Mass balance of Storglaciären 2004/05

Transcription

1 Tarfala Research Station Annual Report 2004/2005 Mass balance of Storglaciären 2004/05 Yvo Snoek Dept. Physical Geography & Quaternary Geology, Stockholm University, Sweden and Department of Physical Geography, Utrecht University, the Netherlands Summary The winter balance was surveyed between April 24 and April 27 by Peter Jansson, Henrik Törnberg, John Hulth, Mattias de Woul and Steffen Holtzkämper. Snow depths were determined by manual probing of 304 points in a 100*100 meter grid network (Appendix 1). During this period, snow density was measured in pits at six locations, of which four in the ablation area and two in the accumulation area (Appendix 2). The density profiles from the snow pits as a function of snow depth were used to converse the snow depths in water equivalent values. The cumulative snow depths were plotted against the cumulative water equivalent values and two empirical relations, a linear fit yielding d = d with an r 2 = and a second w.eq. snow 2 order polynomial fit yielding d = d + w.eq. snow d with an r 2 = , were obtained (Figure snow 1). All measured snow depths were then converted to Figure 1. Cumulative water equivalent (m w.eq.) as a function of snowdepth on Storglaciären, spring A9 1

2 Tarfala Research Station Part A Monitoring Figure 2. Ablation as a function of elevation (the ablation gradient) water equivalent values using these two relationships. The resulting values of water equivalents were interpolated for the entire glacier surface using the kriging interpolation (Appendix 3 and 4). The main difference between both fits is a higher cumulative water equivalent at larger snow depths for the polynomial fit. The total accumulation according to the two empirical relations (linear and polynomial fit) during the winter 2004/05 was 1.64 and 1.68 m w.eq., respectively. The latter value will be used in the further analysis, since the second order polynomial fit showed a higher correlation. During the spring new ablation stakes were established to improve the existing ablation net. In September 2003 a total of 74 stakes were available for determining the summer balance of the glacier (Appendix 5). The ablation in areas with no stakes was extrapolated using two different interpolation methods, e.g. kriging and the ablation gradient. The latter method was obtained using the relation between ablation and elevation, and yielded b = z s m w.eq., r 2 = (Figure 2). The glacier surface Figure 3. Winter, summer and net balance as a function of altitude. 05A9 2

3 Tarfala Research Station Annual Report 2004/2005 Figure 4. Net balance and area distribution as a function of elevation. was subdivided in elevation bands of 20 meters. The mean ablation was calculated for each elevation band. A mean value for the entire glacier was obtained by multiplying the mean ablation by the proportion of the area of the specific elevation band to the total glacier surface area. The summer balance of 2005 according to both methods (kriging and ablation gradient) was 1.65 and 1.61 m w.eq., respectively. The distribution of melt is shown in Appendix 6. The winter and summer balance values yield a net balance volume of 98.86*10 3 m 3, equivalent to 0.03 m w.eq. (Table 1, Figure 3 and 4) according to the kriging method and *10 3 m 3, equivalent to 0.07 m w.eq. for the ablation gradient method. Since the summer balance (and the net balance) of previous years was mostly published using the kriging method, the final summer and net balance values 2004/05 will be represented by the values according to the kriging interpolation (Appendix 7). The equilibrium line, the mean elevation with a b = 0 m w.eq., was calculated n mass balance (m w.eq.) Year -4 Figure 5. The 60-year mass balance record for Storglaciären, including 5 year-averages. 05A9 3

4 Tarfala Research Station Part A Monitoring Elevation (m a.s.l.) July 11 July 19 August 2 August 16 August 29 September net balance (m w.eq.) Figure 6. Transient net balance as a function of elevation. at 1485 m a.s.l. (Figure 3 and 4). The ratio between accumulation area and total area (AAR) was estimated to 44.6%. As can be seen in Figure 5, this year was the first year since 5 years with a slightly positive net mass balance, with both a higher winter balance and a lower summer balance compared to the average in the period (b = 1.45, b = 1.71, b = -0.25). w s n In addition, the transient summer and transient net balance were calculated according to both methods, e.g. kriging and ablation gradient (Table 2 and Figure 6). It should be mentioned that some assumptions had to be done for the kriging method since insufficient ablation data in the accumulation area were available for several dates, e.g. July 11, July 19 and August 29, resulting in failure of the kriging routine. Therefore, several ablation values in the accumulation area were estimated using the calculated ablation gradient for the specific date and a new interpolation, using kriging, was done. However, a negative net mass balance resulted using this assumption for August 29 whereas the final net balance on September 14 was slightly positive. As can be seen in Figure 6, the net balance is higher for August 29 than September 14 above 1500 meters. Keep in mind that the latter one was not calculated with ablation estimates, but only with true ablation observations. 05A9 4

5 Tarfala Research Station Annual Report 2004/2005 Table 1. Mass balance of Storglaciären 2004/05 Elevation Area Winter balance Summer balance Net balance (m a.s.l.) (m 2 ) (m 3 ) (m w.eq.) (m 3 ) (m w.eq.) (m 3 ) (m w.eq.) Table 2. Transient summer and net mass balance of Storglaciären Date Winter balance Summer balance, Net balance, Summer balance, Net balance, m w.eq. kriging m w.eq. kriging m w.eq. gradient m w.eq. gradient m w.eq. July * 1.38* July * 1.06* August August August * -0.07* September * Note: estimates of ablation in the accumulation area were done using the ablation gradient 05A9 5

6 Tarfala Research Station Part A Monitoring Appendix 1 Snow depth survey on Storglaciären 2005 Appendix 2 Density values as a function of snow depth for the different snow pits on Storglaciären in April A9 6

7 Tarfala Research Station Annual Report 2004/2005 Appendix 3 Winter balance map for Storglaciären 2005, according to a linear fit between water equivalent and snow depth (see text) Appendix 4 Winter balance map for Storglaciären 2005, according to a second order polynomal fit between water equivalent and snow depth (see text) 05A9 7

8 Tarfala Research Station Part A Monitoring Appendix 5 Stake locations and ablation values on Storglaciären 2005 Appendix 6 Summer balance map for Storglaciären A9 8

9 Tarfala Research Station Annual Report 2004/2005 Appendix 7 Net balance map for Storglaciären 2004/05 05A9 9