Propagation from Meteorological Drought to Hydrological Drought Using SPI and SPEI Combined with the Adapted Threshold Level Method

Abstract. The hydrological response within a region is often nonlinear, influenced by various factors such as physiographic features, anthropogenic activities, and climate change. Considering the inherent complexity, our study focuses on investigating the propagation from meteorological drought to hydrological drought in the Banabuiú, Castanhão, and Orós reservoirs, located in the State of Ceará, Brazil. To achieve this, we used the Standardized Precipitation Index (SPI) to identify meteorological drought events, which uses precipitation data, alongside the Standardized Precipitation Evapotranspiration Index (SPEI), incorporating potential evapotranspiration in its calculations. For assessing hydrological drought, we used the Adapted Threshold Level Method (ATLM), which calculates the water balance between the supply (represented available reservoir volume) and the demand (including withdrawals for various uses and evaporation losses from the watershed). For all three reservoirs, drought events were characterized by their frequency, duration, severity, magnitude, and drought recovery time across three aggregated time scales of 12, 24, and 36 months. To determine the propagation time from meteorological to hydrological drought, we calculated three indicators, corresponding to time differences between the onsets, peaks, and conclusions of the propagated drought events. Results indicated no significant differences in meteorological drought characteristics between SPI and SPEI methods. However, the SPEI showed higher values for meteorological drought in the Orós watershed. Analysis of drought propagation, employing different methods, revealed no defined pattern for the onset, peak, and end intervals across all tested combinations. Hence, we recommended utilizing all three indicators to enable a more comprehensive analysis of drought events propagated within the watersheds.