Contribution of Alluvial Groundwater to the Outflow of a Mountainous Catchment

Abstract:

Alluvial soils in mountainous regions are typically heterogeneous and cover a few percent of catchment area. As a result, their contribution to outflow is rarely isolated or monitored continuously; alluvial groundwater discharge under river gauging-stations is generally assumed negligible; and hydrological models tend to lump alluvial storage with other units. To date, the role of alluvial aquifers remains unclear: can they contribute to outflow when they cover a limited area? Should they be considered a storage unit per se or a transmission zone? These issues are addressed experimentally through the continuous monitoring of groundwater and river discharge in the narrow alluvial system of a 208 km² perialpine catchment. The contribution of alluvial groundwater to outflow was highest at the outlet of a sub-catchment where subsurface discharge amounted to 16% of the annual outflow, and 80% of daily outflow during the three driest weeks. In this period, the groundwater flux was almost entirely induced by alluvial storage depletion, whereas over the entire system, depletion supported 40% of outflow. Storage fluctuation occurred predominantly in the upstream reach where the unsaturated zone was thickest, and the groundwater heads fluctuated over 6 m. Thus, the study shows that alluvial soils covering a limited area may contribute significantly to the outflow of mesoscale catchments. In such system, hydrological models will likely underestimate downstream river discharge, and have a biased calibration. Technically, however, the estimation of outflow may be improved, as illustrated here, by extending river gauging stations to monitor alluvial groundwater discharge.