Northeast Thailand is a square shaped plateau almost completely surrounded by mountain ranges and divided into two basins (Khorat Basin, Sakon Nakon Basin) by a relatively small mountain range (Phu Phang Range) (Figure 1). These basins are composed of hilly, undulating and flat low-lying regions. In the flat low lying region, large rivers (e.g. Mun River, Chi River) flow along these mountain ranges. According to Koppen’s system, the climate belongs to tropical savanna with an alternation of rainy and dry seasons. In the rainy season, erratic and small rainfall shows two peaks (Figure 2). In the past, a large part of Northeast Thailand was covered with the forests of Dypterocarpacae (Boulbet 1982, Ishizuka 1986). Deforestation has proceeded gradually in old times and rapidly in recent years in parallel with intensification of human activities