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The Selenge basin contributes approximately 50% of the total inflow into Lake Baikal and is thus of high significance for the regional hydrological regime. Our study was conducted in the upper reaches of the basin, where the Selenge river and its tributaries flow through the Mongolian forest-steppe. Monthly and maximum runoff, precipitation, and air temperature data from 12 gauging stations collected between 1978 and 2015 were analyzed to characterize the hydrological regime response to climate change. Concomitant with rising temperatures and increased potential evaporation, river runoff in the Mongolian part of the Selenge basin has decreased from the first interval (1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995) of our study period compared with the consecutive interval from 1996 to 2015. The decrease in runoff throughout the study area was most likely caused by an increase in potential evapotranspiration (and not reduced precipitation or land use changes) for both summer rainfall-and snowmelt-dominated rivers. Annual maximum runoff has also strongly decreased suggesting that reduced flooding is a contemporary threat for Mongolia's riverine ecosystems, probably causing the replacement of wetland and mesic habitats.

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Permafrost is an important component in the ecosystem and plays a key role in soil regime characteristics in high-altitude regions. Thawing depths and mean annual ground temperatures are the main parameters to conduct research on permafrost. Here we present the results of different modeling approaches for estimating thawing depths and mean annual ground temperatures in the Khuvsgul region of Mongolia. The aim of this study was to analyze the modeling approaches and determine what model best simulates the different characteristics of the soils. Moreover, this study investigates the factors that determine the best fit model approaches for certain conditions of the study area. For this study, the Stefan model was applied to estimate thawing depths and the TTOP and Kudryavtsev model approaches were applied for the estimations of mean annual ground temperatures. The estimations were performed at seven observational boreholes in the region. The evaluations show that model results are more sensitive to thermal and physical properties of the soil than the air temperatures for estimating thawing depths and mean annual ground temperatures.

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