No abstract

No abstract

No abstract

ABSTRACT. Due to in c reased demand for electric energy, which in Norway is produced almost entirely by hydro-electric power plants, extensive glacierized basins are now considered for future power produc tion. However, glacier behaviour has a great impact on the annual run-off from these basins, and information about expected a nnual deviations from a "normal" discharge must be obtained from mass-balance studies o n representative glaciers within the basin . For this reason suc h studies are presently performed at eight selected Norwegian glaciers ; the costs involved are in the order of $3 000-$10 000 annually for each o bserved gla cier.The transient snow line can be identified easily on conventional air photographs and its height dete rmined by direct comparisons with reliable topogra phic maps (with contour lines on glaciers) . The resolution in ERTS images is sufficiently good to use the satellite data for a similar height determination of the transient snow line on glacie rs-at least on the larger valley glaciers and, of course, on ice caps.Due to an existing correlation between the height of the tra nsient snow line (at the end of the melt season) and the a nnual specific net mass balance, it is possible to construct a diagram expressing this relationship, provided detailed mass-balance studies have been performed during a period of several years of different mass-balance conditions (ideally comprising both years of negative and of positive net balances ). Determinations of the transient snow-line height based upon ERTS data, obtained at the end of the summer season, can then be used to determine the mass balance for those glaciers whe re the above-mentioned relationship is es ta blished. This would be a great advantage in cases where there is a request for information on the net bala nce only; no separate data on winter balance or summer balance will be obtained by this method.Experiments have been performed to d elineate the transie nt snow line on glacie rs directl y from ERTS digital data (CCT magnetic tape). The results indicate that last winter's snow is characterized by grey levels 41 -48 (of 63 possible grey-scale levels) on MSS-7, whereas exposed glacie r ice is represented by le vels 13-20 in the same multi-spectral band. Snow a nd ice can also b e separated on MSS-4, but the differe nce in grey-scale levels between the two materials is much smaller. RESUME. Les donnees ERTS en glaciologie -un essai pour deduire des bilans glaciaires cl partir d'images de satellite. En raison de la demand e accrue d'energie electrique qui, e n Norvege, est presque entierement d ' origine hydro-electrique, d e grands bassins englaces sont actuellement examines du point d e vue de leurs possibilites d e production future d'energie. Quoiqu'il e n soit, les comporte m e nts des glaciers ont une influence importante sur les ecoulements annuels en provena nce d e ces bassins et une prevision d es va ri a tio ns a nnuelles a ttendues pa r rapport a un de bit " no rmal" peut etre basee sur des e tudes d e bilans da ns ...

Østrem, G. and Haakensen, N., 1999: Map comparison or traditional mass-balance measurements: which method is better? Geogr. Ann., 81 A (4): 703-711.ABSTRACT. A number of Norwegian glaciers were selected in the 1960s for long-term mass-balance measurements, to produce necessary hydrological information for hydropower exploitation. Special large-scale glacier maps were produced for field work and data processing, and some glaciers have been mapped more than once. Thus, comparison of glacier maps can be used to calculate changes in glacier volume for some of the glaciers, provided they are of sufficient accuracy.Conventional mass-balance measurements were carried out on all the selected glaciers. A cumulative calculation of net balances for a series of years is used to indicate the change in a glacier's volume during that period. However, various errors originate in the field, some of which are systematic, particularly on glaciers with large winter accumulation.The present study indicates that certain errors are difficult to define and determine, For the maritime glacier Ålfotbreen, a cumulative mass-balance calculation gives a positive total balance (+3.4 m water equivalent in the period 1968-88), whereas the map comparison indicates a total negative balance (-5.8 m water equivalent). This indicates a discrepancy between the methods, which must be accounted for.Determination of errors in mass-balance measurements is difficult. Sinking of stakes in the accumulation area and the use of sounding sticks (steel probes) in heavy snowlayers cause problems.