gauge caught 17% more than the snowboards. Except for N03 at the forest site, chemical analyses of samples from the tube and the snowboard showed that H, ...
Atmospheric Deposition (Proceedings of the Baltimore Symposium, May 1989). IAHS Publ. No. 179.
M O N I T O R I N G A T M O S P H E R I C D E P O S I T I O N IN CALIFORNIA'S SIERRA NEVADA: A COMPARISON OF METHODS Bruce J. McGurk Hydrologist, Pacific Southwest Forest and Range Experiment Station, Forest Service, P.O. Box 2Jt5, Berkeley, California 94-701, USA
USDA
Neil H . B e r g Hydrologist, Pacific Southwest Forest and Range Experiment Station, Forest Service, P.O. Box 2Jt5, Berkeley, California 94-701, USA
USDA
Danny Marks Assistant Research Hydrologist, Center for Remote Sensing and Environmental Optics, University of California, Santa Barbara, California 93106, USA John M. Melack Professor, Department of Biological Sciences, University Barbara, California 93106, USA Frank Setaro Marine Science Institute, 93106, USA
University
of California,
of California,
Santa Barbara,
Santa
California
ABSTRACT Four methods for measuring atmospheric deposition in the Sierra Nevada of California were compared during the winter of 1986-1987. A large (28 by 122-cm) polyvinyl chloride tube was compared to a Belfort precipitation gauge and to snowboards at both an exposed site, near Mammoth Lakes, and a site in a forest clearing, near Soda Springs. An Aerochem Metrics collector was also included at the forest site. At the exposed site, the tube and the Belfort gauge caught 23% less snow water equivalent than the snowboards. In the clearing, the tube and the Belfort gauge caught 17% more than the snowboards. Except for N 0 3 at the forest site, chemical analyses of samples from the tube and the snowboard showed t h a t H, N 0 3 , and S 0 4 concentrations differed significantly (p < 0.05). Laboratory tests showed no adsorption or desorption of synthetic s t a n d a r d solutions of major ions from the tube. For sheltered sites with occasional midwinter rain, the tube is recommended. For windy sites without midwinter rain, sampling from weekly snowboards provides a better estimate of chemical and snow volume loading. INTRODUCTION Stations t h a t monitor atmospheric deposition must obtain both elemental concentrations and precipitation volume to estimate total chemical loading. Snowfall in the Sierra Nevada of California has relatively low concentrations of chemical constituents compared to rain or to precipitation elsewhere in the USA (Feth et al. 1964, Melack et al. 1982, Woo & Berg 1986). However, because a meter or more of water falls as snow compared to a few centimeters of rain per year in the Sierra, accurate measurement of both volume and chemical concentration of the snow is crucial. Between 71
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Bruce McGurk et al.
1984 and 1987, in the central Sierra Nevada, the mean annual chemical loading from snow was 3.8 times t h a t of rain (Marks et al. 1988). Snow accumulation is difficult to measure accurately because of the influence of wind on low-density snow crystals. Snowfall rates and volumes are the least accurate component of hydrological modeling (Peck 1972), and these difficulties are compounded in mountainous environments where winds are high and terrain is rugged. It is difficult to maintain a gauge at a fixed height above the snow surface and to design a collector t h a t is sensitive to but not overwhelmed by single storms t h a t can deposit more than 75 cm of snow. Few studies have evaluated the undermeasurement by Belfort-type gauges by comparing their catches to measurements of snow on the ground or on snowboards (Goodison 1978, McGurk 1986). Extensive efforts have been made to design and test wind screens or shields to prevent entrainment of snow crystals and the resultant undercatch (Larson & Peck 1974, Goodison et al. 1981). Gauges equipped with shields catch more precipitation than unshielded gauges, but catch efficiency decreases as wind speed increases to 10 m s . A serious undercatch is suspected above 10 m s~ . Because stands of trees reduce wind speeds, forest clearings are preferred for precipitation gauges. Many mountainous areas, however, have little forest cover and have high wind speeds; therefore collection networks may require a range of techniques to obtain accurate information at alpine sites. Also, some areas of the Sierra Nevada receive midwinter rain, so the characteristics of the site may affect the choice between snowboards or tubes. STUDY DESIGN AND METHODS Our study was designed to evaluate a polyvinyl chloride (PVC) tubular collector (Dawson 1986) and to determine if it would collect accurate samples of volume and yield water suitable for chemical analysis. Snow volumes were also measured by high-capacity Belfort weighing-precipitation gauges and five 0.36-m 2 snowboards. The tubes, Belforts, and two boards were measured once per week. A set of three boards was measured once a day if precipitation occurred. Made of Schedule 80 (0.5-cm walls) irrigation pipe, the PVC tubes were 122 cm long and 28 cm in diameter. A cap was glued on one end, and the other end was beveled. The tubes were acid-washed and repeatedly rinsed (AWRR) with deionized water before installation. The tubes and Belfort gauges were equipped with Alter wind screens placed at orifice height. Rinsed tubes were replaced weekly, and tubes with precipitation were allowed to melt indoors at 15 ° C. Meltwater volume and pH were measured at the field site laboratories. The remainder of the sample was refrozen in AWRR linear polyethylene bottles for transport to the Santa Barbara laboratory. Liquid volume was converted to weekly areal depth for comparison with the other measurement techniques. Both the weekly and the 24-h event snowboards were measured at about 0800 h on the weekly change date or after any 24-h interval with precipitation. Depth was measured at all four corners of the board and a 10-cm diameter PVC tube was used to cut cores t h a t were weighed on a top-loading balance. Replicate snow samples were collected with an AWRR 4-cm polyethylene tube. The depth-integrated sample was placed in AWRR 2-liter polyethylene bottles. At the sheltered site, weekly samples were collected from an Aerochem Metrics collector. Field chemistry and volume measurements were made as described for the 24-h boards. This sampler is also used in both the California Acid Deposition Monitoring Program (CADMP) and the National Acid Deposition Program (NADP).
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At the Santa Barbara laboratory, the major cations (calcium, magnesium, sodium, and potassium) were analyzed with a Varian-AA6 atomic absorption spectrophotometer. An air-acetylene flame was used; addition of l a n t h a n u m chloride suppressed chemical and ionization interferences during calcium and magnesium determinations. The organic anions (acetate and formate) and inorganic anions (chloride, nitrate, and sulfate) were measured with a Dionex Model 20101 ion chromatograph employing chemical ion suppression and conductivity detection. Measurement of pH was made with a Ross 8104 combination pH electrode on a Fisher Acumet 805 MP pH meter. Before each trial, the electrode was calibrated with pH 7.00 and pH 4.00 buffers. After thorough rinsing with deionized water, a calibration with a freshly prepared 10~ 4 M HC1 solution was performed. S T U D Y SITES Two sites, the Central Sierra Snow Laboratory (CSSL) and Mammoth Mountain, were selected as representative of portions of the Sierra Nevada where snow is important (Figure 1). CSSL is 1 km east of Soda Springs, California (39 ° 19 26" N, 120 ° 22 W), and is in the mixed conifer-fir zone. The Belfort gauges and the tubes were on two 8-m towers in a 40 x 50 m clearing at 2100 m a.m.s.l. An Aerochem Metrics sampler was on a 7-m tower. Mean annual precipitation is 139 cm (California Cooperative Snow Survey 1987), of which 120 cm is snow. The peak depth is about 3 m of snow t h a t is isothermal near 0 ° C and has a dilute chemical load (Berg 1986). Surface wind speeds are low, and midwinter melt and rain occur. Forest cover may be locally dense but with numerous openings. Much of the central Sierra Nevada has a similar forest cover. The open study site was located at Mammoth Mountain Ski Area (37 ° 28' 16" N, 119° 01 38" W) at 2940 m a.m.s.l, about 3 km west of Mammoth Lakes. Mean annual precipitation is 142 cm (California Cooperative Snow Survey 1987), and a 3 to 4-m accumulation of snow is common. All instruments were mounted on a 6-m platform. This site is similar to much of the alpine zone of the central and southern Sierra Nevada and has high winds and no midwinter rain. QUALITY CONTROL A N D QUALITY ASSURANCE The study design included a program to ensure accuracy and comparability between sites for both volume and chemistry measurements. Identical instruments were used, and adherence to standardized d a t a collection procedures and field analysis protocols was emphasized with the field staff at both sites. Replication of volume measurements and sample collections allowed estimation of procedural variability and of confidence intervals around mean values. Precipitation volume All Belfort gauges were calibrated across their full range both at the s t a r t and t h e end of the season. Snowboards, snow density cutters and tubes, and other equipment for both sites were fabricated and calibrated by the manufacturer or our technicians. Precipitation chemistry All containers and washing procedures were assessed for adsorption and desorption of major ions and for post-washing chemical contamination, respectively. No adsorption or desorption was detected after addition of
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Bruce McGurk et al.
F i g u r e 1 Location map of snow study sites, Central Laboratory and Mammoth Mountain, California.
Sierra
Snow
known synthetic standards. Contaminants were below detection limits, in 9 1 % of 98 ion samples from PVC cylinders, polyethylene bags, and polyethylene bottles after AWRR. Performance at the field sites was assessed by means of field blanks and field audit samples. The blanks were final rinses from the tubes and other samplers which were placed in AWRR bottles and sent to Santa Barbara along with a sample of the deionized water used t h a t day. Audits were for pH or conductivity and were sent to the field sites from Santa Barbara. At Santa Barbara, freshly prepared calibration s t a n d a r d s t h a t bracketed the samples' concentrations and reagent blanks were used in every assay. Charge balance controls were included in each analytical run to determine if a persistent deviation in anions and cations existed. Accuracy was assessed by comparison with certified controls, and precision was estimated by analyzing 5% of the samples in a run in duplicate. PRECIPITATION VOLUME RESULTS Although only 60% of the mean precipitation was deposited in water year 1987, precipitation was recorded during 15 weeks of the 16-week monitoring period. During only 5 weeks at Mammoth and 6 weeks at CSSL did precipitation SWE exceed 4 cm (Figure 2). Also, during 5 weeks at Mammoth and 6 weeks at CSSL, minor amounts of precipitation were detected in the tubes but not on the snowboards. At Mammoth, the towermounted collectors caught significantly less snow water equivalent (SWE) than did the snowboards during both large storms and for the seasonal total (Table 1). The 5-month mean wind speed was 3.3 m s - 1 at Mammoth versus 1.3 m s - 1 at CSSL; this difference may explain the approximate 2 3 % seasonal undercatch by the Belfort and the tube at Mammoth. This undermeasurement is approximately what Larson 8i Peck (1974) predicted
Monitoring Atmospheric Deposition
12/23
1/06
1/27
2/03
2/17
2/24
3/10
3/17
3/24
4/08
75
4/24
Week Ending (Date) in Water Year 1987
12/23 1/06
1/20
1/27
2/03
2/10
2/17
2/24
3/10
3/17
3/24
3/31
Week Ending (Date) in Water Year 1987
Figure 2 Mean weekly precipitation (> 0.5 cm water equivalent) measured by several methods during the 1987 winter at the Central Sierra Snow Laboratory (A) and Mammoth Mountain (B), California. for a shielded gauge collecting snow. At CSSL, the tower-mounted collectors caught significantly more SWE than the snowboards (p < 0.01). The 10.5-cm difference is about halved once the 4.3 cm of rain that occurred during two storms is added to the
Bruce McGurk et al.
76
T a b l e 1 Mean precipitation depths between 17 December 1986 and 8 April 1987 at two sites in the Sierra Nevada, California Site
Central Sierra Snow Laboratory Mammoth Mountain
Belfort gauge
PVC tube
24-h board
Weekly board
70.8
73.4
62. T1
61.12/
42.9
48.6
57.5
60.9
Aerochem Metrics 50.8
-
1 / Aerochem Metrics sampler a t CSSL was not replicated. 2 / Underestimated due to rain.
board depths. Rain during the weeks of 17 February, 10 March, and both April weeks contributed to the comparatively low weekly and 24-h board SWE depths (Figure 2). Analysis of variance on the weekly results from the Belfort and tube yielded no significant differences at either site. At CSSL, the 24-h and weekly boards were also not significantly different. The other combinations of Belfort, tube, and boards had significantly different weekly volumes (p < 0.01). Analysis of the replicates showed t h a t the 95% confidence intervals (CI) around the mean weekly differences for the Belforts, tubes, and weekly and 24-h event boards averaged +0.4 cm and ranged from +0.2 cm to +0.6 — cm. ~~ The Belfort gauges and the tubes caught 42% more precipitation at CSSL than did the Aerochem Metrics sampler (Table 1). The wind screen cannot be fitted to this sampler, and the screen's absence may account for p a r t of this difference. An Aerochem Metrics was used for several years at the windy Mammoth site with little success (Dawson 1986). At CSSL, considerable maintenance is required to free the collector's movable arm when it freezes in place and to empty the shallow (40 cm) buckets during large snow storms. PRECIPITATION CHEMISTRY RESULTS For the PVC tube and weekly boards, analysis of variance for H, S 0 4 , and NO3 identified significant differences (p < 0.05) in concentration between the three constituents except for N 0 3 at CSSL. At both sites, mean concentrations of the replicates for all three constituents were generally greater in tube samples than in weekly board samples. Except for Ft, 9 5 % CI around the difference between the replicates for each constituent at CSSL were much greater (2- to 30-fold) for the tube samples t h a n for the weekly board samples. At CSSL, CI for the weekly boards varied from +0.3 //eq L _ 1 for S 0 4 to +2.4 //eq L - 1 for CI and from +1.1 //eq L - 1 for FT to + 13.7 //eq L" 1 for C l i n the tube. At Mammoth, the"95% CI for the boards and tubes were more similar. For the weekly boards, CI ranged from +0.4 //eq L~ ! for K to +5.5 //eq L _ 1 for Ca and from +0.5 //.eq L~ ! for H to +~2.8 //eq L " 1 for CI in fïïe tube. Variability was greatest for CI, Ca, and Na.~"" Both the concentrations of H, S 0 4 , and N 0 3 .and the snow volumes were generally greater from the tube samples at CSSL than from the weekly boards samples (Figures 2A and 3A). The boards caught snow and the tube
Monitoring Atmospheric Deposition
Tube (neq/L)
Figure 3 Selected chemical concentrations at the Central Sierra Sno Laboratory (A) and Mammoth Mountain (B) as measured by PVC tuband weekly snowboards during 1986-87.
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Bruce McGurk et al.
did not during 2 weeks, and the reverse situation occurred during 6 weeks. The cumulative loading in the tubes during these 6 weeks was appreciable due to relatively high chemical concentrations. Therefore, the seasonal loading of 28.0 meq m - 2 at CSSL, estimated from the tube concentrations and SWES, greatly exceeded the loading of 15.1 meq m - 2 estimated from the weekly board concentrations and SWES. Samples from the tubes had generally higher concentrations than samples from the weekly board at Mammoth (Figure 3B). However, because 25% less snow was caught in the tubes at Mammoth than on the boards (Table 1, Figure 2B), the tube's seasonal loading of 13.0 meq m~~2 was only slightly higher than the 12.1 meq m^ 2 estimated from the boards. Although the tubes captured snow during 5 weeks when the boards did not, the volume was small and, unlike CSSL, the concentration was similar to t h a t found on the boards. At CSSL, the loading estimate from the Aerochem Metrics sampler was 17.5 meq m~~--16% more than the board's loading and 37% less t h a n the tube's loading. Because the Aerochem also caught 28% less SWE than the tube, the volume-corrected loading would be close to the tube value. Because the sampler excludes dry deposition from the precipitation bucket, the larger loadings t h a t result from the use of plastic collectors vs^ boards are probably not related to dry deposition. This study could not determine whether rain, surface melt, or sublimation reduced the board loading. CONCLUSIONS Although the chemical concentration in snow is low compared to t h a t in rain in the Sierra Nevada, the seasonal loading from snow compared to t h a t from rain mandates the monitoring of snow in the Sierra Nevada. The SWE from the shielded Belforts and PVC tubes was the same at both sites, and the weekly depths and the sum of the daily board depths were the same at CSSL. Significant differences were found among the other combinations of the Belfort, tube, and board SWEs. At moderate elevations in the Sierra Nevada, where forest cover exists and rain occurs, we recommend the shielded PVC tube for weekly monitoring of SWE and chemical concentration. In the higher areas of the Sierra Nevada where rain does not occur and forest cover is less common, we recommend sampling by weekly snowboards. They have the added advantage over tubes of not needing a tower, a windscreen, and weekly rinsing with deionized water. A disadvantage of the boards is the laborintensive, detailed procedures t h a t must be followed to obtain accurate depth and density measurements and uncontaminated samples for chemical analysis. The tube did not adsorb or desorb ions during tests with synthetic solutions. However, the comparison of weekly samples showed t h a t the PVC tubes had significantly higher concentrations of most ions than did the snowboards. The reason for this difference is not known, but is worth more research because of its implications in network design. The Aerochem Metrics sampler used in the CADMP and NADP networks is not suitable for snow collection because of its undermeasurement problems, mechanical malfunctions, and small bucket capacity. A C K N O W L E D G E M E N T S We t h a n k Dan Dawson, Dan Whitmore, Jim Bergman, Joe Lipka, Jeff Cook, and Randall Osterhuber for collecting d a t a . Dr. Kathy Tonnessen provided design and operational assistance and manuscript review. The staff at Mammoth Mountain Ski Area provided the instrument tower and area access. The study was funded by the California Air Resources Board Grant A6-078-32, as amended.
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REFERENCES Berg, M.H. (1986) Snow chemistry in the central Sierra Nevada, California. Water, Air, and Soil Pollution. 30, 1015-1021. California Cooperative Snow Survey (1987) Water Conditions Fall Report, Dept. W a t . Resour., Sacramento, CA.
in
California,
Dawson, D.R. (1986) Acid deposition monitoring in an alpine snowpack. Final Report, California Air Resources Board, Contract, A4-038-32. Marine Science Inst., Univ. of California, Santa Barbara, CA, USA. Feth, J.H., Rogers, S.M., & Roberson, G.E. (1964) Chemical composition of snow in the northern Sierra Nevada and other areas. Water Supply Paper 1535J. Washington, US Dept. Interior Geological Survey. Goodison, B.E. (1978) Accuracy of Canadian snow gage measurements. Appl. Met. 27, 1542-1548.
/.
Goodison, B.E., Ferguson, H.L., & McKay, G.A. (1981) Measurement and d a t a analysis. In: Handbook of Snow (ed. by Gray, D.M., and D.H. Male), 191-274, Pergamon Press, Toronto. Larson, L.W., & Peck, E.L. (1974) Accuracy of precipitation measurements for hydrologie modeling. Wat. Resour. Res. 10(4), 857-863. Marks, D., McGurk, B., & Berg, N. (1988) Snow volume comparisons for atmospheric deposition monitoring. Proc, Western Snow Conf., 56, 124-135. McGurk, B.J. (1986) Precipitation and snow water equivalent sensors: an evaluation. Proc. Western Snow Conf. 54, 71-80. Melack, J.M., Stoddard, J.L., & Dawson, D.R. (1982) Acid precipitation and buffer capacity of lakes in the Sierra Nevada, California. In: International Symposium on Hydrorneteorology (ed. by A.I. Johnson & R.A. Clark), 465-472, Am. W a t . Resour. Assoc, Denver. Peck, E.L. (1972) Snow measurement predicament. 244-248.
Wat. Resour.
Res.
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Woo, S., & Berg, N.H. (1986) Factors influencing the quality of snow precipitation and snow throughfall at a Sierra Nevada site. Proc. Cold Region Hydrology Symposium., 201-209, Am. W a t . Resour. A s s o c , Anchorage, AL, USA.
