Effects of sand storage dams on groundwater levels with examples from Kenya

Abstract

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.

Introduction

Water retaining structures intercept or obstruct the natural flow of water in wet seasons and store water for drier periods. Water harvesting technologies, which concentrate precipitation through runoff and storage for beneficial use, have probably been in use since 9000 BC (Oweis et al., 2001). Retaining groundwater is not a new concept either. Groundwater dams (Fig. 1) were constructed on the island of Sardinia in Roman times and by ancient civilizations in North Africa (Nilsson, 1988). A specific type of groundwater dam, sand storage dams (Fig. 1), is well known in the Middle East. Such dams have also been used for water supply in the southwestern United States and northern Mexico since the mid 1800s (Van Haveren, 2004). Other examples come from Namibia (Stengel, 1968). More recent efforts include small-scale projects in many parts of the world, notably India, Africa and Brazil (Barrow, 1999). Such dams store sufficient quantities of water for livestock, minor irrigation and domestic use. The technology might be considered ‘simple’ but ‘effective’, reason why many Non-Governmental Organizations (NGO) consider it an interesting instrument to provide drinking water to poor, rural communities (Nilsson, 1988, Van Haveren, 2004). For many other potentially interested parties, such as government agencies and engineering firms, however, ‘simplicity’ often equals ‘uninteresting’ because they are perceived to be too small, not scientific or of little economic interest. In any case, a hydrological analysis of sand storage dams is not available in the literature.

Basic ideas why sand storage dams store water are easily constructed, including drawing simplified, static water balances, either of the sub-surface storage volume created by the dam or of the catchment area connected to the dam. It is likely that the available effective storage of the dams will be easily filled, although some authors do caution that this only will be the case if the dams are ‘sufficiently recharged’ (Van Haveren, 2004, p. 264). Water use will decrease the water volume and thus the water level upstream of the dam, but to what extent typically depends on the amounts used compared to the size of the dam. What happens with the groundwater system on the long run, when flows between dam reservoir and riverbanks and interactions between recharge and water extraction become important, has not been reported yet. Evaluating the effectiveness of dams is not as simple as measuring volumes of water stored in the sand. Perhaps a certain volume is available at a dam site, but actual flows to, for example, a scoop hole or pump used to fetch drinking water determine the potential yield in a certain timeframe. In turn, these flows are determined by factors like surrounding groundwater levels and soil properties. To understand water fluxes over seasons, including infiltration into the riverbanks and human water use, the temporal and spatial groundwater flow pattern needs to be known. Data from measurements over longer periods (several years) are not available. Only recently measurements in the Kitui (Kenya) area have started (Borst and De Haas, 2006).

To gain insight in the longer-term effects of sand storage dams over periods of several years, our aim was developing a numerical model to understand water flow effects in close proximity to a dam (within several 100 m). Simple as it may be, the model can be used to simulate existing situations and to predict potential successes of sand storage dams before being built. This should be seen as our main scientific result. Obviously, precise performance analysis of sand storage dams requires availability of typical flow characteristics in the river stretch concerned. With the (conceptual) model, different scenarios were evaluated to study under which circumstances sand storage dams do or do not function as intended. The scenarios are based on data from two regions in Kenya, Voi and Kitui. Below the concept of groundwater storage will be discussed, followed by a description of sand storage dams in Voi and Kitui. This descriptive part is followed by a discussion on the hydrological model applied. The geometry, hydrology and water use aspects of the scenarios modeled are explained. In a final paragraph it is discussed how the model supports a better understanding of the hydrological behavior of sand storage dams. Results of the model are compared with available field data.