The sensitivity of a catchment model to soil hydraulic properties obtained by using different measurement techniques

Most studies on the use of physically based hydrological models have identified saturated hydraulic conductivity (Ksat) as one of the most sensitive input parameters. However, Ksat is also one of the most difficult landscape properties to measure accurately, casting doubt on the ability of modellers to estimate this parameter a priori for catchment simulations. Several studies have shown that Ksat estimates are greatly influenced by the measurement method used, primarily because of scale effects. In this paper, we evaluate the effect of Ksat measurement method on catchment simulations aimed at predicting water yield from forested catchments. A series of simulations are conducted using the Topog_Dynamic catchment model, with Ksat estimated by means of the constant head well permeameter, small core (6·3 cm×7·3 cm) and large core (22·3 cm×30 cm) methods. These were applied in a deep, permeable forest soil in which macropore flow has been noted to occur. The three measurement methods yielded very different Ksat estimates and these had a large effect on model results. The model predictions based on small core and well permeameter measurements were extremely poor, as these methods did not adequately account for preferential flow through the soil. The large core estimates of Ksat , which were one to three orders of magnitude higher than the values obtained by the other two techniques, produced good predictions of catchment discharge and known spatial patterns of water table depth. Our results highlight the need for caution when applying soil hydraulic measurements to catchment-scale models.