Plot-scale hydrometric and tracer characterisation of soil water flow in two tropical rain forest catchments in Southeast Asia
This research has endeavoured to characterise soil water flow vectors across a distribution of plots within two undistributed tropical rain forest catchments in Southeast Asia. The research focused on the potentially contrasting flow pathways through two different soil types: the Ferric Acrisol of the Jungle Falls catchment (Singapore), and the Haplic Alisol of the W8S5 catchment (Sabah, Malaysia). The research also sought to determine the uncertainty associated with the flow characterisation methodologies. Flow pathways were determined using two quite separate techniques within each experimental plot. A traditional Darcy-approach (i.e. use of the Darcy-Buckingham equation) was used in combination with an artificial tracer approach, to determine if sound flow predictions can be made from local measurements of hydraulic conductivity and capillary potential.
Analysis of the results indicates that the flow pathways within the two soils contrasted markedly. Tracer evidence suggests that soil water movement within the Ferric Acrisol (Jungle Falls catchment) was predominantly vertical. In the Haplic Alisol (W8S5 catchment), the dominant flow pathway of the tracer was laterally though A and B1 horizons. Often rapid tracer breakthrough velocities were observed, which exceeded the measured soil saturated hydraulic conductivity by several orders of magnitude. This infers the macropores are an important flow route within both the Ferri Acrisol and the Haplic Alisol. In general, the Darcy-based approach failed to predict the velocity and dominant pattern of the tracer flows. This resulted from problems associated with the measurement of the soil parameters needed to solve the Darcy-Buckingham equation. Uncertainty analysis of the measurement and empirical techniques used to derive the Darcy-Buckingham equation parameters indicated that there is serious potential for error, particularly in the derivation of saturated hydraulic conductivity.