Climatic Impacts on Snowpack, Irrigation, and Small Reservoirs for Waterbird Nesting in a Semiarid Rocky Mountain Basin

Abstract In the semiarid western United States, annual variability of mountain snowmelt has long prompted construction of small reservoirs to sustain irrigation through the summer and among years. With relatively consistent, prolonged flooding, these small reservoirs commonly support tall emergent vegetation that is otherwise often limiting to diverse breeding waterbirds. However, increasing water shortages have promoted irrigation methods that apply less water, as well as water transfers from irrigation to urban uses or to mandated deliveries downstream. These initiatives generally overlook wildlife values of irrigation-related wetlands, and approaches to predicting water allocations needed by particular habitats have not been developed. This study examined multidecadal variations in water deliveries to small reservoirs suitable for overwater nesting in the Laramie Basin, Wyoming, and used that variability to project reductions in this habitat resulting from a range of water shortages. Analyses indicate that surrounding high elevations have so far remained cold enough to avoid declines in snowmelt deliveries seen elsewhere. Nevertheless, high snowpack variability and legal obligations to out-of-basin demands make future shortages an important concern. Estimates of the percentage of years when flows to representative wetlands would be inadequate to flood tall emergent vegetation increased from 13‒34% historically to 35‒47% and 70‒78% with decreased snowmelt of 25% and 50% projected by some published models, or by similar reallocations of flows. Given persistent calls for increased water storage for agricultural and municipal uses, further development of small reservoirs in this region may provide opportunities to offset habitat shortfalls in ways that serve multiple purposes.