The hydrologic impact of land cover changes and hydropower stations in the Alpine Rhine basin

Abstract

The effects of water releases and storages by hydropower stations in the Swiss Alpine Rhine basin (4108 km 2) have been investigated and incorporated into hydrological modeling, using the fully distributed hydrological catchment model WaSiM-ETH. This model is driven by observed meteorological data with a grid size of 500 m × 500 m and a temporal resolution of 1 h. Additionally, an analysis of the possible consequences of land cover changes on the hydrology in a subcatchment of the Swiss Alpine Rhine basin (Landquart, 616 km 2) was performed using a combined urbanization-afforestation scenario. Furthermore, a theoretical investigation has been made by examining the effects of a hypothetical change from pastures into forests. The analysis of the runoff in the Swiss Alpine Rhine basin clearly shows the influence of reservoirs. In particular, a significant diurnal and weekly cycle in the release mechanisms can be noticed during low-flow periods in winter. Further, reservoirs function as buffers during flooding events, reducing the runoff peaks. By collecting reservoir level data, an attempt has been made to incorporate release and storage mechanisms of hydropower plants into hydrological modeling. Results show a clear increase in the model performance, especially during the release periods. The analysis of the land cover change scenarios using WaSiM-ETH shows that urbanization may have a significant impact on local flood events, while its effect is negligible further downstream. Changing grasslands into forests results in an increase in evapotranspiration, especially at the valley bottom. Consequently, runoff is reduced, particularly the direct runoff during the growing season. This decline in runoff is dependent on the decrease of soil and root depths with increasing altitude.