The Selenge River basin is located in northern Mongolia, between latitudes 46 and 52 ° N and longitudes 96 and 109 ° E, and has an area of 280 000 km2 . It is the source region for the Lake Baikal basin (area 600 000 km2 ). It contains two main rivers: the Selenge and Orhon (Figure 2).The basin covers seven provinces (Zavkhan, Khubsgul, Bulgan, Arkhangai, Uvurkhangai, Selenge, and Tuv), and contains the capital city, Ulaanbaatar. The population of the basin is about 1 450 000, which is approximately 70% of the total population of Mongolia (Adyabadam, 1999). The average annual temperature is below freezing.

The annual precipitation is about 300 mm (Oyunbaatar, 1999), but the temporal and spatial distribution over the basin is very uneven; during the period 1980–92, it ranged from 147 mm at Murun (no. 231 in Figure 2) in 1980 to 598 mm at Erdenet-Ovoo (no. 236 in Figure 2) in 1985. Although there are no detailed reports of the vegetation distribution for the basin, the World Resources Institute (http://www.wri.org) has reported the land cover for the Lake Baikal basin as 33% forest and 52% grassland.

The basin is shown on a digital map based on the NOAA–GLOBE (global land one-kilometre base ele- vation) dataset (http://www.ngdc.noaa.gov/seg/topo/globe.shtml). Figure 3 shows the distribution of elevation in the Selenge River basin. The lowest elevation, near the outlet, exceeds 600 m, and the highest, in the Hangyan Mountains, exceeds 3000 m. The elevation is between 1000 and 2000 m in about 72% of the basin. To simulate the water flow in channels, a 0Ð1 ° grid map of the river network was derived and, to run the model, meteorological data were interpolated for the whole basin on a 1 ° × 1 ° grid.