Chemical hydrograph separation during snowmelt for three headwater basins in Rocky Mountain National Park, Colorado

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1 Biogeochemistry of Seasonally Snow-Covered Catchments (Proceedings of a Boulder Symposium July 1995). IAHSPubl.no. 228, Chemical hydrograph separation during snowmelt for three headwater basins in Rocky Mountain National Park, Colorado JULIE K. SUEKER U.S. Geological Survey, Federal Center, Denver, Colorado 80225, USA & Environmental Engineering, University of Colorado, Campus Box 428, Boulder, Colorado 80309, USA Abstract Hydrographs from alpine and subalpine streams in Rocky Mountain National Park, Colorado, indicate rapid response to snowmelt input, suggesting that there may be little chance for meltwaters to interact with soil minerals before entering stream channels. The goal of this research was to test the hypothesis that during snowmelt in alpine and subalpine basins stream hydrographs are dominated by contributions from direct surface runoff. Discharge and sodium and chloride concentrations were measured for three headwater basins in the Park from mid-january through mid-october Using sodium as a tracer in a two-component mixing equation, average subsurface water contribution to streamflow during snowmelt was 47 % for Spruce Creek (1300 ha) and 48% for Fern Creek (800 ha) with a range of about 33% to 80% throughout the melt period. In contrast, contribution to streamflow from subsurface sources in Boulder Brook (1000 ha) was estimated to be 90% with a range of 79 to 97% throughout the melt period. Calculated évapotranspiration losses based on chloride concentrations were twice as high in Boulder Brook basin (50%) compared to Fern Creek and Spruce Creek basins (25%). The differences in flow sources and évapotranspiration losses for Boulder Brook compared to the other two watersheds are likely to be the result of geomorphic differences between the basins. Boulder Brook lies in a basin with broad exposed surfaces and with 64% of the basin covered by a deep layer ( > 3 m) of colluvium. Fern and Spruce creeks lie in deeply incised valleys with surficial debris coverage of 20 and 38%, respectively. INTRODUCTION Waters from Rocky Mountain National Park (RMNP), Colorado, are very dilute. Alkalinity concentrations are less than 200 pteq l" 1, and specific conductance values are less than 50 /xs cm" 1 (Baron, 1983). Atmospheric deposition rates of nitrogen in snow in the mountains of northern Colorado are large relative to other parts of the state (Turk etal., 1992). High-elevation basins in northern Colorado may be quite sensitive to acid deposition. To begin to quantify the potential impact of atmospherically derived pollutants in remote basins, we must first understand the flowpaths of precipitation waters through these basins. Due to differences in residence times and materials

2 272 Julie K. Sueker encountered for diverse flowpaths, the chemistry of waters from different sources can vary greatly. Chemical hydrograph separation has been used to trace the routing of water through basins by determining relative contribution to discharge of various source waters during storm events (Fritz et al., 1976; Hooper & Shoemaker, 1986; Hinton et al., 1994). A common finding of these studies is that new water (rain water from the storm) usually contributes less than half of the peak runoff for storms. Caine (1989), Findley and Drever (1992), and Mast et al. (1995) found that new water (snowmelt water) often contributes more than half of the stream flow during snowmelt in alpine basins of Wyoming and northern Colorado, especially after seasonal peak flow has been reached. In this study, chemical hydrograph separation was used to estimate contributions from surface and subsurface sources from early snowmelt through mid-autumn 1994 for three headwater basins in RMNP. Annual hydrographs for basins in the Park are dominated by the melting of snowpacks that accumulate throughout the winter. While stream discharge may increase by an order of magnitude or more during snowmelt, concentrations of dissolved weathering products may be diluted by only a factor of two or three. This suggests that enough snowmelt is routed through subsurface flowpaths to react with soil minerals or to displace more concentrated resident soil waters to mitigate dilution effects of direct snowmelt runoff to surface channels. Streamflow can be separated into surface and subsurface components using concentrations of a chemical tracer and measured discharge in a two-component mixing equation. The surface water component is defined as snowmelt water that is transmitted through the snowpack as overland flow, reaching the stream channel without reacting with soil minerals. The subsurface water component is defined as groundwater and soil water resident in the soils prior to the snowmelt event. Contributions from these two sources can be estimated from with the constraint xisurf ~ ^J-streanv^ stream ~ ^sub''^surf ~ ^sub' ^ stream xisurf "*" xisub where Q is volume flow rate, C is the tracer concentration, and the subscripts stream, surf, and sub represent stream, surface and subsurface, respectively. Estimates of flow source contributions are most accurate when the following four conditions are met: (1) C stream is conservative and does not change in the stream channel, (2) Cj^and C sub are different and can be measured with high precision relative to this difference, (3) the mixing of water from different sources is complete within the channel, and (4) C surf and C sub are constant over time (Caine, 1989). SITE DESCRIPTIONS The three headwater basins in this study are located in RMNP on the east side of the Continental Divide (Fig. 1). They are underlain by granite and biotite schist in varying

3 Chemical hydrograph separation for Rocky Mountain National Park basins 273 Fig. 1 Map of four watersheds in Rocky Mountain National Park showing locations of stream-water gaging stations, weather station, groundwater springs and areas covered by surficial debris. proportions (Braddock & Cole, 1990). Boulder Brook basin is north facing, has an area of about 1000 ha, and ranges in elevation from 2730 to 4350 m. The bedrock unit is composed of about 95% granite and 5% gneiss. Deep colluvium, defined as deposits of rock debris several meters thick or more ranging in size from silt to boulders, covers about 64% of the basin. Fern Creek basin is deeply incised, north-northeast facing, has an area of about 800 ha, and ranges in elevation from 2560 to 3770 m. The bedrock is composed of about 89 % gneiss and 11 % granite. Talus and colluvium cover about 20 % of the basin. Spruce Creek basin, also deeply incised, faces northeast, has an area of about 1300 ha, and ranges in elevation from 2570 to 3940 m. The bedrock consists of

4 274 Julie K. Sueker about 53% granite and 47 % gneiss. About 3 8 % of the basin is covered by talus and colluvium. The talus and colluvium in Spruce Creek and Fern Creek basins is mostly a thin veneer, and deep colluvium is only a small portion of the total surficial debris cover. METHODS Stream gauges are located at the outlet of each of the three basins (Fig. 1). Stage readings were automatically recorded every 15 min during open channel flow (yearround for Boulder Brook, mid-spring to mid-autumn for Fern and Spruce Creeks). Discharge was measured approximately weekly from early May through early September by current meter and rhodamine WT dye dilution for Boulder Brook, and by dye dilution for Fern and Spruce creeks. Stream-water samples were collected in 250-ml or 1000-ml plastic bottles that were triple-rinsed and soaked in deionized (DI) water for at least 24 h prior to sampling. The sampling interval was approximately monthly from mid-january to mid-april and approximately weekly from mid-april to early October. Samples were filtered through a 0.45 /xm polycarbonate membrane filter pre-rinsed with DI water then with sample water. An aliquot of filtrate was acidified below ph 2 with ultra-pure nitric acid for sodium analysis. Sodium concentrations were determined by inductively coupled plasma-atomic emissions spectrophotometry at the U.S. Geological Survey laboratory in Boulder, Colorado. The standard error for duplicate analyses for sodium was less than 5 %. Chloride concentrations were determined on unacidified filtered samples by ion chromatography at the University of Colorado's Environmental Engineering laboratory in Boulder, Colorado. The standard error for duplicate analyses for chloride was less than 7%. Bulk snowpack samples were collected in Boulder Brook and Fern Creek basins at time of peak snowpack accumulation (mid-april) using an aluminum Federal snow core sampler (Fig. 1). Groundwater samples were collected from springs at their point of discharge to land surface three times in Boulder Brook and twice in Fern Creek basin during the summer and fall (Table 1, Fig. 1). RESULTS Daily average discharge per hectare in Boulder Brook was much less than discharge in Spruce Creek and Fern Creek (Fig. 2). Boulder Brook had a calculated total discharge of 2 x 10 6 m 3 for water year 1994 (October 1993 through September 1994). Spruce Creek and Fern Creek water year 1994 discharges were calculated to be 6 X 10 6 m 3 and 5 X 10 6 m 3, respectively. Total annual runoff of 20 cm for Boulder Brook, 50 cm for Spruce Creek, and 62 cm for Fern Creek was calculated by dividing the total annual discharge by the watershed area. There may be significant discharge from Boulder Brook basin in the form of tributary groundwater, but this has not been quantified. Sodium concentrations of stream-water samples were used as C stream in the hydrograph separation. On average, concentrations of sodium were 2 to 3 times higher in Boulder Brook than in Fern and Spruce creeks (Fig. 2). Mean concentrations of sodium in Fern Creek and Spruce Creek were not significantly different from each other

5 Chemical hydrograph separation for Rocky Mountain National Park basins 275 Table 1 Mean sodium concentrations of bulk snow samples, groundwater samples and baseflow stream samples. Bulk snow samples Boulder Brook Fern Creek Spruce Creek mdwater samples Boulder Brook Boulder Brook Boulder Brook Boulder Brook average Fern Creek Fern Creek Fern Creek average Number of Na + Standard samples (fteq l" 1 ) deviation 8 6 * flow samples Boulder Brook Fern Creek Spruce Creek * Snow samples from Boulder Brook and Fern Creek combined at a probability level of 95 % (p < 0.05) based on a two-sample Mest (Helsel & Hirsch, 1991), but were significantly different from Boulder Brook (p > 0.05). Chloride concentrations were significantly higher in Boulder Brook based on a two-sample f-test (p < 0.05), with a volume-weighted mean annual concentration of 4.9 pteql" 1 compared to Spruce Creek and Fern Creek, both with a volume-weighted mean annual concentration of 3.0 /xeq l" 1 (Fig. 2). Concentrations of sodium and chloride in Spruce Creek and Fern Creek followed similar seasonal trends with peak concentrations occurring at onset of snowmelt in mid-april and lowest concentrations during midsummer. While sodium and chloride concentrations decreased by a factor of two from early May to mid-july, discharge varied about an order of magnitude during the same period (Fig. 2). Sodium concentrations in Boulder Brook decreased 30% between early May and mid-july, while discharge varied by a factor of 3.5 (Fig. 2). Concentrations of chloride in Boulder Brook followed a similar seasonal pattern to Spruce and Fern creeks but demonstrated much greater variability. Peak chloride concentration in Boulder Brook occurred in May. Mean concentrations of snowpack sodium values were used for the surface water component, C surf, in the hydrograph separation (Table 1). Values of C surf v/ere assumed to remain constant throughout the period of the hydrograph separation. This assumption may be violated because of preferential elution of ions from the snowpack; however, snowpack sodium concentrations are low compared to stream water and subsurface water sodium concentrations, and elution effects may be averaged out over the snowmelt period due to sequential melting of the snowpacks at progressively higher elevations. Snow samples were not collected from the Spruce Creek basin due to the difficulty of access when snowpacks are present. The mean of all Boulder Brook and Fern Creek snow samples was used as C surf for Spruce Creek. These values agree reasonably well

6 276 Julie K. Sueker o n Jan Feb Mar Apr May Jun Jul Aug Sep Oct 12 9 Jan Feb Mar Apr May Jun Jul Aug Sep Oct m- - - Boulder Brook - - Fern Creek Spruce Creek Jan Feb Mar Apr May Jun Jul Aug Sep Oct Boulder Brook Fern Creek Spruce Creek Fig. 2 Stream-water discharge and concentrations of dissolved sodium and chloride for with volume-weighted mean concentrations of sodium in bulk precipitation of wintertime samples from a nearby National Acid Deposition Program precipitation station in Loch Vale Watershed (Baron et al., 1992) (Fig. 1). Baseflow in the streams prior to the onset of snowmelt was assumed to be supplied only from subsurface sources. The mean of sodium values of stream water samples collected between 13 January and 20 April (baseflow samples) were used as the

7 Chemical hydrograph separation for Rocky Mountain National Park basins 277 concentration of the subsurface component C mb in the hydrograph separation (Table 1). The means of baseflow sodium concentrations for Boulder Brook and Fern Creek are not significantly different from the means of groundwater samples collected during the summer from the respective basins (p < 0.05). Because the values of sodium in Boulder Brook May Spruce Creek 1.2 T 1 - Fern Creek May Jun Jul Total Flow Aug Sep - - Subsurface Flow Fig. 3 Hydrograph separation results for 1994 based on weekly sodium and discharge data. Oct

8 278 Julie K. Sueker baseflow stream samples and summer groundwater samples are not significantly different, C sub was assumed to remain constant during the period of study. Hydrograph separations for Spruce Creek and Fern Creek yielded similar results (Figs 3,4). Contributions from subsurface sources varied between 32 and 77% with a mean of 47% and a standard deviation of 12% for Spruce Creek, and between 34 and 80% for with a mean of 48% and standard deviation of 12% for Fern Creek. These two basins show a strong seasonal relationship in the contribution to streamflow from surface and subsurface sources. Subsurface sources were estimated to provide over half the streamflow on the rising limb of the hydrograph from snowmelt onset until early June. Surface runoff was estimated to dominate streamflow on the falling limb of the hydrograph until melting of the remaining snowpacks diminished in late summer. Results for Boulder Brook were quite different; contributions from subsurface sources were between 79 and 97 %, with a mean of 90 % and a standard deviation of 4 %. Little seasonal variation in streamflow sources was evident in this basin. There are expected to be no internal sources of chloride in these three basins (Braddock & Cole, 1990), therefore, chloride is derived solely from atmospheric ?, 0.6 0A S I S 0) I May Jun Jul Aug Sep Oct s- - - Boulder Brook - - * Fern Creek Spruce Creek Boulder Brook Fern Creek Spruce Creek Fig. 4 Fraction of total streamflow from subsurface sources based on sodium as the tracer in the 1994 hydrograph separation. Daily average discharge is included for comparison. Sep

9 Chemical hydrograph separation for Rocky Mountain National Park basins 279 deposition. Any changes in chloride concentrations are caused by évapotranspiration and sublimation. Using discharge data, an annual volume-weighted mean precipitation chloride concentration of 2.4 /xeq l" 1 (Baron et al., 1992), and volume-weighted mean annual stream-water chloride concentrations of 4.9 /xeq l" 1 for Boulder Brook and 3.0 fxeq l" 1 for Spruce and Fern creeks, évapotranspiration and sublimation losses were calculated to be about 25% in Spruce Creek and Fern Creek basin, and about 50% in Boulder Brook. Evapotranspiration and sublimation losses of about 40 % were estimated for nearby Loch Vale Watershed (Baron & Denning, 1992). DISCUSSION It is expected that the four conditions for a successful hydrograph separation are well met for this study. It is likely that sodium is conservative in the stream channel. The water in RMNP is very dilute and provides little opportunity for instream reactions. The sodium concentrations for C^and C mb are indeed different from each other and can be measured with high precision relative to this difference (Table 1). There are no significant inflows within 500 m of any of the gages and mixing of different source waters in the stream channel is assumed to be complete. Condition four may be less well met than the other three conditions. Values of C sm j are low compared to the other components in the mixing equation and has little effect on the outcome of a hydrograph separation. Doubling C imr^changed the proportions of flow sources by less than 5 %. The least constrained member is C sub. As stated above, baseflow stream and summer groundwater sodium concentrations were not significantly different from each other, and groundwater concentrations were assumed to remain constant. Boulder Brook maintains a steady open channel flow of 30 to 60 1 s" 1 during winter months (December through March), whereas Spruce Creek and Fern Creek winter flows are reduced to less than lis" 1 under ice and snow. This indicates that there is a slow water release mechanism operating in Boulder Brook basin that is not present in the other two watersheds. The Boulder Brook hydrograph shows a damped response to snowmelt input and almost no response to rain storms compared to hydrographs for Spruce and Fern Creeks (Fig. 2). Instantaneous discharge in Boulder Brook for water year 1994 varied from 30 to s" 1 or about one order of magnitude over the year. Instantaneous discharge over the same time period varied from less than 1 1 s" 1 at low flow to 1000 and s" 1 for Fern Creek and Spruce Creek, respectively, for about a three orders of magnitude change. A rainstorm lasting several days in early September provided 3 to 4 cm of moisture to the region producing noticeable increases in streamflow for Spruce Creek and Fern Creek, but no significant increase in the hydrograph was seen for Boulder Brook. Total calculated runoff for Boulder Brook (20 cm) is less than half the total runoff calculated for Spruce Creek and Fern Creek (50 and 62 cm, respectively). The fraction of total precipitation lost to évapotranspiration was estimated to be twice as high in Boulder Brook (50%) compared to Spruce and Fern creeks (25% for both). Using total runoff and évapotranspiration losses, calculated total precipitation is 40 cm for Boulder Brook basin, 67 cm for Spruce Creek basin and 83 cm for Fern Creek basin. Precipitation input to Boulder Brook is lower than precipitation inputs to Fern and Spruce Creeks, and likely reflects differences in snowpack accumulation and évapotranspiration losses among these basins.

10 280 Julie K. Sueker Hydrographie responses to snowmelt in these three headwater basins may be explained by geomorphic differences among the basins. Basin topography and surficial debris cover likely control precipitation input and streamflow. Colluvium several meters deep fills much of Boulder Brook basin (Fig. 1). Pore space in this deep surficial debris may provide a reservoir for the mixing of infiltrating snowmelt water with resident subsurface waters and for the subsequent slow release of this mixed water to surface channels. In Fern Creek and Spruce Creek basins there is much less surficial debris than in Boulder Brook, and much of the debris is present in discontinuous units laying on or at the base of steep slopes (Fig. 1). Meltwaters may pass rapidly through these debris layers to bedrock surfaces and quickly flow to surface stream channels providing less opportunity for water storage and interaction with soil materials. Topography of the basins likely controls total outflow from the basins by affecting snowpack accumulation and évapotranspiration losses. About half of the Boulder Brook basin is above treeline and consists of broad, exposed areas. Boulder Brook basin is situated on the north face of Long's Peak, the highest elevation in RMNP (4350 m), and at times is subject to extremely strong winds. Wind gusts reaching speeds of 30 m s" 1 or more are common in this region during winter months (Baron & Mast, 1992). Much of the snow may be blown out of or sublimed from Boulder Brook basin. Visual inspections of the basin from lower elevations show that much of the rock surfaces are exposed during the winter. High winds across and direct solar radiation on the broad exposed surfaces of Boulder Brook basin may also provide ample opportunity for increased évapotranspiration losses during snowmelt. Fern Creek and Spruce Creek, on the other hand, both flow through deeply incised valleys with surrounding cliffs hundreds of meters in height. Incised basins such as these tend to be deposition zones for snow blowing from the west side of the Continental Divide (Baron et al., 1992). These incised valleys are more protected from high winds and direct solar radiation reducing évapotranspiration losses compared to Boulder Brook. CONCLUSIONS Hydrograph separations calculated using sodium as a tracer in a two-component mixing equation estimated subsurface water contribution to vary between 32 and 77% for Spruce Creek and 34 to 80% for Fern Creek during the 5-month period from mid-may through mid-october Subsurface flow sources accounted for the majority of streamflow on the rising limb of the hydrograph and direct surface runoff provided the maj ority of runoff on the falling limb of the hydrograph until late summer for these two basins. In contrast, contributions of water from subsurface sources dominated streamflow (mean of 90 %) in Boulder Brook during the same 5-month period. Streamflow responses to snowmelt input are likely controlled by geomorphic features in these three basins. Deep colluvium in Boulder Brook basin apparently provides a reservoir for the storage and slow release of snowmelt waters while the steep walls of the deeply incised basins of Fern and Spruce creeks allow rapid flow of snowmelt waters to surface channels. Evapotranspiration losses were 50% in Boulder Brook and 25% in Spruce Creek and Fern Creek. Total estimated precipitation input to Boulder Brook basin (40 cm) was less than input to Spruce Creek and Fern Creek basins (67 and 83 cm, respectively). Broad exposed areas in Boulder Brook basin likely causes reduced snowpack accumulation and increased évapotranspiration compared to Spruce and Fern creeks.

11 Chemical hydrograph separation for Rocky Mountain National Park basins 281 Acknowledgments Funding for this project was provided by Dr. Robert Jarrett with the U.S. Geological Survey in conjunction with work conducted at the Rocky Mountain Hydrologie Research Center, and by global climate change funds provided by Dr. Jill Baron through the National Park Service and the National Biological Service. Many thanks to Stuart Deitrick, Matt Johns, Dave Brethauer, and Al Zeidens for their untiring assistance in the field, to Randy Bowers for his help in the laboratory, and to Dr. Joseph Ryan, Dr. David Clow, and Claudette Trusty for their insightful discussions. REFERENCES Baron, J. (1983) Comparative water chemistry of four lakes in Rocky Mountain National Park. Wat. Resour.Bull. 19, Baron, J. & Denning, A. S. (1992) Hydrologie budget estimates. In: Biogeochemistry of a Subalpine Ecosystem (ed. by J. Baron). Springer-Verlag, New York. Baron, J., Denning, A. S. & McLaughlin, P. (1992) Deposition. In: Biogeochemistry of a Subalpine Ecosystem (ed. by J. Baron). Springer-Verlag, New York. Baron, J. &Mast, M. A. (1992) Regional characterization and setting for Loch Vale Watershed study. In: Biogeochemistry of a Subalpine Ecosystem (ed. by J. Baron). Springer-Verlag, New York. Braddock.W. A. & Cole, J. C. (1990) Geologic map of Rocky Mountain National Park and vicinity, Colorado. Department of the Interior, U.S. Geological Survey, Map Caine, N. (1989) Hydrograph separation in a small alpine basin based on inorganic solute concentrations./. Hydrol. 112, Finley, J. B. & Drever, J. I. (1992) Chemical hydrograph separation usingfieldand experimental data with implications for solute cycling in an alpine catchment. Proceedings of the 7th InternationalSymposium on Water-Rock Interactions (ed. by Kharaka & Maest) (Park City, Utah, July 13-18), vol. 1. Fritz, P., Cherry, J. A., Weyer, K. U. & Sklash,M. (1976) Storm runoff analyses using environmental isotopes and major ions. In: Interpretations of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology : Proceedings of an Advisory Group Meeting, , International Atomic Energy Agency, Vienna. Helsel, D. R. & Hirsch, R. M. (1991) Statistical Methods in Water Resources. Studies in Environmental Science 49, Elsevier, New York. Hinton, M. J., Schiff, S. L. & English, M. C. (1994) Examining the contributions of glacial till water to storm runoff using two- and three-component hydrograph separations. Wat. Resour. Res. 30, Hooper, R. P. & Shoemaker, C. A. (1986) A comparison of chemical and isotopic hydrograph separation. Wat. Resour. Res. 22, Mast, M. A., Kendall, C, Campbell, D. H., Clow, D. W. & Back, J. (1995) Determination of hydrologie pathways in an alpine-subalpine basin using isotopic and chemical tracers. In: Biogeochemistry of Seasonally Snow-Covered Catchments (ei. by K. Tonnessen, M. W. Williams & M. Tranter) (Proc. Boulder Symp., July 1995). IAHS Publ. no Turk, J. T., Campbell, D. H., IngersolI.G. P. &Clow, D. W. (1992) Initialfindingsof synoptic snowpack sampling in the Colorado Rocky Mountains. U.S. Geol. Surv. Open-File Report