1. Introduction
Droughts are by far the costliest natural disaster and threat to water availability in the world [
1]. In recent years, impacts of droughts have been experienced and monitored in many countries; they have led to devastating social, economic and environmental challenges such as increase in famines, energy crisis, reduced water supply and destruction of ecosystems [
2]. Due to climate change, among many factors, it is projected that the intensity and recurrence of weather related disasters such as droughts will increase [
3]. Further, population growth coupled with economic development has continued to put pressure on available water resources [
4]. This prompts the need for prudent water resource strategies, which must incorporate drought management [
5]. A good understanding of spatial and temporal characteristics of droughts at basin, regional and national level is required for efficient water resources management. Additionally, stringent quantitative assessment of droughts aid in reducing societal vulnerability by identifying triggers that apply for successful mitigation and preparedness [
6,
7].
Drought is a severe, recurring natural hazard that results in water deficit across the different components of the hydrologic cycle. It is a phenomenon that occurs in all climatic regions and presents unique challenges due to its distinctive attributes [
8]. Four categories of droughts have been defined based on how the hydrological cycle is affected [
9]: meteorological droughts, agricultural droughts, hydrological droughts and socio-economic droughts. Meteorological droughts occur when less than normal precipitation is received over a prolonged period, whereas agricultural droughts develop whenever there is insufficient soil moisture, thereby affecting forage and crop growth. Hydrological droughts eventuate when a region experiences below average surface and groundwater supplies over time; however, socio-economic droughts are triggered whenever there is failure of supply of goods and services that rely on water availability due to water deficit [
9]. The first three drought definitions are based on a deficiency of different variables of the hydrological cycle and describe physical features of droughts. Even if all droughts originate from a precipitation deficiency, it is not enough to solely depend on this single variable to monitor and characterize drought events because other factors such as relative humidity, temperature and wind also play a vital role. It is therefore important to integrate precipitation and other climatic variables with water information, such as streamflow and soil moisture for effective monitoring of droughts [
5,
9]. Consequently, a comprehensive evaluation of droughts has to encompass meteorological, agricultural and hydrological droughts to have thorough understanding of a drought episode and how it impacts the various sectors [
10]. However, one of the biggest challenges faced is having insufficient data, especially in data scarce regions. In order to overcome this shortcoming, the incorporation of hydrological models in drought studies has gained prominence [
11,
12].
Numerous indicators have been proposed to evaluate droughts such as the Standardized Precipitation Index (SPI) [
13], Standardized Precipitation Evapotranspiration Index (SPEI) [
14], Palmer Drought Severity Index (PDSI) [
15], Soil Moisture Index (SMI) [
16], Reconnaissance Drought Index (RDI) [
17] and Standardized Runoff Index (SRI) [
18]. Among these, PDSI and SPI are the most widely applied [
17]. The complexity of calculation varies among indices, from single input variable for an index such as SPI to two or more variables for SPEI and PDSI, respectively. All these indices have their advantages and limitations but, when a combination of key indicators is applied for assessment, their combined strength compensates for limitations [
8].
In this paper, the Upper Kafue River Basin (UKRB) in Zambia is taken as the study area due to its ecological, economical and geographic importance. Recent research has shown that occurrence of extreme weather events have been on the increase in the country [
19]. UKRB is part of the Kafue River Basin (KRB), which is a sub-basin of the large Zambezi River Basin in Southern Africa [
20]. KRB is the most industrialized region and about 50% of Zambia’s population live there [
21]. It supports the majority of industrial, municipal water supply, agriculture and mining activities. Additionally, KRB is a vital habitat for a wide variety of animals and plants [
20,
22]. Therefore, there is great reliance on water resources, especially surface water but the threat of varying climatic conditions, more specifically droughts have made the region vulnerable to drought conditions and increased competition for the already scarce water resources [
23].
Despite these challenges, the number of studies focusing on droughts in Zambia is limited with varying focus. Due to data scarcity, among other reasons [
24], most studies are either at national or international scale. The known studies applied the theory of runs method to assess precipitation [
25,
26,
27,
28] and streamflow deficits (1906–1969) [
29]. All the studies agree that droughts are becoming a recurrent feature, and that, to minimize impacts, pre-planned measures have to be taken. However, there is still no documented research done at regional scale, in particular, at basin level to investigate droughts, and lack of data is a possible hindrance to application of appropriate methods. This inadequacy in knowledge can be an obstacle in designing effective drought identification, monitoring and mitigation systems, which are a prerequisite for sustainable water resources usage and planning. Thus, need arises to study drought characteristics at a finer resolution and understand river basin dynamics during drought events. Moreover, at the time of this research, no known study was found to have investigated meteorological, agricultural and hydrological droughts simultaneously.
With this state of affairs, the present investigation attempts to: (1) understand characteristics of droughts in humid subtropical UKRB; (2) evaluate the capability of standardized drought indices representing different types of droughts in assessing drought variability during the period 1984–2013; (3) examine the applicability of Soil and Water Assessment Tool (SWAT) model for agricultural and hydrological drought studies in the basin; and (4) assess drought trends and explore the relationship between SPEI and SRI to understand the interaction between meteorological and hydrological droughts. It is expected that results from this study will be important in regional drought management and water resources planning, especially in data scarce regions.
5. Conclusions
In this study, characteristics of droughts were investigated using SPI, SPEI, SRI and SSI. The study concludes that, in UKRB, the average duration for meteorological droughts was 5–8 months, intensity was ≤1.00 and frequencies were above 50.0% compared to agricultural and hydrological droughts (average duration: 13–31 months; intensity: ≥1.02; and frequency: 10.0%–16.67%). All indices could identify the temporal variability of droughts but SPI and SPEI exhibited the best performance by capturing the evolution and severity of all reported droughts. SWAT successfully simulated runoff and soil moisture in the humid subtropical basin hence can be applied for research. Possible drought trends were evaluated using Mann–Kendall on SPEI and SRI. The indices depicted contrasting results but SPEI suggested that the period 1984–2013 had an increase in drought severity trends. Therefore, from a water resources management point of view, this would call for more preparedness and awareness to reduce vulnerability to future drought events. The correlation between SPEI and SRI revealed that SPEI at nine- and ten-month time scale has a strong link with SRI, hence can be used to monitor hydrological droughts when there is limited data.
Overall, the use of different indices representing different components of the hydrologic cycle integrates many factors that affect and trigger droughts, thus can aid in providing a wider understanding of characteristics of droughts on various water sectors, which a single index or method could fail to identify. The use of hydrological modeling in drought study also provides a powerful tool in data scarce regions. Findings of this research will be useful to water resources managers involved in drought planning and mitigation. Moreover, this study forms a basis for more informed application of standardized indices and sets a base to investigate the impacts of anthropogenic activities and climate change on streamflow, and how that affects industrial and municipal water supply, hydropower generation and agriculture activities downstream.