Water Years 2011 and 2012 are a case study in interannual variability of Colorado snowpack formation and ablation driven by vastly different late winter and spring weather conditions, perhaps representing seasonal extremes. In dramatic contrast with the unrelenting storms of Mar/Apr/May 2011, extremely dry late-winter and spring weather in 2012 resulted in low values of peak SWE, very early in the spring.
On the other hand, the past two seasons produced comparatively smaller differences in dust-on-snow conditions, with WY 2012 coming close to matching WY 2011 at the Senator Beck Basin Study Area, our baseline monitoring site. Locally, WY 2012 desert dust concentrations in the Rabbit Ears Pass locale were only somewhat less intense than those observed at Senator Beck Basin and spring 2012 snowpack “ripening” was enhanced by those reductions in snow albedo. The Rabbit Ears Pass CODOS site snowpack became isothermal sometime prior to April 10th, with several dust layers merged at the snowpack surface. Streamflow data from the nearby USGS Yampa River at Steamboat Springs gauge document a substantial advance in the entire snowmelt runoff cycle, and reduced total runoff.
SWE accumulation and ablation for Water Years 2011 and 2012, with dust-on-snow events shown, by date, as brown bars for both water years (as observed at the Senator Beck Basin Study Area).
From the very start, Water Year 2012 snowpack formation at the Rabbit Ears Pass Snotel fell behind average and by December SWE accumulations had fallen into the lower quartile of values for the period of record (see Snotel projection plot below). New record-low values of SWE began in January, briefly rebounded back into the lower quartile in mid-February, then reached peak SWE very early, on March 13th, at just 15.2”, well short of the 1971-2000 average of 30.2” and six weeks earlier than the April 24th average date of peak SWE. Peak SWE in 2011 was over three times that of 2012, at 51.6” of water content, and occurred on May 6th.
During WY 2011 eleven dust events occurred at Senator Beck Basin containing a total of 14 grams of dust per square meter. Most of those major dust layers were found in CODOS snow profiles that season at the Rabbit Ears Pass CODOS site. However, relentless accumulation of late winter and spring snow routinely buried new dust layers and restored high snow albedo values throughout April and May, until the final and perhaps largest event of the season on May 29th.
Last season, during WY 2012, a similar twelve dust events occurred at Senator Beck Basin containing a total of ~10-12 grams of dust per square meter (see Senator Beck Basin Summary – Dust Log discussion). But, in a dramatic switch, late winter and spring 2012 were exceptionally dry, with widely spaced and comparatively small winter storms (see Senator Beck Basin – Winter Storm Log Discussion) leaving dust exposed at the snowpack surface for extended periods, beginning in early March.
In our March 15th snow profile at our Rabbit Ears Pass CODOS site, we found event D4 dust at the surface of the 153 cm (60”) snowpack, with 6 cm of slushy, melt/freeze polycrystals at the snowpack surface. The snowpack retained minimal cold content on this date, with a mean snowpack temperature of -1.0° C. Total SWE in our CODOS site snowpit was 490 mm (19.3”), versus 15.0” at the Rabbit Ears Snotel site (just below its peak SWE of 15.2”, two days earlier).
By April 10th, our next snow profile at Rabbit Ears Pass found the diminished 77 cm (30”) snowpack fully isothermal at 0° C, with wet snow forms throughout. Merged dust layers coinciding with events D4-D8 were at the snowpack surface, possibly including a D9 event, unique to this locale. Total SWE had fallen to 319 mm (12.6”). On our final CODOS site visit on May 2nd we found 56 cm (22”) of snow containing 233 mm (9.2”) of SWE still remaining, saturated by rain the evening before, but with merged dust layers D4-D8 still apparent and substantially reducing snow albedo.
Snowpack on Rabbit Ears Pass, in an open meadow at the headwaters of Walton Creek, on May 2nd, 2012 (left) as compared to a year earlier, on June 9, 2011. This meadow was likely snow-free well before the final D12 dust event of 2012, on May 26th, whereas the final event of spring 2011, D11 on May 29th, fell onto a still very deep snowpack, substantially reducing snow albedo and enhancing snowmelt runoff.
Analysis of Rabbit Ears Pass Snotel data for 2006-2012 snowmelt seasons showing date and quantity of peak SWE, days from peak SWE to “snow all gone” (SAG), total additional precipitation after date of peak SWE, an “adjusted” mean daily rate of snowmelt adding the additional precipitation to the peak SWE total, the maximum five-day moving average of daily melt, and the mean air temperature over the entire snowmelt period, from peak SWE to SAG.
A view of the Rabbit Ears Pass Snotel site approach SAG on May 2nd, 2012. This site has a slightly southwestern aspect. Although part of the snow pillow has melted open (closest to the camera), 14” of snow containing 3.0” of SWE were still being measured over the remainder of the pillow. A small clearing in the forest just beyond the Snotel had lost all snowpack. Our CODOS site, which slopes 4 to the north-northwest, still had 56 cm (22”) of snow containing 233 mm (9.2”) of SWE on this date.
This discussion references CODOS Snotel site data and analyzes rates of snowmelt from Spring 2006 to the present, spanning the period during which we’ve rigorously observed dust-on-snow at our Senator Beck Basin Study Area, at Red Mountain Pass. Since the Snotel network is the only spatially extensive system continuously monitoring snowmelt throughout the Colorado mountains, year-to-year comparisons of Snotel melt rate data may yield insights into dust effects on local watershed-scale processes that our occasional CODOS snowpit measurements can’t reveal.
However, as we often note, many Snotel sites exhibit a radiative regime where surrounding trees reduce the access of incoming solar radiation to snowpack over the SWE measuring snow pillow, and where re-radiation of long wave energy from that vegetation and reduced skyview may inhibit radiant cooling and extend surface snowmelt during nighttime hours. The Rabbit Ears Pass Snotel site exhibits these attributes and, as a consequence, does not experience the maximum effects of dust reductions in snow albedo, and short-wave radiative forcing, as compared to the nearby open meadow and our CODOS snowpit site where solar access is comparatively unimpeded, until the sun is low in the western sky. This difference in dominant radiation regimes (long-wave in shady forests versus short-wave in sunny, open terrain) is routinely seen where snow-free open meadows immediately adjoin forest retaining snowcover.
During WY 2012, three fall and early winter dust storms were observed low in the snowpack at Senator Beck Basin, and at least one of these layers (likely D3) was discernible at Rabbit Ears Pass in our CODOS snow profiles. Later, dust-on-snow events D9-D12 may have had little effect in the Rabbit Ears Pass locale, as very little snowpack was left in open terrain at these sub-alpine elevations. Where snowpack did persist into late May at higher elevations farther north in the Park Range, events D9-12 likely did reduce snow albedo and enhance late-season snowmelt runoff rates.
WY 2012 SWE accumulation and ablation at Rabbit Ears Pass Snotel with upper and lower quartiles of period of record shown in light grey, middle quartiles in dark grey surrounding median trace for period of record, and mean trace for 1971-2000 period.
Because of the early date of WY 2012 peak SWE at Rabbit Ears Pass Snotel, on March 13th, spring 2012 produced a two-step descending limb in the SWE plot rather than the single, steep plunge following the very delayed peak SWE in 2011. In 2012, snowpack ablation gradually accelerated in mid-March until storms in mid-April produced a small rebound in SWE. Then, in late April, under the influence of unseasonably warm temperatures and further reductions in snow albedo from dust, melt rates accelerated again. Overall, the Rabbit Ears Pass Snotel site required 52 days to ablate 19.6” of SWE (including 4.4” of precipitation after peak SWE), averaging 0.34” of SWE loss per day.
WY 2012 USGS Yampa River at Steamboat Springs Stream Gauge data for a 568 sq. mi. drainage area in the upper Yampa River watershed, above 6,695’. Current year and historic data reflect upstream trans- and in-basin diversions for irrigation and for water storage in five reservoirs.
Given the very dry spring and sub-par snowpacks throughout the Northern mountains, below average snowmelt runoff in the Yampa River watershed was unsurprising. The Yampa River at Steamboat Springs hydrograph shows an early start to runoff with substantially above-average flows in March and early April to a peak in late April. An additional surge in early May was followed by a gradually accelerating decline in flows. As was typical throughout the state, by the end of June streamflows had fallen below levels expected two months later, in September.
In addition to the very warm temperatures associated with prolonged periods of dry and sunny weather in late winter and spring 2012, dust-on-snow events D4 (March 6th) and D8 (April 6th) played a role in this early Yampa River runoff, reducing snow albedo and hastening the ripening of the snowpack in late March, to isothermal. Dust then continued absorbing and adding solar energy to the snowmelt energy budget during subsequent periods of exposure at the snowpack surface in April and May. Additional dust events in late May, and perhaps particularly D12 on May 26th, further reduced snow albedo in the scant remaining high elevation snowpack in the Park Range, Flattops, and Elkhead Mountains, hastening the final drying of streams emanating from these Northern mountain systems.