Ninth International Conference on Permafrost ... - IARC Research

Monitoring of the Floodplain Talik Downstream From theUst’-Srednekan ReservoirS.A. Guly, V.M. MikhailovNorth-Eastern Research Station of the Melnikov Permafrost Institute, SB RAS, Magadan, RussiaIn the continuous permafrost area of northeastern Asia,taliks exist only in river valleys and under large lakes. Many ofthem develop due to intensive convective heat exchange withrivers and occupy the entire floodplain. A generally acceptedindicator of such taliks are phytocenoses of thick mixedwoods in which large deciduous trees (Chosenia arbutofoliaand Populus Suaveolens) are often most abundant. Thesewoods stand out sharply against the background of sparsegrowths of trees (almost exclusively larch) dominating inthe lower belt of mountains and over flat interfluves (Fig.1). Many authors, following Vaskovsky (1958), call theman expressive name—tundra-forest—which accentuates thedepression of trees.The largest floodplain talik begins in the Kolyma Rivervalley between Ust’-Srednekan and Seimchan, and extendsdownstream for more than 500 km, somewhat lower thanZiryanka (Fig. 2). Its width varies from 2.5–3 to 4–5 km ormore. Thus, in this reach of the Kolyma River floodplain,a unique natural complex exists, the main constituents ofwhich are the outstandingly huge block of unfrozen groundand the vast woodland north from the taiga forests.In a few years the hydroelectric power station will beput into action 14 km upstream from Ust’-Srednekan. Itis common knowledge that reservoir regulations inducesignificant decreases in temperature and discharge of riversdownstream from dams during summer. Such changescannot but influence the hydrothermal regimes of contiguousaquifers.Convective heat exchange is caused by a two-way waterexchange (developing due to very high alluvium permeability);in each “elementary” river segment, infiltration of waterq into conv ground is accompanied by groundwater recharge. Theresulting heat flux into a floodplain talik ( qconv ) is defined bythe formula (Mikhailov 2002):− +qconv= C(ω T −ω Tf) , ω − (3)− +where C is water ωvolumetric −qconv= C(ω T −ω Tf) ,specific heat; ω − is specific (i.e.,related to the unit river surface area) rate of water infiltration ω +into alluvium; ω + is the same, only of groundwater ω + rechargeinto a river; T is water ω − temperature in a river; and Tf isweighted mean T groundwater temperature (“weights” ω −arefTfpoint rates of recharge). ω + Hereafter all dimensions are in q SI;convspecifically, water exchange characteristics and areqω − ω +measured in m 3 conv/(s∙ m 2 ). Water surface area qconv may vary greatlyTfover short time periods, so the influence of convective ω + T heatfexchange on the thermal regime of a talik is better describedby the product of qconv by a river width (B).Tf qconvCharacteristics of water exchange do not dependsubstantially on river discharge, while B is directly related toit. Therefore, both above-mentioned changes qconv will diminishthe amount of heat supplied to the Kolyma River floodplainwith unpredictable consequences for the talik.The situation has no precedent because never before wasFigure 1. Two typical landscapes of northeastern Asia: mixed foreston a floodplain talik (above) and dominating tundra-forest (below).Figure 2. Locations of the gauging stations on the Kolyma River:U – Ust’-Srednecan; S – Seimchan; B – Baligichan; Z – Ziryanka.89