Summer Rains and Dry Seasons in the Upper Blue Nile Basin: The Predictability of Half a Century of Past and Future Spatiotemporal Patterns

Per-Erik Mellander,Woldeamlak Bewket,Solomon G Gebrehiwot,Annemieke I Gärdenäs,Kevin Bishop,Vanesa Magar

doi:10.1371/journal.pone.0068461

Abstract

During the last 100 years the Ethiopian upper Blue Nile Basin (BNB) has undergone major changes in land use, and is now potentially facing changes in climate. Rainfall over BNB supplies over two-thirds of the water to the Nile and supports a large local population living mainly on subsistence agriculture. Regional food security is sensitive to both the amount and timing of rain and is already an important political challenge that will be further complicated if scenarios of climate change are realized. In this study a simple spatial model of the timing and duration of summer rains (Kiremt) and dry season (Bega), and annual rain over the upper BNB was established from observed data between 1952 and 2004. The model was used to explore potential impacts of climate change on these rains, using a down-scaled ECHAM5/MP1-OM scenario between 2050 and 2100. Over the observed period the amount, onset and duration of Kiremt rains and rain-free Bega days have exhibited a consistent spatial pattern. The spatially averaged annual rainfall was 1490 mm of which 93% was Kiremt rain. The average Kiremt rain and number of rainy days was higher in the southwest (322 days) and decreased towards the north (136 days). Under the 2050–2100 scenario, the annual mean rainfall is predicted to increase by 6% and maintain the same spatial pattern as in the past. A larger change in annual rainfall is expected in the southwest (ca. +130 mm) with a gradually smaller change towards the north (ca. +70 mm). Results highlight the need to account for the characteristic spatiotemporal zonation when planning water management and climate adaptation within the upper BNB. The presented simple spatial resolved models of the presence of Kiremt and annual total rainfall could be used as a baseline for such long-term planning.

Highlights

The Blue Nile Basin (BNB) supplies over two-thirds of the water to the Nile at Aswan in Egypt, and supports a large population living mainly on subsistence agriculture
While temperatures in Ethiopia have increased over the last decades [7], average annual rainfall has not changed over 40 years [8] but there have been inter-decadal variability [6,7,9]
We present a simple spatial model for communicating the length of the rainy period and dry season, as well as the amount of rain falling during the rainy season (Fig. 4, 5 and 6)

Summary

Introduction

The Blue Nile Basin (BNB) supplies over two-thirds of the water to the Nile at Aswan in Egypt, and supports a large population living mainly on subsistence agriculture. The highlands of the upper BNB are vulnerable to the negative impact that a variable climate may have [2] as the effects of climate change on rainfall and temperature in the region will have major implications for both regional food security [3] as well as the transboundary water supply with all that entails for [4] Sudan and Egypt. The limited availability of water resources during the dry season in Ethiopia has been exacerbated by the characteristic climate of the region with a long dry season each year [6] in conjunction with a population increase and human impacts on the landscape, such as erosion associated with land degradation. While temperatures in Ethiopia have increased over the last decades [7], average annual rainfall has not changed over 40 years [8] but there have been inter-decadal variability [6,7,9]

Objectives

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Results

Discussion

Conclusion