In Kenya, sub-surface groundwater dams are constructed at a rapid pace to store water for livestock, irrigation and domestic use. The basic principle of such dams is that water is stored sub-surface instead of at the surface. Evaporation losses and contamination risks are thus greatly reduced. The sub-surface reservoirs are recharged through flash floods originating from rainfall events. A simple groundwater-flow model was developed to understand hydrological processes and flows around the dam. Two different situations in Kenya were studied with the model. The first case in Voi shows how the groundwater levels upstream of the dam and in the adjacent riverbanks are strongly influenced when the sub-surface water is used for relatively intensive irrigation. The second case in Kitui shows the relatively mild effects of household water use on groundwater levels. The model results appear to be realistic. Therefore the model can be used to evaluate the hydrological processes of sand storage dam reservoirs in situations where measured data are scarce.