Rainfall seasonality and timing: implications for cereal crop production in Ethiopia

Rainfall seasonality and timing are key climatic features affecting crop yield in rainfed agriculture (RFA). We evaluated the dynamics of temporal rainfall attributes - the strength of seasonality, the date of onset, cessation, and duration of the rainy season - over RFA areas across Ethiopia for the period 1981-2010 and explored their impacts on cereal crop production (including maize, teff, sorghum, wheat, barley, millet, oats and rice) between 1995 and 2010. First, we quantified the rainfall seasonality using an entropy-based seasonality index, defined the onset and cessation dates of rainy seasons, computed the rainy season duration, and analyzed their interannual variability and trends. Second, we correlated de-trended total cereal production during the Meher (i.e., long rainy) season (April to September) with the anomalies of the temporal rainfall attributes, and we used a univariate linear regression model to estimate the influence of changes in these attributes on crop production. We show that RFA areas in northern Ethiopia are characterized by a highly seasonal and unimodal rainfall regime. The southern parts of the RFA areas are characterized by less seasonal rainfall of bimodal and erratic nature. Cereal crop production during the Meher season, especially in teff and maize-dominated regions, is found to be correlated to both the onset (median rho=-0.32 and -0.37, respectively) and duration (rho=0.34 and 0.19) of the rainy season in the unimodal rainfall regime, whereas it is correlated with the rainfall seasonality (rho=0.21) in regions with a bimodal rainfall. We estimate that on average over RFA areas, a late-onset and shorter rainy season lead to similar to 1.5% and 1.1% crop production losses per pentad (5-day period), respectively.