The arid and semi-arid region of West Africa commonly known as the Sahel is characterized by a strong gradient of decreasing annual rainfall from south to north. Rains fall during a single wet season consisting of short intense storms over a 3-to 5-month period, with about 90 per cent of the rains falling during the months of July, August and September. Total seasonal rainfall ranges from 100 to 650mm in the northern region and from 650mm to over 1000mm in the semi-humid Sudan climate of the south (Ingram et al, 2002). Drought has been a recurrent feature in this region, with early records dating back to the 1680s. Annual rainfall levels decreased in the region over the course of the 20th century, with an increase in inter-annual and spatial variability (Dai et al, 2003; Brooks, 2004) and southward shifts of isohyets by 200km (L’Hôte et al, 2002).