| Summary: | Understanding the spatial and temporal variability of droughts is important to improve drought predictability and ultimately to support drought risk management in agriculture. Several studies were developed, to better understand drought characteristics and variability. For that, the Standardized Drought Index (SPI) was used to identify the main spatial and temporal patterns of drought variability in Portugal, to search for regions with similar drought variability in continental Portugal and significant cycles using a Fourier analysis applied to the SPI, which revelead cycles of 6 and 9.4 years, likely influenced by the North Atlantic Oscillation (NAO) and two stable sub-regions, in the northern and southern regions of Portugal. Moroever, another study using the SPI suggested that for drought monitoring, when using non-stationary time series of precipitation, the reference period used for obtaining the Gamma distribution parameters is important. Altough the SPI is widely used it only uses precipitation, however, for assessment of agricultural drought conditions indices such as the Palmer Drought Severity Index and its modification for the Mediterranean region, the MedPDSI, may be a better option since they consider the interaction between precipitation and evapotranspiration within a soil water balance. For that reason, in this Thesis the different methodologies to estimate evapotranspiration, on a monthly and daily basis, were tested for different climates conditions. The results obtained during the course of this Thesis led to the development of the MedPDSI. The MedPDSI improves upon the PDSI mainly by modifying its soil water balance, adapting it to the Mediterranean climate conditions. MedPDSI was computed using reference evapotranspiration based on reanalysis data. Comparing to the PDSI, with the MedPDSI, droughts are generally identified earlier, are longer and more severe, which is an advantage for drought management, since coping management schemes can be implemented earlier to manage drought impacts and ...
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