S. A. Sharifah Mastura

Analysis of hydrological processes of Langat river sub basins at Lui and Dengkil

Land use changes have seriously impacted the hydrological regimes. This study considered both the upstream and downstream areas of Langat River namely the Lui and Dengkil sub basins, respectively. The important parameters selected for the study were river flowrate, precipitation distribution, and the baseflow (BF) estimation. For the Lui sub basin, the daily flowrate data obtained from 1972 to 2009 (35 years) were used, while for Dengkil sub basin, the data obtained from 1965 to 2009 (44 years) were used. The overall result showed that the monthly mean flowrate of April and November was higher as compared to other months, while the lowest mean flowrate occurred in February and August. Both sub basins showed upward trends in yearly mean flowrate throughout the study period. To obtain a clearer picture on the annual mean flowrate distribution of the two basins, Boxplot variables were plotted. The estimation of the BF was based on the separation method of the United Kingdom Institute of Hydrology (UKIH) smooth minima. Subsequently, BF indexes (BFI) for various years were determined. The Lui subbasin showed almost constant annual BFI, while Dengkil sub basin exhibited downward trend. This indicated that the contribution of the BF on total flow of the Dengkil sub basin reduced over the years. The land disturbances and the increase in the size of imperviousness has certainly reduced the opportunity for infiltration of rainwater into the ground and has most probably decreased the quantum of ground water to recharge the river system during intervening periods between rainfall events. Analysis on the trend of the annual 7-day low flow, however, seems to show increasing trend for both sub basins. This seems to contradict with the decreasing trend of BF, in particular those of Dengkil sub-basin. Finally, this study also included analysis of rainfall based on the 20 to 54 years of available data of the fourteen rain gauging stations within the study areas. The result indicated that the contribution of the total monsoon rainfall in both Lui and Dengkil sub-basins was above 77% of total rainfall received, which is similar to the average monsoon rainfall (81%) of Peninsular Malaysia. The rainfall was classified based on 5 classes as follows: 0 mm/day (no rainfall), 1 to 10 mm/day (light), 11 to 30 mm/day (moderate), 31 to 60 mm/day (heavy) and >60mm/day (very heavy).