Watersheds are complex systems with multiple interactions between physical processes and human-induced socio-economic dynamics. Since the 2000s, numerous flooding and mudslide events have affected the territory in Normandy (France), leading to significant damages. Therefore, a public policy was adopted with the aim to reduce runoff and erosion, it includes: (i) the building of 4,000 hydraulic infrastructures (dams, fascines, hedges, etc.), (ii) the creation of turbidity water-treatment plants and, (iii) the conduction of animation and protection programs on soil and water resources. These investments are co-funded by several local authorities. This original research project aims evaluating the effectiveness of the above-mentioned public policy. Therefore, two complementary approaches are applied: (i) at the regional scale, the investments and damages between 2000 and 2017 were assessed and, (ii) for a pilot small scaled watershed (la Lézarde, 212 km²) a coupled modeling was conducted, taking hydro-sedimentary processes (flood envelopes, diffuse and concentrated erosion, karstic transfers) and associated socio-economic dynamics into account. Our results suggest that over the study period, at the regional scale 500 M\euro were invested to reduce erosion/runoff impacts and, 300 M\euro of damage were caused. Nevertheless, the effectiveness of the public policy since 2000s must be evaluated at the watershed scale using a Cost-Benefit Analysis (CBA) according to two main scenarios: S1 = pre-development (2000), and S2 = post-development (2017). The processes that govern the surface transfer are modeled for different design floods (Q10-50-100) coupling two semi-dynamic models (MikeSHE and Watersed), and the karstic transfer using a deep learning algorithm (Tensorflow). Additionally, three long-term scenarios (until 2050) are modeled taking into account the effects of climate change (RCP scenarios), the change in land use (-33% of grassland areas), and the modification of agricultural practices that limit runoff. These projections provide key elements for decision-makers to guide future public policies controlling runoff and erosion in this territory.

Agricultural drainage networks increase hydrological connectivity from the field to the receiving environments. The response to the issue of surface water quality therefore implies an understanding of the hydrological processes related to drainage, particularly at the field scale. Drainage by tile drains and drainage ditch are the two most studied types at the plot scale. They can be complemented by temporary surface drains to improve the removal of surface runoff. The hydrological processes and functioning of tile-drained fields have been extensively studied at the event scale. However, few studies have been conducted over a full hydrological year and the description of water pathways in the soil generally relies on either exogenous tracer monitoring or irrigation experiments. In addition, only a few studies have been conducted on fields combining tile drainage and temporary surface drainage. In this study, high temporal resolution quantification of runoff from surface and subsurface drainage was conducted for a full year to establish one of the first water balances for a surface and subsurface drained field. Soil water pathways were studied under dry and saturated soil conditions tracing water by measuring stable isotope concentrations (18O and 2H) on rainwater, soil water, and surface and subsurface runoff. Runoff quantifications showed that surface drainage and subsurface drainage respectively evacuate 41% and 32% of the annual cumulated effective rainfall. The water balance highlights the importance of infiltrations to the deep horizons: 46% of the water transferred to the soil is not captured by the subsurface drains. Water tracing showed that rainwater was directly transferred to subsurface drains on dry soil, likely through macropores. On saturated soil, soil water present before the rain remains the main source of water to the subsurface drains, but event-rainwater also reaches the subsurface drains and can constitute up to 25% of the subsurface runoff volume.