Satellite-based mapping of river discharge at very high spatio-temporal resolution over the Ebro and Po basins

The 4DMED-Hydrolog ESA project aims at developing a high-resolution (1km) and consistent reconstruction of the Mediterranean terrestrial water cycle by using the latest Earth Observation (EO) products. We exploit here the synergy between available EOs to better estimate the terrestrial water cycle components (i.e., precipitation P, evaporation E, water storage dS and river discharge RD). The obtained, more accurate, representation of our environment is intended to feed decision support systems, in a changing climate, for a more resilient society. Among the water components, RD is strategic because it integrates many water-related processes. Unfortunately, in situ RD measurements are very sparse spatially. This paper presents a new approach for the mapping (i.e., spatially continuous estimate) of RD based on indirect EOs and a water budget balance constraint. First, satellite estimates of P, E, and dS are corrected, at the basin scale, using RD from a gauge network. Second, the water budget is balanced at the grid level using a horizontal flow direction information from topography. This approach is therefore based on satellite products and in situ measurements, without the use of any dynamical model. This methodology is used over the Po and Ebro basins. We use the new P, E, and dS data products, at high spatio-temporal resolution (1km and daily), developed in the 4DMED project. The resulting RD mapping is evaluated using a leave-one-out experiment, resulting in a mean KGE of 0.6 over the Ebro, to be compared to 0.5 for a river dynamical model such as Continuum. The spatially continuous RD is, by design, closer to the in situ measurements. Such work combining EO datasets to optimize, at high spatial resolution, to optimize our monitoring of the water cycle opens new doors for hydrology, water management, agriculture, as well as natural hazards predictions and response.

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