IMPACT OF CHANGES IN BIOPHYSICAL FACTORS ON STREAMFLOW AND SUSPENDED SEDIMENT IN THE CHI RIVER BASIN

Abstract

Impact of changes in biophysical factors including deforestation or streamflow and water quality have long been controversial in Thailand. This investigation aimed to find out to what extent deforestation or forest conversion in the northeastern region could affect on such phenomena Biophysical factors in terms of drainage area, average basin slope, relief ratio, forest cover area, aneual rainfall and proportion of watershed classes in 11 subwatersheds of Chi River Basin, were employed to determine their effects on annual flow and suspended sediment. Historical recoreded data on runoff discharge and suspended sediment observed by the RID and NEA during 1962 to 1984 were compiled in coresponding to land use charges mapped as forest map by RFD. Watershed class distribution prepared by ONEB was also corperated to those information and statistically analysed by sep-wise regression. Results indicated that of all parameters employed for directing the mentioned impacts, forest conversion showed insignificant influence on changing on annual runoff. The relational equations, however, indicated increasing annual runoff discharge at about 37 MCM / yr. for every 10 percent of decreasing forest cover in the basin. This increasing runoff caused to add more 5,671 tons of annual suspended sediment yield. Results also imply that upstream reservoirs can help increasing both summer and wet flow of this basin.