LAM PHRA PHLOENG DAM, THAILAND - A HIGH RESOLUTION RECORD OF HUMAN ACTIVITY AND CLIMATE VARIABILITY

Lam Phra Phloeng reservoir is one of the most seriously affected by soil erosion related sedimentation in Thailand. Clearing in the catchment from 1974 to 1985 reduced forest area by about 70% (from 531 sq.km to 160.25 sq. km). The dominant crops in the upper catchment are sugar-cane and cassava. After the crop has been harvested the land is tilled and becomes sensitive to sheet erosion. The area suffers periodically from floods as well as draughts. Because of these rapid changes there is an increased sediment load in rivers draining into the reservoir. As a direct result the Lam Phra Phloeng reservoir has become very shallow at the point where major streams enter.

The measurement of sedimentation rates in Lam Phra Phloeng Dam (Water supply catchment Dam) is the co-operative research between the Royal Irrigation Department, Ministry of Agriculture, the Office of Atoms for Peace, Ministry of Science and Technology and the Australian Nuclear Science and Technology Organisation . During a fieldsampling campaign in June 2002, several sediment cores were taken from the Lam Phra Phloeng reservoir. These sediment have been analysed at high resolution for sedimentation rates using the radioisotopic techniques (Pb-210). Also investigated were climatic records and sediment characterisation such as grain size ,in order to estimate the rate and nature of sediment accumulation in the reservoir. Comparison of the data obtained with climatic and hydrological data revealed the relationship between sediment accumulation rate, seasonal changes in rainfall and sub-decadal climate variability. These findings show that a revised life-span of the reservoir as water storage body is necessary and a more permanent crop is advisable to tackle soil-erosion issue, without compromising the economic status of the farmers in the catchment.