Snow Hydrology And Water Resources Western United States

ROGER C. BALES AND DON CLINE

1 INTRODUCTION

Seasonally snow-covered areas of Earth offer special challenges for water resources management, challenges that arise from both hydrologic and social factors. Seasonal snowpacks account for the major source of the runoff for streamflow and groundwater recharge over wide areas of the midlatitudes. For example, in the western United States over 85% of the annual runoff from the Colorado River basin originates as snowmelt. Most of this is from a few small source areas in four western states, mostly above 2700 m, which comprise only 12% of the basin area. Globally, snowmelt runoff from Earth's mountains fills the rivers and recharges the aquifers that over a billion people depend on for their water resources. Future climate variability and change are expected to result in major changes in the partitioning of snow and rainfall and the timing of snowmelt, which will have important implications for water use and resource management in these regions. It is therefore important to understand the processes controlling snowmelt runoff for both water resources as well as other resource management purposes.

2 CURRENT HYDROCLIMATIC CONDITIONS IN THE WESTERN UNITED STATES

Much of the variability in snow cover found in Earth's seasonally snow-covered regions can be found in the relatively well-studied western United States, which

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

has regions ranging from the high-precipitation Pacific Northwest coast to the semi -arid Southwest. The area from the Rocky Mountains to the Pacific Coast can be conveniently divided into seven regions with different hydroclimatic regimes (Fig. 1). Throughout the region, much of the annual streamflow is dircctly attributable to springtime melting of snow accumulation from the previous winter; however, there are also lower-elevation areas within the region that experience snowmelt throughout the winter and spring. Extended winter snowpack water storage in alpine areas, with often gradual melt rates, results in annual hydrographs having rising limbs of characteristically low slope, usually superimposed with small diel fluctuations reflecting daily melt cycles (Fig. 2). Beyond this basic similarity, however, wide differences in the source, delivery, and amount of moisture each region receives, the amount of water typically stored in their snowpacks, and the rate of release of that water result in different streamflow regimes between regions.

The climate of the western United States is dominated by large-scale atmospheric circulation originating over the north Pacific, In winter, the Pacific/North American (PNA) anomaly pattern forms a series of pressure centers of alternating sign stretching across the Pacific into southern Canada and down toward the Gulf of Mexico (Lin et al,, 1990). The PNA typically forms a series of dry longwave ridges and wet troughs across North America with cold polar air masses to the north of the frontal boundary and warm subtropical air masses to the south. The position of the ridges and troughs influences the seasonal moisture cycle. The mountain ranges of the western United States, like all major mountain ranges, strongly influence global atmospheric circulation; in doing so, they affect the seasonal moisture cycle.

Figure 1 Seven hydroclimatic regimes of the western United States [alter Paulson et al. (1991)1.

Water year day

Figure 2 Discharge hydrograph for 19 km2 Marble Fork of Kaweah River, southern Sierra.

Water year day

Figure 2 Discharge hydrograph for 19 km2 Marble Fork of Kaweah River, southern Sierra.

The major source of moisture for all of the western United States is the Pacific Ocean; during fall and winter, orographic lifting and cooling of Pacific air masses laden with moisture results in precipitation either as rain or snow. The coast ranges, Cascades and Sierra Nevada, form a major orographic barrier for the Pacific moisture, causing much of the winter precipitation to fall as rain on the western side of the mountains. Winter precipitation on the eastern side of the Cascades and Sierra Nevada, although less, generally falls as snow in higher elevations. Relatively warm winter temperatures usually result in warm, wet snowpacks that often are nearly isothermal, and susceptible to rapid melting from warm temperatures and rainfall.

In the spring and summer, moisture from the Gulf of Mexico and subtropical Atlantic Ocean becomes important in most of the western states, with the exception of the coastal states. Early spring Gulf and subtropical Atlantic moisture often precipitates as snow, especially at higher elevations. Additional summertime moisture is provided in the southwestern states by subtropical Pacific air masses. With the exception of "land-recycled" moisture from land surface évapotranspiration (Paulson, 1991), these are the three sources of moisture that provide the western United States with precipitation and runoff. Peak snow accumulation and snowpack water storage in most of the region is found at higher elevations and generally occurs in March or April with snowmelt runoff occurring through May to July, depending on elevation and latitude.

Frontal activity associated with low-pressure systems is responsible for much of the winter precipitation in the northern Rocky Mountains, and upslope transport of moisture from east to west is important at lower elevations on the eastern side of the mountains. Summer precipitation, much of which ends up as évapotranspiration in the semiarid western United States, is mostly influenced by convective activity. However, snowpack storage serves as the major water supply for the summer months. The continentality of the northern Rocky Mountain region leads to cold, dry snowpacks. Significant energy is required to raise the temperature of the snowpack to the isothermal and melting stage; as a result the snowpack tends to remain well into spring. Rainfall generally does not contribute sufficient energy to drive snowmelt, until perhaps very late in the season.

High elevations in the central Rocky Mountains receive most of this region's annual precipitation as winter snowfall. Pacific frontal systems bringing most of the winter moisture to this region can arrive from the west, northwest, or southwest, and this influences the distribution of precipitation. Westerly tracks are orographi-cally lifted to some extent by the Wasatch Plateau in Utah and are lifted further by the ranges along the Continental Divide in central Colorado, resulting in the heaviest precipitation west of the Continental Divide. Northwesterly tracks are lifted by the Wasatch Range and the Uinta Mountains in Utah and by the ranges along the Divide in north central Colorado, resulting in heavier precipitation at these locations. Storm tracks arriving from the southwest do not encounter major orographic effects until they reach the San Juan Mountains in southwestern Colorado, resulting in typically heavy winter precipitation in this part of the region from these storm tracks. In general, precipitation declines markedly throughout areas east of the Continental Divide. However, low-pressure systems east of the Divide can bring significant moisture in from the Gulf of Mexico during spring, resulting in sometimes heavy snowfall in the foothills at lower elevations on the eastern side of the Divide. Lower elevation areas of the central Rockies receive considerably less precipitation; most of the region's snowpack storage is concentrated in the higher mountains.

3 MEASUREMENT AND ESTIMATION OF SNOW PROPERTIES Historical Background

Undoubtedly, the main recurring question in snow hydrology in the western United States is: How much snow is out there? Water resources managers forecast the amount of seasonal runoff, based in part on estimates of the amount of snow accumulation, or snow water equivalent (SWE), across a watershed or region and in part on forecasts of future precipitation. Estimates of SWE and snow-covered area (SCA) are used for a variety of purposes that are vital to the economy of a region, including: reservoir management, snow load maps, annual precipitation maps (for planning), drought monitoring, fish and game management, recreation (e.g., skiing, river trips), acid precipitation monitoring, and avalanche forecasting. (See Section 5 by Doesken.)

Historically, the Natural Resource Conservation Service (NRCS) has been charged with coordinating snow surveys, or point measurements of SWE. It also prepares seasonal water supply outlooks in the western United States. Predictions of water availability in the western United States are made by inventorying snowpacks in winter and early spring using measurements at over 2000 snow courses, including about 1000 snowpack telemetry (SNOTEL) sites that provide continuous data. The remaining sites are manual and are visited monthly. Empirical relationships between these observations and measured streamflow are used to forecast streamflow at over 500 points. In California, the California Cooperative Snow Survey (CCSS) coordi nates measurements; it depends on 40 cooperating agencies for data collection. CCSS makes seasonal water supply forecasts, as do many program cooperators; weekly updates are made for major streams (Hart and Gehrke, 1990).

Estimation of the spatial distribution of SWE is challenging because of the many factors that affect its distribution and the small correlation length of the SWE spatial distribution. Topographic heterogeneity and variability in precipitation patterns also present problems in accurately determining the time of maximum accumulation. The simplicity of regression models makes them an attractive means of estimating SWE because of the large amount of work required to directly measure SWE on the catchment scale.

Estimates of the spatial distribution of SWE for the western United States come primarily from two sources. Operationally, the National Weather Service's (NWS's) National Operational Hydrologic Remote Sensing Center (NOHRSC) assimilates in situ and airborne snow survey data with satellite observations of snow cover to estimate SWE distributions throughout the winter and spring (Figs. 3a and 3b). Second, atmospheric models estimate the distribution of SWE based on modeled snowfall and surface energy balance parameters. The NWS SWE estimates are based on an interpolation procedure called snow estimation and updating system (SEUS), which interpolates between observed points to produce gridded SWE estimates. Point SWE estimates come from the NRCS sites and remote sensing. The NOHRSC conducts airborne SWE surveys along 1800 flight lines throughout the United States, many of which are located in the west (Fig. 4). Water contained in the snowpack attenuates terrestrial gamma radiation. The attenuation is measured relative to snow-free conditions to estimate SWE.

Remote Sensing

Remote sensing provides important spatial information about snow that can be used to improve the accuracy and timeliness of hydrologic forecasts for seasonally snow-covered areas, with commensurate gains in water resources management. At present, the only remotely sensed snow information used in operational hydrologic forecasting is the areal extent of snow cover (Fig. 3a). Over the past decade there has been an expanded development of remote sensing as a tool for determining other snow properties, which can be used to assist in estimating snow distributions and snow-melt runoff. There has also been a move toward development of physically based snowmelt models to use with this emerging data, particularly for alpine areas. The coupling of remote sensing and physically based approaches will enable making not only more accurate basin-scale forecasts but will also provide spatially distributed estimates of snowmelt.

The possibilities for detecting snowpack properties are largely determined by the wavelength being recorded by the remote-sensing instrument. Visible and near-infrared wavelengths, because they do not penetrate far into the snowpack, mainly provide information about the surface of the snowpack (e.g., snow-covered area, grain size, and albedo). However, microwave wavelengths can penetrate the snow-

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