The highly variable timing of streamflow in snowmelt-dominated basins across western North America is an important consequence, and indicator, of climate fluctuations. Changes in the timing of snowmelt-derived streamflow from 1948 to 2002 were investigated in a network of 302 western North America gauges by examining the center of mass for flow, spring pulse onset dates, and seasonal fractional flows through trend and principal component analyses. Statistical analysis of the streamflow timing measures with Pacific climate indicators identified local and key large-scale processes that govern the regionally coherent parts of the changes and their relative importance. Widespread and regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place across most of western North America, affecting an area that is much larger than previously recognized. These timing changes have resulted in increasing fractions of annual flow occurring earlier in the water year by 1–4 weeks. The immediate (or proximal) forcings for the spatially coherent parts of the year-to-year fluctuations and longer-term trends of streamflow timing have been higher winter and spring temperatures. Although these temperature changes are partly controlled by the decadal-scale Pacific climate mode [Pacific decadal oscillation (PDO)], a separate and significant part of the variance is associated with a springtime warming trend that spans the PDO phases.

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Abstract:Mountain snowpack and spring runoff are key components of surface water resources, and serve as important, regionally integrated indicators of climate variability and change. This study examines whether mountain snowpack and snowmelt have manifested a consistent hydrologic response to global climatic changes over the past several decades. Prior findings are compared to identify spatial and temporal patterns of trends in the volume, extent, and seasonality of snowpack and melt for key mountain regions. Evidence suggests that both temperature and precipitation increases to date have impacted mountain snowpacks simultaneously on the global scale; however, the nature of the impact is, among other factors, strongly dependent on geographic location, latitude, and elevation. Warmer temperatures at mid-elevations have decreased snowpack and resulted in earlier melt in spite of precipitation increases, while they have not affected high-elevation regions that remain well below freezing during winter. At high elevations, precipitation increases have resulted in increased snowpack. Not all local responses are consistent with the general findings, possibly because of local climatic trends, atmospheric circulation patterns, record lengths, or data quality issues. With continued warming, increasingly higher elevations are projected to experience declines in snowpack accumulation and melt that can no longer be offset by winter precipitation increases. There is a continued research need for hydroclimatic trend detection and attribution in mountains owing to the length, quality, and sparseness of available data from monitoring stations not directly impacted by human activity. THE SENSITIVITY OF MOUNTAIN SNOWPACK AND SNOWMELT RUNOFF TO CLIMATIC CHANGESReductions in mountain snowpack and changes in snowmelt-derived streamflow timing from higher elevations are of considerable concern in arid regions where human water demand already equals or exceeds the renewable water supply today. In these areas, even greater pressures on water resources are projected with growing populations and warmer temperatures. For many arid and semi-arid lowland regions, the surface water is derived from precipitation falling as rain or snow on neighbouring higher elevations. While rain immediately contributes to streamflow, the mountain snowpack serves as a natural reservoir for cold-season precipitation storage, releasing water during the warmer months to the drier and much hotter valleys below. Therefore, mountain snowpack and snowmelt is an important predictor of summer streamflow and constitutes the primary source of water for large populations (Singh and Bengtsson, 2004), and the natural release of water from snowmelt in conjunction with constructed reservoirs is supplying human and ecosystems demands during the season of greatest need. Economically, the seasonal snowpack is more valuable than glacier ice (Krishna, 2005). Thus, for * Correspondence to: Iris T. Stewart, Santa Clara University, Environmental Studies Institute, Santa Clara, CA, ...

[1] We develop a stream temperature model within the Soil and Water Assessment Tool (SWAT) that reflects the combined influence of meteorological (air temperature) and hydrological conditions (streamflow, snowmelt, groundwater, surface runoff, and lateral soil flow) on water temperature within a watershed. SWAT currently uses a linear air-stream temperature relationship to determine stream temperature, without consideration of watershed hydrology. As SWAT uses stream temperature to model various in-stream biological and water quality processes, an improvement of the stream temperature model will result in improved accuracy in modeling these processes. The new stream temperature model is tested on seven coastal and mountainous streams throughout the western United States for which high quality flow and water temperature data were available. The new routine does not require input data beyond that already supplied to the model, can be calibrated with a limited number of calibration parameters, and achieves improved representation of observed daily stream temperature. For the watersheds modeled, the Nash-Sutcliffe (NS) coefficient and mean error (ME) for the new stream temperature model averaged 0.81 and À0.69 C, respectively, for the calibration period and 0.82 and À0.63 C for the validation period. The original SWAT stream temperature model averaged a NS of À0.27 and ME of 3.21 C for the calibration period and a NS of À0.26 and ME of 3.02 C for the validation period. Sensitivity analyses suggest that the new stream temperature model calibration parameters are physically reasonable and the model is better able to capture stream temperature changes resulting from changes in hydroclimatological conditions.

[1] Warmer temperatures are expected to raise mountain stream temperatures, affecting water quality and ecosystem health. We demonstrate the importance of climate-driven changes in hydrology as fundamental to understanding changes in the local water quality. In particular, we focus on changes in stream temperature, dissolved oxygen (DO) concentrations, and sediment transport in mountainous, snowmelt-dominated, and waterlimited systems, using the Sierra Nevada as our case study. Downscaled output from an ensemble of general circulation model projections for the A2 (higher greenhouse gas) emission scenario was used to drive the Soil and Water Assessment Tool with a new integrated stream temperature model on the subbasin scale. Spring and summer stream temperature increase by 1 C-5.5 C, with varying increases among subbasins. The highest projected stream temperatures are in the low-elevation subbasins of the southern Sierra Nevada, while the northern Sierra Nevada, with distinct impacts on snowmelt and subsurface flow contributions to streamflow, shows moderated increases. The spatial pattern of stream temperature changes was the result of differences in surface and subsurface hydrologic, snowmelt, and air temperature changes. Concurrent with stream temperature increases and decreases in spring and summer flows, simulations indicated decreases in DO (10%) and sediment (50%) concentrations by 2100. Stream temperature and DO concentrations for several major streams decline below survival thresholds for several native indicator species. These results highlight that climatic changes in water-limited mountain systems may drive changes in water quality that have to be understood on the reach scale for developing adaptive management options.Citation: Ficklin, D. L., I. T. Stewart, and E. P. Maurer (2013), Effects of climate change on stream temperature, dissolved oxygen, and sediment concentration in the Sierra Nevada in California, Water Resour.