The effects of elevation on surface water fluxes in dry alpine grassland ecosystems were investigated along an elevational transect between 1000 and 2000 m above sea level established in the Vinschgau/Venosta valley, a relatively dry region in the Italian Alps. The GEOtop-dv hydrological model was employed in point-scale mode to model the effects of the elevation gradient on snow water equivalent (SWE), soil water content (θ), evapotranspiration (ET), aboveground biomass (Bag) and water use efficiency (WUE) in different climatic conditions. Results show that SWE decreased strongly with decreasing elevation but was also affected by the interannual variability of meteorological drivers. During warmer years, the magnitude of changes in SWE was mitigated at higher altitudes while exacerbated below 1500 m. θ dynamics indicated that water stress conditions for vegetation currently occur at 1000 m each year, while only a warmer and drier year caused drought at 1500 m, and no water stress was found at 2000 m.
ET, Bag and WUE did not decrease with elevation but showed a maximum at an intermediate elevation around 1500 m because of the contrasting trends of a shorter vegetation season at higher elevations and water stress at lower elevations, where, in fact, irrigation is needed to maintain grassland productivity. A simulation based on long-term climatic conditions in combination with a sensitivity analysis of precipitation change showed that this effect is more pronounced during drier years, while for the wettest years, ET tended to decrease with increasing elevation. Taking these findings together, this study suggests that in relatively dry climatic conditions, mountain areas generally act as ‘water towers’ above 1500 m. Quantifying this critical threshold and its likely future variation under climate change scenarios is a challenge for water resource research in the Alpine region and can help stakeholders in planning future mitigation strategies. Copyright © 2014 John Wiley & Sons, Ltd.
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