Study area
The study area, Nam Choen watershed is located in Northeast Thailand. and covers an area of about 5120 sq.km.
It lies between the latitudes of 16° and 17° N and the longitudes of 101° 24’ and 102° 32’ E (Fig 1). It is a part of
upper Chi basin that is the one of third main basin in northeast region. It is consist of Nam Phom and Nam Choen
catchment which Nam Phom river combine to Nam Choen river and ends at the Ubol Ratana Reservoir (Fig. 2). TheNam Choen watershed is established in the Korat plateau. Almost area is mountainous area in the west with
evergreen forest and dry dipterocarp forest. The area is quite flat in the east which main land use is paddy field in the
rainy season (during September-December) and becomes second crop in the dry season (during February-April). The
foot slope area in the middle is a rolling topography which is used for rubber plantation and mixed orchard.
Geologically, it is underlain by thick sequence of Mesozoic rock which mainly consists of sandstone which gave
sandy soil in this area and limestone. In the lower plain it is composed of alluvial deposits of sand, silt and clay.
The climate in this area is influenced by the northeast and southwest monsoon. The mean annual rainfall is
about 1,116 mm. and the average annual temperature is about 27 0C.
Fig. 1 Location of study area
Fig. 2 The RGB (4,5,3) colour composite image of Landsat TM data acquired during
December 2004