Seasonal compartmentalisation of water in a grassland at 2600 m a.s.l.

Abstract

<p>High Alpine catchments are precious water-resources since they act as natural storage reservoirs, storing water in the snow cover and in the subsurface and thereby providing water during the dry seasons. Thus, a deeper knowledge of the hydrological functioning of these systems is necessary, in particular to make climate change projections. The role of seasonality is crucial in these catchments that generally exhibit a snow-dominated hydro-climatic regime.</p><p>Here we use high-frequency observations of stable isotopes of water to identify the seasonal origin of streamwater in a high-elevation Alpine catchment located in the Valle d’Aosta Region, Italy. We quantify the relative contribution of winter and summer precipitation reaching the stream through the Seasonal Origin Index (SOI<sub>Q</sub>), calculated using the δ<sup>18</sup>O values and the volumes of precipitation and streamflow. Highly negative SOI<sub>Q </sub>values are obtained suggesting that streamwater is mainly composed of winter precipitation. Conversely, the Seasonal Origin Index for evapotranspiration (SOI<sub>ET</sub>), which can be directly inferred from SOI<sub>Q</sub>, returns a positive value reflecting that plants preferentially take up water deriving from summer precipitation.</p><p>These findings allow us to develop a conceptual model of this Alpine system. This conceptual model suggests:</p><ul><li>a deep infiltration component, mainly composed by snowmelt water, reaching the stream through a preferential flow.</li> <li>a shallow infiltration component, predominantly represented by summer rainfall, that dominates the shallow soils and that is used by plants.</li> </ul><p>Therefore, we presume a seasonal compartmentalisation of water in this high-elevation catchment.</p><p>Nevertheless, a previous study in Switzerland revealed SOI<sub>Q </sub>≈ 0 for the Allenbach and Dischmabach snow-dominated catchments, indicating that similar fractions of summer and winter precipitation become streamflow. This different result achieved in systems with an apparently similar functioning highlights the need for a deep insight into the flow paths governing high-elevation catchments and it opens the way for new challenges to understand the hydrological processes hidden behind this difference.</p>