To predict the lifespan of Lam Phra Phloeng Reservoir, in which the capacity has been reduced by sedimentation, both sediment inflow volumes generated from upstream areas of the catchment and deposition in the reservoir were calculated. The rating curve of the relationship between discharge and sediment at station M.145 from 1996 to 2000 obtained using water level discharge and a sediment estimation (LQS) showed that the annual sediment volume resulting from inflow to the Lam Phra Phloeng catchment (820 km2) was 0.36553 Mm3 (million cubic meters). The siltation rate and annual sediment volume in the reservoir were obtained for a series of periods. During the first period, from 1970 to 1983, the annual sediment volume was 2.23 Mm3 and the erosion rate was 2.72 mm/year/km2. This period coincided with a 73.57% decrease in forest area. The second period, from 1983 to 1991, had a lower annual sediment volume of 1.625 Mm3 and a lower erosion rate of 1.98 mm/year/km2. The forest area increased 1.05% during this period. In the recent period, from 1991 to 2000, the annual sediment volume was 0.36553 Mm3 and the erosion rate was 0.445 mm/year/km2. These low rates were associated with a 4.95% increase in forest area. Since the 1960s, the agriculture of Thailand has shifted from subsistence farming to a cash crop culture to develop the social economy. This shift has resulted in the conversion of forests to cultivated lands. As a result, rapid deforestation has occurred, and soil erosion in crop fields has become a serious problem with regard to resource degradation. Soil erosion from crop fields has generally been recognized since the old days.