Understanding changes in annual runoff following land use changes: A systematic data-based approach

A simple modelling framework for assessing the response of ungauged catchments to land use change in South-Western Australia is presented. The framework uses knowledge of transpiration losses from native vegetation and pasture and then partitions the 'excess' water (resulting from reduced transpiration after land use change) between runoff and deep storage. The simple partitioning is achieved by using soft information (satellite imagery, previous mapping and field assessment) to delimit the spread of the permanently saturated area close to the stream. Runoff is then assumed to increase in proportion to the saturated area, with the residual difference going to deep storage. The model parameters to describe the annual water yield are obtained a priori and no calibration to streamflow is required. We tested the model using gauged records over 25 years from paired catchment experiments in South-Westem Australia. Very good estimates of runoff were obtained from high rainfall (> 1100 mm yr^-1) catchments (R² = 0·9) and for low rainfall (<900 mm yr^-1) catchments after clearing (R² = 0·96) but results were poorer (R² = 0·55) for an uncleared low rainfall catchment, although overall balances were reasonable. In the drier uncleared catchments, the within-year distributions of rainfall may exert a substantial influence on runoff response that is not completely captured by the presented model.