A downward approach to identifying the structure and parameters of a process-based model for a small experimental catchment

An intensive field monitoring programme was conducted in 1998 and 1999 in an 84 ha catchment located on the North Island of New Zealand. The data collected includes six soil moisture patterns, 12 soil moisture time‐series, flow at the outlets of two subcatchments of 56 ha and 28 ha, rainfall and other meteorological data. This data set was used in a downward approach to constrain the conceptualisations and the parameters of a terrain‐based distributed model, aiming to simulate the spatial and temporal variability of the soil moisture and the flow response observed in the two subcatchments. The principal mechanism producing runoff was assessed by a preliminary data analysis, involving rainfall, flow and soil moisture time‐series as well as the simulation of infiltration processes at the point scale. Runoff was identified as being mainly produced by saturation excess across the entire monitoring period, despite the high intensity rainfall observed in that area. The model soil‐water‐retention parameters were determined from the soil moisture patterns. The other soil parameters controlling the soil transmissivity were determined by calibration against the observed flow in 1999 in the 56 ha subcatchment, accumulated at the daily scale. The analysis of the flow data at the hourly scale illustrated the need for a more complex subsurface transmissivity function in order to produce lateral storm flow with a larger range of celerity. A simple solution was to modify the decay of the lateral transmissivity with the soil moisture content by adding a second component activated only for soil moisture close to saturation. The additional parameters were calibrated against the observed hourly flow in 1999 in the 56 ha subcatchment. The remaining data were used for validation purposes. This data‐driven, downward approach to identifying the model conceptualisation and parameters resulted in a model capable of reproducing the observed catchment behaviour while minimising model complexity. Copyright © 2003 John Wiley & Sons, Ltd.