Effect of Teak Plantation in Agroforestry System for Soil and Water Conservation and Watershed Rehabilitation   Pitsabu Jutvapornvanit and Somchai Inthasothi   ABSTRACT         Teak plantation in agroforestry system for watershed rehabilitation program was studied during the year 1986-1993. It was implemented in upsteram portion of Yom River in Song district, Phrae province. The two miniwatersheds of Mae Jah-1 and Mae Jah-2 were selected to be the representative watershed. In the first watershed the forest was clearcut in 1986 and planted teak 2 x 4 m.. The second watershed was collected data of natural mixed deciduous forest for two years after that the forest was clearcut and having similar experiment as the first watershed.       The multiple linear regression equations which indicated the significant relationships between annual rainfall, rainfall distribution and percent crown cover were used to evaluate streamflow and sediment load for comparing land use in different periods of teak growth.       It was found that in the first year of teak plantation after land clearing, sediment transportation increased 43% of natural forest. The amount of transported material declined with time and became close to those of natural mixed deciduous forest in 5 years.       Land clearing also affected to hydrological characteristics. It was found that in the first year of teak plantation after land clearing, overland flow was at a high rate when compared to subsurface flow. After a few years overland flow substantially improved which decreased and being in contrast with increasing subsurface flow and also became close to natural mixed deciduous forest during 5-7 years.       It can be concluded from the investigation that reforestation in agroforestry system is able to effectively control soil loss in terms of sediment yield, it also helps regulate water flow in the stream, which is the first step towards watershed rehabilitation.         Keywords : Hydrology, Runoff, Water balance