Hydrological modelling of paired catchments with com peting land uses

We report the preliminary results of a modelling study of surface/subsurface water interactions applied to a paired-catchment project that is primarily aimed at quantifying the hydrologic response of catch- ments with competing land uses. The study focuses on a site in southwestern Victoria, Australia, composed by two adjacent catchments with different agricultural uses (grazing and Eucalyptus globulus(blue gum) planta- tion), but similar topography, geology and soils. The grazing and plantation catchments have a size of 0.48 and 0.76 km 2 , respectively, and their monitoring network comprises groundwater observation bores, stream gauges and a rainfall station. We use the distributed, numerical model CATchment HYdrology (CATHY), which simulates flow in the sur- face, soil, and aquifer zones as well as the hydrological interactions across the atmosphere-land surface- subsurface continuum. CATHY combines a three-dimensional equation for subsurface flow in variably satu- rated porous media with a one-dimensional diffusion wave approximation of the de Saint-Venant equation for surface water dynamics. A particular feature of CATHY controls the switching between atmosphere-controlled and soil-limited evapotranspiration; the switching is regulated by a threshold pressure head ( min) and is used in this study to investigate CATHY's capability to simulate the impact on water fluxes of different types of vegetation. Preliminary simulation results of a 1-year period show that the model is capable to reproduce satisfactorily the hydrological regime of both catchments without the need for a detailed multiparameter calibration. In partic- ular, the surface hydrological response is matched satisfactorily by the model simulations, with a reasonably good fit (Nash-Sutcliffe coefficients of 0.63 and 0.52 for the grazing and plantation catchments, respectively) in terms of daily flow hydrographs. Water table levels observed in the bores proved to be more difficult to match, even though the overall groundwater dynamics is well captured by the model. The use of two different values of min, which control the conversion of potential evaporative demand into actual evapotranspiration, accounts for the impact of the different vegetation covers on the total water bud- get. The two threshold parameters will be accurately calibrated in the next step of this study by matching the simulated fluxes to measurements of actual evapotranspiration. The model will thus allow accurate quantifi- cation and comparison of the catchment scale impacts of different agricultural land uses on surface water and groundwater fluxes.