Surface water balance estimation in a mountainous and forested Mediterranean protected area using remote sensing estimates of evapotranspiration

Drought, as an extreme climatic event with a growing presence worldwide, plays an important role in forestry as well as in the management and conservation of natural areas, especially in the Mediterranean basin. Currently, the abandonment of primary activities because of the lack of economic profitability and the lack of generational relief is detrimental to agroforestry mosaics. The expansion of the forest mass due to the afforestation of former crops and pasture fields favors naturalization, but also leads to an alteration of the water balance at both local and regional levels. In addition, one of the effects of climate change will be an increase in the demand for atmospheric water for natural vegetation caused by the increase in temperatures and the decrease in precipitation and water reserves. This will have an important effect on the increase in wildfire risk as well as water flow decrease to maintain fauna and flora, especially in riparian habitats. Thus, in a global change scenario, the next challenge in the 21st century for the conservation and management of biodiversity and natural resources will be on how to adopt a set of technologies that allow monitoring and estimating water resources at regional scales. Evapotranspiration (ET) plays a significant role in the hydrologic cycle of Mediterranean basins, where surface-atmosphere exchanges due to ET may be more than 70% of annual precipitation. Together with precipitation (P), the surface water balance (P-ET), key parameter in the management and conservation natural resources, can be estimated. Even though ET is a significant component of the hydrologic cycle in this region, bulk estimates do not accurately account for spatial and temporal variability due to vegetation type or topography. The main objective of this study is to estimate the surface water balance at a regional scale on a mountainous protected area, the Montseny Biosphere Reserve, through the analysis of ET remote sensing estimates from 2017 to 2022 in a long-term gauged catchment. To estimate ET, the SEN-ET modelling framework (http://esa-sen4et.org) based on the Two-Source Energy Balance model that allows estimating high-resolution ET daily estimates at a spatial resolution of 20 m by sharpening thermal observations from Sentinel-3 satellites (1km, daily) and optical observations from Sentinel-2 satellites (20m, every 5 days) was applied. To estimate the surface water balance, daily precipitation was obtained by multiple regression analysis of meteorological stations. Preliminary evaluation of ET remote sensing estimates with ET derived from the long-term gauged catchment yielded an RMSE of around 1.5 mm·day^-1 that allowed computing a reasonable surface water balance for this period. Due to a severe drought within the study period, the annual surface water budget showed a decrease pattern. A water balance anomaly analysis showed that 80% of the reserve forest were under a negative anomaly for three consecutive years, pointing out that surface water balance derived from ET remote sensing estimates can be used to improve forest management by focusing on those areas that will become more affected by drought episodes.