Processes regulating watershed chemical export during snowmelt, Fraser experimental forest, Colorado

In the Central Rocky Mountains, snowfall dominates precipitation. Airborne contaminants retained in the snowpack can affect high elevation surface wat er chemistry during snowmelt. At the Fraser Experimental Forest (FEF), loca ted west of the Continental Divide in Central Colorado, snowmelt dominates the annual hydrograph, and accounts for >95% of annual stream water dischar ge. During the winters of 1989-1993, we measured precipitation inputs, snow pack water equivalent (SWE) and ion content, and stream water chemistry eve ry 7-10 days along a 3150-3500 m elevation gradient in the subalpine and al pine Lexen Creek watershed. The study objectives were to (1) quantify the d istribution of SWE and snowpack chemical content with elevation and aspect, (2) quantify snowmelt rates, temperature of soil, snowpack, and air with e levation and aspect, and (3) use change in upstream-downstream water chemis try during snowmelt to better define alpine and subalpine flowpaths. The SW E increased with elevation (P < 0.001) and was greater on NE aspects (P < 0 .05). Subalpine soils were unfrozen. Snowmelt occurred throughout winter at low elevation (3150 m), SE aspect stations. By snowpack peak water equival ent (PWE), the snowpack lost about a third (0.24 m) of its moisture. The su balpine snowpack had warm (> - 3 degreesC) temperatures throughout winter w hich resulted in significant snowpack ion loss. By snowpack PWE in mid May, the snowpack had lost almost half the cumulative precipitation H+, NH4+, a nd SO42- inputs and a third of the NO3- input. Windborne soil particulate i nputs late in winter increased snowpack base cation content. Variation in s ubalpine SWE and snowpack ion content with elevation and aspect, and wind r edistribution of snowfall in the alpine resulted in large year-to-year diff erences in the timing and magnitude of SWE, PWE, and snowpack ion content. The alpine stream water ion concentrations changed little during snowmelt i ndicating meltwater passed quickly through surface porous soils and was wel l mixed before entering the stream. Conversely, subalpine stream water chem istry was diluted during snowmelt suggesting much melt water moved to the s tream as shallow subsurface lateral flow. published by Elsevier Science B.V .