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

Abstract

Abstract. Global warming has increased regional evapotranspiration 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 evapotranspiration.In this paper, we analyze changes in catchment evapotranspiration 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 10 energy, vegetation, and soil moisture as possible drivers. Trend analyses suggest that evapotranspiration has significantly increased in 60 % of the catchments (p≤0.05) with an average increase of 29 ± 14 mm y . Estimates of reference evaporation accounting for changes in stomata resistance due to changes in 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 evapotranspiration and precipitation, as a proxy of soil moisture, yields a sensitivity of 0.30 ± 0.04 mm y −2 increase in evapotranspiration to 1 mm y −2 20 increase in precipitation. A synthesis of the data analyses suggests that 38 ± 13 % of the observed increase in catchment evapotranspiration can be directly attributed to increased atmospheric demand and available energy, 30 ± 12 % to increased vegetation activity, and 32 ± 5 % to increased soil moisture due to increases in precipitation. IntroductionEvapotranspiration (E) is an important process in the water, energy, and carbon cycles and directly controls agricultural 25 productivity and water availability for human purposes. Global warming has increased regional E in many parts of the world in the last decades (Huntington, 2006). However, due to the difficulty of measuring E, especially at large spatial scales, the drivers of changing E are still debated.Decadal changes in catchment E may be inferred from the catchment water balance, as storage changes are usually small over decadal scales. Surprisingly few studies have investigated trends in water balance based evapotranspiration (E wb , despite increases in air temperature (Peterson et al., 1995;Roderick et al., 2009;McVicar et al., 2012). In some instances, this decrease in E pan has been explained by decreases in net radiation and/or wind speed (Roderick and Farquhar, 20 2002;Roderick et al., 2007). In other instances, decreasing E pan has been interpreted as a consequence of increasing actual evaporation (Brutsaert and Parlange, 1998;Brutsaert, 2013). In Europe, most studies found increasing trends of E pan (e.g. Ireland (1963Ireland ( -2005 (Stanhill and Möller, 2008), England (1957-2004 and 1986 (Stanhill and Möller, 2008;Clark, 2013), Greece (1983Greece ( -1999 (Papaioannou et al., 2011), and the Czech Republic (Trnka et al., 2015)). In this paper, we use the term atmospheric conditions to summarize the drivers available energy and atmospheric demand. 3 CO 2 concentration and nitrogen deposition, Mao et al. (2015) found that...

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