Snow-Water-Equivalent (SWE) Measurement

Last modified by Steve Custer or Karl Birkeland 21 March 2006

Required Reading:

Goodison et al., 1978
NRCS Snow Survey Sampling Guide

Optional Reading

Rovansek et al., 1993
Custer, 1991 if you have time.

Introduction

Snow is an important water resource. Because of snow density changes that accompany metamorphism, simple snow depth cannot be used to assess the amount of water in the snow pack. The U.S. Federal snow sampler has been developed to measure snow depth, snow density and snow water equivalent quickly without digging a snow pit. A snow course uses many measurements with a U.S. Federal snow sampler along a slightly sloping line to find the mean snow water equivalent. The problem is this is very time consuming. Rovanseck et al. (1993) have suggested a combination of snow sampler and depth measurements to improve efficiency. An alternative of course is to dig a snow pit, find the mean density, and determine the water equivalent in the snow pack based on the density and depth measurements from a snow pit (not easy or fast if you use several snow pits, but sometimes better than a snow course if snow cover is thin). This laboratory explores these two approaches and the situations in which each is appropriate.

Goals

Learn to locate in the field.
Learn to measure total snow-water (HSW) and density with a U.S. Federal snow sampler.
Learn to measure snow-water with a snow pit and depth measurements using a probe.
Explore advantages, disadvantages, and situations where each of the two techniques are most appropriate.
Explore change in total snow-water (HSW) and snow depth (HS) with elevation.
Explore the difference in total snow-water (HSW) and snow depth (HS) with elevation.

Objectives

Sample snow-water depth at three elevations using U.S. Federal snow sampler and depth measurements on a snow course.
Locate sample site on a map.
Compare three methods (US Federal sampler, snow probe and density, combined).
Assess change in HSW with elevation.

Equipment

Copy of a part of the U.S. Geological Survey Saddle-Mountain 1:24,000 topographic quadrangle.

HTML (Small file, low resolution)

PDF (Large file, high resolution)

Copy of an air photograph of Bridger Bowl.
Altimeters, compasses, U.S. Federal Snow Samplers, avalanche probes, 100 m tapes, density kits, notebooks, pencils.
If snow is thin, bring some of the big density cylinders and the 2 kg springs.

Procedure

Groups:

Break into groups of 4 people. The sample groups and the subgroups should remain close together so questions can be resolved as a class. If practical, one group might work in the trees and one in the adjacent openings to compare SWE in the two settings.

Sample Location:

The class will sample at three different elevations. Use the topographic map to identify the elevation of the base. Set your altimeter at the base . The first sample will be on the apron north of the top of the alpine lift. Find your elevation using the map and your altimeter. At the top of Bucks Run. The third sample will be taken near the Base at the end of the day. Don’t forget to recheck your elevation to see how much the barometer has changed your elevations throughout the day.

Snow Sampling:

There are three ways to sample the snow water equivalent. Try one and two. Use 3 if conditions dictate use (you are familiar with 3).

The first is with a standard snow course which is on a slight slope of relatively uniform snow. The snow course is usually between 130 and 270 m long and is sampled with a U.S. Federal Snow sampler at 30 m intervals. The problem with this approach is that the number of samples is low.
The second method uses both the U.S. Federal snow sampler and additional snow depth measurements be taken between the U.S. Federal Snow samples (Rovansek et al., 1993). The depths are multiplied by the densities measured with the U.S. Federal sample to obtain the snow water equivalent at more sites. For this exercise, each group should measure a minimum of four water equivalents with the U.S. Federal snow sampler and another 12 or more snow depths at each site.
You may find that the snow depths are too low to obtain good snow samples with the U.S. Federal snow sampler. This is particularly problematic at Bridger Base where depths can be low. Should this be a problem, you can dig a 2-3 snow pits, measure snow density, and then measure many depths and apply the density obtained from your snow pit to the depths.

How to Use the U.S. Federal Snow Sampler

Estimate the snow depth.
Assemble the snow tube.
Tare the snow tube and record the tare.
Insert the snow tube into the snow with a twisting motion, but keep the tube vertical. Be sure you get to the soil
Read the snow depth and record it. These samplers are marked in inches of water. NOTE: the snow depth is read outside the tube not inside. The snow inside may be settled or compressed and does not represent the snow depth. Read the snow depth on the outside of the tube when the tube is inserted into the snow.
Remove the snow tube and extract the soil plug.
Weigh the snow tube and snow (the scale is in inches of water). Record the weight.
Subtract the tare to find the HSW.
Calculate the density in g cm^-1 by dividing HSW by HS. This is correct, but looks dimensionally incorrect. You might think about why this works.
Repeat the procedure at the other sample points on the snow course.

*In the past, classes have had some difficulty with the snow core freezing in the snow tube. This can be related to loss of wax on the tube interior (the wax is fresh). Another problem can have to do with dramatically different temperatures in the snow and air. If water droplets freeze in the tube, these droplets become sticking points for the core and partial retrieval will be the result. Here are some hints for preventing this problem:

Wear gloves. No hot bare hands on the metal.

Chill the tube in the snow before use (clean inside) so its temperature can equilibrate with the material being sampled. You have to be careful not to get much water in the tube if you do this.

When moving between sampling points, carry the core in the tube. Empty the tube only when ready to sample immediately.

If you feel silicone is a problem, NRCS experience suggests that silicone for skis is a BAD idea. This material leads to additional sticking. They recommend Turtle Wax.

Encourage instructor to transport outside in a truck rack not in the hot car.

SWE by Snow Pit Density and Depth (Recall Snow Stratigraphy and the Snow Profile Sheet).

Dig a snow pit.
Find the high and low density layers.
Measure the density of each layer with the density kits and calculate the weighted mean density as in previous labs. (rho at the top of the graph paper on the International snow data sheet.) If the snow appears to be uniform in density collect at least 3 preferably 5 densities, each from a different level. You will find this admonition difficult in some settings. If the pack is very thin, measure the density at three to 5 locations with a density kit.
(Note: Since you are using U.S. Federal snow sampler you may also have density this way. Use it.)
Record the densities.
Use a meter stick, avalanche probe or a ram probe pole to measure snow depth at several sampling points along the snow course. (Beware, is the ram probe pole measuring correctly? Hint: Zero is referenced to the ram tip not the pole tip; the pole tip may penetrate the soil.) Record each depth.
Use layer-weighted average snow pit density values from snow pits with probe depths to estimate HSW (HSW = decimal mean density times snow depth)

Product:

There are two products for this exercise, raw data and the report.

Data

This data should include the elevations sampled, the mean SWE from the U.S. Federal sampler and the mean depth using the probes without the US Federal sampler and, if you did a snow pit, the mean density from the snow pit with the probed depths. Include the names of the people in your group. Turn in enough copies of your data tables so that each student gets one on Wednesday in class. Points will be lost if you fail to meet this deadline.

A Report

Introduction:
   Question
   Location
                   Location Map:   Show location of your sampling sites. Include elevation in ft above sea level.
Methods:
   Federal Snow Sampler Goodison et al., 1978)
   Probe and snow pit density (If snow is deep enough use US Federal sampler densities)
   Federal Snow Sampler + probe combined (Rovansek et al., 1993)
Results:
   Data summary of results of three methods (US Federal only; Probe and density only; Rovansek (combined); Pit and Density if needed.
   Data Tables from the field in the appendix.
            Blank Field Data Table
            Blank Field Data Summary Table
            Blank Method Summary Table
   Present a graph of snow depth versus elevation.
   Present a graph of snow water versus elevation.
   Calculate the snow depth gradient from the apron to the base.
   Calculate the snow water equivalent gradient from the graph.
   Calculations in appendix
Discussion:
   How well do methods compare?

US Federal Sampler

Federal sampler combined with additional depths

Snow pit densities and additional depths

   Did snow water equivalent drop with elevation? If so, what is the gradient?
       What is the error in your elevation measurement?
       What is the error in your snow water equivalent measurement?
   If you had problems what were they and how could the be solved?

Grading Rubric

Due Date:

Data next Wednesday in class (one copy from your group for each student).
Report Friday before class outside instructor's office.