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INFILTRATION AND PERCOLATION OF WATER AT SELECTED SITES IN SINGAPORE

GRACE LEE CHENG YEE
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Abstract
Studies on water movement in Singapore have generally been confined to monitoring the process of infiltration, as it occurs under various conditions. Relatively little has been done to monitor the rates and direction of soil water movement during and after infiltration. This study aimed to monitor soil water behaviour, in terms of variation in flow rates laterally and vertically during infiltration. In addition to this, changes in soil moisture were monitored prior to and following infiltration. In view of the above, and the various factors influencing the process of infiltration and percolation, four sites were selected based on the following criteria: 1) Soil type : Only the Harimau and Rengam series were chosen. 2) Vegetal cover reflects the degree of surface disturbance. Two types of vegetation were selected Forested (undisturbed) and Mown grass (disturbed) The main source of data for this study was obtained in the field. Infiltration rates were measured by the ponding process, using the double ring infiltrometer. Flow rates and direction within the soil profile were determined in terms of suction measurements,by means of tensiometers installed to various depths and at various points in the field. Infiltration and suction measurements can both be employed to monitor the rates and direction of soil water movement as explained in chapters 2 and 5. In view of the massive amount of data procured in this exercise, the results were graphically presented in chapter 4. For practical analysis, this manner of presentation provides a visual presentation of soil water behaviour in the form of trends and changes. In addition to this, wetting front diagrams were used to depict water movement with time during infiltration in chapter 5. Results from this study revealed the significance of soil type, vegetal covers and degree of surface disturbance on flow rates and direction. On the basis of soil type, it was observed that differences between vertical and lateral flow rates were less marked in the relatively fine textured Rengam soil. However, very rapid vertical flow was observed at Seletar Lawn. This was found to be the direct effect of high antecedent moisture conditions following a heavy downpour. In particular, the slope gradient at Seletar was pertinent in influencing the direction of flow. The foregoing demonstrated the influence of external factors on soil water movement. The rate and direction of flow was more difficult to predict for sites under the Old Alluvium soil profile in view of its inconsistent nature. The highest and lowest infiltration and perrcolation rates were recorded at Loyang Forested and Loyang Lawn respectively. The study concluded with a recommendation that, a local classification system of flow rates and direction based on varying factors of influence be compiled. It is believed that this can be useful to future related studies when quick and systematic references need to be made.
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GEOGRAPHY
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1985
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