A water balance approach for estimating groundwater recharge rates through high-resolution satellite soil moisture

Groundwater represents a massive portion of the total freshwater available. It is the primary source of water for more than two billion people worldwide and an essential source for agriculture in many areas of the world. It is well known that water resources are expected to face an ever-increasing stress during the upcoming decades because of the combined effects of human exploitation and climate changes, and groundwater is not an exception. Recently, the monitoring of hydrological fluxes through approaches based on the closure of the water cycle budget has been boosted by the availability of multi-source satellite data sets. Under this perspective, this study aims at presenting a novel approach for estimating groundwater recharge rates from satellite surface soil moisture observations through a water balance approach. In order to do this, data from groundwater monitoring networks over selected pilot areas in central Italy have been collected for validation purposes. Several remotely sensed soil moisture products have been evaluated, limiting the selection to latest high-resolution (1 km) data sets, namely an experimental product derived by SMAP (Soil Moisture Active Passive) and developed by Planet Labs, the operational Sentinel-1 soil moisture data delivered by the Copernicus Global Land Service (CGLS) and a newer Sentinel-1 retrieval based on a first-order radiative transfer model (RT1). Preliminary results show a general good agreement between observed and satellite-derived recharge periods, with highest quantitative agreements found for stations monitoring shallower aquifers. Even though further investigation is required, the proposed framework opens the interesting perspective of an innovative hydrological application of satellite soil moisture data and, if successful, it can be potentially upscaled to different targets (i.e., from the regional to the country scale).