Mean hydrographs

Long-term annual mean runoff ought to balance P — ET, but this is not true of means for seasonal and shorter time scales. The most obvious reason is that, for much of the year, the net precipitation is stored in the snowpack. There are also subsurface storages. The typical annual cycle is as follows. In the cold months, runoff is small, and can even cease for small basins. Runoff begins to climb in April or May when the snowpack begins to melt. It then increases sharply, generally with a June peak. This is the same general pattern we would see for any region (e.g., the Colorado Rocky Mountains) where there is significant seasonal snowpack storage. As assessed over the Arctic drainage, approximately 60% of the discharge occurs from April to July (Lammers et al., 2001). However, the shapes of the mean hydrographs (plots of runoff or discharge versus time) vary between different basins in response to temperature as it influences snowmelt and ET, the duration of river freeze-up, the mean water equivalent of the snowpack, the seasonality of precipitation, permafrost conditions, soil moisture and groundwater recharge, runoff storage in wetlands and other factors.

Differences in the mean water-year hydrographs for the Ob, Yenisey, Lena and Mackenzie (Figure 6.14) capture some of these processes. While runoff peaks in June for all basins, the peaks are much sharper in the Yenisey and Lena as compared with the Ob. In part, this manifests lower mean temperatures in the Yenisey and Lena from autumn through spring, meaning a longer period of snow accumulation and delayed spring melt. Once it becomes warm enough in June, the rapid melt of the snowpack gives rise to sharp runoff peaks. Especially for the Lena, the large extent of continuous

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Figure 6.14 Mean hydrographs for the four major Arctic-draining rivers (at the gauge sites closest to the river mouths) expressed in terms of runoff (mm) (adapted from Serreze et al., 2003a, by permission of AGU).

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Figure 6.14 Mean hydrographs for the four major Arctic-draining rivers (at the gauge sites closest to the river mouths) expressed in terms of runoff (mm) (adapted from Serreze et al., 2003a, by permission of AGU).

permafrost fosters rapid movement of meltwater to streams, further contributing to the sharp June runoff peak. Over the period 1970-99, 35% of total discharge at the mouth of the Lena occurs in June and a further 20% in July (Yang et al., 2002). By contrast, especially for the Ob, a large proportion of precipitation is lost through ET, especially in summer, and is not available for runoff. Other effects not considered here include snowmelt storage in lakes, wetlands and aufeis (see Chapter 2) that can subdue peak discharge. Storage in lakes and wetlands is believed to be pronounced for North American rivers such as the Mackenzie (Bowling et al., 2000). River and lake ice is a further storage component.

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