The great contribution of mountain glaciers and seasonal snow cover to the water ... and microwave parts of the spectrum become more widely used in snow hydrology ... Because of wind effects on the catch of snowfall by the gage, winter precipitation ... D with a sampler of 100 cm3 and a spring balance of 5 g resolution.
Snow. Hydrology and Forests in High Alpine Areas (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 205, 1991.
Physical properties of snow cover and estimation of snowmelt runoff in a small watershed in high alpine Tianshan
DAQING YANG, YINGSHENG ZHANG & ZHIZHONG ZHANG Lanzhou Institute of Glaciology & Geocryology, Chinese Academy of Sciences, Lanzhou 730000, P. R. China KELLY ELDER & RICHARD KATTELMANN Center for Remote Sensing & Environmental Optics, University of California, Santa Barbara, CA 93106, USA ABSTRACT The seasonal snow cover in the high alpine area of the Urumqi River basin of the Tianshan Mountains is shallow with the maximum snow depth generally less than 1.5 m and 70 percent of the annual precipitation occurring during the summer months. Snow distribution in the research watershed is extremely uneven because of wind drifting, varying sublimation from the snow surface, and weak melting of the snow on south-facing slopes in winter. Snowpack temperature tends to be quite low. A large temperature gradient is common, and a gradient of-0.58°C cm"1 has been measured in late January. The snowpack melts in late April or early May, and the melt water sustains surface runoff in all of May and sometimes in June. Snowfall in summer generally melts in one to two days and contributes directly to surface runoff from June through August. Daily discharge of the watershed can be accurately estimated from daily air temperature in May.
INTRODUCTION The great contribution of mountain glaciers and seasonal snow cover to the water resources of the northwestern arid and semi-arid regions of China has been widely recognized since the mid-1960s. Some of the physical properties of the seasonal snow cover and avalanches in the subalpine areas of the western Chinese Tianshan and northern Yunnan have been studied (Zhang, 1987; Wang, 1987; Ma and Hu, 1990). Unfortunately, aspects of snow hydrology and snowmelt runoff have seldom been considered in these studies. As remote sensing of the visible, infrared and microwave parts of the spectrum become more widely used in snow hydrology and climate research, knowledge of the physical properties of the snow cover from ground measurements is necessary for interpretation and adjustment of the satellite imagery. Therefore, a project on properties of alpine snow-cover and snowmelt runoff was conducted at the Tianshan Glaciological Station to understand the spatial and temporal changes in the depth, density, and temperature characteristics of the snow cover. Relations between snowmelt runoff, air temperature and precipitation in the arid and semi-arid areas of the Chinese Tianshan were also studied. 169
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STUDY SITE AND INSTRUMENTATION The Urumqi River originates on the northern slope of the Chinese Tianshan and flows northward to the city of Urumqi, the capital of Xinjiang Autonomous Region. The climate of the source area of theriveris typically continental, with a mean-annual air temperature of -5.4 °C and mean-annual precipitation of 420 mm at the Daxigou meteorological station (43.06°N, 86.50°E; 3539 m a.s.l.). When corrected for the systematic errors of wind influence and gage losses for a Chinese standard gage, the actual mean-annual precipitation should be about 560 mm (Yang, ei al., 1989). There are seven smafl glaciers in this region that cover a total area of 5.7 km . Glacier No. 1, with an elevation range of 3720 to 4484 m a.s.l., is the largest of the group and has been the main object of study of the Tianshan Glaciological Station since 1959. The Dry Cirque is a south-facing ice-free watershed surrounded by peaks of about 4300 m a.s.l. elevation on its western, northern, and eastern sides. A hydrometric station2 has been established at the outlet (3804 m a.s.l.) and controls an area of 1.68 km (Fig. 1). The following observations have been carried out since May 1982: (a) Air Temperature (°C). A daily thermograph has been used from May through August, and a weekly-recording instrument has been in place from September through April. Data from the charts are adjusted relative to a manual measurement of temperature at 8:00 every day. (b) Relative Humidity (percent). Daily and weekly hygrographs are used from May through August and September through April, respectively. A hairhygrometer is also read manually to check the recording instruments. (c) Precipitation (mm). A Chinese standard precipitation gage, 20 cm in diameter and 65 cm long, with the orifice 70 cm above the ground is measured daily at 8:00 from May through August. Observations are made weekly during the winter. Because of wind effects on the catch of snowfall by the gage, winter precipitation data must be corrected for use in water balance calculations. (d) Water Stage (m) and Streamflow (m-V1). A float-type water-stage gage is used to measure and record stage hourly. Daily mean discharge is calculated with a rating equation developed from current-meter measurements. In the winter of 1989-90,42 snow stakes were installed between 3800 and 3930 m a.s.l. in the Dry Cirque. Snow depth was measured at each stake every seven to ten days. A snow-density profile was measured in snowpits between stake rows C and D with a sampler of 100 cm3 and a spring balance of 5 g resolution. Three to five samples were measured at each depth interval to reduce the relative error in the density determinations. A small plastic lysimeter (16.5 by 11.2 by 6.7 cm) was placed near stake row C and weighed with a balance of 1 g resolution once per day in the afternoon for several days in March and April to estimate sublimation from the snow surface. Snow temperature was measured at the ground surface and glacier ice surface by two groups of copper electrical-resistance sensors at 0, 0.1, 0.2, 0.3, 0.4, and 0.5 m above the ground. The sensors were calibrated in the laboratory, and adjustments were made in temperature ranges of 0.5°C. The resolution of the sensors was 0.1 °C. RESULTS Formation and Distribution of the Snow Cover The rate of snow accumulation in the long winter is generally low except in late April and early May. Seasonal snow cover begins to form in early October when air temperatures drop below -4°C. In normal years, snow depths range from 0.1 to 0.2 m, because winter is relatively dry with a total snowfall water equivalence as
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Physical properties of snow cover and estimation ofsnowmelt runoff
Snow Depth ]Less Than 0.10m [77770.10 TO 0.30m E=ii0.30 To 0.60m
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FIG. 1 Dry Cirque watershed showing the hydrometric station, instrumentation and snow depths (m) on 15 May 1990.
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low as 60 to 70 mm. However, in April and May, snow depths increase substantially to 0.4 to 0.6 m as precipitation increases. Maximum depth of the snowpack is attained in early May just before the melting season (Fig. 2). About 70 percent of the annual precipitation occurs during the summer months. Snow distribution depends primarily on local wind patterns, vegetation, and topography. Wind speeds in the headwaters of the basin are rarely greater than 4 m s . In the Dry Cirque, the deepest snow is found on the bottom and at the foot of steep slopes. The upper parts of slopes above 4000 m a.s.l. are free of snow because of avalanches and wind scour. Snow depths on south-facing slopes tend to be much shallower than those on slops with a somewhat-northern aspect. The south-facing slope is occasionally bare because of greater exposure to solar energy and consequently greater sublimation and melting than other slopes of other aspects. North-facing slopes maintain a snow depth of 0.1 to 0.3 m during midwinter. There is also a significant difference between snow depths on windward and leeward slopes. Prevailing winds from the south and shading from insolation result in north-facing slopes having the greatest accumulations of snow in the basin.
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FIG. 2 Mean snow depth (m) of the 42 snow-scales, accumulated precipitation (mm) and daily temperature (°C). Snow Density Snow density increases slowly from January to March and reaches a maximum of _;} 310 kg,m at the middle or end of March. It then decreases to a minimum of 215 kg m _ i in April or early May because new snow of low density accounts for 60 to 70 percent of the total snow accumulation. As snow melts in early May, percolation and refreezing of liquid water leads to a sharp increase in snow density. This melting-refreezing process of densification is obviously different from the densification or compaction of the snow layers (Fig. 3). Statistical analysis showed insignificant correlation between snow density and snow depth as was also found by Goodison, ei aj. (1981). The coefficient of variation of snow depth is 3.5 to 4 times that of snow density at Daxigou meteorological station. Snow density itself is variable over even small areas
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Physical properties of snow cover and estimation ofsnowmelt runoff Z
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FIG. 3 Depth (m) and density (kg/m ) of the snowpack between snowscales C and D.
(Cooley, 1988). In the Dry Cirque, snow density profiles measured on the same day were quite different at three example locations (Fig. 4). Such large differences in density complicate the estimation of basin-wide snowpack water equivalence.
(a) foot of north-facing slopes 3940m
(c) foot of east-facing slope, 3920m
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Snow density (kg/m3) profile at various locations in Dry Cirque on 19 May 1990. FIG 4
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Snow Temperature The general pattern in snow temperature from January to March is cold at the bottom layer, coldest at the middle layer and colder at the upper layer and surface (Fig. 5), indicating the transfer of solar energy through the surface of the snowpack and upward heat flow from the ground. When cold air masses pass over the region, snow temperature decreases gradually from the surface to the base of the snowpack so that the temperature pattern becomes cool at the bottom, cold in the middle layer, and coldest in the upper layers. In the middle of April, snow temperatures at various layers are similar to each other, and the temperature profile is almost a vertical line. In late April or early May, the pattern becomes coldest at the bottom, cold in the middle layer, and warm in the upper layer. When surface temperatures reach 0°C and melting begins, the percolating water and release of latent heat during refreezing warm the entire snowpack to 0 °C in a period of five to ten days (Fig. 5).
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FIG. 5 Snow temperature (°C) profile near C3 snow-scale.
Snow temperature gradients at the bottom (0-0.1 m) and middle layers (0.1-0.2 m) can be as low as -0.36°C cm"1 and -0.58°C cm" , respectively, in January and February. Such steep gradients lead to a large amount of water vapor transfer upward and form depth hoar crystals with diameters of 8 to 11 mm. The temperature profile of the snowpack on the glacier is very close to that in the Dry Cirque during January and Feoruary. However, during March and April, the temperatures of the bottom and upper layers of the snowpack on the glacier become 2-4°C and 1-2°C lower, respectively, than the snowpack on the ground in the Dry Cirque. In May, the temperature differences between the two sites are as large as 8.5"C and 4.5°C, respectively, in response to the effect of the glacier on
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Physical properties of snow cover and estimation ofsnowmelt runoff
the overlying snowpack and the lower amount of solar energy received by the generally north-facing glacier. Because of these two reasons, the snowpack on the ice surface starts melting seven to ten days later than the snowpack on the ground. Snow temperatures at different layers are closely related to air temperature at 2:00, 8:00, and 14:00 (Beijing Time) in the Dry Cirque. Generally, the dependence of snow temperature on the thermal condition of the air decreases from the snow surface to the base. Snow temperature in the upper layers is best related to air temperature at 8:00, while the temperature of the bottom layers (0-0.3 m) is best correlated with air temperature at 2:00. This result is reasonable because the response of snow temperature to fluctuations of temperature at the upper boundary of the snowpack decreases gradually from the surface to the bottom. The response time also increases proportionally with depth according to heat transfer theory. Air temperature on Glacier No. 1 has not been measured in winter; therefore, its effect on snowpack temperature is unknown.
Snow Cover Depletion and Snowmelt Runoff The snow cover is depleted by both sublimation and weak melting in winter and then by rapid melting in early spring. Daytime sublimation ranges from 0.03 to 0.7 mm, and condensation during the night is about one order of magnitude smaller than the daytime sublimation. Therefore, a snowpack of 0.05 m on a south-facing slope disappears in 10 to 15 days without precipitation through sublimation and weak melting. Strong melting of the snowpack begins in early May when the maximum air temperature reaches 2-5°C. Surface runoff at the hydrometric station begins two to three days later because the initial melt water is refrozen in the cold snow and at night. Additional water is stored at the base of the snowpack and in the channel system. As air temperature rises into the middle of May, snowmelt runoff quickly attains a peak flow of 0.18 m-V 1 because of the complete melting of the stream ice. Runoff from the winter snowpack then decreases gradually and ends in late May or early June depending on the water equivalence of the snowpack and the rate of melting. The runoff in May accounts for 15 to 20 percent of the annual total. The relation of runoff to air temperature becomes relatively weak in June and July compared to precipitation because the fresh snowfall in summer melts within one or two days after deposition. Runoff varies directly with changes in air temperature during August, with the highest flow corresponding to the maximum temperature. Streamflow then recedes during late August and September. The discharge of the watershed varies sharply from day to day because of changing melt rates, channel storage in a flat area above the stream gage, and ice formation in the channel. Frequent snowfalls in summer maintain saturation of the thin soils and produce rapid runoff following the storms. A relationship between snowmelt runoff and both air temperature and precipitation was developed from regression analysis to simulate streamflow from the Dry Cirque but is not yet in final form. The antecedent daily air-temperature was the most important predictor of daily discharge in May. However, the correlation between precipitation and runoff was greater than that between air temperature and runoff in June and July. Air temperature was again the most important factor in August. Estimates ofsnowmelt runoff in May based on antecedent daily air-temperature closely matched observed streamflow (Fig. 6). Estimates of daily runoff in June and July using the same approach were poor. Calculated discharge in August was similar to measured amounts, although the measured and calculated peaks did not correspond very well. This simple approach to calculating snowmelt runoff was obviously not adequate for all conditions in the study area. In addition to the rate
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FIG. 6 Measured daily discharge (m3/s) from May to August and calculated daily discharge in May.
of snowmelt, the volume and timing of streamflow from the Dry Cirque are also affected by storage and movement of water in the snowpack, substrate and stream channel.
CONCLUSIONS The winter snowpack in the Dry Cirque of the upper Ururnqi River basin is generally less than Î .5 m deep because about 70 percent of the annual precipitation occurs in summer. Snow distribution within this research basin is quite uneven because of wind transport of snow, spatial variations in sublimation and weak melting in winter. Snow density also varies around the basin. Therefore, accurate estimation of basin-wide snowpack water equivalence could not be simply based on depth measurements at a network of 42 snow stakes. Snow depth and density at a 50 to 60 m grid spacing (Cooley, 1988) will be measured during the 1990/1991 snow season. Sublimation and melting will also be observed to allow calculation of the mass balance of the snow cover in winter. Because of the cold and dry climate of the Dry Cirque, the shallow snow cover remains at very low temperatures throughout winter. Temperature gradients as large as -0.58°C cm have been measured in January and control snow metamorphism. Differences in temperature between snow on the ground and snow on a nearby glacier were minor from January through March but became as large as 2-5°C in early May. The lower layers of the snowpack on the ground in May are much warmer than the corresponding part of the snowpack on the glacier because of the thermal inertia of the cold ice. The temperature of the snowpack on the ground in the Dry Cirque appears to be closely related to air temperature. Snow cover in the Dry Cirque begins to melt in early May and sustains streamflow throughout May and often into June. Surface runoff in June and July results mainly from the rapid melting of fresh snowfall. Because there is enough energy available (as indexed by air temperature) during summer to melt snow during clear weather, variability in streamflow is better explained by variations in precipitation than in air temperature during June and July. Streamflow during the recession period in August is better correlated with air temperature than with precipitation. Daily streamflow varies sharply because the winter snowpack melts in only 10 to 25 days and new snow in summer melts in just one or two days, leading to peak flows from the small catchment. Daily discharge could not be adequately simulated from regression equations involving air temperature and precipitation
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Physical properties of snow cover and estimation ofsnowmelt runoff
except in May during the melting of the winter snowpack. Development of a suitable runoff model for the entire period of May through August will require better understanding of the snowmelt process, soil freezing, subsurface storage and routing, and the interaction of groundwater and surface water.
ACKNOWLEDGEMENT This research project has been sponsored by the National Natural Sciences Foundation of China for two years. The logistic support from the Tianshan Glaciological Station during the field work is gratefully appreciated by the authors. REFERENCES Cooley, K. R. (1988) Snowpack variability on western rangelands. Proceedings of the Western Snow Conference 56.1-12. Goodison, B. E., Ferguson, H. L. & McKay, G. A. (1981) Measurement and data analysis. In: Handbook of Snow (ed. by D. M. Gray and D. H. Male), Pergamon Press, Toronto, 220-232. Ma, W. & Hu, R. (1990) Relationship between the development of the depth hoar and avalanche release in the Tianshan mountains, China. Journal of Glaciology 36 (122), 37-40. Wang, Y. (1987) Some physical properties of seasonal snow cover in northern Yunnan and Western Tianshan Mountains. In: Proceedings Second National Conference on Glaciology of the Geographical Society of China (in Chinese), 179-184. Yang, D., Shi, Y., Kang, E. & Zhang, Y. (1989) Research on analysis and correction of systematic errors in precipitation measurement in Urumqi River basin, Tianshan. In: Precipitation Measurement. WMO/IAHS/ETH Workshop on Precipitation Measurement, St. Moritz, Switzerland (ed. by B. Sevruk), 173-179. Zhang, Z. (1987) PreUminary research on temperature regime in seasonal snow cover and its relation to frost penetration depth in Gongnaisi Valley, Tianshan Mountains. Journal of Glaciology & Geocryology (in Chinese), 9 (1), 69-79.
