Drought indices validation: Improving monitoring knowledge on different systems in Spain and the United States

This PhD Thesis focuses in the evaluation of different drought indices on multiple systems and the spatio-temporal response of agriculture, forests and streamflow to drought conditions in two heterogeneous regions, the United States and Spain from the sixties to nowadays. Knowing the importance of selecting appropriate tools for monitoring drought, the performance of seven of the most commonly used drought indices and their ability to capture impacts on vulnerable systems were validated. For this purpose, three multi-scalar drought indices (the SPI, the SPEI and the SPDI) and four uni-scalar Palmer family drought indices (the PDSI, the PHDI, the Z-index and the PMDI) were quantitatively compared. The results obtained from the different analysis conducted demonstrated the superior performance of the SPEI, the SPI and the SPDI in comparison to the PDSIs. Independently on the type of crop, tree species, river basin and the temporal scale considered, drought indices calculated at different time scales have a superior capacity to reflect the different impacts of drought over diverse systems with a wide range of temporal responses to drought associated to specific characteristics that difficult even more this identification. The varying responses of crops to drought indices time-scales observed in crop yields from US and Spain were mainly determined by the resilience of plants to develop strategies to deal with soil moisture depletion and by the resistance of the different types of crops during the sensitive vegetative stages of growth. Similarly, findings from forest sensitivity to drought in forests in Spain showed variations among species and climatic regions highlighting the role of resilience mechanisms to handle with extreme climatic conditions. In addition, seasonal variations predetermined the response of tree species to drought. In general, results suggested a lagged response to drought depending on the part of the tree decay cycle affected, thus secondary growth was found especially sensitive to humid conditions during summer months while photosynthetic activity was affected by drought conditions occurring during spring months. From the propagation of climatic drought to streamflow drought analysis, results suggested a primary response to drought at short-time scales in most of the near-natural basins analysed in the US and Spain. However, seasonal patterns and local differences in the response of streamflow demonstrated the influence of catchment properties (e.g. vegetation cover, land-use, climatic conditions or topographic characteristics such as elevation) on streamflow response to climatic drought. This PhD Thesis provided quantitative evidences about the effectiveness of drought indices for quantification and monitoring purposes, and also improved the knowledge on the sensitivity and spatio-temporal response of different natural systems to the most hazardous and tricky climate phenomenon