Charles A. Troendle

The effect of partial and clearcutting on streamflow at Deadhorse Creek, Colorado

Abstract Two subalpine forest subdrainages of Deadhorse Creek, Colarado, USA were used to demonstrate the comparable impact on water yield of two tree-harvesting practices. Of the 40 ha North Fork subdrainage 36% was clearcut commercially using five-tree height circular openings. In contrast, 40% of the basal area on the 41 ha Unit 8 was removed by partial cutting in the first step of a three-step shelterwood cut. Annual flow and peak flow from the clearcut watershed were increased significantly. The partial cut resulted in a significant increase in total water equivalent in the winter snowpack and an apparent increase in total annual streamflow that was comparable to the clearcut. The timber harvest on the two subdrainages, however, represented only 10% of the total basal area of the larger Deadhorse Creek Watershed in which they are located. Annual flow at the mainstream gage was not significantly increased as a result of harvest.

Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado

Abstract Streamwater samples were collected during 1987–1988 from two adjacent gauged watersheds, the subalpine-alpine East St. Louis and the Fool Creek Alpine, in the Fraser Experimental Forest, Colorado. The study objective was to compare the relationships between streamwater discharge and ion concentration in alpine and alpine-subalpine watersheds at a site receiving low inputs of atmospheric contaminants. Streamwater discharge accounts for much of the variation in ion concentration. Trajectories of time, discharge, and ion concentration suggest that patterns of nutrient flux are controlled primarily by the magnitude of streamwater discharge, and seasonal differences in the relative contributions of snowmelt and soil water. In the subalpine catchment, increased streamwater discharge accounted for most of the decline in concentration of ions, with high concentrations in soil water relative to precipitation. This relationship was not seen in the alpine catchment, probably because of the influence of large diurnal variation in the ratio of snowmelt to soil water. In both catchments, ions with comparatively high concentrations in precipitation and the snowpack relative to soil water showed less concentration decline with increased streamwater discharge. The recurring nature of the trajectories, especially in the subalpine catchment, suggests that the time, discharge, and ion concentration patterns may represent a general characteristic in moderate-sized, undisturbed Rocky Mountain catchments which do not receive high inputs of airborne contaminants.

Change in snowpack, soil water, and streamwater chemistry with elevation during 1990, Fraser Experimental Forest, Colorado

Every 7–10 days during the winter of 1989–1990, we measured precipitation ion input, snowpack water equivalent (SWE) and ion content, soil water and streamwater chemistry along a subalpine to alpine elevation gradient in the Fraser Experimental Forest (FEF), Colorado. The objective was to quantify the effect of elevation and aspect on snowpack ion accumulation and loss, and relate these processes to subsequent change in soil water and first-order stream chemistry during snowmelt. SWE increased with elevation (P < 0.001) and was greatest on NE aspects (P < 0.001). Snowpack ion concentration increased with elevation, with NH4+ and NO3− showing the most consistent (P < 0.05) increase throughout winter. Snowpack base cation (CB) and S042− concentrations were significantly higher on NE aspect transects. In most weeks, the snowpack NH4+ and NO3− content was highest (P < 0.05) in the alpine zone. Peak snowpack NH4+ and NO3− content totalled more than 1 kg inorganic N ha−1. Alpine soil water NO3− concentration during and after snowmelt was greater (P < 0.001) than at lower elevations. However, there was no significant correlation between weekly snowpack ion loss and soil water chemistry. At the FEF, it seems that most snowmelt is routed through the watershed as subsurface flow. Soil processes such as ion exchange, biological uptake, overwinter nitrification and mineralization are probably major factors modifying melt water chemistry. There was no correlation between soil water chemistry and streamwater chemistry, which probably reflected the high variation in distance and time required for soil water to reach the stream.

Effect of subalpine canopy removal on snowpack, soil solution, and nutrient export, Fraser Experimental Forest, CO

Research on the effects of vegetation manipulation on snowpack, soil water, and streamwater chemistry and flux has been underway at the Fraser Experimental Forest (FEF), CO, since 1982. Greater than 95% of FEF snowmelt passes through watersheds as subsurface flow where soil processes significantly alter meltwater chemistry. To better understand the mechanisms accounting for annual variation in watershed streamwater ion concentration and flux with snowmelt, we studied subsurface water flow, its ion concentration, and flux in conterminous forested and clear cut plots. Repetitive patterns in subsurface flow and chemistry were apparent. Control plot subsurface flow chemistry had the highest ion concentrations in late winter and fall. When shallow subsurface flow occurred, its Ca 2+ , SO 4 and HCO 3 - concentrations were lower and K + higher than deep flow. The percentage of Ca 2+ , NO SO 4 2- , and HCO 3 - flux in shallow depths was less and K + slightly greater than the percentage of total flow. Canopy removal increased precipitation reaching the forest floor by about 40%, increased peak snowpack water equivalent (SWE) > 35%, increased the average snowpack Ca 2+ , NO 3 - , and NH 4 + content, reduced the snowpack K + content, and increased the runoff four-fold. Clear cutting doubled the percentage of subsurface flow at shallow depths, and increased K + concentration in shallow subsurface flow and NO 3 - concentrations in both shallow and deep flow. The percentage change in total Ca 2+ , SO 4 2- , and HCO 3 - flux in shallow depths was less than the change in water flux, while that of K + and NO 3 - flux was greater. Relative to the control, in the clear cut the percentage of total Ca 2+ flux at shallow depths increased from 5 to 12%, SO 4 2- 5.4 to 12%, HCO 3 - from 5.6 to 8.7%, K + from 6 to 35%, and NO 3 - from 2.7 to 17%. The increases in Ca 2+ and SO 4 2- flux were proportional to the increase in water flux, the flux of HCO 3 - increased proportionally less than water flux, and NO 3 - and K + were proportionally greater than water flux. Increased subsurface flow accounted for most of the increase in non-limiting nutrient loss. For limiting nutrients, loss of plant uptake and increased shallow subsurface flow accounted for the greater loss. Seasonal ion concentration patterns in streamwater and subsurface flow were similar.