Geoscience Reference
In-Depth Information
Figure 6.20.
Water-year time series (mm) of
runoff (RU), precipitation (P), precipitation
minus evapotranspiration (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).
in the timing of snow melt. Early melt leads to greater discharge in May. Other fac-
tors being equal, less of the snowpack is left during June, reducing runoff in this
month. A late snow melt typically means less runoff in May, and more in June. For
smaller watersheds, variability in discharge will 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 variations in runoff is
ice jams, which block the flow of river. When the jam breaks, there is a rapid dis-
charge of water. Ice jams can sometimes cause catastrophic flooding of downstream
communities.
A final example is provided by water-year time series of runoff, P-ET, ET, and
P for the Lena
(
Figure 6.20
). 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 in 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, the Yenisey, and the Mackenzie, the
correlation between water year runoff and precipitation is quite weak, as is the cor-
relation (shared variance) between runoff and P-ET. But this is not true for the Lena.
Over the 1960-1999 period, the squared correlation between water year runoff and
P-ET and between runoff and precipitation is 0.52 and 0.61, respectively.
These interbasin differences may point to effects such as diversions and impound-
ments, but these are not considered to be major at the present time, at least for water
year means. The primary reason for these differences lies with the extensive per-
mafrost in the Lena. Most of the Lena is underlain by continuous permafrost. As
discussed, the impermeable permafrost layer promotes rapid movement of precipi-
tation into river networks and will dampen interannual variations in groundwater
recharge. The low correlations for the Ob, the Yenisey, and the Mackenzie, where
there is less permafrost, point to recharge effects.
