Chapter 23 - Probabilistic characterization of hydrological droughts in the Blue Nile River, Ethiopia

Abstract

Evidence suggests that drought is a widely occurring phenomenon that causes disaster across Ethiopia including the wet basin of Upper Blue Nile basin. For example, there was a devastating drought in the early 1980s in most parts of the Ethiopian highlands that affected the flows of upper Blue Nile significantly beyond the border of Ethiopia in Sudan and Egypt. In the recent years, upper Blue Nile was part of the prolonged east African drought which ended in 2005. In the 1970s and 1980s, droughts typically occurred, on average, once per decade; presently, droughts are anticipated to occur about once in 2–3 years. Drought has different dimensions related to duration and severity and assessment and characterization of the likely occurrence of the joint hydrological drought probabilities are of great interest in highly water scarce basins for management and operations of existing and upcoming reservoirs in the eastern Nile basin. We explored different Archimedian copula families such as Gumbel, Frank, and Clayton copula, and parameter estimation techniques to investigate hydrological drought characteristics in the Upper Blue Nile basin. Based on the selected copula families of Clayton and Gumbel as the most appropriate copula for sites of interest, we developed joint probability, and contour plots of various years of return periods. The results show the importance of clear definition of drought in terms of duration and severity. As demonstrated in the study, the longest drought does not necessarily correspond to the most severe drought. Similarly, for a desired return period varying drought can occur with respect to their severity and duration. Therefore, for sensitive river basins of transboundary nature with cascading large reservoirs, seasonal drought forecasting tools that consider the probabilities of occurrence of long duration and/or severe droughts jointly or separately helps better management of the eastern Nile basin cascaded reservoir system. Furthermore, the joint probabilistic approach demonstrated here for hydrological drought frequency analysis can be implemented as a bivariate frequency analysis for sizing and planning of large reservoirs in the basin.