Genèse des crues normales dans les petits bassins versants ruraux

Normal runoff generation in small agricultural catchments

Unité Sol et Agronomie de Rennes-Quimper, INRA

Abstract

The small catchments with a Strahler order under 3 are adequate to identify the main processes of storm flow generation. The paper focused on the results obtained on intensive agricultural catchments located in Western France. The endmember mixing analysis (EMMA), applied to two high frequency storm events showed that stormflow is due to different hydrological endmembers, whose contribution varies strongly both within storm event and from one event to another. The processes are highly non-linear. Three points are detailed: the delineation of the contributive area; the importance and variability of surface runoff; the dynamics of groundwater on the hillslope as linked to runoff generation.

- The studies on the delineation of the saturated bottom lands, considered as the runoff contributive areas, to better constrain hydrological models such as TopModel showed: i) that the location of these area is related to the elevation relative to the river level, a modification of the slope computation in the Kirby index is proposed, ii) the increase of the connectivity of the saturated areas in wet periods; iii) the interest of radar data to better constrain TopModel.

- The surface runoff is not very high but does vary depending on soil surface and hydric conditions along the hillslope and during the year. Experimental and numerical simulations showed the effect of the rugosity at different scales on runoff generation.

- The analysis of the seasonal and within-event responses of the water table confirmed the predominant role of the groundwater in storm flow generation in this physical context. Finally, this paper shows how the structure of the landscape, including the different linear networks, can modify the storm flow generation in the physiographic context of Western France. As an example, the network of hedgerows strongly modifies the potential length of surface runoff and the dynamics of the groundwater close to the contributive area.