Channel network simulation models compared with data from the Ashley River, New Zealand

This paper compares the ability of two channel network simulation models to simulate the channel network properties of the Ashley River in the foothil ls of the Southern Alps of the South Island of New Zealand. The basin was c hosen because it contains a large-scale topographic feature, a central ridg e, the simulation of which would provide a measure of each model's ability to handle spatial nonuniformities. The two models assessed were the optimal channel network (OCN) model and a catchment evolution model (SIBERIA) catc hment evolution model. The ability of these models to replicate observed ge omorphic statistics and relationships was assessed. The models were also co mpared against each other to assess their relative suitability for simulati ng the observed geomorphology. Model performance was gauged using the singl e-valued measures of catchment convergence, hypsometric integral, and energ y expenditure by the network; and plots of the width function, the slope of the cumulative area diagram, and the hypsometric curve of basin topography . The effects of different forcings were examined. The basic forcing was on e in which the climate and the geological and tectonic properties of the ba sin were assumed to be unchanging in both space and time. The first variant on the basic forcing looked at what happens when there is a permanent spat ial gradient in the rainfall over a basin with spatially uniform geological and tectonic properties. The second variant considered the effects of cons tant, spatially uniform rainfall on a basin in which spatially variable tec tonic uplift is occurring. Neither model adequately simulated the observed geomorphology when the spatially nonuniform tectonic forcing was ignored. W hen spatially nonuniform tectonic effects at length scales of tens of kilom eters were simulated, SIBERIA performed more satisfactorily. The effect of nonuniform rainfall was found to be small for both models. The performance of the OCN as gauged by single-valued measures improved markedly when energ y expenditure calculations were consistent with the geometric length of flo w paths.