Case studies of groundwater- surface water interactions and scale relationships in small alluvial aquifers

An alluvial aquifer can be described as a groundwater system, generally unconfined, that is hosted in laterally discontinuous layers of gravel, sand, silt and clay, deposited by a river in a river channel, banks or flood plain. In semi-arid regions, streams that are associated with alluvial aquifers tend to vary from discharge water bodies in the dry season, to recharge water bodies during certain times of the rainy season or when there is flow in the river from managed reservoir releases. Although there is a considerable body of research on the interaction between surface water bodies and shallow aquifers, most of this focuses on systems with low temporal variability. In contrast, highly variable, intermittent rainfall patterns in semi-arid regions have the potential to impose high temporal variability on alluvial aquifers, especially for small ones. Small alluvial aquifers are here understood to refer to aquifers on rivers draining a meso-catchment (scale of approximately 101 - 103 km2). Whilst these aquifers have lower potential storage than larger ones, they may be easier to access for poor rural communities the smaller head difference between the riverbed and the bank can allow for cheap manual pumps. Thus, accessing small alluvial aquifers for irrigation represents a possibility for development for smallholder farmers. The aquifers can also provide water for livestock and domestic purposes. However, the speed of groundwater depletion after a rain event is often poorly understood. In this study, three small alluvial aquifers in the Limpopo Basin, Zimbabwe, were studied: (i) upper Bengu catchment, 8 km2 catchment area on a tributary of the Thuli River, (ii) Mnyabeze 27 catchment, 22 km2 catchment area on a tributary of the Thuli River, and (iii) upper Mushawe catchment, 350 km2 catchment area on a tributary of the Mwenezi River. All three are ephemeral rivers. In each case, the hydrogeological properties of the aquifer were studied; the change in head in the aquifer was monitored over time, as well as any surface inflows. Results from each case are compared showing that scale imposes a lower limit on alluvial aquifer viability, with the shallowness of the Bengu aquifer (0.3 m) meaning it has effectively no storage potential. The much higher storage of the Mushawe aquifer, as well as the longer period of storage after a flow event, can be assigned partially to scale and partially to the geological setting.