Potential groundwater influences on simulation of surface energy and water balance over France

Groundwater is a key component of the global hydrological cycle. It sustains base flow in humid climate while it receives seepage in arid region. Moreover, groundwater influences soil moisture through water capillary rise into the soil and potentially affects the energy and water budget between the land surface and the atmosphere. Despite its importance, most global climate models do not account for groundwater and their possible interaction with both the surface hydrology and the overlying atmosphere. At the "Centre National de Recherches Météorologiques", we use the ISBA land surface model to compute the energy and water budget at the land surface and the TRIP river routing model to convert the daily runoff simulated by ISBA into river discharges. Recently, we have developed a simple groundwater scheme into the TRIP model to account for both the groundwater dynamics and the river-groundwater exchanges at low resolution. This new scheme has been evaluated with success in off-line mode over France and at global scale. Main results showed improvement of the simulated terrestrial water storage and simulated base flow. In this study, we propose to explicitly couple this groundwater scheme implemented in TRIP with the ISBA diffusive soil scheme in order to simulate the interactions between the deep water table dynamic and the overlying unsaturated soil. This coupling is evaluated over France at fine (1/12°) and low (0.5°) resolution on the 1989-2009 period. The ISBA-TRIP hydrological system is forced by the SAFRAN atmospheric reanalysis. An original method based on the topography at fine resolution is proposed to represent the upward capillary fluxes at the coarse resolution of climate models. Moreover, the river geometry in TRIP is corrected with respect to previous studies. New relationships defining river geometry are derived from a large database of river water level and annual mean discharges covering the whole France domain. These new parametrizations allow to have more realistic river widths and river bankfull heights. First results show an increase of evaporation together with a decrease of drainage when capillary rises are taken into account. The next step of this work will consist to evaluate this coupling at global scale. The ultimate objective of this study is to evaluate the possible impact of groundwater on the present and future climate in a fully coupled earth system modeling.