2. STUDY AREA
The study area covers the extensive areas of the central part of northeastern Thailand of Chi watershed being used for test area to represent agricultural land use planning in the region. The Chi watershed is drained eastward to Mun river and eventually to the Mekong river in the border of Thai-Lao,PDR. Physiographically, the Chi is formed by the prominent topography in the upstream and flat to gently undulating landscape in the central and downstream of the river. The current land use is restricted to dipterocarp and evergreen forests on the upstream/ mountain, field crops on the well drained soil of the gently undulating areas, and paddy on the flat and low lying areas. The isolated patches of remnant forest are commonly found throughout the Chi. Geologically the Chi is underlain by a thick sequence of Mesozoic sediment, the Korat Group ranging in age from upper Triassic to Tertiary. The extensive alluvial plain is underlain by Maha Sarakham Formation (a Formation of the Korat Group) which was deeply weathered in the Tertiary period and contains considerable quantity of evaporites interbedded with sandstones siltstones and sandy shades. The occurrence of this Formation coincides with the distribution of salt affected soils. The soils on the undulating topography are mainly derived from alluvium of sandstones origin. The mean annual rainfall ranges from 1000-1500 mm. and is increased from the west to the east of the region.