Local recycling alters the balance of interactions between rainfall, evapotranspiration and soil moisture in the semi-arid Ebro basin.

Mediterranean climates experience important climatic variability often causing droughts, whose consequences are especially worrisome in highly human-altered basins such as the Ebro Basin. An accurate understanding of the governing interactions of the water cycle is crucial in this area, which is a basin representative of water-related issues of the Mediterranean area. The HUMID project (CGL2017-85687-R) studies how remote sensing data and models (Quintana-Seguí et al., 2019; Barella-Ortiz and Quintana-Seguí, 2019) can improve our understanding of the alterations of rainfall-evapotranspiration-soil moisture interactions, which is essential to characterize the water cycle in drought-prone regions. Climates in these areas are driven by radiative factors while controlled by water-related ones, but the dominance of certain feedbacks such as the one of evapotranspiration-rainfall can locally modify the water balance and interactions.

Within the complex climatic mosaic of the Ebro basin, there are areas with interesting high levels of local water recycling due to storm tracks of relevance at Iberian and even European scale. However, other areas of the basin barely show any moisture recycling. Since recycling suggests enhanced interaction between evapotranspiration and rainfall, this study explores the differences in the magnitude of rainfall anomalies with evapotranspiration and soil moisture anomalies between areas with low and high recycling. The comparison of the dominance of evapotranspiration-rainfall interaction over the other interactions of the water cycle is evaluated over areas of storm tracks compared to those barely affected by recycling. The comparison is conducted over three climatic types of the Köppen-Geiger classification: BSk, Cfa and Cfb in order to distinguish the relevance of recycling, mostly of local scale, in comparison to the climatic type, influential at the synoptic scale.

High-resolution remote sensing products such as SMOS 1km and MODIS16 A2 ET enable evaluating rainfall, evapotranspiration and soil moisture anomalies with a level of detail suitable for local-scale analysis. Standardized drought indices such as soil moisture deficit index (SMDI) or the evapotranspiration deficit index (ETDI) can be calculated based on SMOS 1km data (2010-2019) and MODIS16 A2 ET 500m. The SPI index is used for rainfall anomalies. To assess the impact of recycling on the rainfall-evapotranspiration and soil moisture interactions we compare the distribution and magnitude of lags between these three variable-specific drought indices at the contrasting regions. The method allows identifying differences in the distribution of lags between the SPI, ETDI and SMDI that differ depending on the vicinity to the storm track. The type of climate shows certain interaction with the effects of local recycling.

The study illustrates the worth of high-resolution remote sensing data to evaluate recycling mechanisms and the anomalies of the land-atmosphere system propagating drought across feedbacks, even at the local scale. This advantage facilitates a better understanding of the climatic variability in semi-arid Mediterranean climates while encouraging developing monitoring tools integrating the particularities of water-limited types of climate.