Case Studies Effect of Single and Multisite Calibration Techniques on the Parameter Estimation, Performance, and Output of a SWAT Model of a Spatially Heterogeneous Catchment Olkeba Tolessa Leta 1 ; Ann van Griensven 2 ; and Willy Bauwens 3 Abstract: Although 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, towell 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. DOI: 10.1061/(ASCE)HE.1943-5584.0001471. © 2016 American Society of Civil Engineers. Author keywords: Calibration; Multisite; Streamflow; Soil and water assessment tool (SWAT); Zenne River Basin; Belgium. Introduction Spatially distributed hydrologic simulators can be used for model- ing different catchment processes, including evapotranspiration, interception, infiltration, percolation, surface runoff, groundwater flow, soil erosion and sediment transport, and nutrients transport. To represent these processes, spatially distributed hydrologic sim- ulators primarily use physically-based mathematical equations. Such simulators also can be used for investigating the impacts of land use change, climate change, agricultural activities, and various management practices on the catchment processes (Bae et al. 2011; Li et al. 2009; Mukundan et al. 2013; Narasimhan et al. 2010; Santhi et al. 2006; Schilling et al. 2008; Singh et al. 2005; Vansteenkiste et al. 2013). Furthermore, distributed hydrologic simulators can help decision makers to better understand environ- mental related problems and design appropriate mitigation mea- sures (Debele et al. 2008; Leta et al. 2014; Shrestha et al. 2013b). As such, they are useful tools for integrated river basin management studies and decision making (Andrews et al. 2011; Migliaccio and Chaubey 2007; Moriasi et al. 2007). As a result of the advances in techniques for data acquisition and measurement, the use of spatially distributed, physically based, and thus complex hydrologic simulators has gained increased attention for integrated river basin management and decision support (Ajami et al. 2004; Arnold et al. 2010). A typical example of such simu- lators is the soil and water assessment tool (SWAT) (Arnold et al. 1998). SWAT is a semidistributed and physically based simulator that is used widely for continuous hydrological, soil erosion, sedi- ment, and nutrients transport modeling (Arnold et al. 2012; Debele et al. 2008; Gassman et al. 2007; Santhi et al. 2001, 2008). The simulator considers the interactions among water, soil, and plants in the upland catchments in a quite detailed way (Gassman et al. 2007). However, simulators such as SWAT have many param- eters that cannot be measured directly in the field, but should be obtained through a model calibration process (Hogue et al. 2000; Li et al. 2010). Even if some parameters of spatially distrib- uted hydrologic simulators could be measured, scaling problems still would have to be solved (Nossent 2012). Therefore, model cal- ibration and parameter estimation are essential tasks to obtain the optimal parameter values that can reasonably match simulations 1 Postdoctoral Research Fellow, Water Resources Research Center, Univ. of Hawaii at Manoa, 2525 Correa Rd., HIG 217, Honolulu, HI 96822 (corresponding author). E-mail: olketole@yahoo.com 2 Professor, Dept. of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium; Associ- ate Professor, Dept. of Water Science and Engineering, UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX, Delft, Netherlands. 3 Professor, Dept. of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium. Note. This manuscript was submitted on January 11, 2016; approved on August 2, 2016; published online on September 30, 2016. Discussion per- iod open until February 28, 2017; separate discussions must be submitted for individual papers. This paper is part of the Journal of Hydrologic En- gineering, © ASCE, ISSN 1084-0699. © ASCE 05016036-1 J. Hydrol. Eng. J. Hydrol. Eng., 05016036 Downloaded from ascelibrary.org by Olkeba Tolessa LETA on 09/30/16. Copyright ASCE. For personal use only; all rights reserved.