April 14th: It has been a busy few days. Yesterday the remaining part of team arrived from Fairbanks. The seven are from Fairbanks and Colorado, all veterans of Arctic snow research, with the exception of Allison, a graduate student from the University of Colorado. We have been working at three things from the start of the campaign, anticipating the arrival of our LiDAR-bearing aircraft tomorrow. These have been:
- Setting up a precise GPS control network that will allow the aircraft to navigate with 10-cm positioning precision,
- Operating our ground-based LiDAR to produce small “bull’s eyes” where we will know the snow surface topography to centimeter accuracy, and
- Measuring snow depth and snow water equivalent (SWE) along the planned flight path without trashing too much snow before the aircraft flies.
With the full team in place and the aircraft due in a day, the pace has been quickening.
One thing we did a couple of days ago by way of preparation was to install GPS base stations. Very early in the planning process we determined that in order to be able to correct the GPS data that would be collected by the Airborne LIDAR, at least one or two high order base stations would need to be established. The National Geodetic Survey classifies benchmarks with a stability rating. There are four ratings: (A) most reliable and expected to not move, (B) probably stable, (C) may hold an elevation but subject to ground movement, (D) questionable or unknown stability. One of the most adventurous aspects of working in the north is finding a survey monument that is considered Level A and is easily accessible for the from the road system. You can find a rebar drilled into a rock outcrop (Fig 1.), or by rooting around high places, maybe find one (as Fig. 2) located on top of Slope Mountain where a communication tower is located
Unfortunately, most of the monuments that can be found easily tend to be on a level D, which are copperweld rods driven to about 5 feet, into loose gravel soils, and their condition can be questionable at best. A good example would be a station call the GAL (short for Galbraith), which is set atop a nearby hilltop near where we will be conducting most of our work. (Fig 3). This monument was placed in 1971 and we found that the survey cap was deformed by some unknown event. Most likely it was hit by an adventurous snowmobiler cresting the hill.
The second most common class of monuments are Level B, which can be found all along the Dalton Highway and were installed in 1974 (Fig 4). These monuments are copper-clad steel rods that are driven to a depth of 10+ feet. Many are subjected to frost jacking.
Level A monuments offer the best stability and are placed in rock outcrops, drilled into bedrock, or massive structures with deep foundations. These tend to be in high, hard to get to places. In our case, we happened to know there was a beautiful monument located just off of the highway in the farthest north rock outcrop along the road near the Sagavanirktok (or the Sag) River. This monument is set in top of a ledge of a rock above a creek (Fig 5).
However, the crew that was sent out to install the base station had a bad GPS position for the monument, which sent the crew digging around the bottom of the creek looking for a non-existent rod and cap. After some time shoveling, we realized no self-respecting surveyor would place a monument in the creek, so we looked up on the ledge rock and found the monument in a few minutes (Fig 6).
There we set up our GPS base station, anchoring it in place using chains to keep the mast in a vertical position. A bank of batteries and a solar panel were connected to provide power for the GPS, which is now collecting position points at a rate of 5Hz.
The other thing we have been doing is getting out in the field measuring a lot of snow depths. This requires riding snowmobiles, and they can leave pretty deep tracks in the snow (Fig. 8). Consequently, we had been concerned about not tracking up any more snow than we had too before the airplane flew over the area mapping the snow with its airborne LiDAR. On the other hand, we knew we needed to a get a jump on the ground-based measurements because the ground-based measurements take so much more time to make than the airborne ones.
Consequently, we had been enforcing a pretty strict discipline on snowmobile tracks: Everyone had to ride in the same track; no one was allowed to carve turns up pristine soft snowfields, and we drove circuitous routes around some of our intensive of sampling areas. This doesn’t sound too restrictive, but one thing about snowmobiling out on the open tundra is that it gives one a real is a sense of freedom. Wide open spaces – no fences – a lot of sky. It is pretty hard to stay in line behind another snowmobile when those wide open slopes beckon.
But to our surprise, the snow was trashed already, and not by snowmobiles. It had been trashed by the caribou. The caribou were in our area in force, and they had been feeding on the tundra in many places (Fig. 9). When caribou feed (Fig. 10), they seek out thin areas of snow and dig the snow away to get at the lichens underneath. This is called cratering (Fig. 11) and it has been the subject of considerable study.
The scientific literature agrees that that caribou select where to crater based on the forage beneath the snow (which they find by smell), the depth, and the hardness of the snow. The goal is to dig with the least expenditure of energy for the most food value. The deeper and harder the snow, the higher the cost. Highly packed snow takes almost four times as much energy per stroke of a hoof than softer snow (like new or recent snow).
Now this is where snow science and biology intersect: Over the past few days we have had been noticing that there was a thick wind slab buried in the middle of the snowpack (see blog for April 9th). This wind slab had been produced by a wind storm some time during the winter. When we finish analyzing our data we will know when that occurred.
The density of the layer was about 0.5 g/cm3, which is almost as high as seasonal snow can be (without melting or mechanical packing). This density is equivalent to about half air half ice. The layer was thick in some places, thin in others, and even absent in some locations. We stopped in many of the cratered spots and it appeared to us (though without any statistical rigor) that the caribou had generally found a thin area of snow, where they began cratering and feeding. They had dug out from where they started, but when they encountered the slab layer, they ceased, abandoned that cratering area, and found a new one. Some of the literature we briefly looked at supports this idea… but other papers were less certain.
One thing is certain, in this cold and austere environment, where finding enough food to stay alive and warm is difficult, small things like efficient cratering matters. This is a land where there is a fine line between survival and death.
Co-author on this post: Art Gelvin is the lead Technician at CRREL-Alaska, a surveyor, and experienced Arctic hand. He resides in Fairbanks, Alaska, where he has lived since 1985. He can be reached at Arthur.B.Gelvin@usace.army.mil.
Previously in this series:
Alaskan North Slope Snow LiDAR Campaign: SnowSTAR-2012