Recurrent droughts, climate change and climate variability have significantly strained water supplies worldwide, with arid and semi-arid areas in southern Africa being the most affected (Bhaga et al., 2020). The impact of drought is most noticeable in surface water resources, which are threatened by significant withdrawals due to increased demand, inadequate conservation and poor land-use practices, among other factors (Debnath et al., 2022). Concerns about water scarcity due to droughts and climate variability have increased in recent decades. This concern comes at a time when governments are looking for innovative ways to combat and adapt to the effects of climate change.
Droughts are a natural consequence of climate variability and change and are accompanied by a severe decrease in precipitation over a prolonged period, which leads to a change in the water balance (Huang et al., 2017; Slette et al., 2019). In addition to precipitation deficits, excessive evapotranspiration rates and the overuse of water resources can also contribute to droughts (Bhaga et al., 2020). Droughts can occur in any climate zone, regardless of the region's normal precipitation amounts and patterns (Slette et al., 2019). Unlike other natural disasters, the beginning and end of a drought are difficult to predict (Park et al., 2017) because of their variability in location and timing (Wilhite et al., 2007). The effects of droughts are, therefore, difficult to quantify. Climate-related events such as the El Niño have the potential to make droughts both more frequent and more severe (Verner et al., 2018; Rivera et al., 2021).
Drought is predicted to increase in frequency and severity on a regional and global scale, putting even more strain on water supplies (Zhang et al., 2017). Some studies claim that the impact of droughts on ecosystem surface water resources and availability has not been sufficiently researched (Davis & Hirji, 2014; Bhaga et al., 2020). Droughts have an impact on both the quality and quantity of water resources. Regarding water quality, reduced runoff in surface water bodies is associated with decreased concentrations of nutrients, organic matter and sediments (Bhaga et al., 2020). The stability of wetlands could be threatened by decreased flows, which in turn would affect the local flora and fauna. Groundwater is also affected by droughts in several ways. Apart from the declining groundwater levels and reduced water availability resulting from droughts, saline water can also enter the groundwater system (Sorensen, 2017). In the event of drought, less water may be available to maintain and support a variety of social, ecological, and economic functions. Mitigating the social, ecological and economic effects of drought and developing more resilient societies is a global concern (Hagenlocher et al., 2019). Due to the effects of climate variability and change, an accurate and up-to-date drought assessment is crucial for proper planning and implementation of the necessary mitigation measures (Sun et al., 2020).
Drought is a hazard that may adversely affect human survival. Therefore, close monitoring and characterisation of droughts is needed to minimise drought-related damage. Conventional drought monitoring focuses on field measurements of streamflow, surface and groundwater levels, and soil moisture content, all of which directly indicate a lack of water availability (Rivera et al., 2021). Various drought indices have been developed, each with specific objectives and based on specific datasets, to assess past droughts and conduct drought monitoring. Despite the diversity of these indices, when focusing on the mechanisms that cause droughts, a set of consistent indices may be sufficient to explain the many characteristics of droughts (Su et al., 2017).
Many drought indices are used for hydrometeorological drought monitoring, including the Palmer Drought Severity index (PDSI) (Sheffield et al., 2012), Surface Water Supply index (SWSI) (Mishra & Singh, 2010), Standardised Precipitation Index (SPI) (Haied et al., 2017), Effective drought index (EDI), reconnaissance drought index (RDI) and standardised precipitation evapotranspiration index (SPIE) (Memon & Shah, 2019). Water, soil, groundwater, runoff, temperature, precipitation, PET and streamflow are among the hydrometeorological factors utilised to generate the bulk of these indices (Park et al., 2016). The application of drought indices is often region-specific because of the complicated nature of drought conditions (Kchouk et al., 2021). For example, the PDSI is extensively used in the United States, while the RDDI is used in Australia (Jiao et al., 2019). The SPI is a commonly used drought statistic to study meteorological drought. The SPI defines droughts as deviations from normal precipitation patterns (McKee et al., 1993). The World Meteorological Organisation (WMO) has proposed the SPI as the main meteorological drought indicator for monitoring and tracking drought conditions (Svoboda et al., 1987). The RDI incorporates the influence of evapotranspiration demand produced by temperature fluctuation into the SPI framework, which combines the properties of the PDSI and SPI (Memon & Shah, 2019). The SPI and the RDI are used in this research to examine the severity of hydrometeorological droughts. The SPI is used to capture meteorological drought based on precipitation. In contrast, the RDI captures hydrological drought by including evapotranspiration. The RDI simulates the water balance technique by using precipitation and evapotranspiration. Evapotranspiration is essential since it is the most critical variable responsible for surface water loss (Dimitriadou & Nikolakopoulos, 2021).
Several studies assessing meteorological and hydrological droughts using RDI or SPI have been conducted in different regions of the world. For example, meteorological drought studies using the RDI method have been conducted in Iran (Sharafi & Ghaleni, 2021), Sudan (Atiem et al., 2022), China (Abro et al., 2022), Lithuania (Nazarenko et al., 2022), and Sri Lanka (Abeysingha & Rajapaksha, 2020). However, few studies have explained the linkages between meteorological and hydrological droughts due to their inherent complexity (Dikshit et al., 2022).
Drought frequency and intensity have increased in recent decades due to climate change (Saharwardi et al., 2022; Peng et al., 2022; Piraino et al., 2022; Uwimbabazi et al., 2022). Meteorological, hydrological, agricultural and other forms of drought have severe impacts on water-dependent sectors (Zarei et al., 2021). It is critical for agricultural-based countries such as Zimbabwe to identify historical hydrometeorological drought trends and frequencies. At the subnational and national levels, information on the patterns of hydrometeorological droughts is essential for effective water management and for the development of drought early warning systems (Zarei & Mahmoudi, 2022). Very few studies have analysed hydrometeorological drought patterns in specific catchments in Zimbabwe. Most drought studies have tended to focus on agricultural droughts. For example, studies by Mupepi & Matsa (2022) and Sharara et al. (2022) focused on agro-meteorological droughts in regions in Zimbabwe.
The Upper Mzingwane sub-catchment is important for agricultural production and water supply to Bulawayo (Zimbabwe's second largest city). However, it seems no study has investigated hydrometeorological drought trends in this sub-catchment. Maviza & Engelbrecht (2021) assessed the precipitation trends in the Upper Mzingwane sub-catchment. However, their work did not extend to an assessment of the hydrometeorological droughts to the water resources as shall be done in this study. The present study seeks to fill this gap by analysing hydrometeorological drought variations in the Upper Mzingwane sub-catchment. The study applies the two well-known drought indices to areas for which little information on drought trends and frequency is available. The outcome of this study makes a scientific contribution to the literature on climate change and drought studies by presenting one of the first case studies to characterise historical hydrometeorological drought trends in the Upper Mzingwane sub-catchment. It will also aid in providing successful drought management in the study area.
It is against this background that this study assessed droughts in the Upper Mzingwane sub-basin using hydrometeorological-derived drought indices to understand their frequency and severity. The study also evaluated the correlation of the SPI and RDI results, and the impact of droughts on water resource availability.