Increased erosion in a pre-Alpine region contrasts with a future decrease in precipitation and snowmelt

Abstract

Climate change is affecting the hydromorphological system. In many places, changes in sediment dynamics are closely correlated to changes in precipitation frequency and magnitude. However, in nivo-pluvial regimes, the hydromorphological response to climate is more challenging to predict as it is not only the amount and occurrence of precipitation that is changing. The changes in precipitation type (i.e., snow or rain), snow accumulation, and snowmelt rates will also have a significant effect on the catchment net precipitation (composed of direct precipitation plus snowmelt contribution), and this may affect overland flow, erosion, stream discharge, sediment transport, and deposition. We investigated the impacts of climate change on hydrology and geomorphology in a small catchment (Emme, 127 km2) located in the Swiss pre-Alps by simulating the difference between the hydromorphological response to net precipitation in the present climate and in three climate scenarios at the end of the century using the CAESAR-Lisflood landscape evolution model. For the most extreme climate scenario (RCP8.5), simulations showed that despite the reduction in net precipitation (by 7 %) and discharge (by 4 %), sediment yield at the outlet of the catchment increased by 6 %. This is not only because precipitation falls more as rain than snow during the cold months, but also because heavy precipitation is expected to intensify. On a seasonal scale, we found that the amount of net precipitation, discharge, and erosion will increase in winter at the end of the century, while it will decrease in spring. In all three climate scenarios, net precipitation is projected to decrease in summer, but sediment yields may both decrease or increase. Autumn is the season with the greatest changes in erosion, while net precipitation remains constant or only slightly increases. Furthermore, we found that erosion and deposition patterns are changing spatially, with more erosion in mid-elevations and more deposition in valleys. Although our results are specific to the study site, we expect similar trends in other catchments of the pre-Alpine region.