High-resolution evapotranspiration and soil moisture data from remote sensing (SMOS1km and MODIS16A2ET) and the land-surface model SURFEX/ISBA helps identify the onset and propagation of Mediterranean droughts at weekly scale.

Abstract:

The Mediterranean climate dominates the Iberian Peninsula, a region where flash droughts represent a high percentage of total droughts. The Ebro River with its large basin, abundant flow, socio economic importance and climatic complexity in between the Atlantic, Continental and Mediterranean climatic influences requires careful drought monitoring at short temporal scales with high spatial resolution.

Fortunately, there are remote sensing products and land-surface models providing high-resolution datasets of multiple climatic indicators, of which evapotranspiration and soil moisture are of especial interest for short-term drought analysis. In this study we adopt the MODIS16A2ET dataset (Mu et al., 2011) derived from the NASA Terra satellite and the SMOS1km (Merlin et al., 2012) product derived from the SMOS ESA mission to generate weekly time series of drought indexes to analyse the onset of short-term droughts in the basin.

The weekly scale drought indexes adopted are the Evapotranspiration Deficit Index and Soil Moisture Deficit Index (Narasimhan and Srinivasan, 2005). Based on the lag-analysis of these indexes between them and with the SPI (McKee et al, 1993) (recalculated at weekly scale for the aggregation periods SPI-1, -3, -6 -12 months), the ETDI identifies early stages of drought while the SMDI shines on analyzing drought persistence.

Additionally, the Météo-France land-surface model SURFEX/ISBA (Durand et al, 1993; Noilhan and Planton 1989; Habets et al., 1999a; Boone et al., 1998) is included in the comparison to calculate the ETDI, SMDI and SPI indexes with an alternative dataset to those of SMOS1km and MODIS16A2ET. Thus, we compare the agreement between the remote sensing and land-surface model time series of drought indexes to assess the potential of this LSM as drought early-warning system.

To conclude, the study highlights the importance of studying drought evolution at sub-monthly scale in climates of high variability like the one of the Mediterranean region, especially for areas prone to flash droughts like the Ebro Basin. The study outlines how the increasingly availability of remote sensing and land-surface model´s data facilitates combining drought indexes of different scales and scope to improve our understanding about drought evolution in complex regions such as the Ebro River Basin.