Dealing with imperfect data to integrate farm dams and agricultural water uses in hydrological modelling

Farm dams are a controversial solution for ensuring the supply of water for summer agricultural crops. Their potential impact on catchment hydrological balance and river ecology is still an open question. In France, research work is mostly focused on small catchments, where it is possible to collect information. However, it is necessary to study the cumulative impact of farm dams at a larger scale and in contrasting (geographical, agricultural, geological, and climatic) contexts to guide the development of national regulations. To this end, large-scale distributed hydrological modelling integrating representations of farm dams and agricultural uses of water is a solution. This requires spatialised data on agricultural practices (e.g. types of crops, irrigated areas), farm dams (e.g. their locations, sizes, connections to the river network), and their uses (e.g. agricultural, industrial, recreational). These data are not always available, and if they exist, additional challenges are posed by the partial nature of the data, lack of documentation, and varying or overly coarse resolutions. The work undertaken here proposes a methodology to exploit these imperfect databases and to optimise the representation of the territories heterogeneity for hydrological modelling. We collected recent public data (last 20 years) for two major French river basins, the Rhône and the Loire (~100,000 km² each). Agricultural statistics (crops and irrigation) from national census, have an overly coarse spatial resolution and therefore need to be spatially redistributed. National inventory of water bodies (locations, surface) built using satellite imagery, contains limited data on the volumes and uses of water bodies. Annual water abstractions database (locations, volumes, uses, water origins), where the use of farm dams are heterogeneously documented. The first step reconstructs crop and irrigation surfaces at a fine spatial resolution over the domain of study in order to allocate variables in the model units, using a dedicated optimisation algorithms. The second step synthetises and allocates an equivalent farm dam to the model units, crossing informations from the national inventory embedded by local databases. The third step connects irrigated areas to water abstraction origin (farm dams, rivers or groundwater) using statistics rebuilt for both catchment. The spatial reconstruction and allocation of irrigation (i.e. farm dams) is validated by comparing the model units with the original data. In addition, the integration of farm dams and agriculture water use is validated by exploring the hydrological variables simulated with the J2000 hydrological model, in comparison with abstraction volumes. This modelling approach will enable the assessment of the impact of farm dams and their uses on catchment hydrology. It will also evaluate the capacity of farm dams to meet irrigation demands at different spatial scales, while accounting for the uncertainties associated with the imperfect nature of the databases.