Snow in the global environment is altered by human activity in many ways, some intentional and others inadvertent. Among the deliberate changes made to snowpacks are blowing-snow management to benefit agriculture and transportation, forest management to protect and enhance water yields from snowpacks, urban snow management to improve transportation and habitation in developed areas, albedo modification to enhance snowmelt rates, and ski-area management for enhanced and safe recreational opportunities. The major inadvertent change that may impact snowpacks is the possibility of climate change that is being forecast for the twenty-first century. This final chapter generally discusses these human–snow interactions from the viewpoint of the snow hydrologist.

All management of snow should be based upon a thorough understanding of the processes involved at a given site and how they may be varied to accomplish a given goal. Furthermore, the management recommendations must consider acceptability, cost, ease of use, risks, maintenance requirements, and long-term environmental impacts. Many processes affecting snow that can be altered to meet management goals have been described in earlier chapters.

Blowing-snow management

Blowing-snow management generally has the objective of creating snow drifts in useful or non-problematic locations by using some type of barrier to the wind. Barriers take the form of constructed snow fences, rows of living shrubs or trees, and crop stubble. Farmers have realized for centuries that seeds and seedling plants survive and grow faster when located in furrows that accumulated small drifts of snow that supplemented soil moisture.

Measurement of snow at time of fall

Precipitation gauges

Several complications arise when using precipitation gauges to measure snowfall that are not as much of a problem when measuring rainfall. Gauge catch losses are more severe for snow than rain, so that precipitation gauge shields are much more of a necessity. Additionally, subfreezing temperatures during winter, and usually during a snowfall event, dictate that an antifreeze solution be used to prevent collected water from freezing (an evaporation suppressant might be necessary to minimize evaporative losses); and, to insure that bridging of snow over the gauge orifice does not happen, the gauge is sometimes heated. The positioning of the snow-precipitation gauge on the landscape must also be considered in order to get a more representative catch of snowfall.

Nonrecording gauges

There are two classes of nonrecording gauges, those read on a daily or more frequent basis and storage gauges that are read only after a period of time has elapsed, like once a month or once a season. A subtype of the daily nonrecording gauge is the gauge that is only read after a snowfall event has been observed. If a gauge is to be read on a 6- or 24-hour interval, antifreeze, orifice heating, and evaporation suppressants are not as critical as for a storage gauge. For those nonrecording gauges, the funnel and collection tube, used for rainfall events, are removed from the gauge and snow falls directly into the outside container.

Introduction

Recent innovations in computing techniques and the need for modelling snowpack energy exchange processes for large diverse basins have lead to consideration of topographic and forest effects. Both topography and forests can have a major impact on snowpack radiation energy exchange. Effects on convective exchange are less well known. Methods for adjusting the basic theory of snowpack energy exchange for the effects of topography and forests are considered in this chapter. Topographic effects on precipitation and snow accumulation and forest effects on snow interception were previously discussed in Chapter 3.

Topographic influences

Topography controls the elevation, slope and aspect, and exposure of snowpack surfaces that can significantly influence energy exchange and melt across a watershed. Since many watersheds with significant snowpacks are quite mountainous, correction for the effects of topography becomes important when trying to develop spatially accurate models of snowmelt-runoff processes. Slope inclination and aspect angles modify the exchange of direct-beam and diffuse shortwave radiation and longwave radiation. Topographic relief can also lead to shading of surfaces by adjacent terrain. Precipitation amounts and types, wind speeds, temperatures, and humidities that are used in computing the energy budget for snowpack surfaces are also influenced by elevation and slope orientation. Snowpack exposure to prevailing winds and typical storm patterns can be influenced by topography and can greatly affect snowpack energy exchange. Lapse rates can be used to adjust temperature data for effects of elevation, but corrections for topographic effects on other meteorological parameters important to energy exchange are more problematical.

One of the most fundamental aspects of snow hydrology is an understanding of the processes that lead to snowfall and the eventual distribution of a snowpack on the landscape. The factors that lead to the formation of snowfall are generally discussed in the beginning sections of this chapter. Since snowfall once formed, unlike rain, is quite easily borne by the wind and redistributed across the landscape before finally coming to rest to form a snowpack, the basic principles controlling blowing snow are also reviewed. Finally, the interception of snow by vegetation, that can have a profound effect on the amount and timing of snow that accumulates into a snowpack beneath a plant canopy, is described in the last section of the chapter.

Snowfall formation

The occurrence of snowfall in a region is generally dependent upon several geographic and climatic factors: latitude, altitude, the distance from major water bodies, and the nature of regional air mass circulation (McKay and Gray, 1981). General discussions of the climatic factors affecting snowfall formation and precipitation are given by Sumner (1988) and Ahrens (1988). Latitude and altitude largely control the temperature regime of a region and dictate where it is cold enough for snowfall to occur. Virtually no snowfall occurs in low latitudes where the heat balance at the Earth's surface causes air temperatures to average well above freezing during winter. Since air temperature declines with altitude, higher altitudes generally mean lower air temperatures and greater opportunity for snowfall to occur.

Introduction

Hydrologic models used to predict streamflow can be generally classified as either deterministic or stochastic and as either lumped-parameter or distributed (Beven, 2000). Deterministic models predict a single value of streamflow from a given set of input variables, while stochastic models predict a range of possible outcomes based upon the statistical distributions of input variables. Nearly all of the snowmelt models used to continuously predict streamflow from snowmelt are deterministic, but a type of statistical model has been historically used to great advantage to predict seasonal totals of streamflow using measured snowpack and precipitation data each spring. These models are in widespread use in the western United States to forecast spring runoff.

Deterministic models can be further categorized as either lumped-parameter or distributed. Lumped-parameter models consider the entire watershed as a homogeneous unit, while distributed models break the watershed into subunits or grid points based upon a variety of criteria. Early snowmelt models were of the lumped-parameter type, but many distributed-snowmelt modelling studies have been conducted in the past decade. In terms of snowmelt prediction, the lumped-parameter type of model assumes that the snow cover and melt rates are uniform across the entire basin. These so called point-snowmelt models are still in use today and are generally found imbedded in most deterministic, distributed models. Distributed-snowmelt models generally adapt simple point-snowmelt models for conditions that vary from point to point across basins.

Introduction

Biogeochemical cycling of nutrients and pollutants in the environment is significantly affected by the occurrence of snowfall and snowpacks. Snowpacks can be viewed as reservoirs of chemicals that, unlike substances dissolved in rainfall, can be largely stored for significant periods of time during winter until melting occurs. Snowpacks reflect the chemical nature of the original snowfall events or wet deposition that accumulated to create them as well as the dry deposition of chemicals occurring as aerosol droplets, particles, and gases in the atmosphere deposited on the snowpack surface during non-precipitation periods. Both naturally cycling chemicals and pollutants in the environment end up in the snowpack in this manner. Snowpack chemistry may also be affected by interactions with plant canopies during interception and the activity of organisms that find a home in the snow. Once melting or rain-on-snow events occur, chemicals are redistributed and released from the snowpack non-uniformly due to fractionation processes leading to relative enrichment of the initial liquid-water releases. In polluted environments, early spring fish kills have been attributed to effects of these initial concentrated acidic snowmelt-runoff events that caused toxic levels of dissolved aluminum to occur in streams. Studies have also shown that some ions are preferentially eluted or leached from the snowpack in different ratios than found in the snow itself. Stable isotope fractionation also occurs in snow during winter and can be used to study phase changes important in snow hydrology.

The importance of snow

There is such a vast difference in the physical properties of snow and other natural surfaces that the occurrence of snow in a drainage basin can cause significant changes in the energy and water budgets. As an example, the relatively high albedo of snow reflects a much higher percentage of incoming, solar, shortwave radiation than snow-free surfaces (80% or more for relatively new snow as opposed to roughly 15% or less for snow-free vegetation). Snow may cover up to 53% of the land surface in the northern hemisphere (Foster and Rango, 1982) and up to 44% of the world's land areas at any one time. On a drainage basin basis, the snow cover can vary significantly by elevation, time of year, or from year to year. The Rio Grande Basin near Del Norte, Colorado, is 3419 km2 in area and ranges from 2432 m a.s.l. at the streamgauge up to 4215 m a.s.l. at the highest point in the basin. Figure 5.1 compares the snow-cover depletion curves obtained from Landsat data in 1977 and 1979 in elevation zones A (780 km2; 2432–2926 m), B (1284 km2; 2926–3353 m), and C (1355 km2; 3353–4215 m) of the Rio Grande Basin. In a period of only 2 years from April 10, 1977, to April 10, 1979, a great difference in seasonal snow-cover extent was experienced. Landsat data show that 49.5% or 1693 km2 were covered by snow on April 10, 1977.

Perceptions of snow

There is a general lack of appreciation by society of the importance of snow to everyday life. One good example of this is found in the Rio Grande Basin in the southwestern United States and Mexico. The Rio Grande, the third longest river in the United States, is sustained by snow accumulation and melt in the mountain rim regions which provide a major contribution to the total streamflow, despite its flowing right through the heart of North America's largest desert (Chihuahuan). Because the majority of the population in the basin resides in a few large cities in the Rio Grande Valley, which are all located in the desert (see Figure 1.1), there is little realization on the part of the urban residents that snowmelt far to the north is an important factor in their lives. This same situation is true in many arid mountainous regions around the globe. Where agricultural water use predominates, however, the importance of snow for the water supply and food production is more widely known, at least by farmers and ranchers and the rural populace.

The importance of snow during and in the aftermath of a snowstorm is immediately evident because of its significant effect on transportation (see Figure 1.2). The effects of snowstorms on wagon trains (in the past), railroads, and motorized transport are widely documented by Mergen (1997). Except for very small countries, the effect of a snowstorm on transportation is localized and does not affect the entire country.

Once snow has accumulated on the landscape, the individual ice grains can be rapidly transformed or metamorphosed into a structured snowpack. Metamorphism ultimately influences the thermal conductivity and liquid permeability of the snowpack that in turn influence the snowpack temperature regime and storage and release of liquid water. Snowpacks vary seasonally from low density, subfreezing snowpacks capable of refreezing any liquid-water inputs to isothermal, dense snowpacks that rapidly transmit liquid water to the ground below. In this chapter, important processes controlling snowpack metamorphism and the conduction of heat and liquid water in snow are described along with some methods hydrologists use to describe snowpack condition.

Snowpack metamorphism

Snow crystals that fall from the atmosphere undergo changes in size and shape over time as they become part of the snowpack. Owing to their large surface area to volume ratio, crystals are quite unstable in the snowpack and can transform to rounded or faceted ice grains. Processes causing metamorphism of ice grains and snowpacks vary between dry snow and wet snow. Metamorphism within the snowpack is important to hydrologists because it can ultimately lead to changes in thermal conductivity and liquid permeability that influences the snowpack energy budget and release of meltwater. The following discussion of snowpack metamorphism is largely based upon writings by Perla and Martinelli (1978) and McClung and Schaerer (1993). Major snowpack processes for snowpack metamorphism are summarized in Table 3.1.