Can stream architecture inform on subsurface properties to characterize intermittence ?
Résumé
Groundwater (GW) and streams are intimately linked. Transient groundwater storage is the main driver for streamflow during extended periods of droughts. The connection between GW and streams is more resilient when the drainage timescale is longer, i.e. with slower GW flows and low hydraulic conductivity. Eventually, a gaining stream can be transformed into a losing one when the GW table level drops below the streambed elevation, in which case surface and subsurface bodies become disconnected. Identifying the processes controlling fluxes of GW into stream boundaries has been identified as one of the main challenges by the hydrological community (Blöschl et al., 2019). The emergence of new surface observations of stream network dynamics, provides new opportunities to address this challenge. In this work, we explore the information content of surface stream mapping (perennial and intermittent rivers) to characterize subsurface hydraulic properties. To this end, we set up a process-based parsimonious model to relate the recharge assessed independently by climate models and the subsurface hydraulic properties to the structure of the stream network and its evolutions. The model uses the water balance of the SURFEX regional land surface model as an input of a 3D groundwater flow model. The river networks directly result from the interception of the GW table with the topography. We investigate about thirty watersheds in Brittany and Normandy covering a variety of climatic, geomorphological and geological conditions by calibrating observed perennial stream referenced in the national river database (BD Topage). Results are twofold: First, the perennial stream network is highly sensitive to the ratio of hydraulic conductivity to recharge. For each lithology, estimated permeabilities are found consistent with previous estimates reported in similar context. Secondly, hydraulic conductivities defined in the previous steady-state approach are used to simulate observed transient stream discharge and expansion/contraction dynamics of the stream network (Onde database). This surface-based information on the seasonal expansion and contraction of intermittent stream network contributes to constrain the aquifer storage capacity. The use of surface observations on stream to characterize near subsurface properties might bring up new opportunities to provide predictions for ungauged basins with absence of stream discharge data. Applied with other remote sensing products, this method could contribute to identify the spatial distribution and dynamic reorganization of intermittent rivers. It would be especially useful in a context of climate change, as reduced recharge would changes the organization, fragmentation and hierarchy of groundwater flow systems.