A COMPARATIVE STUDY OF SNOWMELT-DRIVEN WATER BUDGETS IN ADJACENT ALPINE BASINS, NIWOT RIDGE, COLORADO FRONT RANGE

The critical zone in alpine and subalpine headwaters areas delivers drinking water to urban corridors near mountainous areas of North America. Snowmelt runoff from alpine areas typically accounts for over 80% of annual flow, but water budgets are not well quantified nor well understood in detail. Redistribution of snow by wind, the difficulty of estimating water losses from sublimation and evapotranspiration, and groundwater gains and losses associated with unconsolidated subsurface material make streamflow and water budget measurements challenging. We investigate two adjacent catchments, Martinelli and Saddle, both about 0.25km²at ~3500m, on Niwot Ridge in the Colorado Front Range. Nearby Long-Term Ecological Research (LTER) monitoring stations indicate mean annual precipitation is between 1210 and 950mm. Mean annual runoff is ~310mm at Martinelli and ~230mm at Saddle, based on 12 and 28 years of gaging records, respectively.

Saddle basin is not fed by an extensive seasonal snowpack, but records indicate ablation-season discharge is still closely related to snowmelt in the basin. Martinelli shelters a ~6m thick snowpatch in 8 ha of the basin, even in a low snow year. During much of the ablation season, snowpack mass density (ρ) is 0.5g cm^-3 and ablation rates are ~100mm day^-1. Since vegetation is shallow-rooted or nonexistent in Martinelli, evapotranspiration (ET) may not be a major factor. Saddle is more heavily vegetated, but only the lower reaches are wooded; ET may be as much as 260mm. Runoff during 2012 was ~270mm yr^-1 at Martinelli and ~35mm at Saddle. By monitoring snowpack area changes and longitudinal discharge, we were able to demonstrate that at low flow at least 30% of snowmelt discharge at Martinelli and over 50% at Saddle becomes subsurface flow and is unrecorded by the gages. We believe similar hydrologic budgeting can be applied to a wide range of small alpine catchments with similar “losing stream” characteristics.