Abstract
Abstract. Water management in sub-Saharan African river basins is challenged by an uncertain future climatic, social and economical patterns potentially causing diverging water demands and availability, and by multi-stakeholder dynamics, resulting in evolving conflicts and tradeoffs. In such contexts, a better understanding of the sensitivity of water management to the different sources of uncertainty can support policymakers in identifying robust water supply policies balancing optimality and low vulnerability against likely adverse future conditions. This paper contributes an integrated decision-analytic framework combining an optimization, robustness, sensitivity, and uncertainty analysis to retrieve the main sources of vulnerability to optimal and robust reservoir operating policies across multi-dimensional objective spaces. We demonstrate our approach on the lower Umbeluzi river basin, Mozambique, which an archetypal example of sub-Saharan river basin, where surface water scarcity compounded by substantial climatic variability, uncontrolled urbanization rate, and agricultural expansion are hampering the Pequenos Libombos dam's ability to supply the agricultural, energy, and urban sectors. We adopt an Evolutionary Multi-Objective Direct Policy Search (EMODPS) optimization approach for designing optimal operating policies, whose robustness against social, agricultural, infrastructural, and climatic uncertainties is assessed via robustness analysis. We then implement the generalized likelihood uncertainty estimation (GLUE) and PAWN uncertainty and sensitivity analysis methods for disentangling the main challenges to the sustainability of the operating policies and quantifying their impacts on the urban, agricultural, and energy sectors. Numerical results highlight the importance of a robustness analysis when dealing with uncertain scenarios, with optimal non-robust reservoir operating policies largely being dominated by robust control strategies across all stakeholders. Furthermore, while robust policies are usually vulnerable only to hydrological perturbations and are able to sustain the majority of population growth and agricultural expansion scenarios, non-robust policies are sensitive also to social and agricultural changes and require structural interventions to ensure stable supply.
Highlights
The availability of freshwater is a limiting factor to food production, energy generation, and industrial consumption around the globe (Hermoso, 2017; Zampieri et al, 2018)
We implement an integrated decision-analytic framework combining optimization, robustness, sensitivity, and uncertainty analysis to better understand the major sources of uncertainty for water supply strategies in the lower Umbeluzi river, Mozambique
The results provide important insights on the robustness and vulnerability of reservoir operation to exogenous perturbations in managing multiple conflicting objectives
Summary
The availability of freshwater is a limiting factor to food production, energy generation, and industrial consumption around the globe (Hermoso, 2017; Zampieri et al, 2018). Investing in new infrastructure is still the predominant option to expand the storing and conveying capacity of freshwater, in sub-Saharan Africa and southeast Asia, which are areas that have a larger untapped potential (Fields et al, 2009). This hard path water solution is evoking contentious debates for the considerable environmental and social costs of damming rivers (Moran et al, 2018), reaching a point that the efficient operation of existing infrastructure, rather than planning new ones, is becoming critical to balance the tradeoffs between supply and demand (Gleick and Palaniappan, 2010). Understanding the impact of those uncertainty sources on reservoir operation is, key for developing robust operating policies that support policymakers towards sustainable river basin management
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