Anne-Laure Cognard-Plancq

The role of forest cover on streamflow down sub-Mediterranean mountain watersheds: a modelling approach

The impact of forest cover on streamflow has been studied in sub-Mediterranean mountain watersheds using the daily time step conceptual rainfall/runoff model GRHUM. Certain characteristics (pertaining to the soil, vegetation or atmosphere) are used to compute model parameters that describe the behaviour of the various interfaces with respect to water exchange by evapotranspiration. This model enables the simulation of runoff and soil water content evolution. A snow function has been added to the initial version of the model to simulate the hydrological behaviour during the snow melt periods. This model has been applied to the Mont-Lozere experimental watersheds, for which hydro-meteorological data have been available since 1981. A site where a gradual felling of the forest was prescribed between 1987 and 1989 (the Latte watershed) is compared to an unexploited forest site (the Sapine watershed), used as reference. The period covered by the study of the Latte site has been divided into three parts, on the basis of changes in land cover. For both watersheds, the model produced good results over all three periods. It is also possible to identify the role of forests on hydrological behaviour by simulating discharge from the Latte watershed during the felling period using adjusted pre- and post-felling model parameters. This exercise has shown that felling induces a significant increase in high flow rates. A similar approach conducted for the Sapine reference watershed made it possible to evaluate the validity of our model predictions (accurate to within 10% of true values); the model bias being mostly due to climatic differences between the three simulation periods. Taking into account this model bias, results suggest that felling may increase runoff by 10%.

Combination of a crop model and a geochemical model as a new approach to evaluate the sustainability of an intensive agriculture system

By combining a crop model (STICS) and a geochemical model (PHREEQC), a new approach to assess the sustainability of agrosystems is proposed. It is based upon aqueous geochemistry and the stepwise modifications of soil solution during its transfer from the surface till aquifer. Meadows of Crau (SE France), irrigated since the 16th century, were field monitored (2012-2015) and modelled. Except for N, the mineral requirements of hay are largely covered by dissolved elements brought by irrigation water with only slight deficits in K and P, which are compensated by P-K fertilizers and the winter pasture by sheep. N cycle results in a very small nitrate leakage. The main determinants of the chemical composition changes of water are: concentration by evaporation, equilibration with soil pCO2, mineral nutrition of plants, input of fertilizers, sheep grazing, mineral-solution interactions in superficial formations till the aquifer, including ion exchange. Inverse modelling with PHREEQC allows for quantifying these processes. For groundwater, measured composition fit statistically very well with those computed, validating thus this approach. This long-term established agrosystem protects both soil and water resources: soil nutritional status remains constant with even some P and (minor) K fixation in soils; long-term decarbonatation occurs but it is greatly slowed by saturation of irrigation water by carbonate; P fixation in soil protects groundwater from eutrophication.