Long-term water stress and drought assessment of Mediterranean oak savanna vegetation using thermal remote sensing

Abstract

Abstract. Drought is a devastating natural hazard, difficult to define, detect and quantify. Global meteorological data and remote sensing products present new opportunities to characterize drought in an objective way, and to extend this analysis in space and time. In this paper, we applied the surface energy balance model SEBS (Surface Energy Balance System) for the period 2001–2018, to estimate evapotranspiration and other energy fluxes over the dehesa area of the Iberian Peninsula, with a monthly temporal resolution and 0.05° pixel size. A satisfactory agreement was found between the fluxes modelled and the measurements obtained for three years by two flux towers located over representative sites (RMSD\u2009=\u200921\u2009W\u2009m−2 and R2 of 0.76, for all energy fluxes and both sites). The estimations of the convective fluxes (LE and H) showed higher deviations, with RMSD\u2009=\u200926\u2009W\u2009m−2 on average, than Rn and G, with RMSD\u2009=\u200915\u2009W\u2009m−2. At both sites, annual ET was very close to total precipitation with the exception of a few wet years in which intense precipitation events, producing high run-off, were observed. The analysis of the anomalies of the ratio of evapotranspiration (ET) to reference ET (ETo) was used as an indicator of agricultural drought on monthly and annual scales. Hydrological years 2004/2005 and 2011/2012 stood out for their negative values, with the first one being the severest of the series, the impact observed on vegetation coverage and grain production. On a monthly scale, this event was also the longest and most intense, with peak negative values in January–February and April–May of 2005, explaining its great impact on cereal production (up to 45\u2009% reduction). During the drier events, the changes in vegetation ground cover over the months, with a preponderant presence of grasslands compared with those in which only oak trees were active, allowed a separate analysis of the strategies adopted by the two strata to cope with water stress. These results indicate that the drought events characterized for the period did not cause any permanent damage on the vegetation of dehesa systems. The approach tested has proved useful to provide insight into the characteristics of drought events over this ecosystem and will be helpful to identify areas of interest for future studies at finer resolutions.