Uncertainties in long-term management of water resources

Abstract

A reliable supply of water to communities, industry and agriculture is crucial for a healthy population, and a successful economy. Long-term management of water resources poses significant challenges for decision makers due to uncertainties. Natural variability in hydrological processes, as well as future changes in climate, land use, demography and other socio economic factors are placing increased pressure on water resources and pose a threat to water security. The release of probabilistic climate information, such as the UKCP09 scenarios, provides improved understanding of some uncertainties associated with climate model projections. This has motivated a more rigorous approach to dealing with other uncertainties in order to understand the sensitivity of investment decisions to future uncertainty and identify adaptation options that are as far as possible robust. To understand the implications of this range of uncertainties, a novel integrated systems model has been developed that couples simulations of weather under current and future climates, catchment hydrology, and the water resources system. This systems model was used to assess the likelihood and magnitude of water scarcity. Uncertainty and sensitivity analyses were undertaken to assess the implications of uncertainties on water scarcity, and to subsequently identify water resources management options that are robust to these uncertainties. The integrated systems model has been applied in the Thames catchment which supplies the city of London, UK. This region has some of the lowest rainfalls in the UK, the largest and fastest growing population, and is therefore particularly sensitive to water availability. Results indicate that by the 2080s, when accounting for all uncertainties considered here, there may not be a considerable change in total amount of rainfall relative to the control period (1961-1990). However as a result of an increase in temperature, the annual mean PET is expected to increase by 26.6%. Based on the results, a 24.0% and 1.3% reduction in annual mean daily flow and subsurface storage are projected to occur in the Thames catchment respectively. Moreover, a 1083.0% increase in the total number of drought days relative to the control period (1961-1990) is expected under current population and climate trends by 2080s. Water scarcity in London is most sensitive to climate and population change, and so investment in monitoring to reduce these uncertainties would help improve the robustness of investment decisions. A portfolio of adaptation measures, that includes a combination of desalination plant with capacity of 150 Ml/d, constructing a new i reservoir with 100 Mm3 capacity and 40.0% reduction in leakage, is required to reduce the drought risks. However, sensitivity testing shows that measures taken to reduce per capita water demand are more robust to future uncertainties than major engineering interventions.