A downward structural sensitivity analysis of hydrological models to improve low-flow simulation

► We detail a downward approach for improving a model for low-flow simulation. ► We compare several model structures on a large set of catchments and propose an improved model version. ► We compare the model with existing ones, with satisfactory results. Better simulation and earlier prediction of river low flows are needed for improved water management. Here, a top–down structural analysis to improve a hydrological model in a low-flow simulation perspective is presented. Starting from a simple but efficient rainfall–runoff model (GR5J), we analyse the sensitivity of low-flow simulations to progressive modifications of the model’s structure. These modifications correspond to the introduction of more complex routing schemes and/or the addition of simple representations of groundwater–surface water exchanges. In these tests, we wished to improve low-flow simulation while avoiding performance losses in high-flow conditions, i.e. keeping a general model. In a typical downward modelling perspective, over 60 versions of the model were tested on a large set of French catchments corresponding to various low-flow conditions, and performance was evaluated using criteria emphasising errors in low-flow conditions. The results indicate that several best performing structures yielded quite similar levels of efficiency. The addition of a new flow component to the routing part of the model yielded the most significant improvement. In spite of the close performance of several model structures, we conclude by proposing a modified model version of GR5J with a single additional parameter.