Effects of experimental uncertainty on the calculation of hillslope flow paths

Measurement uncertainty is a key hindrance to the quanti_cation of water ~uxes at all scales of investigation[ Predictions of soil!water ~ux rely on accurate or representative measurements of hydraulic gradients and _eld! state hydraulic conductivity[ We quanti_ed the potential magnitude of errors associated with the parameters and variables used directly and indirectly within the DarcyÐBuckingham soil!water!~ux equation[ These potential errors were applied to a _eld hydrometric data set collected from a forested hillslope in central Singapore\ and their e}ect on ~ow 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 _eld!state hydraulic conductivity derivation were very large[ Borehole "Guelph permeameter# and core!based "Talsma ring permeameter# techniques were used to measure _eld!saturated hydraulic conductivity[ Measurements using these two approaches di}ered by up to 2=8 orders of magnitude\ with the di}erence becoming increasingly marked within the B horizon[ The sensitivity of the shape of the predicted unsaturated hydraulic conductivity curve to 24) moisture content error on the moisture release curve was also assessed[ Applied moisture release curve error resulted in hydraulic conductivity predictions of less than 29=1 orders of magnitude deviation from the apparent conductivity[ The ~ow pathways derived from the borehole saturated hydraulic conductivity approach suggested a dominant near!surface ~ow pathway\ whereas pathways calculated from the core!based measurements indicated vertical percolation to depth[ Direct tracer evidence supported the latter ~ow 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[ Copyright Þ 1999 John Wiley + Sons\ Ltd[