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

Most studies on the use of physically based hydrological models have identi fied saturated hydraulic conductivity (K-sat) as one of the most sensitive input parameters. However, K-sat is also one of the most difficult landscap e properties to measure accurately, casting doubt on the ability of modelle rs to estimate this parameter a priori for catchment simulations. Several s tudies have shown that K-sat estimates are greatly influenced by the measur ement method used, primarily because of scale effects. In this paper, we ev aluate the effect of K-sat measurement method on catchment simulations aime d at predicting water yield from forested catchments. A series of simulatio ns are conducted using the Topog_Dynamic catchment model, with k(sat) estim ated by means of the constant head well permeameter, small core (6.3 cm x 7 .3 cm) and large core (22.3 cm x 30 cm) methods. These were applied in a de ep, permeable forest soil in which macropore flow has been noted to occur. The three measurement methods yielded very different K-sat estimates and th ese had a large effect on model results. The model predictions based on sma ll core and well permeameter measurements were extremely poor, as these met hods did not adequately account for preferential flow through the soil. The large core estimates of K-sat, 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 tab le depth. Our results highlight the need for caution when applying soil hyd raulic measurements to catchment-scale models. Copyright (C) 1999 John Wile y & Sons, Ltd.