Hydrologic validation of remote sensing soil moisture product

Soil moisture represents a fundamental quantity to understand the interaction between the land surface and the atmosphere. As widely-known, it can be estimated through modelling approaches, in situ measurements and satellite sensors. Recently, coarse resolution satellite-derived soil moisture data from active and passive microwave sensors are becoming more readily and freely available. Moreover, some studies have already demonstrated the potential of these data for improving the applications for which soil moisture represents an important state variable such as flood forecasting, landslide movement prediction, numerical weather prediction, agricultural management, to cite a few. However, the robust validation of these observations still represents a key issue. In fact, the huge differences in the spatial support between in situ (1 dm2) and remote sensing (~1000 km2) measurements along with the low penetration depth of satellite sensors make the evaluation of their accuracy with classical approaches not straightforward. Alternatively, application-driven techniques, i.e. based on the evaluation of soil moisture products in terms of their capability to improve a specific purpose (e.g. floods, landslides), can be employed. In this study, two application-driven approaches for the hydrological validation of satellite soil moisture products are described and applied. In the first method the capability of satellite sensors to estimate the soil wetness conditions before a storm event was assessed by using rainfall-runoff data. In particular, by solving for soil moisture the relationship linking the generation of runoff with rainfall and soil moisture data allows to estimate a catchment-scale soil moisture 'observation' from rainfall-runoff data. In the second approach, by inverting the soil water balance equation, a simple analytical relationship for estimating rainfall accumulations from the knowledge of soil moisture time series is obtained. Therefore, rainfall observations can be directly adopted to evaluate the reliability of different soil moisture products. The wide availability of rainfall and runoff observations makes these two approaches potentially applicable at a large scale and has the advantage to directly offer a better understanding of the effective use of satellite soil moisture products for hydrological applications. The two methods are applied by considering the soil moisture products derived from two different satellite sensors, the Advanced Scatterometer (ASCAT) and the Advanced Microwave Scanning Radiometer (AMSR), and from the reanalysis ERA-Land dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). Rainfall-runoff data were collected for the whole Italian territory in order to perform a state-wide robust intercomparison e validation of the different products. First results reveal the reliability of the two methods that show consistent results among them and allows discriminating the accuracy of the different products in the different regions of Italy.