Key components of the water cycle such as rainfall, evaporation and river flow vary naturally over all time scales – from minutes to millennia. Man’s activities are modifying them and this will continue as CO2 levels rise and our use of water increases. If we are to respond to these changes successfully, we must first quantify both the amount of water that will be available and what the future demand will be. Most importantly, we must begin to consider availability and demand as a single issue.
WATCH has done this by bringing together climate, water cycle, and water resource experts. It has pooled their knowledge and skills, and has established effective frameworks for co-operation. For the first time, these communities have agreed common terminology and protocols for data exchange, providing a foundation for work at the global scale. What started-off as a scientific-handshake has developed into a movement with genuine momentum. It will benefit water management in the 21st century, and will influence international research projects for years to come, underpinning the development of evidence-based inter-governmental policy-making.
From basic physics we know that air temperature will increase with increasing concentrations of greenhouse gases. Higher temperatures over the oceans will increase evaporation and the humidity held in the atmosphere. This is likely to lead to higher rainfall over the whole globe and the further likelihood of more intense rainfall. Regionally, these changes in rainfall will depend on shifts in weather patterns, but there is already general agreement that Mediterranean climates. will become drier and the northern latitudes will be wetter.
Such statements are highly significant. They are also incredibly broad. To manage their implications, we require more details of actual water availability in the future. This information can be provided by climate and hydrological models, but we must appreciate the uncertainty in model projections, and we must establish a culture of on-going model improvement. As a starting point, WATCH recognised that we must develop a thorough understanding of the water cycle in the recent past.
Taking the output data from climate models to drive hydrological models on a global scale seems an obvious thing to do. However, up until now, the different space and time scales at which the two communities work has been a significant stumbling block.