Water balance in a west Greenlandic watershed

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1 Northern Research Basins Water Balance (Proceedings of a workshop held at Victoria. Canada. March 2004). IAHS Publ. 290, Water balance in a west Greenlandic watershed CHRISTIAN HELWEG ASIAQ Greenland Survey, PO Box 1003 DK-3900 Nuuk, Denmark chh@,asiaa.gl Abstract An initial water balance has been set up for the Pisissarfik watershed on the west coast of Greenland. The balance shows that precipitation data are problematic and that more detailed studies are needed on évapotranspiration and storage before a more trustworthy balance can be achieved. Key words évapotranspiration; Greenland; hydrology; Pisissarfik; precipitation; storage; water balance INTRODUCTION This paper concerns the water balance in the Pisissarfik watershed, Greenland. Pisissarfik is a sub-watershed of the Tasersuaq watershed, which is located on the central west coast of Greenland, approximately 40 Ion from the coast (Fig. 1). The centre of Pisissarfik watershed is at 52 W48' 67 N08'. The nearest town is Sisimiut, and the Tasersuaq watershed is studied because of its hydropower potential in relation to the town. The Pisissarfik watershed was envisioned to serve as a reference watershed for Tasersuaq once hydropower was established. The Pisissarfik is situated in the midst of the Tasersuaq watershed and resembles the rest of the Tasersuaq in that it has approximately the same hypsography, is roughly the same distance from the coast, and is a valley situated in the same northeast-southwest general direction as several of the other sub-watershed valleys. The Pisissarfik is about 39 Ion 2 (or 5%) of the Tasersuaq watershed, which totals 758 km 2. WATERSHED DESCRIPTION Geography The watershed boundary has been determined from topographic maps, aerial photography and field observations (Nielsen, 1999). The watershed is 13 km long and 2.5 km wide. The main part of the watershed is a valley that runs in a northeastsouthwest direction, generally sloping toward the southwest from 1200 m a.s.l. down to 450 m a.s.l., but which also has higher topographic areas in the extreme southwest end. The discharge exits the valley in the southwestern direction into Lake 450; but the flow in the lake changes to the northwest, which is perpendicular to the general direction of the Pisissarfik valley. There are two glaciers in the extreme northeast of the catchment (Fig. 2). The northeast end of the valley is generally less steep than the southwestern parts in the lower end of the valley. The southeast side of the valley holds several interconnected small lakes or ponds. Small streams run from the smaller

2 Christian Helweg Fig. 2 The contributing area of the Pisissarfik catchment. The obvious discrepancy between the catchment boundary and the topography boundary in this map is due to the inaccuracy of the topographic map shown. The catchment border has been developed from a more precise map and from field observations.

3 Water balance in a west Greenlandic watershed 145 Table 1 Topographic data for the Pisissarfik catchment on the west coast of Greenland. Topography Land cover Catchment area km 2 Ice 2.85 km 2 Highest point 1200 m a.s.l. Rock Major part Station altitude at 475 m a.s.l. Sediment with vegetation Minor part outlet Lakes 0.5 km 2 Aspect classes Hypsography North 20% Area Altitude East 20% 50% m a.s.l. West 26% 25% m a.s.l. South 34% 25% m a.s.l. lakes down to the stream draining the main valley. The largest lake in the catchment is Lake 450. The area of Lake 450 is approximately km 2. The volume has not been determined. The total area of lakes in the catchment can roughly be estimated from maps to be around 0.5 km 2 or 1.3% of the catchment area. The geology of the watershed is dominated by rock massifs that are generally gneiss with broad bands of amfibolite (GEUS geological maps). Most of the watershed area has bedrock at the surface; sediments are mostly present in the areas near the main stream. In the lower parts of the catchment there is some peat soil. Sediments in the watershed are mainly moraine deposits (Hag et al, 2001). Some topographic data are given in Table 1. Vegetation The areas with sediment, in valley bottoms, are generally covered with low grasses, mosses and low shrubs, while the major part of the catchment, which is rock, has no significant vegetation. Permafrost There is generally permafrost in the catchment, and the active layer is estimated to be around 1 m deep, based on nearby measurements. In the middle of June the frost free layer is around m, while it reaches its maximum in August (Hag et al., 2001). There has been no measuring or mapping of permafrost extension or depth so far, but in areas 2 more north (69 28'N) ASIAQ has measured permafrost depths of m. Glaciers The watershed includes parts of two small glaciers. However, when evaluated from the available maps, discharge from these glaciers does not only run into the Pisissarfik watershed but also into other watersheds. The ice-covered part of the catchment due to these two glaciers, is approximately 2.63 km 2 plus 0.22 km" which equals a total of

4 146 Christian Helweg 2.85 km"; this corresponds to 7.3% of the area (calculation based on the 1: KMS vector map with GIS software). The extent of melt on the glacier has not been investigated. MEASURED PARAMETERS Monitoring Hydrological and meteorological data was collected at Station 464, and placed at the outlet of the Pisissarfik watershed (Fig. 3). Station 464 was set up in 1992 and is situated at a small lake, 475 m above sea level. Water level was measured in the lake and a stage-discharge relationship was established just below the outlet of the lake. The parameters shown in Table 2 were monitored. Traditionally the precipitation gauges used at remote hydrological stations by ASIAQ have been unscreened probably due to the problems of transport and lack of correction formulas for screened gauges and this is also the case at Station 464. The station is generally only visited once a year due to its remote location. During the visits, sensors are calibrated or adjusted if faulty.

5 Water balance in a west Greenlandic watershed 147 Table 2 Monitored parameters at station 464 Pisissarfik. All parameters are measured year round. Parameter Precipitation, unscreened Belfort Gauge Water level (lake) Water temperature (lake -3.5 m) Wind speed (6 m) Since 1998 Wind direction (6 m) since 1998 Air temperature (2 m) Air humidity (2 m) Soil temperature (-0.5 m) since 1998 Incoming shortwave radiation (2 m) since 1998 Outgoing shortwave radiation (2 m) since 1998 Sampling interval 3 h 3 h 3 h 5 min 5 min Table 3 Monthly average, minimum and maximum temperatures in C measured at station 464 in the Pisissarfik catchment. Month Average ± Standard deviation Max Min Jan ± Feb ± Mar ± Apr -6 ± May -2.2 ± Jun 5.7 ± Jul 9.4 ± Aug 6.4 ± Sep 2.1 ± Oct -4.8 ± Nov -10 ± Dec 11.6 ± Temperature The climate in the catchment is arctic. The temperatures measured at Station 464 in the recent period of are shown in Table 3. The winter is from around October to somewhere around the end of April. In winter temperatures are generally well below zero, but warm spells with temperatures above zero can occur throughout winter. Such warm spells are often followed by severe freezing and often lead to the formation of ice on the snow. The winter is followed by a melting season of about one month. After the melting season, when almost all snow is gone, the summer lasts for about 4 months, until 1 October. The temperature sensors in Lake 450 show that the lake freezes around 1 October every year and that the ice has diminished enough for the air temperature to be reflected in the water temperature around the end of May. Wind Wind data has only been collected since 1998, but measurements indicates that wind speeds at Station 464 generally are low. Approximately 60% of the time, the average wind speed is below 2 ms" 1.

6 14 8 Christian Hehveg Table 4 Discharge and precipitation for the hydrological years of Period Discharge (mm year" 1 ) Measured precipitation at St. 464, uncorrected (mm year"') Corrected precipitation in catchment (mm year" 1 ) 1 Oct 1996 to 1 Oct Oct 1997 to 1 Oct Oct 1998 to 1 Oct Oct 1999 to 1 Oct Residual (Evaporation storage and error) (mm year" 1 ) Precipitation Precipitation has been collected at Station 464 since 1992, but typical for the measurement of precipitation in Greenland, only a fraction of the areal precipitation seems to be caught by the precipitation gauges used. This is especially true in winter, when precipitation comes as snow. It is normal that only a part ofthe precipitation is caught. Goodison (1978) measured the snow catch for an unscreened Belfort precipitation gauge as a function of wind speed. His correction has been applied to the Pisissarftk precipitation data for years where both air temperature and wind speed have been collected and when air temperatures were below 1 C (snow). However, no correction factor is known for rain caught by the unscreened Belfort gauge, and a factor of 1.5 has thus been applied when temperature was above -1 C (rain). For the few years where wind data is available together with precipitation data, analysis gave a yearly average factor (both snow and rain) of approximately 2.0. Thus the yearly uncorrected precipitation for all years was simply multiplied by 2.0 to correct for undercatch. For the water balance calculation, the precipitation was furthermore adjusted in the catchment by adding an additional 3% for every 100 m of altitude. The results are shown in Table 4. Snow There has been one campaign of snow depth measurement, from 15 to 21 April Snow depths were sampled at 28 locations in and around the catchment. The sites were selected to represent various altitudes, hillside orientations and topographic features (valleys/crests). Statistics were not used in selection of the sites. Snow depths were quite varied, with much snow in valleys and depressions and little snow on windexposed hilltops and crests, but had an average depth of 1.11 m. No strict relationship with altitude could be determined. As only one campaign was conducted, it is not possible to determine whether the uneven distribution is due to uneven snowfall or redistribution, but it is typical of midwest Greenlandic catchments that snow cover is quite uneven. Two snow pits were dug in the area and water equivalents were determined. The average snow density in April 1998 was 370 kg m" 3. The water equivalent of the total snow cover in April 1998 was calculated to be around 410 mm (Nielsen, 1999). In spring most of the snow melts, but in some years snow pockets may survive through

7 Water balance in a west Greenlandic watershed 149 the summer. In 1998, snow was melting from 27 May 1998 to 23 June 1998 at Station 464 at 475 m a.s.l. At Station CEB at 1130 m a.s.l., snow melting began 13 July 1998, or several weeks later, than at the lower station (Boggild et al, 1998). The complete snowmelt process took one week more at the high station (Hag et al, 2001). In 1999, outgoing short wave radiation measurements indicated that snow at Station 464 had melted away around 28 June Discharge A stage-discharge relation has been determined for the stream leaving Lake 450: Q = 29.75(A ) 164, PO.0001 and r = (1) where Q is discharge (m 3 s" 1 ) and h is stage (m) (Hag et al, 2001). Using this relationship together with the collected stage values gives the discharge shown in Fig. 4. The annual discharge is shown in Table 4, along with measured and corrected values of precipitation. Missing stage data is usually due to breakdown of equipment and, as the station is only visited once a year, a breakdown shortly after a visit can result in almost a full year of missing data. UNMEASURED PARAMETERS Evapotranspiration Evapotranspiration has not been measured, but in a modelling study done for 1998, a value of 160 mm year" 1 was obtained (Hag et al, 2001). Although many investigations a K Fig. 4 Discharge from the Pisissarfik watershed, outlet of lake 450. The ticks with year below represent the middle of that year (30 June/1 July).

8 150 Christian Helweg for hydropower have been conducted in western Greenland by the Greenland Survey (ASIAQ), the focus has been strictly on runoff in specific potential hydropower catchments, and almost nothing has been done on evaporation. However, considering that only a small part of the catchment is covered with plants, transpiration is expected to be small. Evaporation is also expected to be small, as evaporation from bedrock is minor and only around 1.3% of the catchment is covered by lakes and ponds. Storage Storage of water is expected mainly to be in the peat soil located in the lower parts of the catchment and in the active layer of the sediments in areas along the stream. No determination of water stored in sediments and rock crevices has been made so far, but the year-to-year storage is expected to be small due to the limited capacity of rocks and permafrost soils. Storage in glaciers has not been investigated, but older aerial photography from the surrounding area seemingly shows some local glaciers not present today, indicating that at least some glaciers are disappearing and therefore indicating a negative storage. However, specifically for the Pisissarfik watershed, a change in glacial distribution could not be determined from available aerial photography. Snow pockets may survive the summer, but the water equivalent of these is expected to be small. ADDITIONAL DATA Previously, for a short period, there was an additional station (CEB) in the Pisissarfik catchment at 1130 m a.s.l., where short wave incoming and outgoing radiation, temperature, wind speed and direction, as well as snow depth, was measured. At the outlet of the Tasersuaq catchment, Station 106 has been running for about 30 years. In addition, there is a climatic station in the town of Sisimiut with many years of data. There are only few maps available for this area: Kort og Matrikelstyrelsen (KMS) 1: (available as vector map) based on aerial photography from 1943 and Compukort Guide map for walking, West Greenland, Sisimiut 1: , 1996, based on aerial photos 1: from WATER BALANCE The water balance has not been determined very accurately to date. This is due to difficulties in correcting precipitation and unavailability of évapotranspiration data. The precipitation and discharge are given in Table 4. Using the normal water balance equation, the difference (residual) is due to storage, évapotranspiration and error in this case. It is clear that the yearly discharge data does not directly reflect the measured precipitation for the same period, thus indicating that there is some year-to-year difference in storage, possibly due to glaciers. Other explanations are that there is large variation in evaporation or that precipitation or discharge data do not represent actual

9 Water balance in a west Greenlandic watershed 151 conditions. For 1998, the model value of évapotranspiration is 160 mm year" 1 (Hag et al, 2001); this only explains about half of the P-Q difference. The remaining 140 mm must then be explained by storage and/or error. Due to the reasons given above, a year-to-year storage of 140 mm is not likely, and error in the catch and correction of precipitation is deemed a more likely explanation. Likewise, it is unlikely that the large variation in the annual residual term, from the 1997 value of 98 mm year" 1 to the 1999 value of 433 mm year" 1, could be due to storage and variation in évapotranspiration. However, more studies are badly needed to qualify this notion. CONCLUSIONS The Pisissarfik watershed is the only continuously monitored watershed in west Greenland where parameters such as wind and incoming and outgoing shortwave radiation are monitored. It is thus a good candidate for further enhanced research and modelling. The watershed had a specific runoff of mm year" 1 in the period 1997 to The results from the catchment show that precipitation undercatch is substantial, and, together with the lack of information on évapotranspiration and storage, this makes the water balance highly uncertain. Thus more detailed studies of this catchment are needed for the water balance to be improved. A better understanding of the water balance of west Greenlandic watersheds would be most valuable, e.g. in the local planning of hydropower and in climate change impact prediction. REFERENCES Boggild, C. E., Knudsen, M. B., Knudby, C. J., Pedersen, M. H., Starzer, W. & Thomsen, H. H. (1998) Modelsimuleringer af afstromninge fra et arktisk bassin et pilotstudie til vurdering af arktiske vandressourcer. GEUS 43, Goodison, B. E. (1978) Accuracy of Canadian snow gauge measurements. J. Appl. Met. 17, Hag, M. P., Bille-Hansen, J. & Olesen, O. B. (2001) Arkva II Arktiske vandressourcer-fysisk baseret modelvcerktoj , Greenland: Greenland Survey, ASIAQ. Nielsen, J. D. (1999) Vandressourcen i arktis, snesmeltningsberegninger i Pisissarfikoplandet, Vestgronland, ved hjajlp af G1S og energibalancemetoden. PhD thesis (unpublished data).