Diversity of hydrologic responses to summer and winter warming in the Columbia River basin

Increased temperatures will lead to fundamental changes in the seasonal distribution of streamflow and the management of water resources, especially in the western United States. Understanding the potential of these changes is, however, complicated by the wide range of projections from climate models, which can obscure the basin-specific hydrologic characteristics that control hydrologic sensitivities to climate change, and a coarse spatial scale that does not easily translate to local water management applications. We report a set of controlled experiments that evaluate, using the Variable Infiltration Capacity (VIC) land-surface hydrologic model at one-sixteenth degree latitude and longitude resolution, the sensitivities of annual and seasonal streamflow over the Columbia River basin to imposed temperature changes. Other work aligned with this effort has shown that western U.S. basins, including the Columbia basin as a whole, are more sensitive to warming in the summer than in the winter in terms of annual streamflows. We further examine the spatial character of these sensitivities in the Columbia using sub-basin level (eight digit Hydrologic Unit Code scale, or cataloging unit) responses in the United States and similar sub-basin level (National Hydro Network Work Unit) responses in Canada. Within the Columbia River basin and Puget Sound region, there are 226 of these watershed units, with an average area of 3000 km², and these watershed units have a great diversity of hydroclimatology, vegetation, soils, and topography. We contrast the range of hydrologic sensitivities across the watershed units with that of the Columbia River basin as a whole. We find that the difference in hydrologic response between summer and winter warming is controlled in substantial part by the relatively high elevation, cold Canadian portion.