Hydrological and meteorological observations. at the proglacial lake of Rhonegletscher

Transcription

1 Glacier Field Course in Switzerland /2016 Hydrological and meteorological observations at the proglacial lake of Rhonegletscher Roisu Yamasaki 1, Eva de Andrés Marruedo 2, Cayetana Recio Blitz 2 1 Graduate school of environmental science, Hokkaido University. 2 Numerical Simulation in Science and Engineering (GSNCI) ETSI de Telecomunicación. Universidad Politécnica de Madrid. 0

2 1. Introduction Rhonegletscher is a glacier of the Swiss Alps, in the canton of Obergoms VS (Figure 1). It has been very popular for visitors due to mainly two reasons. First, it is the water source of one of Europe's largest river; the Rhone river. Second, it is easily accessible from the Furka pass road, situated at about 2300 m a.s.l. From 1874 to 1915, one of the most complete surveys was carried out in Rhonegletscher, and the first detailed topographic map of a glacier was accomplished (Mercanton, 1916). The results of these efforts, this glacier gains world-wide reputation in the glaciological community. Figure 1: Map of Rhonegletscher location. (Omoto and Ohmura, 2015) Recently, due to global warming, Rhonegletscher is retreated and made glacial lake (proglacial lake) at the front of Rhonegletscher (Figure 2). Proglacial lake accelerates ablation of glacier and it has a risk as the lake bursts by increasing water level, in case a large ice block collapses into water (Tsutaki et al., 2011). Figure 2: Rhonegletscher and proglacial lake (September, 2016). 1

3 As shown in Figure 3, over the last 150 years, Rhonegletcsher has retreated from Gletsch to Belvèdere, and some studies suggested 30% of mass lost (Huss et al., 2008a). Figure 3: Plane figure showing the retreat of the Rhone Glacier between 1602 and Red line indicates shoreline of Rhonesee observed in September, (Omoto and Ohmura, 2015) Currently, this glacier is located at an altitude between 2300 to 3500 m a.s.l., with the highest part oriented northward and limited by Eckstock peak; the mountain reaches 3557 meter. Also, this glacier is enclosed in a valley formed by Gästenhörner at the western side and Galestorn at the east. The glacier surface covers almost 16 km 2, with a total volume estimated in km 3 (2007) and the value is smaller recently. (Table 1) Table 1: Surface area and ice volume of Rhonegletscher for different years for which a digital elevation model of the glacier surface exists (Farinotti et al., submitted). There is a proglacial lake placed at the terminus and it was produced by ice melting water filling a basin in front of the glacier. From bed topography studies, this lake is expected to grow, reaching a potential lake volume amounts of m 3 of water, 2

4 with an averaged depth of 22 m and a maximum of 71 m (Zanho, 2004). What will lead to change the configuration and the recession of Rhonegletscher? For example, we can think that its retreat may be somewhat related with the global warming trend, since the higher atmospheric temperatures, the more melting would be expected. Another circumstance which encourages ice melting would be the change of snowfall, i.e., decrease snowfall causes less accumulation. But, have these questions already been studied in Rhonegletscher? Some numerical models have been run under different scenarios, trying to answer this kind of questions and trying to reveal the estimations of Rhonegletscher retreat rates (Sugiyama et al., 2007) as well as the drift in the seasonal runoff peak, which is supposed to vary from July-August to May-June (Huss et al., 2008b). As we have seen, Rhonegletscher is a very well-studied and monitored glacier, with a big dataset that allows us the opportunity to keep going further in order to understand important and crucial aspects of alpine glacier behavior and trends. Our fieldwork has been carried out for several days in September, every year since Therefore, our motivation becomes from the aspects that follows daily fluctuations. Although many parameters have been surveyed on the glacier such as albedo, elevation of GPS-referenced points, and ph of surface melt waters, etc., we focus on measurements of the proglacial lake and atmospheric environments, in order to give a more comprehensive perspective. Apart from keeping increase the dataset, the aims of this study relay on approximating lake-glacier dynamics in terms of: (1) identifying daily variations in height of lake-level surface; (2) determining circulation patterns by observing iceberg movement; (3) co-relating parameters in order to reveal possible triggers. 3

5 2. Method We did three observations (metrological observation, logging of glacial lake variability, photogrammetry).the setting point of each observation is shown in Figure 4. All of the observations were logged from about 9/2 18; 00 to 9/4 10; 00. The detail of each observation is following. Logging of proglacial lake Photogrammetry Figure 4: The setting point of each observation. Metrological observation A) Metrological observation (Vaisala WXT510) Vaisala Weather Transmitter WXT510 (Figure 5) is a multi-sensor instrument that measures six weather parameters. This equipment consists of the sensors, and by connecting it to a data logger, it allows us to obtain continuous measure of climatic components listed below (Field manual for Swiss glacier course, 2006). We set this equipment on the land at east side of the glacier. Wind speed and direction (horizontal) Wind speed and direction are measured using the array of three equally spaced ultrasonic transducers (three small poles) on the top of the instrument. Liquid precipitation The precipitation is by the sensor under the metal plate that covers the top of the instrument, detecting the impact of individual raindrops. Barometric pressure and Temperature and relative humidity Barometric pressure, temperature and humidity measurements are done by the sensor module in the inner part of the instrument, behind the radiation shield. 4

6 Figure 5: WXT510 Cut away view (WXT510 users guide) B) Logging of lake variability. We used HOBO U20 (Figure 6) water level and water temperature logger for recoding water pressure, water temperature and air temperature. The measurement was 5 min intervals. The logger for measuring water pressure and temperature was fixed to a bar (Not to move in water) and it was set at lake shore (Figure 7, 8). The logger for measure of air parameters was set on the land near the water logger (Figure 8). Figure 6: HOBO U20 water level and water temperature logger. One used for recoding water pressure, another used for recoding air pressure. Figure 7: Logger for water parameters was fixed to a bar. 5

7 Logger for water Logger for Air Figure 8: Installation of the logger. Since this loggers cannot measure directly the change of water level, we used following the equation (hydrostatic pressure) to calculate the change in the water level from water pressure and air pressure. h=(pw-pa)/ρg h; water level [m] Pw; water pressure [kpa] Pa; air pressure [kpa] ρ; density of water (= 1000 [kg/m 3 ] ) g; Acceleration of gravity ( = 9.8 [m/s 2 ] ) C) Photogrammetry We used two interval cameras (GARDENWATCHCAM V1.0) to observe the movement of icebergs floating on the proglacial lake (Figure 9). This technique is able to determine the geometric properties of the icebergs and spatial situations. The cameras were set on the land at the east side of the lake (Figure 10). Both two cameras photographed the lake and the terminus part of the glacier, but the angle was different (Figure 11). Photographs were taken with 5 min intervals except for evening when the camera stops working. After shooting, we made videos by connecting the photographs and we analyzed the movement of icebergs through the observation period. 6

8 Figure 9: Internal camera fixed to land. Y X Cameras Figure 10: The location of cameras for photogrammetry and photo area of each the cameras (X, Y; Figure 11). (modify Omoto and Ohmura, 2015) Y X Figure 11: The photo area. (X, Y is defined in Figure 10) 7

9 3. Results A) Metrological observation Wind was blowing during the entire sampling period, with speed varying from 0.5 to 3 m/s (Figure 12). Northeast was main wind direction (Figure 13). Note that the time in Figure 12 is given in Swiss local time (Time in following figures is same). Figure 12: Wind speed record (metrological observation, 1 st to 4 th September 2016) Figure 13: Dominant wind component (metrological observation, 1 st to 4 th September 2016) Highest air temperatures were at 15:00 h 16:00 h, corresponding with the lowest relative humidity (Figure 14). Air temperature ranged from 6 ºC in the early morning (6:00 h) to 17 ºC and relative humidity from 37 to 83 %. Air temperature and humidity have inversely correlated patterns. 8

10 Figure 14: Humidity and Air temperature (metrological observation, 1 st to 4 th September 2016) Atmospheric pressure was measured both at the metrological observation site and at the lake-shore station. In Figure 15, we observed higher pressures at lake shore (788.8 to kpa) compared to those at the metrological observation site (781.8 to kpa), which is due to the elevation difference between the sites. Both places registered a decreasing trend in time. Figure 15: Air pressure at metrological observation site and lakeshore station. B) Logging of lake variability Minimum atmospheric temperatures were of about 6 ºC between 6:00 h 9:00 h in the morning and continuously increasing to reach 17 ºC at around 17:00 h in the afternoon (Figure 16, green line). Water temperature also started to increase from 0.8 ºC at 9:00, reaching its maximum of almost 4 ºC at 14:00 h to 15:00 h in the afternoon (Figure 16, red line). 9

11 Both atmospheric and water pressure were observed by data loggers in the lake-shore station. To calculate the water level height above the underwater logger, atmospheric pressure was removed and hydrostatic equation applied as described in the method section. In terms of pressure, the underwater logger recorded values from 79 to 83 kpa, which are translated to 0 to 40 cm of water column above the sensor, i.e. from surface to m depth. Maximum peaks take place around 18:00 h and minimum at 9:00 h (Figure 16, blue line). We can see that all of these parameters follow a similar daily pattern, starting to increase at about 9:00 h in the morning. An interesting point of this observation is that the water temperature peaked earlier than air temperature and water level, which will discuss later. Figure 16: The change of air and water temperature and water level of the lake. The water level is relative to the depth of the sensor. C) Photogrammetry We have collected images for three days, from September 2 to These photographs showed the movement of icebergs and suggested that the movement seems to be periodic. These behaviors are more precisely explained below (Figure 17-20). Since the photogrammetry was started at 14:15 of September 2, small icebergs are constantly moving but the move is small. These small icebergs are located at this time in the remotest of the glacier tongue area which is the end of glacier (left in 10

12 Figure 17). This movement continues until 16:40 h at the same day, when suddenly the main iceberg (which is larger) pivots on its axis. The main iceberg pivots without moving until 20:30 h. Figure 17: The image at 20:30 h of Sep.2 At 20:30 h, the main iceberg stops pivoting. And just at that moment, the small icebergs move slowly. These icebergs move up to an intermediate area of the lake while passing near the shore. At night, the camera was not scheduled to collect images, so the first image of the next morning (at 05:15 h of September 3, Figure 18) found small icebergs still at the same place (in the intermediate zone lake) and they barely move around the place. But between 05:15 h and 10: 45h, small icebergs began to move closer to the glacier tongue (Figure 19). Figure 18: The image at 5:15 h of Sep.3 11

13 Figure 19: The image at 12:00 h of Sep.3 After this movement, small icebergs moved to the inner area of the lake while drawing a curve, between 11:41 h and 14: 10 h (Figure 20). And they returned to initial position (the other side of the glacier tongue; left in Figure 20). Figure 20: The direction of the small icebergs movement Small icebergs continue to move until 16: 40 h when the main iceberg pivots again. Icebergs behavior is repeated over the three days. They moved approximately at the same times and moved along the same path in the lake. 12

14 4. Discussion I. Metrological observation and logging of lake variability Wind is controlled by the orientation of the glacier valley, which channelizes the air masses, resulting in main wind direction of about 60 N (Figure 13). Because the metrological observation was carried out near the glacier, it probably was influenced by land air mass and glacier air mass depending on wind direction. Air temperature and humidity showed anti-correlated sinusoidal curves (Figure 14). This is because warmer air becomes more unsaturated. Atmospheric pressure at both stations agrees with the elevation difference, being higher at lower altitudes (Figure 15). The decreasing tendency in both curves indicates that a low-pressure system was approaching, and this also accords with the observation of clouds during the last day. Next, we discuss Figure 16. The air temperature peak at the meteorological observation takes place earlier than at the lake-shore station. This may suggest that lake-shore station is affected higher influence of glacier and/or evaporation effect of water. At 9:00 h in the morning, the lake-shore station begins to be exposed to sunlight. It is observed that both upward trend curves of water temperature and water level start going up at almost the same time but it is after air temperature curve does. This means air is warmed earlier than water, and this agrees with physics laws in terms of heat capacity. Therefore, water level is supposed to rise because more insolation is received on glacier surface, which means air temperature increase, advancing the surface melting, then runoff into the lake increases. Another discussion in Figure 16 is the phenomenon which water temperature stops increasing and starts decreasing about three hours before air temperature or water level do (Figure 16a). Similar observation result was reported in preceding study (Figure 21). This phenomenon is not consistent with what is expected from heat capacity. Because the heat capacity of water is larger than that of air, water temperature increase and decrease more slowly than the change of air temperature, but these observations show inverted phenomenon. 13

15 Figure 16a: Water (red) and air temperature (green), and water level (blue). The red and yellow circles indicate the peaks in the water and air temperature, respectively. Figure 21: The change of water temperature and air temperature at same region in September 2015 (Ishii and others, 2015). Blue and red line shows water and air temperature respectively. The red circle point is faster than yellow circle as well as Figure 16a. Here, we discuss a hypothesis that may support such this inconsistent behavior. Since the logger is located at surface level, it is plausible that water surface may be warmed by solar radiation, since its albedo is relatively low (5-8%). However, water reaches its maximum density at 4 ºC, and taking into account that meltwater from the glacier is at the freezing/melting point of 0 ºC, it is feasible to think that water from glacier melting will tend to reach the upper layers of the lake and the 4 ºC-warmed water laying in the surface will move downwards to occupy the bottom layers. Indeed, Ishii and others, (2015) reported the lake had thermocline at 6m depth. As a result, the surface water temperature of lake dramatically changed. 14

16 II. Movement of icebergs Throughout the days, as photogrammetry shows constantly icebergs moving, there is subglacial discharge (entering new melting water) from the surface and/or bed of Rhonegletcher into the lake. From field data, we see the highest water level of the lake is at 18:00 h and the level rise is periodic (Figure 22). We consider this periodic change of water level corresponds to the discharge behavior from the glacier, which means a maximum discharge flow at 18:00 h. Figure 22: The change of water level in the lake. Note that the main iceberg pivots between 16: 00 h and 20:00 h. On the other hand, small icebergs move in the whole day, which suggests there are continues subglacial discharge. The relationship between the time of main iceberg pivoting and the time of highest water level imply that the main iceberg movement is controlled by the discharge behavior which is suggested by the change in water level. Therefore, the magnitude of this discharge is not enough for moving the main iceberg, but only several hours, it can influence the iceberg and occurs the pivoting. Furthermore, we compare the movement route of icebergs in this research area with earlier study and it shows a similar pattern (Figure 23). Glacier Iceberg Figure 23: A schematic diagram for horizontal movement of water in lake (modify Ishii and others, 2015). The number shows the order of water movement. 15

17 Iceberg movements are very similar from year to year. Figure 23 shows that the main water flow (number (1) in Figure 21; the same below) is pushed by glacier tongue and it probably corresponds to the point of maximum subglacial discharge flowing into lake. This main flow moves small icebergs quickly and pushes them against the opposite side of the glacier tongue, then hits the wall of the lake. The wall backs up the main flow (number (4)) and the flow returns along the shores of the lake by inertia of water (number (5), (6)). The icebergs also return along with the flow of water. Contrastively, only for a certain time, the main iceberg is influenced by the main flow (it implies in number (2), (3)). This behavior is cyclic; it is repeated again and again. 5. Conclusion and Future We measured the change of water level with environmental parameter (ex. air temperature, water temperature ). Our date shows that increasing air temperature causes more ice melting and it leads higher water level in the proglacial lake. Also, the icebergs movement in the lake permits us to understand how ice melted water circulates in the lake and to suggest the relationship between the change of environmental parameter and the change of discharge behavior. We consider this daily variations study is so important to better understand the Rhonegletscher behavior and need to continue this observation every year. As the a reason for that, monitoring the daily variations of air and water temperature, height water level and iceberg movements could help us to understand and estimate potential trends in Rhonegletscher dynamics. 6. References Farinotti, D., Huss, M., Bauder, A., Funk, M., and Truffer, M. (2009). A method to estimate ice volume and ice thickness distribution of alpine glaciers. Journal of Glaciology. (submitted). Huss, M., Bauder, A., Funk, M., and Hock, R. (2008a). Determination of the seasonal mass balance offour Alpine glaciers since Journal of Geophysical Research, 113(F1):F Huss, M., Farinotti, D., Bauder, A., and Funk, M. (2008b). Modelling runoff from highly glacierized alpine catchment basins in a changing climate. Hydrological Processes. doi: /hyp Mercanton, P. L. (1916). Vermessungen am Rhonegletscher, Mensurations au Glacier du Rhône, Neue Denkschriften der Schweizerischen Naturforschenden Gesellschaft, 52. Sugiyama, S., Bauder, A., Funk, M., and Zahno, C. (2007). Evolution of Rhonegletscher, Switzerland, over the past 125 years and in the future: application of an improved flowline model. Annals of Glaciology, 46:

18 Tsutaki, S., Nishimura, D., Yoshizawa, T., & Sugiyama, S. (2011). Changes in glacier dynamics under the influence of proglacial lake formation in Rhonegletscher, Switzerland. Annals of Glaciology, 52(58), Zahno, C. (2004). Rhonegletscher in Raum und Zeit: Neue geometrische und klimatische Einsichten. Diplomarbeit an der VAW/ETH-Zürich, (unveröffentlicht). 小元久仁夫, 大村纂. (2015). 急速に後退するスイスのローヌ氷河. 地学雑誌,124(1), 石井, 柑谷, 島田. (2015). ローヌ氷河の全縁湖における氷塊の挙動と水位変動 年スイス実習レポート. 17