Abstract Kenya's current 4% rate of population growth requires cultivation of food crops by smallholder farmers in ever drier zones of semiarid lands. Rainfall is limited, variable and unpredictable, but maize, widely known for susceptibility to drought, remains the staple and favorite food crop. A case study is presented in which “effective rainfall” for Katumani Composite B-type maize, grown at Katumani, Machakos District, is evaluated for each of the 48 wet seasons in the 24-year record. The newly developed analysis takes into account rainfall, evaporation, soil depth and water holding capacity, and growth characteristics of the crop influencing water uptake and yield. Conclusions are: 1) The analysis evaluates suitability of a given crop for production in any location where rainfall and evaporation records are available. 2) Dates of onset of the rains at Katumani are sortable into periods termed “early” (implies expectation of high to medium water adequacy for maize production), “late” (medium to low ...