Fai Fung

Water availability in +2 ◦ C and +4 ◦ C worlds

While the parties to the UNFCCC agreed in the December 2009 Copenhagen Accord that a 2°C global warming over pre-industrial levels should be avoided, current commitments on greenhouse gas emissions reductions from these same parties will lead to a 50 : 50 chance of warming greater than 3.5°C. Here, we evaluate the differences in impacts and adaptation issues for water resources in worlds corresponding to the policy objective (+2°C) and possible reality (+4°C). We simulate the differences in impacts on surface run-off and water resource availability using a global hydrological model driven by ensembles of climate models with global temperature increases of 2°C and 4°C. We combine these with UN-based population growth scenarios to explore the relative importance of population change and climate change for water availability. We find that the projected changes in global surface run-off from the ensemble show an increase in spatial coherence and magnitude for a +4°C world compared with a +2°C one. In a +2°C world, population growth in most large river basins tends to override climate change as a driver of water stress, while in a +4°C world, climate change becomes more dominant, even compensating for population effects where climate change increases run-off. However, in some basins where climate change has positive effects, the seasonality of surface run-off becomes increasingly amplified in a +4°C climate.

Coping with the curse of freshwater variability

Institutions, infrastructure, and information for adaptation Coping with variable and unpredictable freshwater resources represents a profound challenge to climate adaptation. Rainfall, snowmelt, soil moisture, and runoff can vary from zero to large quantities, over a range of time scales and in ways not well predicted by climate models. Extreme floods and droughts are the most obvious manifestations, but hydrologic variability can also have chronic impacts. Water security involves managing these risks so that they do not place an intolerable burden on society and the economy (1). We discuss interlinked roles of institutions, infrastructure, and information in managing those risks.

Using Large Climate Ensembles to Plan for the Hydrological Impact of Climate Change in the Freshwater Environment

We explore the use of large ensembles of climate scenarios to inform climate change adaptation in response to hydrological impacts on the freshwater environment, using a sensitive chalk river in south east England to illustrate the approach. The climateprediction.net experiment provides large ensembles of transient climate series from 1920 to 2080. We use 246 transient climate series in the CATCHMOD rainfall-run-off model to develop large ensembles of plausible river flows for the River Itchen. This transient ensemble allows the exploration of how flows may change through the twenty-first century, and demonstrates the range of possible consequences for freshwater ecosystems, based on invertebrate community impacts. Hydrological modelling of flow sequences including abstraction allows the continued effectiveness of river support from groundwater to be assessed. A new environmental impact matrix considers the response of the freshwater ecosystem in the Itchen, concentrating particularly on macro-invertebrates. Through the century increasing numbers of models fail the flow targets, with a minority of models suggesting flows that would lead to irreversible change to the invertebrate community. The large ensemble provides a richer picture of the range of possible change, allowing managers to explore a range of different responses. The approach used is illustrative, but demonstrates that large ensembles may be of great value in improving the understanding of the possible impact of climate change, provided that they can be communicated effectively to decision-makers.

Managing hydroclimatic risks in federal rivers: A diagnostic assessment

Hydroclimatic risks and adaptive capacity are not distributed evenly in large river basins of federal countries, where authority is divided across national and territorial governments. Transboundary river basins are a major test of federal systems of governance because key management roles exist at all levels. This paper examines the evolution and design of interstate water allocation institutions in semi-arid federal rivers prone to drought extremes, climatic variability and intensified competition for scarce water. We conceptualize, categorize and compare federal rivers as social–ecological systems to analyse the relationship between governance arrangements and hydroclimatic risks. A diagnostic approach is used to map over 300 federal rivers and classify the hydroclimatic risks of three semi-arid federal rivers with a long history of interstate allocation tensions: the Colorado River (USA/Mexico), Ebro River (Spain) and Murray–Darling River (Australia). Case studies review the evolution and design of water allocation institutions. Three institutional design trends have emerged: adoption of proportional interstate allocation rules; emergence of multi-layered river basin governance arrangements for planning, conflict resolution and joint monitoring; and new flexibility to adjust historic allocation patterns. Proportional allocation rules apportion water between states based on a share of available water, not a fixed volume or priority. Interstate allocation reform efforts in the Colorado and Murray–Darling rivers indicate that proportional allocation rules are prevalent for upstream states, while downstream states seek reliable deliveries of fixed volumes to increase water security. River basin governance arrangements establish new venues for multilayered planning, monitoring and conflict resolution to balance self governance by users and states with basin-wide coordination. Flexibility to adjust historic allocation agreements, without risk of defection or costly court action, also provides adaptive capacity to manage climatic variability and shifting values. Future research should develop evidence about pathways to adaptive capacity in different classes of federal rivers, while acknowledging limits to transferability and the need for context-sensitive design.