Watershed analysis of the Semenyih River Basin, Selangor, Malaysia

The Semenyih River Basin has undergone various degrees of land use changes since the last decade, particularly assosiated with urbanization and industrialization. The total area of the basin is 266.60 km² and contains 36 subcatchments with sizes ranging from 1.37 to 35.57 km². An integrated study was carried out to determine the various factors of land use changes that may affect the stability of the watershed and its subsequent impacts on water quality. Rock and soil samples were analyzed in a petrographic and XRF methods and sieve analysis for soil. Rainfall-run-off relationships were created to elucidate the hydrologic responses and to help the graphical analysis. Stream flow pattern of particular years were grouped to show their flow variations. 18 water quality parameters were performed from 11 sampling locations at the Semenyih River that were also used for the hydrological measurements. The water quality analysis involved 4 in-situ parameters and 14 laboratory parameters. The relationships of hydrology and water quality variables were determined by regression and correlation analysis. The above three criteria were used together with land use and population density factors to develop the basin classification system. The geology of the study area consists of five rock formations with the dominant feature being granitic rocks. The soil in the study area comprises seven soil series and five types of soil texture. Seven categories of land use were identified and forests constitute the largest land use. Rainfall-runoff relationships based on hydrologic response analysis showed that urbanization in Semenyih town contribute to significant surface runoff compared to the other land uses. Graphical analysis indicated that the Semenyih dam regulates the flow of the Semenyih River. The principal categories of pollution sources were from domestic activities, industries, manufacturing activities and land clearing activities. The result indicated that water quality deterioration due to urban wastes was significant (WQI 56 to 48). The means values for eight water quality parameters (turbity, TDS, NO3, NH3-N, SO4, BOD, E. coli and COD) increased by 30% to 50% (WQI 53 to 36) during wet periods. The mean concentration of the water quality parameters analyzed between dry and wet periods was significantly different (p smaller than 0.05). The results of the Semenyih River Basin classification was established for the 36 sub-catchments and indicated taht 12 forested sub-catchments can be classified as "good", six agriculture and forested sub-catchments as "fair", ten agriculture and settlements as "slightly disturbed" and eight urban sub-catchments classified as "disturbed". Finally, based on this study it can be concluded that the condition of the Semenyih Basin is slightly disturbed. In near future, more land will be developed, due to increase in infrastructure development, population and industrial activities that will increase of the pollution level in Semenyih River.