Nesbitt, Ian M.; Adv. David P. Dethier
Department of Geosciences, Williams College (2013)doi: 10.13140/RG.2.2.33197.20966
The Critical Zone, which extends from the top of the weathered bedrock to the tops of the tallest vegetation in alpine and subalpine headwater areas, delivers fresh water to urban corridors near mountainous areas of North America. Snowmelt runoff from alpine basins 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 from outside the basin make streamflow and water budget measurements challenging. I investigated two adjacent 0.25 km2 catchments, Martinelli and Saddle streams, both at ~3500 m, on Niwot Ridge in the Colorado Front Range. Mean annual runoff is ~230 mm (25% of mean annual precipitation) at Saddle basin and ~310 mm (30% of mean annual precipitation) at Martinelli basin, based on 12 and 28 years of gaging records, respectively. Saddle stream is not fed by a late-lying snowpack, but records indicate that ablation-season discharge is still closely related to snowmelt in the basin. Martinelli basin shelters a ~6 m 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 0.5 g cm-3 and ablation rates are ~100 mm day-1. Since vegetation is shallow-rooted or nonexistent in Martinelli, evapotranspiration (ET) is probably not a major factor. Saddle basin is more heavily vegetated, but only the lower reaches are wooded; ET is likely < 260 mm annually. Specific runoff measured at the gage during 2012 was ~270 mm at Martinelli and ~35 mm at the Saddle gage. By monitoring snowpack area changes and longitudinal discharge, we were able to demonstrate that at least 30% of annual precipitation in Martinelli basin and 10% in Saddle basin bypasses the gage as subsurface flow. Short-term yield calculations indicate that approximately 2.5% of precipitation discharges from the basin as measurable surface water within a five-day period; the rest recharges groundwater or becomes immeasurable subsurface flow. For comparison, a nearby 2.3 km2 glaciated basin, Green Lake 4, discharges 50% of the water that falls on it within the same five-day period. Measured water yields from small, unglaciated alpine catchments thus should be viewed with caution.
The Critical Zone is the biologically active zone of the terrestrial world, which extends from the top of the weathered bedrock to the tops of thetallest vegetation. In alpine areas, the Critical Zone is a vital resource that delivers fresh water to populated areas downstream. Snowmelt from alpine basins accounts for over 80% of annual water budgets in the mountain west of North America, but water budgets at these source areas are not well quantified nor well understood. A combination of a number of factors influencing the water budgets of these headwaters areas (redistribution of snow by wind, evaporation, use by vegetation, and groundwater contributions) make measurements challenging. I investigated two adjacent 0.25 square kilometer watersheds, called Martinelli and Saddle, on Niwot Ridge in the Colorado Front Range. Mean annual runoff at the measurement gage is equivalent to 230 millimeters of precipitation (approximately 25% of mean annual precipitation) at Saddle, and 310 millimeters (~30% of mean annual precipitation) at Martinelli. These estimates are based on 12 and 28 years of gaging records at Saddle and Martinelli respectively. Saddle stream is not fed by snowpack late in the melt season, but records show that discharge is still closely related to snowmelt. Martinelli basin shelters a ~6 meter thick snowpatch in 8 hectares of the basin, even in low snow years. During much of the melt season, snowpack density is 0.5 grams per cubic centimeter and ablation rates are ~100 millimeters per day. Since vegetation is shallow-rooted or nonexistent in Martinelli, evapotranspiration (the evaporation of water from the basin due to use by vegetation) is probably not a major factor in the water budget. Saddle basin is more heavily vegetated, but only the lower areas of the stream are wooded. Evapotranspiration at Saddle is likely less than 260 millimeters annually. In 2012, runoff was ~270 millimeters at Martinelli and ~35 millimeters at the Saddle gage. By monitoring the snowpack and longitudinal discharge, we were able to demonstrate that at least 30% of discharge at Martinelli and 10% of discharge at Saddle bypasses the gages as subsurface flow. Short-term calculations over a five-day period indicate that approximately 2.5% of precipitation discharges from the basins as surface flow, while the rest either recharges groundwater or discharges as immeasureable subsurface flow. For comparison, a nearby formerly-glaciated basin (Green Lake 4) discharges approximately 50% of the water that falls on it within the same five-day period. Results indicate that measured surface water yields from small, unglaciated alpine catchments should be viewed with caution.