The effects of downscaling method on the variability of simulated watershed response to climate change in five U.S. basins.

Simulations of future climate change impacts on water resources are subject to multiple and cascading uncertainties associated with different modeling and methodological choices. A key facet of this uncertainty is the coarse spatial resolution of GCM output compared to the finer-resolution information needed by water managers. To address this issue, it is now common practice to apply spatial downscaling techniques, using either higher-resolution regional climate models or statistical approaches applied to GCM output to develop finer-resolution information for use in water resources impacts assessments. Downscaling, however, can also introduce its own uncertainties into water resources impacts assessments. This study uses watershed simulations in five U.S. basins to quantify the sources of variability in streamflow, nitrogen, phosphorus, and sediment loads associated with the underlying GCM compared to the choice of downscaling method (both statistically and dynamically downscaled GCM output). We also assess the specific, incremental effects of downscaling by comparing watershed simulations based on downscaled and non-downscaled GCM model output. Results show that the underlying GCM and the downscaling method each contribute to the variability of simulated watershed responses. The relative contribution of GCM and downscaling method to the variability of simulated responses varies by watershed and season of the year. Results illustrate the potential implications of one key methodological choice in conducting climate change impacts assessments for water - the selection of downscaled climate change information.

MODELING STREAMFLOW AND WATER QUALITY SENSITIVITY TO CLIMATE CHANGE AND URBAN DEVELOPMENT IN 20 U.S. WATERSHEDS.

Watershed modeling in 20 large, United States (U.S.) watersheds addresses gaps in our knowledge of streamflow, nutrient (nitrogen and phosphorus), and sediment loading sensitivity to mid-21st Century climate change and urban/residential development scenarios. Use of a consistent methodology facilitates regional scale comparisons across the study watersheds. Simulations use the Soil and Water Assessment Tool. Climate change scenarios are from the North American Regional Climate Change Assessment Program dynamically downscaled climate model output. Urban and residential development scenarios are from U.S. Environmental Protection Agency's Integrated Climate and Land Use Scenarios project. Simulations provide a plausible set of streamflow and water quality responses to mid-21st Century climate change across the U.S. Simulated changes show a general pattern of decreasing streamflow volume in the central Rockies and Southwest, and increases on the East Coast and Northern Plains. Changes in pollutant loads follow a similar pattern but with increased variability. Ensemble mean results suggest that by the mid-21st Century, statistically significant changes in streamflow and total suspended solids loads (relative to baseline conditions) are possible in roughly 30-40% of study watersheds. These proportions increase to around 60% for total phosphorus and total nitrogen loads. Projected urban/residential development, and watershed responses to development, are small at the large spatial scale of modeling in this study.

Nutrient and particle load estimates to Lake Tahoe (CA-NV, USA) for Total Maximum Daily Load establishment.

The Lake Tahoe Total Maximum Daily Load (TMDL) requires detailed methodologies to identify sources of flows and pollutants (particles and nutrients) for estimating time-variant loads as input data for the Lake Tahoe clarity model. Based on field data and a modeling study, the major sources of pollutant loads include streams (three subdivisions of this category are urban, nonurban, and stream channel erosion), intervening zones (IZs) (two subdivisions of this category are urban and nonurban), atmosphere (wet and dry), groundwater and shoreline erosion. As Lake Tahoe remains well oxygenated year-round, the contribution of internal loading from the bottom sediments was considered minor. A comprehensive quantitative estimate for fine particle number (< 16 µm diameter) and nutrient (nitrogen and phosphorus) loading is presented. Uncertainties in the estimation of fine particle numbers and nutrients for different sources are discussed. Biologically available phosphorus and nitrogen were also evaluated. Urban runoff accounted for 67% of the total fine particle load for all sources making it the most significant contributor although total urban runoff was only 6%. Non-urban flows accounted for 94% of total upland runoff, but the nitrogen, phosphorus and fine sediment loadings were 18%, 47% and 12%, respectively of the total loadings. Atmospheric nitrogen, phosphorus, and fine particle loadings were approximately 57%, 20%, and 16%, respectively of the total loading. Among streams and IZs, IZ 8000, Upper Truckee River, Trout Creek, Blackwood Creek, and Ward Creek are the top fine particle, nitrogen and phosphorus contributors. The relative percentage contribution of inorganic fine particles from all sources based on annual average for the period 1994-2008 on size classes 0.5-1, 1-2, 2-4, 4-8, and 8-16 µm are 73%, 19%, 5%, 2%, and 1%, respectively. These results suggest clear priorities for resource managers to establish TMDL on sources and incoming pollutants and preserving lake clarity.