CONTRIBUTION OF RUNOFF TO STREAMFLOW IN A HUMID-TROPICAL FOREST, WITH EMPHASIS ON THE VARIABLE SOURCE AREA CONCEPT

Abstract

Process studies in hillslope hydrology have advanced since the 1930s. Since then, the Variable Source Area Concept has taken pre-eminence in the field, with many mathematical models that lead to a plethora of background information that yield simulations of processes under various parameters. However, most studies have been undertaken in temperate areas, and there is a paucity of hillslope hydrological studies in the humid-tropics. In the local context, many studies have documented runoff processes and/or related studies but a majority of them have been investigated under various kinds of land uses. Clearly, not many have taken the primary rainforest as a study site to determine runoff generation. This study is an attempt to determine specifically the pathways rainwater takes before entering the stream in a primary forest. It is confined to the study of relative contributions of runoff processes to streamflow, as encompassed in the Variable Source Area Concept. The Variable Source Area Concept envisages that source areas of runoff contributions are limited in a real extent, with the nearby riparian areas being the most probable sources of runoff generation. The source areas expand and contract according to rainfall conditions. To investigate the phenomena, 12 piezometers were installed at the adjacent riparian area of a first-order stream in the primary forest in Bukit Timah Nature Reserve. These piezometers were used to measure the water-table fluctuations during storms in order to elucidate information on the saturated zones near the adjacent channel. In addition, soil hydraulic properties such as infiltration rates & capacity, saturated hydraulic conductivity at the sites were obtained. The latter data provides an explanation of the implications of soil physical properties on runoff generation. During the 10 storms studied, piezometric responses support the contention that the saturated areas near riparian zones expand during storm events. For most storms, the piezometers furthest away from the stream expanded the most. At Site 1, where the flat, sandy plain is, saturation overland flow was observed. The shallow water-table environment, coupled with the nature of the subsurface material creates conditions for saturation overland flow to occur.