Reply on RC2

Abstract

To supplement the use of hydraulic tests and assess catchment-scale hydraulic conductivity (K), we propose a methodology for shallow aquifers only based on the Digital Elevation Model (DEM) and on the observation of the stream network. The methodology requires the groundwater system to be a main determinant of the stream density and extension. It assumes that the perennial stream network is set by the intersection of the groundwater table with the topography. The topographical structures and the subsurface hydraulic conductivity divided by the recharge rate K / R determine the groundwater table depth and the development of the stream network. Using a parsimonious 3D groundwater flow model, we calibrate K / R by minimizing newly defined distances between the simulated groundwater seepage zones and the observed stream network. Deployed on 24 selected headwater catchments from 12 to 141 km2 located in north-western France, the method successfully matches the stream network in 80 % of the cases and provides catchment-scale hydraulic conductivities between 9 x 10-6 and 9 x 10-5 m s-1 for shallow aquifers sedimentary and crystalline rocks. Results show a high sensitivity of K to the density and extension of the low-order streams and limited impacts of the DEM resolution as long the DEM remains consistent with the stream network observations. With the emergence of global remote-sensing databases combining information of high-resolution DEM and stream network, this approach will contribute to assess hydraulic properties of in shallow headwater aquifers.