Why has catchment evaporation increased in the past 40 years? A data-based study in Austria

Regional evaporation has increased in many parts of the world in the last decades, but the drivers of these increases are widely debated. Part of the difficulty lies in the scarcity of high-quality long-term data on evaporation. In this paper, we analyze changes in catchment evaporation estimated from the water balances of 156 catchments in Austria over the period 1977–2014 and attribute them to changes in atmospheric demand and available energy, vegetation, and precipitation as possible drivers. Trend analyses suggest that evaporation has significantly increased in 60 % of the catchments (p ≤ 0.05) with an average increase of 29±14 mm yr−1 decade−1 (± standard deviation) or 4.9±2.3 % decade−1. Pan evaporation based on 24 stations has, on average, increased by 29±5 mm yr−1 decade−1 or 6.0±1.0 % decade−1. Reference evaporation over the 156 catchments estimated by the Penman–Monteith equation has increased by 18±5 mm yr−1 decade−1 or 2.8±0.7 % decade−1. Of these, 2.1 % are due to increased global radiation and 0.5 % due to increased air temperature according to the Penman–Monteith equation. A satellite-based vegetation index (NDVI) has increased by 0.02±0.01 decade−1 or 3.1±1.1 % decade−1. Estimates of reference evaporation accounting for changes in stomata resistance due to changes in the NDVI indicate that the increase in vegetation activity has led to a similar increase in reference evaporation as changes in the climate parameters. A regression between trends in evaporation and precipitation yields a sensitivity of a 0.22±0.05 mm yr−2 increase in evaporation to a 1 mm yr−2 increase in precipitation. A synthesis of the data analyses suggests that 43±15 % of the observed increase in catchment evaporation may be directly attributed to increased atmospheric demand and available energy, 34±14 % to increased vegetation activity, and 24±5 % to increases in precipitation.