Assessment and characterisation of hydrometeorological droughts in the Upper Mzingwane sub-catchment of Zimbabwe

Analysis of frequency and severity of droughts is critical for assessing the availability of surface water in ecosystems and for water resource planning. This study investigated hydrometeorological drought in the Upper Mzingwane sub-catchment using the standardised precipitation index (SPI) and reconnaissance drought index (RDI). Precipitation, minimum and maximum temperature data from three stations in the sub-catchment were used to calculate RDI and SPI for the period 1990-2020. Two-tailed Mann-Kendall and Pettitt tests were conducted to identify trends and breaks in the RDI and SPI values. The results showed that the sub-catchment was mainly affected by mild (62%) and moderate (20%) droughts. Significant increasing trends were observed for the calculated 6- and 12-month SPI and RDI values (p < 0.05), indicating a decreasing trend in long-term hydrological droughts. No trend was observed for the 3- and 6-month SPI and RDI values (p > 0.05). The Pettitt test did not detect any breaks in the RDI and SPI values. SPI and RDI showed strong correlation coefficients (0.93-0.99) over similar time periods. Thus, both indices behaved in the same manner across multiple time intervals (3, 6, 9 and 12 months), but RDI was more sensitive to climatic conditions because its formulation incorporates potential evapotranspiration. Approximately 14-16 droughts of varying severity occurred in the sub-catchment from 1990 to 2020. Despite increasing precipitation pattern, the catchment was still vulnerable to drought and this could undermine the ability of the catchment to deliver requisite ecosystem services such as water provision. Thus, drought forecasting, and information dissemination will be crucial to help stakeholders better prepare for future droughts in the sub-catchment.

Automated summary

This study investigated the frequency and severity of droughts in the Upper Mzingwane sub-catchment using the SPI and RDI. The results showed that the sub-catchment was mainly affected by mild and moderate droughts, with a decreasing trend in long-term hydrological droughts. Both indices behaved similarly, but RDI was more sensitive to climatic conditions.