Snow, Hjdmtog) and Forests in IS0i Alpine Areas (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 205,1991.
Areal modelling of snow water equivalent based on remote sensing techniques
J. MARTINEC Federal Institute for Snow and Avalanche Research CH 7260 Weisfluhjoch/Davos , Switzerland K. SEIDEL, U. BURKART & R. BAUMANN Institute for Communication Technology ETHZ Image Science Division, CH 8092 Zurich, Switzerland ABSTRACT Average water equivalents of the seasonal snow cover are evaluated from periodic snow cover monitoring by remote sensing satellites for 9 regions and 5 elevation zones of the Alpine basin Felsberg (3250 km2, 560-3614 m a.s.l.). Regional trends in snow accumulation are illustrated and the representativeness of point measurements is assessed. The method can also 5e applied to simulate the snow accumulation in ungauged areas and in a future changed climate.
INTRODUCTION The snow accumulation in alpine areas depends not only on the elevation above sea level, but also on regional variations. The knowledge of the typical snow distribution is necessary for the evaluation of snow reserves and for the assessment of snow loads on structures to be expected with a given probability. The density of snow gauging networks is usually not sufficient for areal evaluations and their representativeness uncertain. A method is presented for complementing snow data by evaluating areal average snow water equivalents from periodical snow cover mapping.
CHARACTERISTICS OF THE STUDIED AREA
In the catchment area of the upper Rhine at FELSBERG (3250 km2,560-3614 m a.s.l.) the seasonal snow cover was periodically mapped during the snowmelt season from Landsat-MSS, SPOT-XS and NOAA/AVHRR satellite data. The basin was divided in 5 elevation zones and in 9 regions. The size, elevation range and mean hypsometric altitude of the resulting partial areas are listed in TABLE 1. The boundaries of the regions are shown in FIG. 1. In previous work a method has been described to analyse multispectral satellite recor dings with respect to snow coverage. Resulting snow cover area measurements became available even if the satellite images are obscured by a reasonable amount of clouds. With the aid of a Digital Terrain Model (DTM) it is in these cases possible to use a so phisticated snow cover interpolation algorithm (Seidel et al, 1983).
AREAL WATER EQUIVALENT FROM SNOW COVERED AREAS The importance of the areal evaluation of the snow accumulation has been recently emphasized from a new aspect: a report on impacts of climate change states that instead of relying on index points, better methods of monitoring the spatial distribution of
121 /. Martinec et al. 122
TABLE 1 Elevation range, mean hypsometric altitude and size of the regions and elevation zones Region Zone À Zone B Zone C Zone D Zone E Total (1) Tavanasa + Sedrun altitude (m a.s.l.) - 1277-1600 1600-2100 2100-2600 2600-3210 1277-3210 hypsometric altitude - 1492 1900 2344 2732 - area (km2) - 19.84 91.94 149.10 62.08 322.96 (2) Cadi, Surselva altitude (m a.s.l.) 690-1100 1100-1600 1600-2100 2100-2600 2600-3614 690-3614 hypsometric altitude 940 1360 1860 2340 2740 - area (km2) 46.26 115.92 121.48 117.60 44.03 445.29 (3) Lugnez/Vals/Safien altitude (m a.s.l.) 600-1100 1100-1600 1600-2100 2100-2600 2600-3340 600-3340 hypsometric altitude 904 1372 1880 2355 2938 - area (km2) 79.32 141.60 196.49 218.35 73.14 708.90 (4) Rheinwald altitude (m a.s.l.) - 1300-1600 1600-2100 2100-2600 2600-3370 1300-337 hypsometric altitude - 1500 1900 2360 2770 - area (km2) - 16.08 65.89 91.16 40.89 214.02 (5) Schams altitude (m a.s.l.) 700-1100 1100-1600 1600-2100 2100-2600 2600-3020 700-3020 hypsometric altitude 990 1350 1860 2331 2693 - area (km2) 15.38 27.50 34.00 38.92 7.44 123.24 (6) Domleschg altitude (m a.s.l.) 560-1100 1100-1600 1600-2100 2100-2600 2600-3200 560-3200 hypsometric altitude 765 1390 1834 2260 2745 - area (km2) 105.92 95.13 84.88 32.79 6.45 325.00 (7) Avers altitude (m a.s.l.) - 1235-1600 1600-2100 2100-2600 2600-3190 1235-3190 hypsometric altitude - 1495 1931 2380 2720 - area (km2) - 78.70 57.63 136.86 65.63 267.99 (8) Oberhalbstein altitude (m a.s.l.) 930-1100 1100-1600 1600-2100 2100-2600 2600-3340 930-3340 hypsometric altitude 1028 1400 1910 2355 2745 - area (km2) 1.98 36.98 84.94 142.64 57.32 323.86 (9) Albula/Davos "Tiefencastel" altitude (m a.s.l.) 837-1100 1100-1600 1600-2100 2100-2600 2600-3418 837-3418 hypsometric altitude 1000 1460 1890 2363 2727 - area (km2) 13.40 63.18 153.27 225.06 76.87 531.78
seasonal snow cover need to be developed (Street and Melnikov, 1990). The satellite monitoring of snow cover appears to be a useful tool for this purpose. The areal extent of the seasonal snow cover gradually decreases during the snowmelt season as illustrated by two Landsat images of the FELSBERG basin in FIG. 2. In addition, FIG. 3 shows the so-called modified depletion curves of snow coverage. These curves are derived by plottimg snow covered area (S) against totalized computed daily snowmelt depth (£HW)- The procedure is explained in detail elsewhere (Hall and Martinec, 1985). It is evident that the snow accumulation in the region 2 on 1 April 1982 was bigger than in the region 9 because a larger snowmelt depth was necessary to bring the snow covered area toward zero. Actually, the area below each curve is directly proportional to the average water equivalent over the whole region (zone C) on 1 April 1982, as was explained elsewhere (Martinec and Rango, 1987). In this manner, the areal water equivalents were evaluated for all partial areas. The results are listed in TABLE 2. 123 Areal modelling of snow water equivalent based on remote sensing
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FIG. 1 Division of the Felsberg basin (3250 km2, 560-3614 m a.s.l.) into 9 regions.
29-MAR-1985 FIG. 2 Snow cover maps from Landsat images of the FELSBERG basin showing the decrease of snow covered area from 29 March to 1 June 1985.
100 - ZONEC 1982 80 i H, =60.54 cm
2 Hs =33.20 cm 60
en 40 9 20
i i i i • i i i i f I I 0 40 80 120 160 200 240 280 320 360 400 EHw[cm] FIG. 3 Modified depletion curves of snow coverage for the regions 2 and 9, starting date 1 April 1982. /. Martinec et al. 124
TABLE 2 Areal water equivalents for 9 regions and 5 elevation zones of the basin Felsberg on 1 April 1982 and 1 April 1985.
Region Zone A Zone B Zone C Zone D Zone E (1) Tavanasa + Sedrun HW 1982 (cm) - 38.3 57.7 99.8 137.2 HW 1985 (cm) 29.5 40.8 93.0 127.6 (2) Cadi, Surselva HW 1982 (cm) 18.5 27.7 60.5 139.3 176.5 HW 1985 (cm) 11.6 33.8 36.7 100.9 138.2 (3) Lugnez/Vals/Safien HW 1982 (cm) 16.1 25.3 43.9 82.6 82.8 HWJ985 (cm) 20.1 30.4 32.4 63.5 70.3 (4) Rheinwald HW 1982 (cm) - 25.0 48.1 91.3 127.9 HW_1985 (cm) 26.3 52.8 112.5 133.3 (5) Schams HW 1982 (cm) 14.2 16.8 46.0 77.7 71.0 HW 1985 (cm) 18.2 18.8 34.3 72.1 100.9 (6) Domleschg HW 1982 (cm) 2.5 19.3 41.8 84.0 95.6 HW_1985 (cm) 3.0 25.5 29.8 56.4 61.0 (7) Avers HW 1982 (cm) - 12.5 37.8 66.0 78.3 HWJ985 (cm) 15.0 33.4 76.0 104.5 (8) Oberhalbstein HW 1982 (cm) 14.9 40.0 39.8 69.0 75.1 HW 1985 (cm) 5.3 19.8 24.2 66.5 88.2 (9) Albula/Davos "Tiefencastel' HW 1982 (cm) 12.8 14.1 33.2 80.3 100.8 HW_1985 (cm) 12.6 14.4 18.3 68.2 95.6
As could be expected, the values increase (with small exceptions) with the altitude. This relation is illustrated by FIG. 4 for the regions 2 and 9. In the western part of the FELS BERG basin, the snow accumulation appears to be bigger than in the eastern part in the whole altitude range, as already indicated by FIG. 3 for the medium elevation zone.
REPRESENTATIVENESS OF POINT MEASUREMENTS If the relations as shown in FIG. 4 are derived for all regions, it is possible to compare the data from snow cover mapping with direct measurements of the snow water equiva lent. For example, Hw = 100.6 cm was measured at Weisfluhjoch, 2540 m a.s.L, on 1 April 1982. Since this station is located in the region 9, Hw = 94 cm can be read off in FIG. 4 for the same elevation of 2540 m. Similarly, the remaining comparative values have been derived as summarized in TABLE 3. Point measurements as well as areal values confirm that on 1 April, the snow accumu lation in 1985 was smaller than in 1982, with the exception of region 4 in which there was more snow in 1985. The station Disentis seems to underestimate the snow cover in this area and altitude. However, the maximum snow cover accumulation at this elevation occurs already in February or March so that on 1 April the measured value may have been influenced by special melting conditions at the site. The station Biischalp measures higher values than the areal evaluations. The differences for other stations are either 125 Areal modelling of snow water equivalent based on remote sensing
3600 |2 i9 m as.l. E i 2600 J ^y 2100 2000 C 1600 B 1100 lo / ,' p A 1 1 SLF /£ ZNo. «1-1814 50 100 150 200 Hw[cm] I April 1982 FIG. 4 Relation between the altitude and the snow water equivalent on 1 April 1982 for the regions 2 and 9 small or become reversed in the other year. More years would be necessary to access the representativeness of these stations. At all events, snow reserves in the Felsberg basin can hardly be evaluated from the stations in TABLE 3 alone, although this is a well established and maintained snow gauging network. Reasonable estimates are possible by using the areal water equivalents and the respective areas.
On 1 April 1982, the snow cover represented a water storage of 2029-106 m3 (62.2 cm runoff depth) and on 1 April 1985,1812-106 m3 (55.5 cm runoff depth). The natural runoff (with reservoir manipulations corrected) was 3162-106 m3 in April-September 1982 and 2815-106 m3 in April-September 1985. Taking into account precipitation during the snowmelt period and the losses, these runoff volumes indicate that the
TABLE 3 Point measurements of the snow water equivalent Hw compared with the areal average at corresponding altitudes.
Station [m a.s.l.] Partial Hw [cm]: 1 April 1982 Hw [cm]: 1 April 1985 Area Measured Areal Measured Areal Weissfluhjoch 2540 9 100.6 94 60.5 84 Buschalp 1960 9 60.1 40 31.9 24 Davos 1560 9 41.9 18 16.6 15 Bivio 1770 8 35.1 34 28.8 25 Zervreila 1735 3 47.1 38 36.4 34 San Bernadino 1630 (4) 37.9 33 39.3 34 Splugen 1460 4 21.1 23 29.5 25 Sedrun 1420 2 39.4 30 20.1 28 Disentis 1170 2 8.4 22 9.5 19 J. Martinec et al. 126
computed water storage was realistic in both years. In addition, this comparison also shows that, if there was any difference in snow accumulation in forested areas of the basin, it did not significantly influence the results.
GRAPHICAL REPRESENTATION OF THE AREAL SNOW WATER EQUIVALENTS In FIG. 5 and 6, each selected range of areal snow water equivalents is represented by a gray tone to illustrate the regional distribution of the seasonal snow cover. Naturally, the mountain relief is clearly indicated since the snow accumulation increases with the altitude. In addition, regional anomalies can be recognized. However, it appears that there may be variations of these anomalies in certain years.
<25 25-50 51-75 76-125 >125 In order to eliminate the altitude effect from the regional abundance or lack of snow, the areal snow water equivalents at 2000 m a.s.l. (see FIG. 4) are shown in FIG. 7 in three gray shades. At this altitude the snow accumulation on 1 April 1982 decreases from west to east. In 1985, the general pattern of the snow distribution appears to be similar, as shown in FIG. 8. However, in the middle stripe of the Felsberg basin, there was more snow in the north than in the south in 1982 and this distribution was reversed in 1985. CONCLUSIONS The periodic mapping of seasonal snow cover during the snowmelt season by satellites 127 Areal modelling of snow water equivalent based on remote sensing ±985 <25 25-50 51-75 76-125 >125 A Chur r'A ../\y ,v ••..••• (X-Vj 'fccft^ 10 km V s •w? 1982 <50 50-60 >60 i —B 1985 <40 40-55 >55 enables the average areal water equivalent of snow to be evaluated for selected regions and elevation zones. The results reveal the regional distribution of snow, altitude gradients of snow accu mulation and variations of the snow distribution in different years. By comparing the avarage areal values with point measurements, it is possible to assess whether snow gauging stations represent snow conditions in their respective localities. The regional distribution of snow in terms of the areal water equivalent was evaluated from satellite data without any calibration or adjustment by terrestrial measurements. Therefore the method is also useful in mountain areas with inadequate or non-existent snow gauging networks. Further it can be used to simulate the regional distribution of the seasonal snow cover for different scenarios of a changed climate. ACKNOWLEDGEMENT This paper brings results from a research project at the Institute for Communication Technology, Image Science Division, Swiss Federal Institute of Technology (ETHZ), Zurich, Switzerland. This project is supported by the Swiss National Science Foun dation, the Electric Company of Northeast Switzerland (NOK), TeleColumbus Ltd. and by the Swiss Federal Institute for Snow and Avalanche Research (EISLF) at Davos- Weisfluhjoch. REFERENCES Hall, D.K. and J. Martinec, "Remote Sensing of Ice and Snow.", Chapman & Hall Ltd. New York, 1985. 129 Areal modelling of snow water equivalent based on remote sensing Martinec, J. and A. Rango (1987) Interpretation and utilization of areal snow cover data from satellites. IGS Symposium Cambridge 1986, Annals of Glaciology, Intern. Glaciology Society, 9, pp. 166-169. Seidel, K., J. Lichtenegger and F. Ade (1983) AugmentingLANDSAT MSS Data with Topographic Information for Enhanced Registration and Classification. IEEE Trans, on Geoscience and Remote Sensing, GE-21, pp. 252-258. Street, R.B. and P.I. Melnikov (1990) (Co-Chairmen), Seasonal snow cover, ice and permafrost (28 contributors), Chapter 7 in "Potential Impacts of Climate Change, Report for IPCC by Working Group", WMO-UNEP, pp. 7-35, World Meteorological Organization, Geneva.