Water Management encourages submissions from practitioners and researchers across a broad spectrum of issues in the management of water resources. The four papers in this issue represent a neat transect across that spectrum – from the impact of flow resistance in corrugated pipes to planning for conjunctive water use at the river basin scale in China, via planning for sanitation systems in sub-Saharan Africa and best-practice operational design for run-of-river power plants. Techniques include hydraulic experimentation, socially-informed economic planning and mathematical modelling of optimisation algorithms, and involve authors based in six different countries from both universities and the private sector – a conclusive display of the intellectual energy expended in providing well-founded water management across our planet.
Our first contribution focuses on varying roughness coefficients attributable to different flow regimes in corrugated PVC pipes (Kouchakzadeh et al., 2011). Such pipes are commonly used in sewer and subsurface drainage networks and understanding different roughness conditions under increasing discharge in gradually varied, spatially varied and pressurised flow is important in estimating the discharge capacity of the pipe network. Different experimental laboratory configurations were devised to investigate each flow condition in two different diameter pipes, recording results using piezometers and high-resolution digital photography. Results demonstrate clearly that roughness coefficients increase from pressurised to gradually varied flow, and increase again by 40–50% in spatially varied flow as compared to gradually varied flow. Thus, the need to employ an appropriate roughness coefficient under different flow regimes in corrugated pipes is demonstrated.
Scaling up from pipes to entire sanitation systems, our second contribution discusses the critical factors in transitioning from on-site sanitation to off-site waterborne sewerage systems in secondary towns around Lake Victoria in sub-Saharan Africa (Mattholie, 2011). Achieving better sanitation is critical in reducing child mortality linked to waterborne diseases and was a key component of the United Nations millennium development goals for 2015, but sanitation issues have often lagged behind water supply improvements. The study determines several ‘tipping points' that can favour the transition from on-site to sewerage-based sanitation systems. Critical factors likely to favour off-site sanitation include higher levels of daily water consumption in the host population and higher population densities. Filters include the concept of ‘wastewater density' and also capital cost, wherein the cheaper cost of simplified and small-bore sewerage may make off-site sewerage cost-effective at far lower population densities than conventional sewerage systems. Various location-specific factors also need to be considered. Tentative guidelines are devised for sewerage feasibility that integrate consumption, population density and wastewater density – the thresholds in such guidelines will vary according to the unit cost of various sewerage solutions and must accommodate population growth forecasts.
Cost factors are also prevalent in our third contribution, concerning optimisation of operating designs for run-of-river power plants (Bozorg Haddad et al., 2011). The essential challenge in such plants is to maximise the annual difference between generated energy and operating costs over the project horizon, subject to various performance and hydraulic constraints. Optimising the annualised benefits is important in making run-of-river solutions more cost effective at a time when the environmental and social impacts of various other energy sources (including large reservoirs) are under increasing scrutiny. Optimisation of various factors, including the plant's penstock design and the number, type and operation of the turbines in relation to the prevailing flow regime and the plant's construction cost, is a large-scale, non-linear problem. The authors compare the use of their honey bee mating optimisation approach (HBMO: simulating processes akin to the lifecycle of a bee colony) to an analytical approach using Lagrange multipliers for a case study in Iran. They conclude that the HBMO algorithm is a more effective approach and achieves a better solution while requiring a smaller penstock diameter, thus reducing project costs.
Our final contribution also concerns optimisation, in this case applied to the conjunctive use of surface and groundwater resources in the 42 000 km2 semi-arid Shiyang river basin in Gansu province, China (Geng et al., 2011). Optimisation is required because recent irrigation practice led to a rapid decline in groundwater tables: water use in the middle and lower basin was estimated to have exceeded upper basin runoff by approximately 40%. Based on the concept of integrated water resources management, the authors argue that, as irrigation is by far the largest user of water both locally (86% of water use) and globally, the efficient, equitable and sustainable use of water resources relies largely on improved irrigation management. The approach combines a surface water optimisation model, set to a given irrigation demand and resulting in a residual requirement for groundwater abstraction, with a linked groundwater optimisation model. The models are based on genetic algorithms, argued to provide robust and computationally efficient solutions. The approach achieved a more equitable allocation of irrigation water than those achieved by a previous method, but illustrated frequent irrigation deficits because the new approach integrates constraints resulting from the requirement to assure sustainable groundwater resources. The implication is that equitable water allocation without compromising groundwater resources will require demand management of irrigation supplies, that is, a reduction in irrigated area that will necessitate taking some difficult resource management decisions.
