Abstract

Measurement uncertainty is a key hindrance to the quantification of water fluxes at all scales of investigation. Predictions of soil-water flux rely on accurate or representative measurements of hydraulic gradients and field-state hydraulic conductivity. We quantified the potential magnitude of errors associated with the parameters and variables used directly and indirectly within the Darcy - Buckingham soil-water-flux equation. These potential errors were applied to a field hydrometric data set collected from a forested hillslope in central Singapore, and their effect on flow pathway predictions was assessed. Potential errors in the hydraulic gradient calculations were small, approximately one order of magnitude less than the absolute magnitude of the hydraulic gradients. However, errors associated with field-state hydraulic conductivity derivation were very large. Borehole (Guelph permeameter) and core-based (Talsma ring permeameter) techniques were used to measure field-saturated hydraulic conductivity. Measurements using these two approaches differed by up to 3-9 orders of magnitude, with the difference becoming increasingly marked within the B horizon. The sensitivity of the shape of the predicted unsaturated hydraulic conductivity curve to ±5% moisture content error on the moisture release curve was also assessed. Applied moisture release curve error resulted in hydraulic conductivity predictions of less than ±0.2 orders of magnitude deviation from the apparent conductivity. The flow pathways derived from the borehole saturated hydraulic conductivity approach suggested a dominant near-surface flow pathway, whereas pathways calculated from the core-based measurements indicated vertical percolation to depth. Direct tracer evidence supported the latter flow pathway, although tracer velocities were approximately two orders of magnitude smaller than the Darcy predictions. We conclude that saturated hydraulic conductivity is the critical hillslope hydrological parameter, and there is an urgent need to address the issues regarding its measurement further.