Hydrological drought analysis- occurrence, severity, risks : the case of Wabi Shebele River Basin, Ethiopia

This study analyses hydrological drought with due emphasis to ungauged catchments. Identification of hydrological drought and methods of unveiling its intrinsic multi-variate characteristics are investigated. The severity of drought has been examined using a multitude of methodologies. Quite often, absence of recorded long time streamflow data hinders a reliable drought analysis and understanding of the phenomenon in the past. Signatures of water stress are imprinted on tree rings. In this study streamflow reconstruction is achieved by coalescing proxy data from riparian tree rings and climatic indices. The generated data are used for extracting the multivariate features of this extreme hydrological event. The case study is demonstrated in Wabi Shebele river basin in Ethiopia. The methods proposed here are applicable to other similar river basins. Extracting hydrological drought entailed defining a possible threshold level. Threshold levels of low exceedance probability are found to be appropriate unlike the commonly used higher exceedance probabilities in temperate climate. It was corroborated that the frequency of some notable recent droughts as revealed with data generated from proxy records well matches that of instrumental data. Stochastic simulation of hydrological drought is done using SARIMA models from time series of instrumental monthly streamflow records in the study area. The nonlinear dependency between severity and duration of hydrological drought is studied using copula models. Different copula families and parameter estimation techniques are evaluated. Joint and conditional probabilities of severity and duration of drought in the area is specified from the derived relationship to provide empirical insight on the nature of the extreme events. It is demonstrated that droughts have distinct spatial patterns regarding temporal evolution and variability in the study area in which the geographic and climatic characteristics have a high degree of diversity. The relative strengths in association between the climatic, morphometric and geologic features of the catchment to the base flow estimates are weighted and a plausible relationship is produced. However, in view of the tremendous spatio-temporal heterogeneity of climatic and landscape properties extrapolation of response information or knowledge from gauged to ungauged basins remains fraught with considerable difficulties and uncertainties. Catchment characteristics can be related to low flows thus are used to delineate hydrologically homogeneous pools. Severity-area-frequency analysis of drought in the area using nonparametric kernels shows high variability of drought events within the pools formed. Associated relative risks of drought are also scrutinized using multicriteria analysis. No single variable is sufficient to portray the complexity of the vulnerability of an area to drought

Hydrological Simulation in a Rift-Bounded Lake System and Implication of Water Abstraction: Central Rift Valley Lakes Basin, Ethiopia

The Katar and Meki subbasins play a significant role in supporting the livelihoods of people in the region. However, the subbasins are currently under heavy human pressures, mainly associated with the ever-increasing human population and the subsequent intensification of irrigated agricultural activities. The aims of this study are to quantify the water balance components of the Katar and Meki rivers using the Soil and Water Assessment Tool (SWAT) model and to assess the implication of water abstraction on river hydrology. The Katar and Meki subbasins were discretized into 107 and 87 micro-subbasins, which were then subdivided further into Hydrologic Response Units (HRUs) of 683 and 658, respectively. Hydro-meteorological data from 1997 to 2014 were used for model setup, calibration, and validation. Nash–Sutcliffe Efficiency (NSE), coefficient of determination (R2), and Percent Bias (PBIAS) were used for model performance evaluation. The results of the simulation revealed NSE = 0.68–0.83, R2 = 0.72–0.85, and PBIAS = 1.6–22.7 during calibration and validation. More than 65% of the simulated flow was bracketed with the 95PPU for both subbasins, with the thickness of the 95PPU in the range of 0.90 to 1.41 calibration and 1.15 to 1.31 validation, which indicates that the overall performance of the water balance model can be rated as “very good”. The results of the water balance show that evapotranspiration (ET), surface runoff (Qs), and groundwater discharge (Qgw) were large in the Meki subbasin, while percolation (PERC) and water yield (WYLD) were large in the Katar subbasin. The model estimated 140 and 111 mm of average annual WYLD for the Katar and Meki subbasins, respectively, and the Katar subbasin is a major contributor of water to Lake Ziway. A total volume of 19.41 million cubic meters (MCM) of water is abstracted from Katar and Meki rivers for irrigation and domestic use, which significantly reduces Lake Ziway’s level by 4.5 cm (m). If the current trend of development continues, 149.92 MCM water will be abstracted each year from the lake environment and will reduce the lake level by 1.72 m. It is suspected that the Katar and Meki rivers are likely to cease to exist after a few decades and that Lake Ziway will also dry out