Specific features of winter runoff formation in the southern Lena river basin
V.S.Vuglinsky (1), M.L.Markov (1)
(1) State Hydrological Institute
Winter runoff in the upper reaches of the Lena river basin is formed in the permafrost zone accompanied by a severe climate. In most rivers winter runoff is observed during one or two winter months; then it is ceased due to subsurface water depletion and its accumulation in big ice mounds ( or aufeis events ). Within the Baikal Range and Stanovoe Plateau where taliks are observed and water from cracks rises on the surface, winter runoff is permanent in most rivers Meanwhile, small watercourses with drainage areas less than 500 sq.km freeze up to the bottom. The nature of rivers freeze-up indirectly depends on the water availability in the river and its drainage area. Local hydrogeology and depths of subchannel deposits where subchannel runoff is formed also affect the river freeze-up greatly. It has been established that rivers with mean annual runoff no more than 2 l/s/sq.km on the main part of the study territory and no more than 3 l/s/sq.km on the Stanovoe Plateau may be frozen up to the bottom. In some rivers, e.g. in the upper Aldan and Olekma rivers ice holes may be formed during mild winters and aufeis events may be observed in one and the same site.
The winter low-water period is observed in most rivers in the middle of October and it is over late in April or early in May. Mean duration of the winter low-water period in the tributaries of the upper Lena river is usually 190-200 days long. Maximum winter low-water runoff is typical of the rivers running down the Stanovoe and Patomskoe Plateaus where it may exceed 60 mm. In the rivers of the Trans-Baikal region and Upper Aldan river runoff during winter low-flow period may equal 10-20 mm.
A rapid freeze-up of thin and loose deposits in valley bottoms early in winter and an intensive increase of the ice cover thickness on rivers reducing the conveyance capacity of soils and river channels and producing an additional hydraulic resistance to the water flow are specific features of the study region with a long winter period and low air temperatures. This event, so-called gcryogenic barrageh is most typical of small rivers; it makes the subsurface water drawdown in winter less intensive if compared with the warm season. On the contrary, in the lower river reaches the water drawdown in winter is great if water inflow to channel from the upper reaches is limited or missing. During the warm season melt and subsurface water from river heads flows down and reaches lower links of the river network where it is lost for infiltration into river banks and for aquifer recharge, thus making water discharge increase in river channels slower. Thus, a gcryogenic h river runoff control is made.
Some case studies are given in the paper on the specific features of winter river runoff in the upper reaches of the Lena river; conditions of the gcryogenic barrageh formation and its effect on river runoff redistribution are described.
