Friday, April 5, 2013

Field Day: Snow sampling and the coming of spring

Don’t sink. Don’t sink. Don’t…sunk.

We’re floundering around in waist deep snow, our snowshoes buried and our packs throwing us off balance. The snow mixing bucket and data clipboard in our hands add to the struggle of regaining our footing, but we could not be more thrilled. The sky is blue, the sun is warm and spring is on its way. The snow melting and soaking into our boots and clothes makes that clear. One more week, and our outing would have been almost pointless. Collecting snow samples with no snow would be impossible.

We scramble to our feet, the wet snow helping us to slide down the slight slope to the road. We could not have asked for a more perfect day.

Wil Chapple mixes a snow sample.
This field day is part of a WyCEHG project focused on the isotope signatures of snow to help determine where snow melt is actually going. Does the runoff end up in streams and watersheds, like Libby Creek, where we are today, or is it stored in the ground and later used by trees? This is one of the questions Dr. Dave Williams and EPSCoR Undergraduate Fellow Wil Chapple are trying to answer. To do so, snow samples at specific sites, picked because of their soil structure, must be methodically collected. Today, our team of four heads to the Snowy Range Mountains, west of Laramie, to measure and collect the snow Wil needs to continue work on his project and which will be used to inform other aspects of WyCEHG’s work.

Water runs in little rivers down Barber Lake Road where we pull off. The snow banks that usually cover the road until May are melting in a hurry. Our collection sites are half a mile and a mile up the road. Elizabeth Traver (ET), manager of the Surface and Subsurface HydrologyLab (SSHL), and Wil accompany Dr. Williams and me to the first site, to show us how to correctly measure the snow and collect the samples. We are using GPS units and snow core samplers to pinpoint our locations and determine the snow water equivalents (the depth of liquid water of the snow at a spot). The snow core sampler is comprised of a scale with a hook and a long metal tube with centimeters marked on the side and a sharp, slightly pointed end for digging into the ground.  In addition, we have a clipboard with data sheets, plastic sampling bottles, a 100 meter measuring tape, a stake, a bucket and a trowel.

The snow core sampler.
We start by poking the stake firmly into the snow, attaching the measuring tape and walking it 50 meters across the top of the slope we have decided to work on. This terrain was created from rock and sediment deposited during the Bull Lake Glaciation period, approximately 140,000 years ago. Because of its age, this site has a different soil structure, texture and chemistry than that on nearby hills which were created during the much more recent Pinedale glacial event. Our goal at this site is to stay within this soil type during our measurements, because Dr. Williams and Wil believe that soil type affects water movement in this basin. In order to understand this dynamic, all of our measurements must be within the same soil.

After Dr. Williams walks the tape measure out 50 meters, we measure the weight of the long metal tube: 0.71 kg. ET then pushes the tube down vertically in the snow until it hits the ground below. We take note of the snow depth: 42 centimeters. With gloved hands, so as not to warm the tube, ET pushes down on it. This digs the sharp metal end of the tube into the ground. When she pulls the tube out, there is no dirt in the bottom. This means we didn’t hit the ground enough, so our measurements are not accurate. We have to try it again.

This time the snow depth is 46 centimeters and when ET pulls the tube out, deep brown dirt is clumped into the bottom. Perfect!  We can take our next measurement, which is the compressed depth snow: 22 centimeters. Carefully, ET digs the dirt out of the bottom of the tube, trying not to scrape her fingers too much on the sharp edges. Once all the dirt is out, we measure the weight again: 0.81 kg. This final weight will allows us to calculate the snow water equivalent and snow density. Every ten meters we will take these same measurements. At 0, 50, 100, and 150 meters we will collect a snow sample for isotope analysis.

To do so, we dump the snow in the tube into our mixing bucket and Wil stirs it around with the trowel. The snow we collect in the tubes is from different storms, all with slightly different isotope signatures. If we just scoop snow off the top of the bank, we only get a signature for that layer of snow. So, to get a bulk isotope signature for that exact spot, the layers must be mixed together. Once mixed, we stuff it into a properly labeled plastic bottle.

With the method down, Dr. Williams and I head 10 meters down the line while ET and Wil slide their way back to the road to go further up into the mountains to visit two other sites, one in Pinedale glaciation terrain and one in pre-Pinedale glaciation terrain.

A measurement site where one snow sample was taken
By the end of the first 50 meters, we have a system down. Dr. Williams take the measurements, I record the data, and we sink through the snow to our next spot. At meter 50, we collect another sample, and then it’s time to take measurements down slope. This time, Dr. Williams measures 100 meters down, making as straight of a line as possible. The aim is for our measurements to be taken in an L shape, with 50 meters across the slope and 100 meters down the slope.

As we slip and slide down, our measurements become more varied. Whereas the snow depth across the slope was similar at each measurement point, down slope the snow depth ranged from almost 70 centimeters to just 12. This snow was wet and in many places it had already collapsed significantly before we took the measurements. Dr. Scott Miller, Co-Principle Investigator for WyCEHG, told us this would be the case. There is more heterogeneity down than across a hill.

At 100 meters down (and 150 meters total), we are almost out of snow. Large patches of soil, covered with pine needles expose the impact of the warm weather over the last few weeks. For some Laramie-ites, tired of the winter, this may be a welcome sign of summer to come, but for watershed and water resource managers this is an alarm bell going off. The snowpack, according to the Water Resource Data System at UW, is at only 82% for the entire state. With warm weather moving in already, this could be an ominous sign for the coming summer. It is hard to tell if the snow melt from this winter will be enough to keep wildfires at bay and water flowing down the streams. This is why WyCEHG’s work is important. Without a complete understanding of the water system, making accurate estimates about seasonal conditions, water availability and water in general is difficult. With WyCEHG’s data and research, water resource managers can be armed with the tools they need for more accurate predictions about the seasons to come.

One site down, one site to go. Dr. Williams and I head up the road to take measurements from a site formed during the Pinedale glaciation event.

By Kali S. McCrackin
Photos by Kali S. McCrackin

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