Assessing SWAT models based on single and multi-site calibration for the simulation of flow and nutrient loads in the semi-arid Onkaparinga catchment in South Australia

•Single and multi-site SWAT calibration approaches were compared.•SWAT can realistically capture the low flow conditions of the semi-arid Australian catchment.•Improved simulation of nutrient loads were achieved by the multi-site calibration approach.

Distributed catchment models such as SWAT (Soil and Water Assessment Tool) are widely used to assess catchment characteristics and facilitate informed decisions for safeguarding water quantity and quality. This study applied SWAT to simulate monthly stream flow and loadings of total suspended sediment (TSS), total nitrogen (TN) and total phosphorus (TP) for five monitoring stations within the Onkaparinga catchment, and tested the models’ performance based on single-site or multi-site calibration.The results showed that multi-site calibration did not improve simulations of flow and sediments compared to single-site calibration. However, simulation results for TN and TP loads improved in both rural and urban sub-catchments of this catchment. Uncertainty analysis revealed that there is high uncertainty in model simulation of TSS by both strategies. The study has demonstrated: (1) the capability of SWAT to simulate realistically the extreme flow conditions of the semi-arid Onkaparinga catchment; (2) the benefit of local monitoring data for more realistic simulations of nutrient loads by means of the multi-site calibration of SWAT as pre-requisite for scenario analysis of spatially-explicit management options.