Surface of mean annual runoff

Recall that runoff is discharge divided by the catchment area. It has units of water depth. Gauge data can be used to construct surfaces of runoff. Plate 3 is the mean annual runoff

Mean Annual Runoff Depth China

Plate 3 Mean annual runoff surface for the Arctic drainage over the period January 1960 to December 1989. White areas within the southern Ob Basin represent internal drainage basins. Large areas along the Arctic Ocean coast and the Canadian Arctic Archipelago (shown as white) are ungauged. The lines over the Arctic Ocean indicate sea basins into which different river systems drain (from Lammers et al., 2001, by permission of AGU). See color plates section.

Plate 3 Mean annual runoff surface for the Arctic drainage over the period January 1960 to December 1989. White areas within the southern Ob Basin represent internal drainage basins. Large areas along the Arctic Ocean coast and the Canadian Arctic Archipelago (shown as white) are ungauged. The lines over the Arctic Ocean indicate sea basins into which different river systems drain (from Lammers et al., 2001, by permission of AGU). See color plates section.

surface over the period January 1960 to December 1989 for the monitored part of the Arctic drainage. Following Arnell (1995), calculations of runoff were made for all "interstation areas". These were obtained by subtracting all upstream discharge values from the discharge value at the next downstream gauge. Runoff was then calculated by dividing the interstation discharge by the interstation drainage area. The monthly interstation runoff values for each gauge were then distributed to the interstation regions to provide a gridded field.

The spatial resolution of the runoff surfaces that can be obtained depends on the density of the gauge network, which varies greatly. For large interstation areas, inconsistencies can be introduced because of the long transit time of water moving downstream in the river system. This was mitigated in Plate 3 by aggregating monthly runoff into annual sums. Flow diversions and impoundments (dams and irrigation channels) can significantly alter the natural drainage. Known problem areas include the Nelson/Churchill rivers in western Canada, the La Grande hydroelectric complex east of James Bay, the southern Ob, the main stem of the Yenisey River and the lower Lena. Ye et al. (2003) provide a comprehensive assessment of problems in the Lena. However, at least for annual discharge, anthropogenic effects are considered to be as yet fairly minor for the Artic drainage as a whole as compared to other parts of the world (Lammers et al., 2001). Some strong impacts have been documented with respect to winter discharge (see Chapter 11).

Annual runoff is highest over the mountainous regions of central and eastern Siberia and along the Rocky Mountains in western Canada, the European part of Russia and the eastern Hudson Bay drainage in Quebec. Areas of low runoff include the southwestern Ob Basin and the south-central part of the drainage in Canada. The mean annual runoff as averaged across the Arctic drainage is calculated at 212 mm yr—1. There should be a correspondence between mean annual runoff and mean annual P — ET as computed from the NCEP/NCAR reanalysis. Annual P — ET fields interpolated to the Arctic drainage as defined in Plate 3 do capture the general features of observed runoff. There is also considerable disagreement, however, related to the different resolutions of the data sets and to errors in the reanalysis fields.

The ungauged areas of the Arctic drainage can be assessed by comparing Plate 3 and Figure 6.12. The ungauged areas include much of coastal Eurasia, the Eurasian Arctic islands, northern Alaska and the Canadian Arctic Archipelago. It remains a challenge to estimate runoff from these ungauged areas and assess their contribution to the freshwater budget of the Arctic Ocean.

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