ORIGINAL PAPER Dry spell, onset and cessation of the wet season rainfall in the Upper Baro-Akobo Basin, Ethiopia Asfaw Kebede 1 & Bernd Diekkrüger 2 & Desalegn C. Edossa 3 Received: 29 December 2014 /Accepted: 25 April 2016 # Springer-Verlag Wien 2016 Abstract In this study, maximum dry spell length and num- ber of dry spell periods of rainy seasons in the upper Baro- Akobo River basin which is a part of the Nile basin, Western Ethiopia, were investigated to analyse the drought trend. Daily rainfall records of the period 1972–2000 from eight rain gauge stations were used in the analysis, and Mann-Kendall test was used to test trends for significance. Furthermore, the begin- ning and end of the trend development in the dry spell were also tested using the sequential version of Mann-Kendall test. Results have shown that there is neither clear monotonic trend found in dry spell for the basin nor significant fluctuation in the onset, cession and duration of rainfall in the Baro-Akobo river basin. This sufficiently explains why rain-fed agriculture has suffered little in the western part of Ethiopia. The predict- able nature of dry spell pattern may have allowed farmers to adjust to rainfall variability in the basin. Unlike many parts of Ethiopia, the Baro-Akobo basin climate variability is not a limiting factor for rain-fed agriculture productivity which may contribute significantly to national food security. 1 Introduction In countries like Ethiopia, where agriculture serves as a back- bone of the economy as well as the main source of food pro- duction and income for about 80 % of the labour force and 47.68 % of the gross domestic product (GDP) (Araya and Stroosnijder 2011; World Bank 2011), the availability of water resource is quite essential. However, the sector is mainly de- pendent on rain-fed agriculture. For the management of rain- water and agriculture as well as for the evaluation of drought risk, the seasonal rainfall has to be evaluated in terms of dura- tion, onset and cessation of the rainy season and the dry spell lengths based on past records. In Ethiopia, the drought years of 1965, 1972–1973, 1983–1984, 1987–1988 and 1997 resulted in low agricultural production and affected millions of rural poor farmers, pastoralists, domestic and wild animals, with se- rious degradation of the environment (Seleshi and Zanke 2004). There have been reports of rainfall variability and occurrence of drought in northern, eastern and southeastern parts of Ethiopia (Araya and Stroosnijder 2011; Tilahun 2006; Tilahun 1999). Most of the studies carried out in various parts of Africa indicated the importance of a clear understanding of the length and number of dry spells and their probabilities for assessing recurring droughts and their impacts on agricultural produc- tivity (Muita et al. 2012; Sivakumar 1992). Many authors define a dry spell with different threshold values of rainfall on a dry day. In this study, rainfall amount of 1 mm per day was used as the threshold, often 0.1 mm used with respect to the common precision of rain gauges. Some studies employed a threshold of 1.0 mm, on the assumption that rainfall less than this amount is evaporated off directly (Mathugama and Peiris 2011). However, the definition of thresholds depends on the purpose of the study and methodology used. Seleshi and Camberlin (2006) analysed dry spells based on the maximum number of consecutive days with rainfall less * Asfaw Kebede asfaw649@gmail.com 1 Institute of Technology, Haramaya University, Dire Dawa, Ethiopia 2 Hydrology Research Group, Institute of Geography, University of Bonn, Bonn, Germany 3 Department of Civil Engineering, Central University of Technology, Free State (CUT), 20 Pres Brand Street, Private Bag X20539, Bloemfontein 9300, South Africa Theor Appl Climatol DOI 10.1007/s00704-016-1813-y