Variability

Precipitation, evapo-transpiration and river discharge are all highly variable on monthly and shorter time scales. Figure 6.15 provides one example. This figure shows for two years (1980 and 1981) daily time series of precipitation for the Lena Basin, based on averaging data for all monitoring stations within the basin. Also given is the mean

6 i- t r—i—r—r ri it r r—m—m—i—i—t—i r l l—l—r

JFMAMJJASONDJFMAMJJASOND

1980

1981

Figure 6.15 Daily averaged precipitation over the Lena for 1980 and 1981 (solid line) along with the mean annual cycle (dotted line) over the period 1960-91 (from Serreze and Etringer, 2003, by permission of John Wiley and Sons).

JFMAMJJASONDJFMAMJJASOND

1980

1981

Figure 6.15 Daily averaged precipitation over the Lena for 1980 and 1981 (solid line) along with the mean annual cycle (dotted line) over the period 1960-91 (from Serreze and Etringer, 2003, by permission of John Wiley and Sons).

annual cycle, based on monthly means for the period 1960-92. It is clear that the mean annual cycle, even on the large scale of the Lena, is superimposed on highly variable precipitation on a day to day basis. As would be expected, the largest daily precipitation events tend to be associated with well-defined cyclonic systems (Serreze and Etringer, 2003).

Another example is provided by monthly discharge at the mouth of the Lena River over the period 1936-95 (Figure 6.3). One of the key features is the large year-to-year variability from May through October. This relates to variations in the water equivalent of the snowpack, temperature as it influences snowmelt, and the relative frequency of widespread and intense precipitation events (such as those just discussed). The variability is especially pronounced in May, pointing to variations in the timing of snowmelt. Early melt leads to greater discharge in May. Other factors being equal, less of the snowpack is left during June, reducing runoff in this month. A late snowmelt typically means less runoff in May, and more in June. For smaller watersheds, we would expect variability in discharge to be more pronounced - for example, a single summer precipitation event over a small watershed in the Lena can result in a pronounced hydrograph peak, which may be reflected only weakly at the mouth of the Lena some time later. Another factor that can lead to variations in runoff is ice jams, which block the flow of the river. When the jam breaks, there is a rapid discharge of water. Such ice jams can sometimes cause catastrophic flooding of downstream communities.

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Jan Feb Mar Apr May Jun J.Ii Aug Sep Oct Nov Dec

Month

Figure 6.3 Monthly discharge (m3 s—1) at the mouth of the Lena over the period 193699. For each month, the plot shows the discharge for all years (from Yang et al., 2002, by permission of AGU).

Jan Feb Mar Apr May Jun J.Ii Aug Sep Oct Nov Dec

Month

Figure 6.3 Monthly discharge (m3 s—1) at the mouth of the Lena over the period 193699. For each month, the plot shows the discharge for all years (from Yang et al., 2002, by permission of AGU).

Figure 6.4 Water-year time series(mm) of runoff (R), precipitation (P), precipitation minus evapo-transpiration (P - ET) and ET for the Lena. ET is calculated as a residual from P and P - ET. P is adjusted for estimated gauge undercatch and P -ET is from aerological estimates using NCEP/NCAR reanalysis data (adapted from Serreze et al., 2003a, by permission of AGU).

Figure 6.4 Water-year time series(mm) of runoff (R), precipitation (P), precipitation minus evapo-transpiration (P - ET) and ET for the Lena. ET is calculated as a residual from P and P - ET. P is adjusted for estimated gauge undercatch and P -ET is from aerological estimates using NCEP/NCAR reanalysis data (adapted from Serreze et al., 2003a, by permission of AGU).

A final example is provided by water-year time series of R, P — ET, ET and P for the Lena (Figure 6.4). P — ET is based on aerological estimates from the NCEP/NCAR reanalysis. The precipitation data contain adjustments for estimated gauge undercatch. ET is again estimated as a residual. Runoff is based on records in R-ArcticNET. The time series of annual P and P — ET are highly correlated, as they are in the other major river basins. In the Ob, Yenisey and Mackenzie, the correlation between water-year R and P is quite weak, as is the correlation between R and P — ET. But this is not true for the Lena. Over the period 1960-99 the squared correlation (shared variance) between water-year R and P — ET and between R and P is 0.52 and 0.61, respectively.

These inter-basin differences may point to effects such as diversions and impoundments, but these are not considered to be serious at the present time, at least for water-year means. The primary reason for these difference lies with the extensive permafrost in the Lena. Most of the Lena is underlain by continuous permafrost. As discussed, the impermeable permafrost layer promotes rapid movement of precipitation into river networks and will dampen interannual variations in groundwater recharge. The low correlations for the Ob, Yenisey and Mackenzie, where there is less permafrost, point to recharge effects.

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