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


Love, Dave and Hamer de, Wouter and Owen, Richard J.S. and Booij, Martijn and Uhlenbrook, Stefan and Hoekstra, Arjen Y. and Zaag van der, Pieter (2007) Case studies of groundwater - surface water interactions and scale relationships in small alluvial aquifers. In: 8th WATERNET/WARFSA/GWP-SA Symposium, 31 October-2 November 2007, Lusaka, Zambia, 31 October-2 November 2007, Lusaka, Zambia.

Abstract: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.
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Engineering Technology (CTW)
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