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Peasant farming is the most important economic activity in many parts of Africa’s semi-dry and dry sub-humid environment, and a key impediment to its success has remained largely the erratic rainfall regime, especially since the great Sahelian drought of 1969-1973. This variability and the response it has engendered have motivated the development of schemes for determining these phases of the rainfall regime. This paper reviews some of the existing methodologies for determining the effective onset date of the rain (the most critical to the farmer), highlights some of their drawbacks and proposes a new rainfall-only scheme (Intra-seasonal Rainfall Monitoring Index-IRMI) which seeks to address the identified gaps. Results of a test of efficacy have shown IRMI’s characterization of rainfall seasons between 1970 and 1999 completely in agreement with what has been documented in a large body of literature on this subject. In addition, results have shown that though onset dates appear to have occurred earlier in the 1990s than in the 1980s and 1970s, inter-seasonal variability actually got progressively worse from the 1970s to the 1990s underscoring the urgent need to put in place reliable forecast schemes in aid of peasant farming. Consequently, the scheme should be adopted for determining the effective onset of rains and commencement of intensive planting across the region.