A survey of policy makers and scientists indicates water supply is the most pressing environmental issue facing the United States, researchers said Wednesday. A survey conducted by the University of California, Davis, and Britain's University of York found providing enough water to sustain human populations and ecosystem resilience was ranked as having the greatest potential to increase the effectiveness of policies related to natural resource management in the United States. The survey asked managers, policymakers and scientists to rank 40 research questions that most reflected the needs of those with jurisdiction over natural resources. The survey, published in the journal BioScience, was released as California is mired in its worst drought in nearly half a century. Other questions that were ranked as of high importance to policy focused on methods for measuring the benefits humans receive from ecosystems; the effects of sea-level rise, storm surge, erosion and precipitation on coastal ecosystems and human communities; and the effect on carbon storage and ecosystem resilience of different management strategies for forests, grasslands, and agricultural systems. Still, water supply ranked above all, the researchers found. "Our results suggest that participatory exercises such as this are a robust way of establishing priorities to guide funders of research and researchers who aim to inform policy," UC Davis researcher Erica Fleishman said. "The consensus in priorities is even more striking as California's current drought leads to unprecedented reductions in water supply and delivery."