Glacier Mass Balance and Regime: Data of Measurements and ...

were found between published and recalculated values of glacier area, seasonal components, and annualmass balances. The results of these comparisons are given in Table 2.1.Data on mass-balance variables versus altitude are basic for calculation of variables for an entireglacier, and it is unfortunate that it has not become the rule to publish all such results of measurementsalong with results averaged over entire glacier area.Results presented in Appendices 2 and 3 were basically obtained using the glaciological method.Still, there are many modifications of the method in use, at least eleven, according to ∅strem andBrugman (1991). It is difficult (if not impossible) to reconstruct what modification were used to getvalues for any particular glacier and year. The original definitions of variables have generally beenpreserved in Appendices 2 and 3; these indicate the system used in field work and data presentation. Inthese appendices, b n, b w, b s, means the annual, winter and summer balances in either the stratigraphic,fixed-date, or combined systems (Mayo et al, 1972). In several cases the annual values of snowaccumulation, ablation, mass balance (c t, a t, b a) are given. Data obtained by the “alpine system” aregiven as a c(net accumulation), a a(net ablation), and b n(net balance). Obviously, substantial differencesexist between seasonal variables as shown above, but b ais nearly equal to b n, by the definition of themethod.2.5. HOW PRECISE CAN MASS-BALANCE DATA BE?The usual approach to the estimation of data quality is to compare data obtained by one methodwith one or more independent measurements. One common approach is to verify data obtained by theglaciological method by comparing them with the results of repeated geodetic/topographic surveys. Thishas been done for several glaciers in the world. The comparisons shown in Table 2.2 suggest that, ingeneral, the two methods only give comparable results for a few cases (glaciers and periods).Discrepancies between the two methods may have different signs (plus and minus) for different glaciersand for different periods. For some glaciers the discrepancy is large (Blue, Storbreen), but for others it isvery small (Careser, Djankuat). There are several reasons discussed in detail by Golubev et al., 1978;Andreassen, 1999; Conway et al, 1999; Krimmel, 1999; ∅strem and Haakensen, 1999; and others. Oneimportant conclusion from these comparisons and discussions is that the topographic method may serveas a control for many cases. Obviously two methods are needed to get sufficient data control.The case of Alfotbreen is of special interest. The calculated mass balance for this glacier appliesto about 25% of the entire Alfotbreen ice cap area (∅strem and Haakensen, 1999). The accumulation38