RUNOFF GENERATING PROCESSES IN A MOUNTAINOUS HEADWATER IN THETRANSITION ZONE BETWEEN STEPPE AND TAIGA IN NORTHERN MONGOLIA

Northern Mongolia faces a variety of water-related problems that are related to both, the harsh natural conditions and the lack of structures to control the distribution and protection of water. The Kharaa River Basin (KRB), which is located north of Mongolia's capital Ulaanbaatar, was chosen as a model region for the development and implementation of an integrated water resources management (IWRM) project called MoMo (Model Region Mongolia). The aim of IWRM is to provide insight into both, societal structural and eco-systematic conditions as a whole. Besides an improvement of the structures, especially in the urban sector, all relevant water related processes are captured. This includes the identification of “water towers” and the underlying runoff generating processes. The KRB stretches from the steppe towards the western Khentii Mountains. Thus, it covers a wide range of environmental conditions and can be considered to be representative for many regions within Mongolia. The Sugnugr Basin situated within the western Khentii Mountain ranges is one of the most important tributaries of the Kharaa. The climate is highly continental and semi-arid; average annual precipitation in the mid-reaches remains below 400 mm. With a fraction of 90 %, precipitation is predominantly falling during the summer half year (May to October). Air temperature in 2011 and 2012 was -2°C and -3°C, with monthly air temperatures ranging between 28°C in January and 16°C in July. The Sugnugr Basin is situated in the transition zone between steppe and taiga, and is part of the discontinuous permafrost zone. Thus, it is characterized by a heterogeneous mosaic of herbaceous steppe vegetated south-exposed slopes and taiga vegetated north-exposed slopes that exhibit permafrost. Above the tree line of approximately 2300 m a.s. l. alpine scree prevails. In 2004 and 2007 widespread forest fires destroyed considerable amounts of the taiga, especially in the headwaters. Forest regrowth under the prevailing natural conditions can last 200 years and more. The following presented runoff generating processes are based on comprehensive and detailed field studies in an anthropogenic unaffected headwater of the Sugnugr. The results indicate hydrological processes in the study area to be controlled by the dominant vegetation cover. It has been shown that steppe vegetated south-exposed hillslopes only sporadically contribute to river runoff during intense precipitation events. Infiltration rates into the soil matrix are small and evenly distributed and no signs of preferential flow were found. In general, these sites were characterized by dry conditions and soil moisture content occasionally reached the permanent wilting point during the summer months. A majority of river runoff must hence be generated by taiga vegetated north-exposed hillslopes. Especially the thick organic surface layer and the depth of the active layer have been shown to control runoff response. On the catchment scale, hydrograph recession analysis revealed a quick transfer of event water on top of a thin active layer towards the rivers in early summer. In the course of summer, active layer depth and thus storage capacity is increasing, resulting in enhanced recession periods and increased low-flow runoff. Simultaneously, it could be shown that hillslope runoff only occurs after a threshold of 5 % increase of the relative volumetric water content of the upper soil zone, including the organic surface, is reached. Wildfires alter this system sustainably. As the insulating organic surface cover is removed, soil temperatures have been shown to increase, while permafrost subsided to deeper depth. This resulted in a network of preferential flow paths on top of the frost table, which effectively transfers event water towards the adjacent river without substantial retention. Land-cover change scenarios were simulated with the conceptual hydrological model HBV-D. As a consequence of a wildfire, with all taiga stands being burned, monthly runoff was simulated to decrease during the winter half year, while it increased during the summer half year. However, results of the field studies suggest that the simulated increase in summer runoff following wildfire does not lead to greater water availability, as stormflow runoff increased while low flow runoff decreased. The results support the conservation of the alpine headwaters, especially from deforestation and wildfire in order to safeguard the current amount of surface water quantity. This gains further importance regarding the flourishing Mongolian economy and the related increasing water demands.