Are more advanced hydrological models necessary to help water managers adapt to climate change?

A current challenge in hydrological modelling is to provide water resource managers with projections of future water resources under various climate change, land use and water management scenarios. This will help them to develop climate change adaptation strategies specific to their territories. The hydrological models used to simulate these projections can be complicated and time-consuming to implement at a local level. Therefore, the challenge lies in providing water managers with modelling tools that are simple enough to implement, yet realistic enough in their representation of processes to simulate the correct hydrological response.

CACTUS (CustomizAble CaTchment model for water Use Scenarios) is an interactive tool that allows users to customise the characteristics of a simplified catchment, then run simulations using the distributed hydrological model J2000 and visualize the simulation results. In order to make the tool operational and quick to implement, several simplifications had to be made : (i) the shape of the catchment and river network is predefined and discretized into a fixed number of grid cells (185) and reaches (14), (ii) the input climate data are estimated for a specific localization based on a limited number of reference time series, (iii) the Penmann-Monteith formula to represent potential evapotranspiration has been replaced by a formula that depends only on latitude and temperature (Oudin formula), (iv) the model is not calibrated and the parameters are selected from standard values found in the literature, (v) climate change scenarios are produced by perturbing the climate variables of the present period (delta on the seasonal temperature, cumulative precipitation and number of rainy days). Thanks to these simplifications, the catchment can be configured in about ten to twenty minutes and a 40-year simulation can be run in a few seconds.

To evaluate the accuracy with which CACTUS can simulate catchment hydrology and its response to climate scenarios, simulation results obtained with this tool were compared with hydrological projections from an ensemble of 7 hydrological models and 4 climate projections (Sauquet et al., 2025), in 5 contrasted French catchments (1 high mountain basin, and 2 lower mountain basins, 1 agricultural basin in plains, 1 peri-urban basin). The comparison shows that CACTUS simulates hydrological regimes and changes in hydrological indicators (in terms of both signs and magnitudes) that fall within the uncertainty range of the benchmark 7-models ensemble. This demonstrates that quick-to-implement, simplified hydrological models can provide water managers with a valuable primary level information for initiating an exploration of adaptation strategies.

Sauquet, E., Evin, G., Siauve, S., Aissat, R., Arnaud, P., Bérel, M., Bonneau, J., Branger, F., Caballero, Y., Colléoni, F., Ducharne, A., Gailhard, J., Habets, F., Hendrickx, F., Héraut, L., Hingray, B., Huang, P., Jaouen, T., Jeantet, A., Lanini, S., Le Lay, M., Magand, C., Mimeau, L., Monteil, C., Munier, S., Perrin, C., Robelin, O., Rousset, F., Soubeyroux, J.-M., Strohmenger, L., Thirel, G., Tocquer, F., Tramblay, Y., Vergnes, J.-P., and Vidal, J.-P.: A large transient multi-scenario multi-model ensemble of future streamflow and groundwater projections in France, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1788, 2025