ET calculated as a residual

With fields of monthly P — ET from the aerological approach and gridded fields of monthly P from station networks, one can obtain fields of estimated ET. Such estimates of course contain the errors in both P and P — ET. Plate 2 illustrates results from this approach applied to the Arctic terrestrial drainage (see Figure 2.8). P — ET is based on means calculated from NCEP/NCAR data over the period 1960-89, while the precipitation fields are based on station records for the same period with the Legates and Wilmott (1990) bias adjustments. Serreze et al. (2003a) provide details.

Estimated ET for July is largest (>70 mm) over west-central Eurasia and the southern part of the North American domain. Monthly totals over northern Canada and northeast Eurasia are much smaller (10-40 mm). For some areas, especially in the cold months, residual ET is negative. At face value, this implies net deposition of water vapor at the surface. While rime deposition occurs in winter, it is unlikely to result in climatological values of negative ET. Indeed, sublimation during blowing snow events can represent a significant loss of snow mass during winter (Kane et al., 1991; Hinzman et al., 1996). These results indicate that either/or: (a) bias adjustments not withstanding, estimated winter P is still too low, (b) calculated P — ET is too large.

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