Field experiments in the semi-arid regions of Kenya have shown that soil water conservation techniques can result in increased maize grain yields. The degree of benefit in a particular season is dependant on the rainfall amount and distribution. However, the results of field experiments are limited to a few years of observations at specific locations and it is therefore difficult to make generalized conclusions about the benefits in years of differing rainfall patterns.The PARCH model has been calibrated for Katumani Composite ‘B’—a local variety of maize used in the Machakos district of Kenya—and validated against observed grain and dry matter yields from experimental plots. Historical daily rainfall data were collected for Katumani Research Station, in the semi-arid region of Kenya, during the period 1961 to 1994. Seasonal totals for the short rains (October to February) and long rains (February to August) were calculated and nine years were selected as representative of ‘wet’, ‘average’ and ‘dry’ seasons for the long and short rains, respectively.The PARCH model was then used to simulate the maize grain yield under three soil water conservation scenarios: (i) a typical situation where 30% of rainfall above a 15 mm threshold is lost as runoff, (ii) runoff control, where all rainfall infiltrates, and (iii) runoff harvesting, which results in 60% extra ‘rainfall’ for rains above 15 mm. The soil was taken to be a sandy clay loam which is typically found in the region. Two planting densities of 4·4 and 8·8 plants m−2were used to simulate normal and high levels of management. Planting dates were determined from the 30% runoff scenario and were fixed for the other scenarios to avoid confounding the results.The results showed that runoff control and runoff harvesting produce significant yield increases in ‘average’ years in both the long rains and the short rains. However, in ‘dry’ years there were only small yield increases in the short rains and negligible benefit in the long rains. In ‘wet’ years there were no significant yield increases due to water conservation in either season.Clearly, these results are a simplification of the real situation where water conservation strategies may allow earlier planting or be accompanied by increased planting densities, both of which may result in yield increases. However, this work demonstrates the usefulness of appropriate crop growth models in evaluating a wider range of crop management strategies under a realistic range of climatic conditions than would be possible in the field.