Several studies demonstrate that the hydrographs of many western rivers are changing in response to climate change, with one of the most pronounced changes being earlier runoff. This trend is most pronounced in low elevation basins. For example, in many basins of the Pacific Northwest, annual spring snowmelt is now occurring more than 20 days earlier than a half-century ago. This trend is less evident in high elevation watersheds, but even in those locations, continued global warming is expected to eventually modify streamflow patterns. Coincident with these changes are shifts in the timing and magnitude of water demands. The "water rights and climate change" project is focused on how changes in the timing of spring snowmelt does or does not create problems in the administration of western (prior appropriation) surface water rights that are defined in part with respect to seasonal characteristics, either generally through terms such as "irrigation season" rights, or more specifically in rights that use explicit calendar dates to describe the start and end of diversion (or storage) seasons.
