The European Space Agency Climate Change Initiative Soil Moisture (ESA CCI SM) product provides soil moisture estimates from radar satellite data with a daily temporal resolution. Despite validation exercises with ground data that have been performed since the product’s launch, SM has not yet been consistently related to soil water storage, which is a key step for its application for prediction purposes. This study aimed to analyse the relationship between soil water storage (S), which was obtained from soil water balance computations with ground meteorological data, and soil moisture, which was obtained from radar data, as affected by soil water storage capacity (Smax). As a case study, a 14-year monthly series of soil water storage, produced via soil water balance computations using ground meteorological data from northeast Portugal and Smax from 25 mm to 150 mm, were matched with the corresponding monthly averaged SM product. Linear (I) and logistic (II) regression models relating S with SM were compared. Model performance (r2 in the 0.8–0.9 range) varied non-monotonically with Smax, with it being the highest at an Smax of 50 mm. The logistic model (II) performed better than the linear model (I) in the lower range of Smax. Improvements in model performance obtained with segregation of the data series in two subsets, representing soil water recharge and depletion phases throughout the year, outlined the hysteresis in the relationship between S and SM.
This research was partially funded by the Foundation for Science and Technology (FCT,
Lisbon, Portugal), grant number UIDB/00690/2020. Furthermore, A.C.R.’s contribution to the
research was financially supported, first, by the Instituto Politécnico de Bragança through the Double
Diploma MSc programme in Environmental Technology with the Technological Federal University
of Paraná, Brazil, and second, by the EU FEDER Fund, through the POCTEP programme funding the
TERRAMATER research project (grant 0701_TERRAMATER_1_E).
