A Remote-sensing Perspective on Evapotranspiration Trends in the Alps.

Abstract:

This study analyzes 20 years of changes in evapotranspiration (ET) over the entire Alpine region by exploiting MODIS timeseries, with the aim to detect significant trends and examine their main drivers. The Alps are considered the “water towers of Europe” because of the ecosystem services that they provide, sustaining socio-economic security in large lowland areas. Worryingly, Alpine climate is changing fast, with temperature increasing more than twice the global average during the 20^th century. This can affect the hydrological cycle, not only by causing the decrease of snow and glacier cover, but also by altering ET. In fact, ET is a key component of the water budget, as it feeds back to the atmosphere 60-80% of precipitation. Recently it has been shown that both actual ET and the evaporative demand of the atmosphere are changing in several regions, including some Alpine catchments. However, decadal studies at Alpine scale are still missing, and the drivers of these changes are not fully understood. Given the impacts that changes in ET can produce on the hydrological cycle and on the human activities that depend on water availability, it is important to quantify the entity of these changes and estimate their spatial distribution. Remote sensing offers the possibility to monitor ET on wide areas. This work exploits the 2000-2020 timeseries of the MOD16A2/A3 operative product at 500 m x 500 m spatial resolution, which is first validated at Alpine eddy-covariance stations, and then statistically analyzed for trend detection. The trend variability with altitude and land-cover is investigated. Furthermore, the influence of atmospheric demand and vegetation activity, for which Leaf Area Index (LAI) is considered here as a proxy, is studied. First results show a significant positive yearly ET trend in 32% of the Alpine vegetated area, up to 6.4 mm year^-1 at altitudes between 1000 and 1500 m. Monthly ET is increasing up to 1.3 mm month^-1 year^-1 in summer months, especially for grasslands and shrublands. Trends in yearly atmospheric demand explain up to 80% of ET increase, but they are occurring only in limited areas (6% of the area with positive ET trend). Conversely, LAI trends are less influential, but they are concurrent with ET trends all over the Alpine region. Next step will be the analysis of the influence of water supply on ET trends.