An examination of the hydrological system of a sand dam during the dry season leading to water balances

To address water scarcity in semi-arid regions, rainfall and runoff need to be captured and stored locally before they are lost to the sea. This can be done using a sand dam which consists of a reinforced wall constructed during the dry season across a seasonal riverbed. However it is unclear whether their main utility is to store water in the sand that is also trapped behind them, or to facilitate aquifer recharge. This paper aims to answer this question by the calculation of a water balance in three sand dams in Kenya to quantify the amount of water transferred between the sand dam and the surrounding aquifer system. The components of the water balance were derived from extensive field monitoring. Water level monitoring in piezometers installed along the length of the sand deposits enabled calculation of the hydraulic gradient and hence the lateral flow between the different reaches of the sand dam. In one sand dam water was gained consistently through the dry season, in one it was lost, and in the third it was lost almost all the time except for the early dry season in the upper part of the trapped sand. In conclusion sand dams should not be treated as isolated water storage structures.