EFFECT OF SLOPE WITH VEGETATION ON THE FLOW RATE OF SOIL WATER IN THE RENGAM SOIL SERIES

Abstract

Flow rate determines the amount of water that enters the soil and within the layers of soil. In this study, we are concerned with both the surface and subsurface flow rate. The rates for these can be changed a great deal depending on the surface conditions of the soil. This then calls for a need to investigate the factors that would augment changes to the condition of the soil and eventually the flow rate. Considerable information is available on hydraulic flow rate into and within soil under different variables like vegetation, soil texture etc. However, not enough information is available for the variability of these rates with position of slopes under various types of vegetation. This study attempts to determine the effect of slope variable under different vegetation conditions on a specific soil in Singapore. Only one soil is chosen to eliminate the variable of soil type. In fulfilment of this aim, five vegetation covers namely the primary forest, secondary forest, lalang, grass cover and bare ground, and with each vegetation cover, slope position i.e. upper slope, mid slope and slope base have been selected to test for the infiltration rate and saturated hydraulic conducitivity. The infiltration rate was measured using the constant head single-ring infiltrometer and the saturated hydraulic conductivity by the Guelph permeameter. As expected, the results indicate that infiltration rate varied with depths as well as with vegetation types, slope positions and gradients. The infiltration rate ranged widely from the highest of 386.606 cm/hr to the lowest of 0.532 cm/hr. Likewise, the saturated hydraulic conductivity also varied extensively from 0.139 cm/sec to 0.00000378 cm/sec or even lower values which were not able to be measured by the instrument used. In general, both the rates decrease with declining density of vegetation and increasing depth below ground surface. This conforms with results conducted by other studies. The highest infiltration rate is found at the upper slope and the lowest at the slope base. The slope base being the least steep part of the slope makes re-distribution of moisture slowest here. Together with contribution of throughflow, there is a greater tendency to attain high moisture content at the slope base which inhibits high infiltration rate. This is also enhanced by the presence of certain clay minerals here and not at the upper slope which would further hinder infiltration capacity. On the other hand, saturated hydraulic conductivity is lowest at the uppermost part of the slope and higher at either the mid slope or slope base. As the slope base is mostly saturated, the subsurface flow will tend to be higher. At locations of slope which is at the upper slope where moisture content is lower, the hydraulic conductivity will also incline to be of a lower velocity.