Long-term meteorological drought characterization in the Sao Francisco watershed, Brazil: A climatic water balance approach

Partially located in the Brazilian Semiarid region, the Sao Francisco watershed (SFW) is a strategic hydrological system for power generation, agricultural development and the integration of the national territory. Droughts are recurrent in the Brazilian Semiarid, and climate change projected scenarios indicate their intensification in the SFW, which may compromise the access to its resources. In this context, the objective of this study was to characterize long-term spatial and temporal meteorological drought patterns over the SFW while accounting for its different climates. We used gridded Climate Research Unit Time Series rainfall and potential evapotranspiration (PET) data for the 1942-2019 (78 years) period to retrieve a monthly climatic water balance-based drought index. The climatological aspects of drought incidence in the SFW were assessed through linear trend analysis, the estimation of the relative effects of precipitation and PET trends in drought occurrence, and the identification of the main interannual variability modes controlling the triggering of meteorological drought through wavelet functions. Results showed that water deficit periods are becoming more frequent and intense, affecting larger areas in the middle (tropical climate) and lower (semiarid climate) portions of the SFW. Months with water surplus in these regions are also becoming less frequent. In the upper (humid subtropical climate) SFW, evidences show that water deficit periods are becoming more frequent, with an expansion of areas affected by droughts. In this subregion, increasing PET trends played a more important role than precipitation trends in the propagation of drought. Finally, low-frequency teleconnection mechanisms such as the El Nino-Southern Oscillation were found to drive the occurrence of drought in most of the SFW, while higher frequency oscillations (such as the Atlantic Meridional Mode) play a role in the lower latitudes of the basin. In the upper SFW, results indicated that interannual variability might be driven by oscillations with stronger intraseasonal components.