Project at a Glance
Title: Snow, snowmelt, rain, runoff and chemistry in a Sierra Nevada watershed
Principal Investigator / Author(s): Dozier, Jeff
Contractor: Center for Remote Sensing and Environmental Optics, UC Santa Barbara
Contract Number: A6-147-32
Research Program Area: Ecosystem & Multimedia Effects
Topic Areas: Acid Deposition, Ecosystem Impacts
Snow Accumulation and Distribution of snow water equivalence (SWE) was measured in the Emerald Lake watershed located in Sequoia National Park, California, by taking hundreds of depth measurements and depth profiles at six locations during the 1986, 1987, and 1988 water years. Elevations range from 2,800 to 3,416 m, and the total watershed area is about 120 ha. A stratified sampling scheme was evaluated by identifying and mapping zones of similar snow properties based on topographic parameters that account for variations in both accumulation and ablation. Elevation, slope, and radiation values calculated from a digital elevation model were used to determine the zones. The topographic parameters (slope and elevation) do not change between survey dates, but the radiation data vary temporally, providing a physically justified basis for the change in SWE distribution through time. Field measurements of SWE were combined with the physical attributes of the watershed and clustered to identify similar classes of SWE. The entire basin was then partitioned into zones for each survey date. Optimal sampling schemes are calculated based on the observed variance in SWE found in each zone, Although results do not identify which of the classification attempts is superior, net radiation is clearly of primary importance, and slope and elevation appear to be important to a lesser degree. The peak accumulation for the 1986 water year was 2.0 m SWE, about twice the 50-year mean. The peak accumulation for 1987 was 0.67 m SWE, and for 1988 was 0.63 m SWE, both about half the 50-year mean.
A water balance developed for the Emerald Lake basin illustrates the absolute and relative magnitudes of the main water transfers in the catchment over two hydrologic years (1986 and 1987). For the combined water years, total precipitation (367 cm) - total losses to the atmosphere (80 cm) = total streamflow (283 cm) + error (4 cm). Snow dominated the water balance, accounting for 95 percent of the precipitation and subsequent streamflow. Snowpack accumulation was the principal hydrologic process from November through March, and snowmelt was the main activity from April through June. Evaporation from snow was the principal water loss to the atmosphere, accounting for about 80 percent of the total evaporation. Groundwater storage and release account for only a small portion of the total quantity of water in the annual water balance of this largely impermeable basin.
We have developed adequate rating curves (stage-discharge relationships) for the outflow and the two major inflows (1 and 2). These three channels were continuously monitored using automatic data-logging devices. Minor inflows were monitored with many manual observations.
The total annual volume of water flowing out of the Emerald Lake basin over the complete period of record (October 1983 to September 1987) ranged from 670,000 m3 to 2.6 million m3. The maximum volume during water year 1986 was more than three times the minimum Snow, Snowmelt, Rain, Runoff, and Chemistry UC Santa Barbara ABSTRACT page 4 volume during water year 1985. The total volume of Emerald Lake is about 160,000 m3. Equivalent depths of water averaged over the basin were 214 cm in water year 1986, 68 cm in water year 1987 and 58 cm through mid-June 1988. Annual streamflow even during the low year was more than twice the national average of 23 cm.
Hydrographs clearly show that the majority of runoff occurred during the months of snowmelt. More than three-quarters of the annual runoff occurred in the months of April through July. Under optimum combinations of conditions favoring high rates of snowmelt runoff, peak discharges approached 1 m3 s-l during 3 days in 1986. The minimum flow in water year 1986 was about 180 m3 day-l. The minimum for the entire period of record were below 20 m3 day-' and occurred in mid-February to mid-March of 1985 and September and October of 1987.
Climate and Energy Exchange at the Snow Surface A detailed evaluation of surface climate and energy exchange at the snow surface is presented for the 1986 and 1987 water years. Each form of energy transfer - radiation, sensible and latent heat flux, soil heat flux, and heat flux by mass advection is evaluated to determine its magnitude and importance in the seasonal energy and mass balance of the snowcover. During snowmelt, radiation accounts for between 75 and 90% of the energy available for melt. Sensible and latent heat transfer during this time are of approximately equal magnitude, but are usually of opposite sign, and therefore cancel. Calculated sublimation during the entire snow season accounted for the loss of about 20% (approximately 50 cm SWE) of the mass of the snowcover in 1986, and about 35% (approximately 23 cm SWE) of the mass of the snowcover in 1987.
Topographic Distribution of Solar Radiation Among the energy fluxes controlling snow metamorphism and snowmelt in mountainous drainage basins, solar radiation has the largest topographically caused variation. A two-stream atmospheric radiation model calculates solar radiation over alpine terrain in two broad wavelength bands - visible and near-infrared - and a spectral model for the albedo of snow is parameterized to the same wavelength bands to estimate net solar radiation. A least-squares fit to surface measurements finds the necessary atmospheric attenuation parameters, and the topographic variables are calculated from digital elevation data.
The spatial and temporal distribution of solar radiation is characterized by low spatial variance at low magnitudes in the winter, higher spatial variance in the early spring, and low variance at high magnitudes in the late spring and early summer.
Chemistry of Wet Deposition and Snowmeit Runoff The annual volume-weighted concentration of solutes in wet deposition at the Emerald Lake watershed, for water years 1985 through 1987, was equal to or less than 5 ueq L-l for each of the major ions. H- and NH4+ each account for about 18% of the total ionic content, followed closely by N03- (17%), SO42- (14%) and Cl- (11.5%). The remaining portion is divided among Ca2+, Na+, K+ and Mg2+. The organic anions CH3COO and HCOO-comprise 25% of the total anionic content of wet deposition. Dry deposition to the snowpack does not appear to be important during the winter season. Rainfall is acidic, with a H+ concentration about 6-fold greater than pure water in equilibrium with atmospheric Snow, Snowmelt, Rain, Runoff, and Chemistry UC Santa Barbara ABSTRACT page 5 carbon dioxide. Snowfall supplied 90% of the solute flux to the basin in 1985 and 1986.Rain supplied 66%. of the solute flux in 1987.
Interactions among the solutes retained and released from the snowpack, energy flux throughout the basin, and hydrologic pathways are all important to hydrochemistry during snowmelt runoff. Solutes in the initial fraction of snowmelt runoff are five- to ten-fold more concentrated than the bulk concentration of solutes in the snowpack, an ionic pulse. Spatial and temporal variations in the initiation and intensity of snowmelt prolong the time period of the ionic pulse in the basin. NO3- concentrations in streamwater during snowmelt are elevated 100% to 200% above winter concentrations of NO3-. The source of the elevated NO3- concentrations in streamwater is snowmelt runoff. SO42- concentration in streamwater during snowmelt runoff is attenuated with respect to SO42- concentrations in meltwater. Hydrogen ion concentration in streamwater during snowmelt runoff indicates strong interactions between runoff and biogeochemical processes: 80% of the H+ stored in the snowpack in 1986 was removed before reaching Emerald Lake; 90% was removed before reaching the lake in 1987. Snow, Snowmelt, Rain, Runoff and Chemistry UC Santa Barbara.
For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893
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