Cumulative impact of farm dams on catchment water balance and streamflow dynamics at the regional scale. A numerical experiment using a distributed hydrological model.

Climate change increases the risk of water scarcity due to a higher probability of droughts and heat waves, even in temperate countries. A currently controversial adaptation strategy deployed by farmers is the multiplication of small dams to intercept water during the winter months (either from hillslopes or headwater streams), and store it through the summer months to secure irrigation and cattle watering. However, the impact of such practices on catchment water balance and streamflow dynamics is difficult to assess, because of the lack of reliable data but also the lack of models able to represent these devices. In the framework of the J2000 distributed hydrological model, we developed a simple component representing farm dams in mesoscale to regional scale catchments. The model was applied to the Rhône catchment in France (~ 100000 km²), using a database of known locations of farm dams to assess the impact of these dams on catchment water balance components and several streamflow dynamics indicators. Several scenarios were simulated, under present climate, according to various parameterizations such as absence / presence of dams but also varying the density and dimensions of the reservoirs, as well as their infiltration properties and drainage areas. The results show that the impact of such dams is potentially high, but is also highly dependent on the parameterization scenarios, thus confirming the need for more complete land and water uses databases.