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This paper summarises innovative research into the assessment of longterm groundwater recharge from flood events in dryland environments of the Kuiseb (Namibia) and the Buffels (South Africa) rivers. The integrated water resource management (IWRM) policies and institutions affecting the exploitation of groundwater resources in each of these developing countries are compared. The relatively large alluvial aquifer of the Kuiseb River (∼240 Mm3) is recharged from irregular floods originating in the upper catchment. Reported abstraction of 4.6 Mm3 per year is primarily consumed in the town of Walvis Bay, although the groundwater decay (pumping and natural losses along the period 1983–2005) was estimated in 14.8 Mm3 per year. Recharge is variable, occurring in 11 out of 13 years in the middle Kuiseb River, but only in 11 out of 28 years in the middle-lower reaches. In contrast, the Buffels River has relatively minor alluvial aquifers (∼11 Mm3) and recharge sources derive from both lateral subsurface flow and floodwater infiltration, the latter limited to a recharge maximum of 1.3 Mm3 during floods occurring once every four years. Current abstractions to supply the adjacent rural population and a few small-scale, irrigated commercial farms are 0.15 Mm3 yr−1, well within the long-term sustainable yield estimated to be 0.7 Mm3 yr−1. Since independence in 1990, Namibia’s water resource management approach has focussed on ephemeral river basin management of which the Kuiseb BasinManagement Committee (KBMC) is a model. Here, some water points are managed independently by rural communities through committees while the national bulk water supplier provides for Walvis BayMunicipality from the lower aquifers. This provides a sense of local ownership through local participation between government, NGOs and CBOs (community-based organisations) in the planning and implementation of IWRM. Despite the potential for water resource development in the lower Buffels River, the scope for im

Flood water infiltrates ephemeral channels, recharging local and regional aquifers, and it is the main water source in hyperarid regions. Quantitative estimations of these resources are limited by the scarcity of data from such regions. The floods of the Kuiseb River in the Namib Desert have been monitored for 46 years, providing a unique data set of flow hydrographs from one of the world’s hyperarid regions. The study objectives were to: (1) subject the records to quality control; (2) model flood routing and transmission losses; and (3) study the relationships between flood characteristics, river characteristics and recharge into the aquifers. After rigorous quality-testing of the original gauge-station data, a flood-routing model based on kinematic flow with components accounting for channel-bed infiltration was constructed and applied to the data. A simplified module added to this routing model estimates aquifer recharge from the infiltrating flood water. Most of the model parameters were obtained from field surveys and GIS analyses. Two of the model parameters—Manning’s roughness coefficient and the constant infiltration rate—were calibrated based on the high-quality measured flow data set, providing values of 0.025 and 8.5 mm/h, respectively. This infiltration rate is in agreement with that estimated from extensive direct TDR-based moisture measurements in the vadose zone under the Kuiseb River channel, and is low relative to those reported for other sites. The model was later verified with additional flood data and observed groundwater levels in boreholes. Sensitivity analysis showed the important role of large and medium floods in aquifer recharge. To generalize from the studied river to other streams with diverse conditions, we demonstrate that with increasing in infiltration rate, channel length or active channel width, the relative contribution of high-magnitude floods to recharge also increases, whereas medium and small floods contribute less, often not r

A study on flood water infiltration and ground water recharge of a shallow alluvial aquifer was conducted in the hyperarid section of the Kuiseb River, Namibia. The study site was selected to represent a typical desert ephemeral river. An instrumental setup allowed, for the first time, continuous monitoring of infiltration during a flood event through the channel bed and the entire vadose zone. The monitoring system included flexible time domain reflectometry probes that were designed to measure the temporal variation in vadose zone water content and instruments to concurrently measure the levels of flood and ground water. A sequence of five individual floods was monitored during the rainy season in early summer 2006. These newly generated data served to elucidate the dynamics of flood water infiltration. Each flood initiated an infiltration event which was expressed in wetting of the vadose zone followed by a measurable rise in the water table. The data enabled a direct calculation of the infiltration fluxes by various independent methods. The floods varied in their stages, peaks, and initial water contents. However, all floods produced very similar flux rates, suggesting that the recharge rates are less affected by the flood stages but rather controlled by flow duration and available aquifer storage under it. Large floods flood the stream channel terraces and promote the larger transmission losses. These, however, make only a negligible contribution to the recharge of the ground water. It is the flood duration within the active streambed, which may increase with flood magnitude that is important to the recharge process.