2. STUDY AREA
The study area encompasses an extensive portion of the central part of northeastern Thailand. It
includes the Chi Watershed, which is being used as a test area for agricultural land use planning in
the region (Fig.1). The Chi watershed is drained to the east by the Mun river and eventually by the
Mekong river at the Thai-Lao, PDR border. Physiographically, the Chi Watershed is formed by the
strong topography in the upstream portion and flat to gently undulating landscapes in the central
and downstream portions of the river. The land cover consists of dipterocarp and evergreen forests
in the upland mountain zone, field crops on the well drained soils of the gently undulating areas,
and paddy rice on the flat and low lying areas. Isolated patches of remnant forest are commonly
found throughout the Chi Watershed. Geologically the area 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 the Maha Sarakham Formation (a formation of the Korat Group),
which was deeply weathered in the Tertiary period and contains considerable quantities of
evaporites interbedded with sandstones siltstones and sandy shales. The occurrence of this
formation coincides with the distribution of salt affected soils. The soils on the undulating
landscapes are mainly derived from alluvium of sandstone origin. The mean annual rainfall ranges
from 1000-1500 mm. and increases from the west to the east portions of the region.