Do droughts and floods pose similar risks to Sahel staple food crop?

With its arid and semi-arid climate, the African Sahel is a region highly drought-prone and vulnerable given its reliance on meagre natural resources for food security. However, the region recently also experienced flood occurrences. Against the background of food security, effects of floods and drought upon crop yield requires thorough investigation. To investigate the risk that flood and droughts pose to food crop we considered: 1) the case of the Sudanese Sahel in the eastern part of the region given its representation of one-third of the total area of the region, 2) traditional rainfed farming systems as the most fragile system in terms of marginality and regular experiences of challenges, including weather shocks and 3) sorghum as a main staple crop in the area. To identify risk and quantify its magnitude during the years when yield losses were encountered, we used gridded climate datasets, dynamic (yearly) land-use datasets and an unusually long national sorghum statistics for the last half a century. We expressed risk in % as hazard x vulnerability x 100%. Using a drought index for the growing season based on the ratio of rainfall to potential evapotranspiration, hazard is expressed here as a function of: i) severity of drought, ii) dry spell and iii) time frequency of drought. For the 51-year period from 1970 – 2020, 26 risk years were identified representing both hydrological extremes – floods and droughts. A risk year is defined here as a year with yield loss below the detrended yield data. In the decade 2011-2020, seven years were identified risk years. Four of those (2011, 2012, 2013 and 2015) were drought years and three (2017, 2019 and 2020) were exceptionally wet years. Sorghum yield varied significantly as a function of risk. Variations in the risk index explain 97.5% of the variation in sorghum yield, with a 1% increase in the risk leading to a decline of yield by 14.5 kg/ha. Nevertheless, the traditional farming sector achieved several high yield levels during the 26 risk years, namely 2020, 2019, 2017 and 2015 were years with the 3^rd, 4^th, 5^th and 7^th highest yield levels. Our findings show that: a) the traditional farming system experienced a high degree of vulnerability to hydrological extremes during the decade 2011-2020 and b) drought remains the most relevant hydrological risk whereas floods cause a small risk and may even favor yield.