Abstract:Environmental flows (E flows) are now a standard part of sustainable water management globally but are only rarely implemented. One reason may be insufficient engagement of stakeholders and their priority outcomes in the E flow-setting process. A recent environmental flow assessment (EFA) in the Kilombero basin of the Rufiji River in Tanzania concentrated on a broad-based investigation of stakeholders' use and perceptions of the ecosystem services provided by the river. The EFA process generally followed the Building Block Methodology but with an enhanced engagement of stakeholders. Engagement began with the involvement of institutional stakeholders to explain the purpose of the EFA and to elicit their priority outcomes.Extensive interactions with direct-use stakeholders followed to investigate their uses of and priorities for the rivers. Results were used by the EFA specialist team in choosing flow indicators and defining measurable environmental objectives. The specialists then met to reach a consensus of the flow requirements. The EFA results were lastly reported back to stakeholders.During the Kilombero EFA we learned that stakeholders at all levels have a good awareness of the natural services provided by a healthy river and can contribute to the setting of environmental objectives for the rivers and floodplain. These can be factored into the biophysical assessments of river flows required to maintain habitats, processes, water quality and biodiversity. It is therefore important to allocate significant resources to stakeholder engagement. It now remains to be seen if enhanced stakeholder engagement, including the increased understanding and capacity built among all stakeholders, will increase support for the implementation of the recommended flows.
Abstract. This paper presents a salt intrusion model for the Pungue estuary with the aim to determine the minimum discharge required to prevent the salt intrusion from reaching the water intake situated 82 km from the estuary mouth. The Pungue river is shared between Zimbabwe and Mozambique and has a large variation in precipitation and runoff. The mean monthly discharge can be as low as 8 m3/s and as high as 893 m3/s. The second largest city of Mozambique, Beira, relies on the Pungue for its water supply. In the dry season it frequently occurs that the water intake has to be ceased because the salinity of the Pungue is too high. The salt intrusion model used in this paper is based on a fully analytical and predictive theory which is confronted with measurements of salt intrusion and estuary topography. The paper presents the collection of estuary characteristics and the salt water intrusion measurements that were obtained by field measurements in 1993 and 2002. Using these data the salt intrusion model has successfully been applied. During salinity intrusion measurements in the dry season of 1993 it was observed that sand banks in the middle zone of the estuary prevented the salt water from intruding further upstream, resulting in lower salinity levels upstream than the theoretical salt water intrusion model predicts. This effect occurs during ebb of neap and average tides and can reduce the salt water intrusion by 10 km. The model indicates that in a natural situation a minimum monthly discharge of 12 m3/s is required to maintain acceptable salinity levels during high water and spring tide near the water intake. The actual water discharge upstream of the water intake has to be higher, since this minimum discharge does not take into account the water abstracted for irrigation and/or urban water supply. Current water abstractions lead to salt water intrusion near the water intake at approximately 10% of the time. The model indicates that an additional water abstraction of 5 m3/s will lead to an increase in salt water reaching the intake at 10% of the time. During neap tide the sand banks act as a temporary natural salt intrusion barrier reducing the chance of salt water reaching the water intake.
Incorporation of environmental flow releases from reservoirs has proven to be challenging due to fear of losses to existing water uses. Moreover environmental flow requirements (EFR) have not often been operationalized. This study compares the possibility of implementing dynamic EFR based on natural flows lagged against an upstream unregulated gauging point with static EFR. It simulates different scenarios with a high flow release in the wet season and analyses its impacts on hydropower production. This method accounts fully for the natural variability of environmental flows, implying less pressure on existing water uses during relatively dry years. Joint operation of two cascading dams vs. individual operation for EFR was also explored. These approaches were tested for the Zambezi River basin in Southern Africa using a water resources model, WAFLEX. Historic data on reservoir water levels, releases and power generation of the hydropower schemes were synthesized. Combining these yielded a validated series of monthly flow data for a 28 year period . The results show that Kariba and Cahora Bassa reservoirs face a reduction in power produced when they would annually release an environmental flow. However, the dynamic EFR method entails smaller hydropower losses. Joint environmental flow operations will reduce overall basin power production more than if Cahora Bassa alone would release an environmental flow. However, such joint operation would be more beneficial to the ecosystem.
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