Can we relate flood and low flow behavior to spatial distribution of thick quaternary deposits? Case study of the 14 km2 alpine Poschiavino catchment, Switzerland

Abstract:

Better understanding of water storage timescales of soils and quaternary deposits may improve flood prediction and low flow estimation in mountainous catchments. Even steep slopes can react damped to precipitation events and sustain baseflow during dry periods due to large storage. It remains an open question to what extent short term storage mechanisms that dampen flood runoff are also responsible for sustaining baseflow. Therefore we explored how flood and low flow behavior relate to spatial organization of storage potential in the upper Poschiavino, a 14km²basin with strongly contrasting subcatchments.

Winter months provide good opportunities for studying flow recession in alpine catchments because there is little groundwater recharge from rainfall and snowmelt. Therefore, discharge time series were obtained for different nested subcatchments in 7 campaigns throughout the 2013/14 winter season. Stream water electrical conductivity and various ion composition where measured to identify different drainage types and their origin.

To study the effect of storage on low flow, sediment cover type and thickness were mapped. This allowed classifying storage potential throughout the catchment. Alongside, contribution to flood formation was evaluated for different slopes using a recently developed tool for geomorphology-based classification of dominant runoff formation processes in mountainous terrain.

We found substantial spatial variation in drainage timescales and contributed volumes between the different subcatchments (54mm vs. 200mm discharged in four months). Subsurface flow and point source contributions complicate small scale studies of recession flow, suggesting this process should be studied at subcatchment rather than hillslope-scale. The recession analyses combined with time series of ion composition allowed detecting different drainage timescales and an estimation of storage volumes. The variability of low flow discharge and differences in recession behavior can be attributed to the mapped storage potential. Flood runoff behavior could be linked to the mapped distribution of storages, but also requires understanding of drainage mechanisms not only during recession but also during flood formation. Short and long term storage do not necessarily share the same mechanisms but can be related in some areas (strong runoff response - substantially lower winter discharge). Our observations suggest that understanding storage and drainage behavior of areas with large storage potential helps assessing catchment-scale flood and low flow problems.