Average actual cereal yields are a result of both inter-seasonal droughts and intraseasonal dry spells that occur during the cropping season. During dry spells crops cannot effectively use available soil nutrients. In order to decrease the gap between actual and potential cereal yields there is need to minimise the intra-seasonal dry spells. Analyses of water balances for cereal production in the semi-arid areas show that much water is inaccessible to the crop due to poor rainfall distribution. A step towards intra-seasonal dry spell mitigation, the results of rainfall analyses of Machakos District, Kenya are presented in this paper. Rainfall amounts, frequencies and runoff are examined. The main objective is to identify occurrence of intraseasonal dry spells that lower yields on farms. The paper also discusses when these dry spells could have been alleviated through supplemental irrigation of the crop, using a small-scale water harvesting system.