Assessment of drought characteristics using SPEI and VHI in Tanzania and their associated climate factors

Severe drought conditions can have tremendous impacts on ecosystem operation. However, the assessment of drought characteristics derived from vegetation conditions and water availability indices has received little attention in Tanzania. This study employed the standardised precipitation evapotranspiration index (SPEI) and vegetation health index (VHI) to examine drought characteristics across the country during 1981–2016. Theil-Sen and Mann-Kendall tests were employed to analyse the spatial and temporal trends of drought indices. Correlation and regression analyses were also conducted to examine the relationship between climate factors (El Niño Southern Oscillation and Indian Ocean Dipole) and drought indices. Results reveal that Tanzania experienced multiple droughts during the study period, with eight droughts occurring in the October-December (OND) season and nine droughts in the March-May (MAM) season based on country-averaged SPEI-derived drought conditions. The MAM droughts generally had less magnitude and spread compared to the OND drought conditions, which can be attributed to the prolonged impacts of wet characteristics from the preceding season. Meanwhile, VHI demonstrated high sensitivity to the prolonged effects of wet/dry conditions and vegetation resilience. Overall, the drought characteristics from 1981 to 2016 indicate that most areas of Tanzania experienced frequent mild to moderate droughts amounting to up to 26 months, with relatively low intensity. However, localized regions, especially in northwestern and southern areas for SPEI and northeastern areas for VHI, were hotspots for severe and extreme droughts of shorter duration but higher intensity. Both the SPEI and VHI showed generally consistent observations of drought characteristics across the country, though some disparities were evident for the most severe drought events. In addition, an increasing drought trend was exhibited over South and East during OND and North, South, and East during MAM. Furthermore, it is found that more drought events are linked to the tropical oceanic regions mediated through La Nina occurrences over the Pacific Ocean and negative IOD events over the Indian Ocean during OND, while the subtropical oceanic regions over the southwestern South Atlantic, Indian and Pacific Oceans are important for the drought conditions during MAM. These findings provide meaningful guidance in minimising drought impacts and enhancing resilience to droughts in Tanzania.