Water and salt balance modelling to predict the effects of land-use changes in forested catchments. 1. Small catchment water balance model

A long‐term water balance model has been developed to predict the hydrological effects of land‐use change (especially forest clearing) in small experimental catchments in the south‐west of Western Australia. This small catchment model has been used as the building block for the development of a large catchment‐scale model, and has also formed the basis for a coupled water and salt balance model, developed to predict the changes in stream salinity resulting from land‐use and climate change. The application of the coupled salt and water balance model to predict stream salinities in two small experimental catchments, and the application of the large catchment‐scale model to predict changes in water yield in a medium‐sized catchment that is being mined for bauxite, are presented in Parts 2 and 3, respectively, of this series of papers. The small catchment model has been designed as a simple, robust, conceptually based model of the basic daily water balance fluxes in forested catchments. The responses of the catchment to rainfall and pan evaporation are conceptualized in terms of three interdependent subsurface stores A, B and F. Store A depicts a near‐stream perched aquifer system; B represents a deeper, permanent groundwater system; and F is an intermediate, unsaturated infiltration store. The responses of these stores are characterized by a set of constitutive relations which involves a number of conceptual parameters. These parameters are estimated by calibration by comparing observed and predicted runoff. The model has performed very well in simulations carried out on Salmon and Wights, two small experimental catchments in the Collie River basin in south‐west Western Australia. The results from the application of the model to these small catchments are presented in this paper.