A Satellite-based Approach for Total Runoff Estimation: Stream Project

STREAM -SaTellite based Runoff Evaluation And Mapping, is an ESA project investigating the feasibility to derive runoff estimates from existing spaceborne missions. The purpose of the project is to develop and validate a solid "observational" approach alternative to existing model-based runoff estimates, that exploits space-only observations of Precipitation (P), Soil Moisture (SM) and Terrestrial Water Storage Anomalies (TWSA) for deriving Total Runoff. The first two variables will provide the event flow, i.e., the fast component of total runoff highly responsive to precipitation variability, while TWSA will be used for obtaining its complementary part, the baseflow, i.e., the return flow from groundwater. The project is motivated by the limitations of existing procedures estimating runoff, which can be grouped into model-based and observation-based approaches. On the one hand, model-based approaches provide outputs highly model dependent and characterized by large uncertainties. On the other hand, observation-based approaches, in particular hydro-geodetic approaches, generally propose a GRACE-based approach in which total runoff is computed only for large basins (>150000 km²) and only at monthly time step (limited by GRACE's spatial-temporal resolution).

STREAM tries to address four scientific objectives: 1) the development of a simple and operational procedure suited to estimate the event-flow component from satellite P and SM observations; 2) the development of a robust procedure to estimate baseflow from TWSA observations; 3) the study of the optimal way to integrate the event-flow component, produced by smaller scale P and SM, with the large scale baseflow variations linked to TWSA; 4) the identification of the best satellite-based P, SM and TWSA products suitable for developing the STREAM runoff estimates. For that, different satellite-based P, SM, TWSA products will be considered.

First attempts of STREAM total runoff estimates will be pursued for the period 2003-2017 at 5 pilot basins across the world (Mississippi, Amazon, Niger, Danube and Murray Darling) characterized by different physiographic/climatic features. First results for Mississippi and Danube basins proved the potentiality of satellite rainfall and soil moisture observations to estimate runoff at daily time scale.