Multi-variable and multi-site calibration and validation of SWAT in a large mountainous catchment with high spatial variability

Many methods developed for calibration and validation of physically based distributed hydrological models are time consuming and computationally intensive. Only a small set of input parameters can be optimized, and the optimization often results in unrealistic values. In this study we adopted a multi-variable and multi-site approach to calibration and validation of the Soil Water Assessment Tool (SWAT) model for the Motueka catchment, making use of extensive field measurements. Not only were a number of hydrological processes (model components) in a catchment evaluated, but also a number of subcatchments were used in the calibration. The internal variables used were PET, annual water yield, daily streamflow, baseflow, and soil moisture. The study was conducted using an 11-year historical flow record (1990-2000); 1990-94 was used for calibration and 1995-2000 for validation. SWAT generally predicted well the PET, water yield and daily streamflow. The predicted daily strearnflow matched the observed values, with a Nash-Sutcliffe coefficient of 0.78 during calibration and 0.72 during validation. However, values for subcatchments ranged from 0.31 to 0.67 during calibration, and 0.36 to 0.52 during validation. The predicted soil moisture remained wet compared with the measurement. About 50% of the extra soil water storage predicted by the model can be ascribed to overprediction of precipitation; the remaining 50% discrepancy was likely to be a result of poor representation of soil properties. Hydrological compensations in the modelling results are derived from water balances in the various pathways and storage (evaporation, strearnflow, surface runoff, soil moisture and groundwater) and the contributions to strearnflow from different geographic areas (hill slopes, variable source areas, sub-basins, and subcatchments). The use of an integrated multi-variable and multi-site method improved the model calibration and validation and highlighted the areas and hydrological processes requiring greater calibration effort. Copyright (c) 2005 John Wiley & Sons, Ltd.