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Top1. Introduction
Hydrological drought is a condition on land characterized by water quantity that falls below a defined truncation level, that impacts ecosystems and society in numerous ways. It is a water-deficit condition within the surface, sub-surface and ground water flow systems, usually caused by deficit precipitation and or extreme climatic conditions. Hydrological drought adversely affects different aspects of human socio-economic development. For instance, hydrological drought may cause water insecurity, food insecurity, poor irrigation and crop yield and erratic hydropower generation (Belayneh and Adamowski, 2013). The characteristics of hydrological drought which are defined by magnitude, severity, duration and frequency can be studied at a basin scale. Hydrological drought impacts adversely on large areas and large human population. This drought may be triggered by climate change and or variability (Mondal and Mujumdar, 2015). Hydrological drought is considered to be a ‘creeping hazard’ since it develops gradually, it is not easily detected, covers extensive areas and it lasts for a long period of time with adverse impacts on water resources, ecological systems, and socio-economic development (Liu et al., 2015; Van-loon, 2015). According to Van-loon and Laaha (2015), hydrological drought has the most significant effects across different sectors compared to other types of droughts.
Hydrological drought may be categorized into surface and ground water droughts (GD). The Surface Water Drought (SWD) is caused by direct reduction in precipitation that subsequently leads to low surface runoff. The SWD is also caused indirectly by reduced groundwater discharge to surface water resources. This may be attributed to reduced flow of groundwater into surface flow in influent rivers and springs. In some instances, increase in groundwater on specific areas within a basin for an effluent river contributes to the SWD. The common indicators of SWD are reduced river flows, low water levels in reservoirs and lakes. SWD results from a combined interaction of meteorological drought, water resources development infrastructure and operational management.
On the other hand, Groundwater Drought (GD) is caused by significantly low quantity of water in aquifers that may be due to reduced recharge. The recharge normally takes place through permeation and inflow from sub-basins (Adindu et al., 2013). The GD may be assessed by measuring the volumetric ground water storage. However, these data are not readily available in most river basins. Thus, aquifer level is considered to be a better indicator than the volumetric ground water storage. The GD is also determined from the evaluation of its secondary effects such as base flow into rivers. Ground water is a vital source of water supply especially in river basins where surface water exhibits a high temporal variability.
Hydrological drought has been assessed using a number of drought indices. A drought index is a function that uses water-related input variables such as precipitation, streamflow, resrvoir volume, dam inflow and ground-water level for quantifying drought. Some of the common indices for quantifying hydrological drought include surface water supply index (SWSI) (Shafer and Dezman 1982), Regional Streamflow Deficicency Index (RSDI) (Stahl, 2001), Aggregated Drought Index (ADI) (Kayantash and Dracup, 2004), Hybrid Drought Index (HDI) (Kayantash and Dracup, 2004), Streamflow Drought Index (SDI) (Modarres, 2007), Groundwater Resources Index (GRI) (Mendicino et al., 2008) and Water Balance Drought Derived Index (WBDI) (Vasiliades et al., 2011). Based on data avaulability in the upper Tana River basin, this research adopted SWSI and SDI for evaluation of hydrological drought. The SWSI was developed in Colorado state by Shafer and Dezman (1982) as an indicator of water availability in the United States of America. Since then, the index has been modified and applied in a number of studies across the world. SDI is one of the most important types of drought that affects a number of activities depending on surface water resources (Nikbakht et al., 2013). Apart from evaluating and comparing hydrological drought using the SWSI and SDI, forecasting of the drought events was considered key to effective formulation of mitigation measures.