Effects of land use patterns on soil and water quality in Khlong U-Taphao basin

Amidst the growing concern about land use change, this article examines changes in land use, investigates effects of such changes on soil erosion and evaluates the impacts of soil erosion on soil and water quality with specific reference to the Khlong U-Taphao Basin, Southern Thailand. During the period from 1967 to 1996, erstwhile existing forests in the basin declined by 77 percent contributing to a 60 percent increase in rubber plantation area. The rapid depletion of natural forests was largely attributed to the expansion of rubber plantation area and illegal logging. Results obtained from runoff plot experiments established in rubber plantation areas showed that soil losses in plots where traditional farming practices were experienced were seven times greater than in the natural forest areas. Rates of soil loss in farmers' plots could be reduced by 73 percent, 42 percent and 45 percent with the application of straw mulch, legume cover and vetiver grass, respectively. Mulching with rice straw was more effective than cover crops or grass strips in reducing runoff and soil erosion. Findings also revealed that erosion in the farmed plots significantly decreased soil organic matter content, concentration of total nitrogen and plant available water resulting in a reduction in soil quality. Generally, the values of electrical conductivity, total suspended solids, turbidity, nitrate notrogen and orthophosphate were initially low in the river draining the forest and progressively increased in a downstream direction. In contrast, the highest concentration of dissolved oxygen was recorded in the upstream forest part of the river and progressively decreased in a downstream direction. Increases in total suspended solid and turbidity, and high nutrient loading in the river lead to the degradation of river water quality. Strategies, which would enhance the basin's environmental quality, are discussed.