||Snow Surveys and Water Supply Forecasting
Snowpack and the Water Supply
Centralized Forecasting System
The beauty of snow
is fascinating, and millions of Americans enjoy the snow-covered landscape
as a playground. But beyond its esthetic and recreational appeal,
snow plays a vital role in our lives as a primary source of the water
supply in the Western United States.
Increasing and often conflicting demands for water in the West have
heightened public awareness of the need for sound management decisions
concerning water. Although the West's high mountain ranges hold a
vast snowpack that provides 50 to 80 percent of the year's water supply,
nature cannot be relied upon to provide an uninterrupted, dependable
supply of meltwater to meet all the downstream requirements. To moderate
this variability, reservoirs and canals have been built to serve the
growing needs of agriculture, industry, and communities. But successful
water management begins with an adequate knowledge of the primary
source of water in the West: snow.
Obtaining accurate and timely information on the extent and water
content of the mountain snowpack requires specially trained people
and unique equipment. The Federal, State, and private cooperative
snow survey program directed by the U.S.
Department of Agriculture's (USDA) Natural
Resources Conservation Service (NRCS) has met those needs since
the mid-1930's and continues to evolve in response to increasing demands
of water users. With a computerized data collection network and forecast
system, the program also fills many other requirements for hydrological
and climatological data useful in natural resources management and
This article describes the cooperative snow survey program. It is
intended to provide the general public as well as water resource professionals
with a better understanding of the importance of snow, snowpack surveys,
and water supply forecasting in natural resources management.
||Mountain Snowpack and the Water Supply
To the casual observer, the process by which we get water from
the mountain snowpack is simple: the weather cools as winter approaches
and precipitation changes from raindrops to snowflakes. Snow accumulates
in winter, and with warming of spring and early summer it melts,
Melting snow produces streamflow –
a vital source of water for people living in the West.
In reality, the relationship between the snowpack and the amount
of snowmelt runoff is complex. It depends on many factors, primarily
moisture content of the soil, ground water contributions, precipitation
patterns, fluctuation in air temperature, use of water by plants,
and frequency of storm events. These factors change throughout the
year and from year to year. Their relative importance varies depending
The stage is set for the snow-water year even before the first snowflakes
fall. The amount of moisture that accumulates in the soil early
in winter, before the snowpack develops, will affect runoff the
following spring. Dry soils tend to absorb more of the meltwater
than wet soils. The amount of moisture that is absorbed depends
on soil characteristics as well as precipitation. Wind, air temperature,
storm frequency, and the amount of moisture in the atmosphere determine
the accumulation of the snowpack. How the snowpack accumulates affects
its density (amount of water per unit volume of snow) and texture
(crystalline structure). Density increases as the snowpack becomes
deeper and the lower layers are compressed. Wetness of the snow
also affects density. Compression affects the crystalline structure
of the snowpack. Density and crystalline structure affect how fast
the snowpack melts and how much water it yields.
Air temperature and availability of atmospheric moisture determine
how wet or dry the snow is. Typically, the west slope of the Cascade
Range, in response to the Pacific Ocean's strong influence, receives
heavy, wet snow. One foot of that snow, newly fallen, can produce
up to 1.5 inches of water. In other areas, such as the Wasatch Mountains
in central Utah, the snow is much drier. It is light and powdery
– excellent for skiing – and 1 foot of fresh snowpack
might contain only an inch of water.
Winds can redistribute the snow into drifts. Drifts differ from
the surrounding snowpack in texture and density because of the weight
of additional snow. On unsheltered snowpacks, high winds can evaporate
the snow cover at temperatures lower than 32° F – a process
called sublimation. Mountain snowpacks do not melt steadily. Melting
varies according to weather, ground temperature, and exposure to
the sun's rays. A snowpack begins to melt when its temperature from
top to bottom equalizes at 32° F. Before reaching this isothermal
state, the snowpack has different temperatures at different depths.
Ground temperature, air temperature, and exposure to incoming solar
radiation affect how quickly it becomes isothermal. South-facing
slopes and open areas receive the most solar radiation and have
the highest melt rates.
The Western United States requires a dependable supply of reasonably
priced, good-quality water if the economy is to prosper and the
quality of life is to remain high. Vast areas that receive just
a few inches of annual rainfall produce bountiful crops, but only
with irrigation. Decisions on the types of crops to plant, the number
of acres, and irrigation scheduling all depend on reliable forecasts
of the year's water supply. Much of the power for cities as well
as agriculture and industry is generated by hydroelectric energy.
Water is truly the life blood of the West.
Because of the very low annual rainfall in
the West, many areas such as this field of alfalfa depend on irrigation.
Wise management of existing water resources in the United States
is essential. Water management, however, is complex even under the
best of circumstances. Supply, demand, and cost are subject to the
climate and to numerous economic and social influences, domestic
and international. The decisions made early in the year, based on
the best available information, often require significant revision
as more data become available.
The Columbia and Colorado rivers are two examples of extremely complex
snowmelt-fed river systems. The area draining into the Columbia
River comprises about 258,000 square miles, which includes 40,000
square miles in Canada. Along the river, Federal agencies have built
30 major dams for power generation,
flood control and irrigation storage. The Columbia and its tributaries
support a wealth of fish and wildlife, including several species
of fish such as salmon, which live in the sea but spawn in the river's
fresh water. Barge traffic on the river is a major link in the area's
transportation network for marketing agricultural and other products.
Because many communities and industries and millions of acres of
agriculture depend directly on this river system for survival, effective
and timely management is critical.
Like the Columbia, the Colorado River also begins in high mountain
country. It drains about 247,000 square miles. Huge population
centers in southern California and Arizona consume enormous quantities
of water, as do the expanding agricultural developments, and
demands for water of the Colorado are intense. As in the Columbia,
numerous storage facilities have been constructed, impounding
snowmelt water to produce electricity, irrigate farms, supply
water to cities and towns, and prevent floods. Unlike the Columbia,
however, the Colorado picks up dissolved salts as it flows through
ancient deserts and areas shaped by prehistoric inland seas.
Heavy withdrawal of water, evaporation, and irrigation return
flows can increase salt concentration downstream and thereby
lower the quality of the water. Because multistate agreements
and compacts regulate the quality and quantity of streamflow
on the Colorado River, accurate management of streamflow and
water use is imperative.
Most smaller river basins throughout the West also have management
requirements for limited water resources that are just as important
for their users. Management decisions are vital every year for
big rivers or small, but the years of vast surplus and extreme
shortage intensify the demands for management excellence and
the importance of snow surveys.
Since 1935, most of the American West has relied on the U.S. Department
of Agriculture's cooperative snow survey program for predictions
of meltwater runoff. This program is a Federal, State, and local
partnership directed by the Natural Resources Conservation Service.
Its survey activities encompass Arizona, Colorado, Idaho, Montana,
Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. Alaska
and southern Canada are partners also. California has an independent
Snow surveys in the West date back to around 1906, when the University
of Nevada's Dr. James Church laid out the first western snow course.
Dr. Church also invented key sampling procedures and equipment.
The next three decades saw a proliferation of snow survey activity
throughout the West. In some States, independent power or irrigation
companies spearheaded the surveys; in other States, universities
or State engineers were in charge.
The snow surveyors are well-equipped for a full day's work.
Federal leadership of snow survey activities came as a result of
the unprecedented western drought of 1934. Agricultural leaders
requested USDA's help in forecasting water supplies for the ensuing
crop-growing season. Because many of the watersheds and streams
were interstate, Federal help was needed to coordinate the surveys
and to develop uniform procedures and equipment for surveying and
To find out how much water will be available in summer, snow surveyors
from NRCS and the other cooperating agencies collect data from some
1,600 snow courses several times each winter. They determine the depth
and the water content of the snowpack and estimate the amount of runoff
from the mountain watersheds.
In 1977, NRCS began developing a network of automated radio telemetry
data sites for collecting snow survey data. This snowpack telemetry
(SNOTEL) network provides NRCS offices with daily or more frequent
information on streamflow potential. The information is especially
valuable during periods of flood or drought.
The information collected by the telemetry system
and snow surveyors is translated into water
supply forecasts that NRCS State offices issue monthly from January
to June in cooperation with the National Weather Service. Major sectors
of the Western economy – agriculture, industry, and recreation
– base their plans on these forecasts.
Snow Surveys - Manual
The surveyors are approaching a typical snow course marker. This
is one of the nearly 1,600 they or others will encounter several
times each winter
Manual surveys require two-person teams to measure snow depth and
water content at designated snow courses. A snow course is a permanent
site that represents snowpack conditions at a given elevation in
a given area. A particular snowpack may have several courses. Generally,
the courses are about 1,000 feet long and are situated in small
meadows protected from the wind.
Measurements generally are taken on or near the first of every
month during the snowpack season. The frequency and timing of these
measurements varies considerably with the locality, the nature of
the snowpack, difficulty of access, and cost. On occasion, special
surveys are scheduled to help evaluate unusual conditions. The manual
surveys involve travel and work in remote areas, often in bad weather,
but reliable data are obtained.
Manual Survey Procedure.
1. The surveyor makes certain that the tube is clear of all snow
and soil before taking the snow core sample. The team uses
a strong, light-weight, graduated aluminum tube and a weighing
2. One surveyor measures the snow depth while the other records data.
From 5 to 10 measurements are taken at regular intervals along
a snow course. Snow depth is measured by pushing the tube
down through the snowpack to the ground surface and extracting
3. In taking an accurate snow core sample, the surveyor must verify
that the tube has reached ground level by examining the base
of the tube and finding soil.
4. After clearing out the soil from the tube, the surveyor determines
the amount of water in the snowpack by weighting the tube
with its snow core and subtracting the weight of the empty
tube. An average of all samples taken is calculated and used
to represent the snow course.
that are too hazardous or costly to measure on the ground can be
equipped with depth markers that can be read from aircraft. Snow
depth can be measured in this way with a high degree of accuracy.
Although the amount of water in the snowpack is not measured, it
can be reliably estimated from the observed snow depth.
Snow Surveys - SNOTEL
Even though the data from the snow courses provide a valuable body
of information, the typical schedule for manual surveys results
in weeks with no specific insight into the condition of the snowpack.
In that time, intense storms may be adding an abnormally large amount
of snow or rain; perhaps an unseasonable warm spell at high elevation
is resulting in a rapid melt with ensuing flood hazards.
Snow surveyors and water managers realized early in the development
of the program that timely forecasting and management decisions
required more frequent measurements and additional information.
They also needed a way to survey particularly remote and hazardous
snowpacks. SNOTEL's automatic sensing and data transmission were
Sensing Devices. A typical SNOTEL remote site
consists of measuring devices and sensors, a shelter house for the
radio telemetry equipment, and an antenna that also supports the
solar panels used to keep batteries charged. A standard sensor configuration
includes snow pillows, a storage precipitation gauge, and a temperature
sensor. The snow pillows are envelopes of stainless steel or synthetic
rubber, about 4 feet square, containing an antifreeze solution.
As snow accumulates on the pillows, it exerts pressure on the solution.
Automatic measuring devices in the shelter house covert the weight
of the snow into an electrical reading of the snow's water equivalent
– that is, the actual amount of water in a given volume of
This drawing depicts a typical remote SNOTEL
site. Pressure pillows are used for measuring snowfall, a storage
precipitation gauge provides current information about conditions
at the site, and a temperature sensor measures the existing temperature.
The precipitation gauge measures all precipitation in any form
that falls during the year. The temperature sensor determines the
minimum, maximum, and average daily readings.
Additional sensors can be incorporated into a particular site for
measuring wind speed and direction, soil temperature, snow depth,
and a variety of other weather and environmental aspects. The configuration
at each site is tailored to the physical conditions, the climate,
and the specific requirements of the data users.
Telemetry. SNOTEL uses the principle of radio
transmission by meteor burst. Radio signals are aimed skyward where
the trails of meteorites reflect or reradiate the signals back to
The meteor burst technique allows communications between two locations
as much as 1,200 miles apart. Two master stations – at Boise,
Idaho, and Ogden, Utah – cover the 10 Western States, an area
of about 1 million square miles. By cable, the master stations feed
the data to SNOTEL's Centralized Forecasting System in Portland,
Oregon. The Alaska Meteor Burst Communication System (AMBCS) for
snow surveys is similar. All remote SNOTEL sites are interrogated
daily on a regular schedule. Additional interrogations can be conducted
on demand, and any special reporting requirements can be programmed
into the site's microprocessors. In the Alaskan system, hourly interrogations
are conducted, and the data are made immediately available to cooperating
Quality Control. The sites are designed to operate
unattended for 1 year in severe climates. Each site receives preventative
maintenance and sensor adjustment annually. The reliability of each
SNOTEL site is verified by ground truth measurements taken during
regularly scheduled manual surveys. These readings are compared
with telemetered data to check that values are consistent and compatible.
Any values found to be beyond specified limits are carefully examined
and edited to ensure a continuous, high-quality record. Every year
each site's performance is compared against established performance
standards. Sites not meeting rigid criteria undergo a thorough field
evaluation to correct any site deficiencies.
Billions of sand-sized meteorites enter the atmosphere daily. As
each particle heats and burns in the region 50 to 75 miles above
the Earth's surface, its disintegration creates a trail of ionized
gases. The trails diffuse rapidly, usually disappearing within a
second, but their short lifespan is adequate for SNOTEL communications
to be completed.
The process has three major steps: (1) master stations request data
from remote sites; (2) sites respond by transmitting their current
data; (3) and finally a master station acknowledges receipt and
signals the site transmitter to stop. This complex exchange, taking
place in a fraction of a second, is possible thanks to microprocessors.
||The Centralized Forecasting System
The snow survey program has a Centralized Forecasting System (CFS),
which is automated for handling information related to water supply
forecasting such as streamflow, precipitation, snow depth and snow
water equivalent, and reservoir data. These data are available for
the current water year (October 1 through September 30) and for
historical water years.
CFS was developed and is operated by the NRCS National
Water and Climate Center (NWCC) in Portland, Oregon. CFS is
the primary focal point for snow survey data analyses, streamflow
forecasting, data exchange, and product dissemination. It serves
as the delivery system to make snow survey and related planning
information available to local conservation districts and NRCS offices
where it is incorporated into their conservation application programs.
CFS also provides access to hydrologic data and interpretative products
for a wide variety of governmental agencies and the general public.
The systems can be accessed by most computers, and it is menu driven
for ease of use.
The data in CFS are important for natural resources management planning.
These data reside in an automated database consisting of monthly
data for 1,700 snow courses, 600 stream gauges, 300 reservoirs,
and 1,200 precipitation stations as well as daily data from 550
SNOTEL sites and 2,000 climatological stations. Data are exchanged
routinely with the National Weather Service and numerous agencies
as well as private entities.
An irrigation water supply ditch carries snowmelt water to the fields.
The 10 Western States and Alaska publish a monthly Water Supply
Outlook Report which is generated by CFS. Special reports can be
created and stored that include data for specific SNOTEL sites and
during specific time intervals.
Additional CFS programs relate snow survey streamflow forecasts
to irrigation planning at the farm level. These programs incorporate
crop consumptive use data and irrigation planning routines from
NRCS State Irrigation Guides.
For more information on CFS, contact the National
Water and Climate Center.
The major reason for the snow survey program with its extensive
data collection network has always been the forecasts of annual
streamflow volume at specific points along a river system. These
forecasts are a vital input to water management. Irrigation, reservoir
operation, domestic water use, power generation, fisheries management,
and flood control are typical of the activities dependent on streamflow.
Reservoirs such as Lake San Cristobal in Colorado are dependent
Others are concerned with the actual measurements rather than forecasts,
and the management of certain resources such as wildlife and range
can be tied directly to these data. Traditionally, information has
been distributed by NRCS in each State through the monthly mailing
of printed water supply outlook reports from January through May.
supply outlook products for the Western United States (including
snowpack, precipitation, and streamflow forecast maps) produced
jointly by the NRCS and the National Weather Service are available
from their respective web sites for the same period. The final product
for the water year is an annual snow data summary. Snow data are
maintained in a national archive.
Range management can be tied directly to annual streamflow volume
The modern snow survey program, with real-time data provided by SNOTEL
and CFS, is delivering a broader range of more timely information
than is possible with printed reports. And the information is keyed
to the specific needs of NRCS and conservation district offices and
an expanding user community: news media, civic organizations, emergency
agencies, recreation manager, and others.
Resources management agencies such as USDA's Forest Service, U.S.
Department of the Interior's Bureau of Land Management and Bureau
of Indian Affairs, or State departments of fish and game and forestry
require up-to-date water supply information. CFS presents opportunities
for NRCS to work cooperatively with these agencies to accomplish soil
and water conservation objectives.
Demands are increasing for the often limited water supply in the western
river systems, and forecasts must be as current and reliable as possible.
The computer access provided through CFS not only makes the latest
data instantly available, but it provides many standard and customized
analysis procedures to support specific needs for information.
||Adapted from Natural
Resources Conservation Service, Snow
Surveys and Water Supply Forecasting: Agriculture Information Bulletin