Effect of Single and Multisite Calibration Techniques on the Parameter Estimation, Performance, and Output of a SWAT Model of a Spatially Heterogeneous Catchment

AbstractAlthough the soil and water assessment tool (SWAT) is a physically based hydrologic simulator, it has many parameters that cannot be measured directly in the field, but must be obtained through a model calibration process. Model calibration is thus an essential task to obtain the optimal parameter values, which match simulations with observations as closely as possible. This study used the Zenne River Basin (Belgium) as a case study, which experiences high spatial heterogeneity in terms of geological formation, groundwater recharge, and rainfall-runoff responses. Therefore, the objectives of this paper were to calibrate the SWAT model on the basis of different calibration techniques and identify which technique is suitable for such a heterogeneous basin so that the calibrated SWAT can be used as a tool for integrated management of the Zenne River Basin. Prior to calibration, the sensitive parameters were identified on the basis of a detailed sensitivity analysis (SA) of the Latin hypercube one-factor-at-a-time (LH-OAT) technique and increased sampling intervals. Then, SWAT was calibrated by using single-site calibration (SSC) at the watershed outlet; sequential calibration (SC), calibration from upstream to downstream; and simultaneous multisite calibration (SMSC), where data of two flow gauging stations were simultaneously used in a single calibration. It was found that at least 200 sampling intervals should be considered for the LH-OAT SA method to obtain converged rankings of SWAT parameters. In addition, to well capture the spatial variability of heterogeneous catchment and achieve stable sensitivity ranking, simultaneous multisite SA technique is important. Streamflow findings suggested that the SC and the SMSC techniques provided very good results and significantly improved model performance, but the SSC results were merely satisfactory. Though the results of SC and SMSC techniques were similar, the SMSC was selected over SC because it simultaneously handles the entire catchment spatial variability by assigning different parameter values and allows data information communication among stations in a single calibration. It was thus concluded that simultaneous multisite calibration should be considered for catchments with a high spatial variability like the Zenne River Basin.