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...

/pdf/changes-toward-earlier-streamflow-timing-across-western-2xsi9gfcp7.pdf

Changes toward Earlier Streamflow Timing across Western North America

Fluctuations in spring climate in the western United States over the last 4–5 decades are described by examining changes in the blooming of plants and the timing of snowmelt–runoff pulses. The two measures of spring's onset that are employed are the timing of first bloom of lilac and honeysuckle bushes from a long-term cooperative phenological network, and the timing of the first major pulse of snowmelt recorded from high-elevation streams. Both measures contain year-to-year fluctuations, with typical year-to-year fluctuations at a given site of one to three weeks. These fluctuations are spatially coherent, forming regional patterns that cover most of the west. Fluctuations in lilac first bloom dates are highly correlated to those of honeysuckle, and both are significantly correlated with those of the spring snowmelt pulse. Each of these measures, then, probably respond to a common mechanism. Various analyses indicate that anomalous temperature exerts the greatest influence upon both interannual and secul...

Changes in the Onset of Spring in the Western United States

Changes in the onset of spring in the western United States

The water resources of the western United States depend heavily on snowpack to store part of the wintertime precipitation into the drier summer months. A well-documented shift toward earlier runoff in recent decades has been attributed to 1) more precipitation falling as rain instead of snow and 2) earlier snowmelt. The present study addresses the former, documenting a regional trend toward smaller ratios of winter-total snowfall water equivalent (SFE) to winter-total precipitation (P) during the period 1949–2004. The trends toward reduced SFE are a response to warming across the region, with the most significant reductions occurring where winter wet-day minimum temperatures, averaged over the study period, were warmer than 5°C. Most SFE reductions were associated with winter wet-day temperature increases between 0° and 3°C over the study period. Warmings larger than this occurred mainly at sites where the mean temperatures were cool enough that the precipitation form was less susceptible to warming trends. The trends toward reduced SFE/P ratios were most pronounced in March regionwide and in January near the West Coast, corresponding to widespread warming in these months. While mean temperatures in March were sufficiently high to allow the warming trend to produce SFE/P declines across the study region, mean January temperatures were cooler, with the result that January SFE/P impacts were restricted to the lower elevations near the West Coast. Extending the analysis back to 1920 shows that although the trends presented here may be partially attributable to interdecadal climate variability associated with the Pacific decadal oscillation, they also appear to result from still longer-term climate shifts.

http://tenaya.ucsd.edu/~cayan/Pubs/92_Knowles_J_Clim_2006.pdf

Trends in Snowfall versus Rainfall in the Western United States

Spring snowmelt is the most important contribution of many rivers in western North America. If climate changes, this contribution may change. A shift in the timing of springtime snowmelt towards earlier in the year already is observed during 1948-2000 in many western rivers. Streamflow timing changes for the 1995-2099 period are projected using regression relations be- tween observed streamflow-timing responses in each river, measured by the temporal centroid of streamflow (CT) each year, and local temperature (TI) and precipitation (PI) indices. Under 21st century warming trends predicted by the Parallel Climate Model (PCM) under business-as-usual greenhouse-gas emissions, streamflow timing trends across much of western North America suggest even earlier springtime snowmelt than observed to date. Projected CT changes are consistent with observed rates and directions of change during the past five decades, and are strongest in the Pacific Northwest, Sierra Nevada, and Rocky Mountains, where many rivers eventually run 30-40 days earlier. The modest PI changes projected by PCM yield minimal CT changes. The responses of CT to the simultaneous effects of projected TI and PI trends are dominated by the TI changes. Regression- based CT projections agree with those from physically-based simulations of rivers in the Pacific Northwest and Sierra Nevada.

/pdf/changes-in-snowmelt-runoff-timing-in-western-north-america-1xt0sdk6q0.pdf

Changes in Snowmelt Runoff Timing in Western North America under a `Business as Usual' Climate Change Scenario

Since about 1950 there has been a trend in the California Sierra Nevada toward a decreasing portion of the total annual streamflow occurring during April through July, while the streamflow during autumn and winter has increase. This trend not only has important ramifications with regard to water management, it also brings up the question of whether this represents a shift toward earlier release of the snowpack resulting from greenhouse warming. Therefore, the observed record has been examined in terms of relative influences of temperature and precipitation anomalies on the timing of streamflow in this region. To carry out this study, the fraction of annual streamflow (called the fractional streamflow) occurring in November-January (NDJ), February-April (FMA), and May-July (MJJ) at low, medium, and high elevation basins in California and 0regon was examined. Linear regression models were used to relate precipitation and temperature to the fractional streamflow at the three elevations for each seas...

Climatic Fluctuations and the Timing of West Coast Streamflow

Many previous studies of snow accumulation and ablation have been based on snow course data obtained at monthly intervals, using April 1 snow-water equivalents (SWE) as surrogates for the total seasonal accumulation and for the maximum seasonal snowpack on the ground. This article uses daily snow pillow data to determine how well the April 1 SWEs represent the total accumulation and the maximum snowpack within the entire Rocky Mountain region and in individual subregions. April 1 SWE tends to underestimate the peak SWE by ∼6 cm (12%). April 1 SWE provides a more accurate estimate of the total seasonal precipitation, with mean errors of approximately −1.7 cm (4%) for most of the study area and approximately +2 cm (6%) in the eastern portion. The error incurred by the use of April 1 SWE in the estimation of peak SWE does not vary systematically with elevation, but errors in the estimation of total seasonal precipitation do.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000WR900256

Use of April 1 SWE measurements as estimates of peak seasonal snowpack and total cold-season precipitation

We examine the influence of climate parameters on seasonal streamflow in watersheds over a range of elevations in California and Oregon. Effects of precipitation, temperature, and snow water content (SWC) are diagnosed using linear regression models and categoric composites. Most of the models explain over 60–80% of the seasonal streamflow variability. The models and the composites provide insight into the climate influences which drive the individual watersheds. Low (warmer) basins have little snow and little memory of prior seasons' climate variability. High (cooler) basins, with more snow, have longer memories. Precipitation has the greatest influence on streamflow variations in spring. Temperature is important in spring in the mid and high elevations. By late spring, SWC accounts for nearly all of the summer streamflow variation at mid and high elevations, but earlier in the runoff season, precipitation and temperature add variance. The variations in surface climate parameters, including streamflow, are generally controlled by atmospheric circulation anomalies with a spatial scale much larger than those in watersheds. This explains why little skill was lost by broadening the scale of temperature and precipitation predictors to regional climate areas.

The influence of precipitation and temperature on seasonal streamflow in California

The radiation balances of melting snowpacks for three seasons at an open site at the Central Sierra Snow Laboratory near Soda Springs, California were examined. The snow covers were examples of below-normal, near-normal and much-above-normal water equivalents. Two of the snow covers melted under generally clear skies in late spring while the other melted under cloudier conditions and at a time when less extraterrestrial radiation was available. Moreover, the snow covers were of very different densities, thereby allowing examination of a possible relationship between that characteristic and albedo. No such relationship was observed. Despite the dissimilarities in the conditions under which melt occurred, the disposition of solar radiation was similar for the three melt seasons. Albedos and their rates of decline through the melt season were similar for the three seasons. Absorbed solar radiation and a cloudiness index were useful predictors for daily net radiation, accounting for 71% of the total variance.

Radiation Balances of Melting Snow Covers at an Open Site in the Central Sierra Nevada, California

Snowpack is an important source of water supply in the western United States. This study examines the seasonal variability of an extensive history of snow observations over the western United States and Canada. It links variations in snowpack to variations in atmospheric circulation, surface temperature, and precipitation.

Edward Aguado

Papers

Climatic Fluctuations and the Timing of West Coast Streamflow

Use of April 1 SWE measurements as estimates of peak seasonal snowpack and total cold-season precipitation

The influence of precipitation and temperature on seasonal streamflow in California

Radiation Balances of Melting Snow Covers at an Open Site in the Central Sierra Nevada, California

Winter climate variability and snowpack in the West