Runoff Hydrograph Analysis of Small Rivers in Lake Hovsgol Area
D. Oyunbaatar, Brad Hiller and Puntsag Tamir
Hovsgol Lake is a deep, pristine lake located in northern Mongolia on the southern edge of the taiga forest and the boundary of continuous permafrost. There are 101 rivers flowing into Lake Hovsgol. Within the Hovsgol Lake Long Term Ecological Research Study (LTER), there are six river valleys on the eastern side of the lake which have been included in the study and have conducted basic hydrological observations. The river valleys, in order from north to south are: Turag River, Shangnuul River, Noyon River, Sevsuul River, Dalbay River and Borsog River. The catchment area of the selected rivers vary from 81.9 to 226 km2 and mean basin elevation is located at an altitude of 1880-2030 m. On average about 55 % of the cathments of the selected rivers are covered by forest. Basic hydrological data obtained from field measurement and also by permanent staff gauges with calibrated pressure transducers.
The objectives of the hydrology component of the study were to develop a comprehensive understanding of the hydrological regime of the eastern shore of Hovsgol Lake within the context of terrestrial ecosystem functions, to reveal peculiarities of runoff formation factors, temporal and spatial distribution. Using data for 2004 and 2005, an attempt was made to estimate a water balance for each of the river basins, on monthly base. The water balance for the study area for June, 2004, produced the following data: the total amount of rain fall was 63.1 mm, of which 46.3 mm was estimated as being evaporated and the remaining 16.8 mm contributed to surface and ground water runoff and soil moisture. At two major automatic meteorological stations in Turag and Dalbay River catchments, rainfall and air temperature were recorded. Some historical data was obtained from the Mongolian Hydrometeorological Network for confirmation and comparison.
This paper mainly covers hydrograph analysis with some rainfall pattern and basic water balance analysis. Data used in this analysis were mainly obtained during the summer of 2005. Due to lower than average rainfall in June, the water levels in all rivers gradually decreased until the first week of July. Significant rainfall events in mid-July caused water level rises of 10-20 cm to occur. River water levels had a clear response to local rainfall events as these events provided the main runoff sources to all the study rivers. Four days of consecutive rainfall which totaled 42 mm in August in the ShagnuulRiver basin, produced maximum discharge of 9.66m3/sec after 3 days. Similarly 13 mm daily rain in Borsogo river caus ed about 30 cm water level rises which maximum rise among the selected rivers.
However, the water level response was different for each river basin as it was dependent on such factors as basin morphology, vegetation/soil cover and local geological conditions. For example, the Turag River basin contains a forested area of 37% and has the highest rainfall maximum discharge, while the Dalbay River basin contains nearly 70% forest cover and the rainfall maximum discharge was the smallest for the observed period. This trend indicates the importance of forests within a catchment to act in a regulating capacity during flooding events.
In terms of rainfall amount, in summer of 2005 from June to August received approximately 35% less than the long-term regional mean rainfall amount. There was clear relationship between rainfall and altitude. The relationship between the warm period sum of the rainfall and altitude, demonstrated an increase of between 18-25 mm per 100 meter increase in altitude within the study area. For the month of July, the average rainfall in the area was 70.7 mm. From this total rainfall amount, 66.2 mm (~94%) was lost as basin evapotranspiration and only 4.5 mm (~6%) contributed to river runoff.
Finally, it is recommended to continue hydrological studies on flow regime and water resources in connection with other runoff formation factors such as soil moisture, permafrost and local geological specific.