Ninth International Conference on Permafrost ... - IARC Research

Recent Rise of Water Level in Lake Hovsgol in the Permafrost Zoneof Northern Mongolia: Trends and Causal FactorsKazuo TakedaObihiro University of Agriculture and Veterinary Medicine, Obihiro, JapanHiroji FushimiUniversity of Shiga Prefecture, Hikone, JapanTatuo KiraLake Biwa Environmental Research Institute, Ohtsu, JapanIntroductionLake Hovsgol is the largest freshwater lake in Mongolia(Table 1), located on the southern fringe of the east Siberianpermafrost zone that supports the so-called “light taiga,” orthe deciduous conifer forests dominated by larch (Larix spp.).Once the larch vegetation is destroyed due to increasing forestfires and the outbreak of pest insects, direct sunshine raisesground surface temperature, resulting in deeper thawing ofpermafrost in summer.According to observations at the station of the NationalAgency for Meteorological (1940–2006) in Hatgal village(on the southernmost shore of Lake Hovsgol) during 67years, the annual mean temperature has gone up at the rateof 3°C per century with mean annual precipitation of only200~400 mm/yr. Also noticed is that the water level of thelake has risen by 100 cm during the latest 37 years (MurunMeteorological Station 1970–2006).As for the cause of such recent rise in water level, it issometimes suggested that global climate change might beresponsible (Kumagai 1998, Kumagai et al. 2006). Namely,the inflow of thawed fossil water from permafrost layers andglaciers brought about by global warming is supposed to be themain source of increasing lake water. This hypothesis, however,lacks concrete evidence because of the absence of continuousreliable records of groundwater supply into the lake.This paper deals mainly with another causal factor relevantto the lake water level rise, viz. the bottleneck structure at thehead of the Egiyn Gol (= river), the sole outlet of the lake.Environmental information related to the water budget of thelake is presented based on field survey around the lake.Study MethodsField surveys were conducted at 12 sites (Fig. 1),including 57 observation points during 3 years since 2000.Items observed were depth of active later (at most sites),soil properties such as moisture content, and grain sizecomposition (at some limited number of sites). To find thedepth of the active layer, a temperature profile was measuredalong a vertical hole made by striking an iron rod into theground. The 0°C isotherm depth was then estimated byextrapolating the profile curve downward. This procedurewas done after late August, when the active layer depthapproached the seasonal maximum.About 1.5 km southward from Hatgal at Site L (Fig. 1), theEgiyn River starts draining lake water through a very narrowand shallow bottleneck route, produced by the deposits carriedby a tributary river. The structure of riverbeds of the two joiningstreams, so important for the control of the lake’s water budget,was carefully surveyed. Further, as some of groundwater, theflow rate of an inflow stream originating from Har usnii Springabout 700 m from the lake (Site C, Fig. 1) was measured.ResultsOn most alluvial beds deposited around the lake, groundsurface soil layers generally consisted of silt and clay,fairly rich in mixed round gravel, and relatively dry witha volumetric water content of 10–20%. The estimatedmaximum depth of the active layer in late summer mainlyamounted to 1.5–4.0 m under grasslands (pasture), while itwas mainly 1.4–2.0 m under larch forests. The water contentof surface soil under larch forests on north-facing slopeswas around 22%, whereas it was only 13% on slopes facingthe other three directions. However, once such a foreststand had been burned or clear-felled, the depth of activelayer increased up to 1.9–5.4 m, owing to increased solarirradiation on the exposed ground.At Site L, a large tributary river, Ulgen Sair, joins theuppermost stream of Egiyn River (Fig. 2). The stream bedof Ulgen Sair is far larger than the Egiyn stream, about 400m wide at the confluence, and is entirely filled with fullyeroded round gravel. Surface water flow does not exist undernormal weather, but in case of concentrated heavy rainfall,vast mounts of debris are carried down by flood water anddeposited on and around the confluence with the Egiyn River.For instance, in July 1971, the debris deposits following aheavy rainfall of 71 mm/day (at Hatgal) entirely buried theEgiyn stream, completely cutting off the outflow from the lake(Batsukh et al. 1976). Similar events took place twice in 2003Table 1. Physical dimensions of Lake Hovsgol (Kurata 1993).Location50°27′–51°37′N, 100°51′–101°47′EAltitude (m) 1,645Surface area (km 2 ) 2,770Maximum depth (m) 267Mean depth (m) 138Volume (km 3 )* 383Length of shoreline (km) 414Drainage basin area (km 2 ) 4,940Forest (km 2 )** (2,365)Pasture (km 2 )** (1,559)Mountain (km 2 )** (1,016)* Data (Kumagai 1998); ** Values measured in map.307