Since the 1990th Sasol, a local Non-Governmental-Organization (NGO), builds sand storage dams, to mitigate the consequences of droughts in the semi-arid county of Kitui, Kenya. Sand storage dams are dams constructed in ephemeral rivers which increase the storage capacity of the riverbeds. The water stored by sand storage dams enables local people to generate income through irrigated agriculture and brick making. By exploiting the stored water people also influence the groundwater flows and tables through water extraction and evapotranspiration, runoff and infiltration through the cultivation of land. This two way feedback between society and water system is further influenced by local circumstances like water quality, slopes and local initiatives. In this study the long term effects of sand storage dams on hydrology and water use were assessed in the Kiindu catchment near Kitui town. It combines observed human agency and simulated effects of sand storage dams on groundwater levels to depicture what is called in socio-hydrology the coevolution of society and water system after the construction of sans storage dams. During a fieldtrip the study area, the local circumstances and the exploitation of water sources were assessed by means of trans-sectional walks and semi-structured interviews. The results show the interaction of local circumstances, society and water system led to different local developments even in a small catchment like the Kiindu catchment. How water is extracted and used depends on catchment properties. If the riverbanks are broad and lots of water from the riverbed can be extracted like in the downstream of the study area, people do irrigated agriculture on the riverbanks. But in such areas it is hard to find locations where sand dams can be founded on rock. Therefore sand dams are endangered by erosion of the riverbanks. In areas with steeper slopes and narrow and thin riverbeds, irrigated agriculture cannot be applied in the vicinity. Here people need to invest in equipment to utilize the water stored by the sand dams. But in these areas there are more locations where sand dams can be founded on rocks. Therefore the sand dams are more robust. Other factors that influence the utilization of water from the catchment are water quality and the availability of alternative water sources. To investigate how the effects of the sand storage dams on the groundwater tables may influence the availability of water and therefore the water use, a groundwater model was developed. With the ‘pmwin’ groundwater model, which is based on the finite-difference groundwater model MODFLOW, the influence of local catchment properties on the performance of the sand dams were assessed and changes in flow directions around sand dams analysed. The simulations show that the slopes of the stream and the thickness of the riverbed have a major impact on the amount of water stored by the sand dams. Because the sand dams which store the largest amount of water lie in the area where most of the irrigated fields are located, the modelling results give a possible explanation for the observed development. The study shows that the sand dams in general lead to larger amounts of water available in the catchment. Therefore they improve living conditions of the local communities. However the effects of the sand dams differ between the areas. In some areas they only help the communities to satisfy their domestic water demand, while in other areas people have sufficient water to generate income through irrigated agriculture. But sand storage dams may also fail and their effect abate afterwards. Thus the study shows that there is not ‘The Effect’ of the sand storage dams on the development of a catchment. To understand why sand storage dams have a certain effect it is important to take the local circumstances into account by assessing their effects. One should also be cautious and choose a flexible implementation and operation approach that takes the local circumstances into account if the experience with sand storage dams is transferred from one area to another. Finally, if there is not ‘The Effect’ of sand storage dams, how do developments of catchments look like after the construction of sand storage dams? Based on the information obtained in the field, hypothetical socio-hydrological narratives for small sub areas in catchment, which need to be proved in further research, are formulated to show how these sub-catchments may have developed.