Effects of experimental uncertainty on the calculation of hillslope flow paths

Measurement uncertainty is a key hindrance to the quantification of water f luxes at all scales of investigation. Predictions of soil-water flux rely o n accurate or representative measurements of hydraulic gradients and field- state hydraulic conductivity. We quantified the potential magnitude of erro rs associated with the parameters and variables used directly and indirectl y within the Darcy-Buckingham soil-water-flux equation. These potential err ors were applied to a field hydrometric data set collected from a forested hillslope in central Singapore, and their effect on flow pathway prediction s was assessed. Potential errors in the hydraulic gradient calculations wer e small, approximately one order of magnitude less than the absolute magnit ude of the hydraulic gradients. However, errors associated with field-state hydraulic conductivity derivation were very large. Borehole (Guelph permea meter) and core-based (Talsma ring: permeameter) techniques were used to me asure field-saturated hydraulic conductivity. Measurements using these two approaches differed by up to 3.9 orders of magnitude, with the difference b ecoming increasingly marked within the B horizon. The sensitivity of the sh ape of the predicted unsaturated hydraulic conductivity curve to +/- 5% moi sture content error on the moisture release curve was also assessed. Applie d moisture release curve error resulted in hydraulic conductivity predictio ns of less than +/- 0.2 orders of magnitude deviation from the apparent con ductivity. The flow pathways derived from the borehole saturated hydraulic conductivity approach suggested a dominant near-surface flow pathway, where as pathways calculated from the core-based measurements indicated vertical percolation to depth. Direct tracer evidence supported the latter flow path way, although tracer velocities were approximately two orders of magnitude smaller than the Darcy predictions. We conclude that saturated hydraulic co nductivity is the critical hillslope hydrological parameter, and there is a n urgent need to address the issues regarding its measurement further. Copy right (C) 2000 John Wiley & Sons, Ltd.