elevation ranges from 6 to 26 mm per 100 m. The ETA rates differ up to 15%, with a decrease of 18–28 mm per 100 m. About 30% of the annual precipitation is evaporated and this implies that even larger correction factors of precipitation are required to satisfy the runoff. The method is demonstrated in basins in the north central Austrian Alps.