[1] The automatic calibration software Parameter Estimation (PEST) was used in the hydrologic calibration of Hydrological Simulation Program-Fortran (HSPF), and the results were compared with a manual calibration assisted by the Expert System for the Calibration of HSPF (HSPEXP). In this study, multiobjective functions based on the HSPEXP model performance criteria were developed for use in PEST, which allowed for the comparison of the calibration results of the two methods. The calibrated results of both methods were compared in terms of HSPEXP model performance criteria, goodness-of-fit measures (R 2 , E, and RMSE), and base flow index. The automatic calibration results satisfied most of the HSPEXP model performance criteria and performed better with respect to R 2 , E, RMSE, and base flow index than manual calibration results. The results of the comparison with the manual calibration suggest that the automatic method using PEST may be a suitable alternative to manual method assisted by HSPEXP for calibration of hydrologic parameters for HSPF. However, further research of the weights used in the objective functions is necessary to provide guidance when applying PEST to surface water modeling.

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Fecal contamination of surface waters is a critical water−quality issue, leading to human illnesses and deaths. Total Maximum Daily Loads (TMDLs), which set pollutant limits, are being developed to address fecal bacteria impairments. Watershed models are widely used to support TMDLs, although their use for simulating in−stream fecal bacteria concentrations is somewhat rudimentary. This article provides an overview of fecal microorganism fate and transport within watersheds, describes current watershed models used to simulate microbial transport, and presents case studies demonstrating model use. Bacterial modeling capabilities and limitations for setting TMDL limits are described for two widely used watershed models (HSPF and SWAT) and for the load−duration method. Both HSPF and SWAT permit the user to discretize a watershed spatially and bacteria loads temporally. However, the options and flexibilities are limited. The models are also limited in their ability to describe bacterial life cycles and in their ability to adequately simulate bacteria concentrations during extreme climatic conditions. The load−duration method for developing TMDLs provides a good representation of overall water quality and needed water quality improvement, but intra−watershed contributions must be determined through supplemental sampling or through subsequent modeling that relates land use and hydrologic response to bacterial concentrations. Identified research needs include improved bacteria source characterization procedures, data to support such procedures, and modeling advances including better representation of bacteria life cycles, inclusion of more appropriate fate and transport processes, improved simulation of catastrophic conditions, and creation of a decision support tool to aid users in selecting an appropriate model or method for TMDL development.

ABSTRACT. As a continuation of efforts to provide a common background and platform for development of calibration and validation (C/V) guidelines for hydrologic and water quality (H/WQ) modeling, ASABE members worked to determine critical topics related to model C/V, perform a synthesis of a previously published special collection of articles and other relevant literature, and provide topic-specific recommendations based on the synthesis as well as personal modeling expertise. This article introduces a special collection of nine research articles covering key topics related to calibration and validation of H/WQ models. The topics include: terminology, hydrologic processes and model representationydrologic and water quality (H/WQ) models consist of interrelated assemblages of mathematical equations that represent complex physical, chemical, and biological processes governing the hydrology and fate and transport of sediments, nutrients, pesticides, and bacteria. H/WQ models are widely used to evaluate the impacts of changes in land use, land management, climate, and conservation practices on soil and water resources. Reliable models provide valuable information for making sound management, policy, and regulatory decisions. Comprehensive calibration and validation (C/V) of models is essential to obtain the "right outcomes" for the "right reasons" (Holling, 1978;Kirchner, 2006). Important components of proper C/V include selecting an appropriate model, setting up the model properly, establishing valid C/V strategies (Refsgaard and Henriksen, 2004), and capturing available datasets to represent the appropriate spatial and temporal scales (Kirchner, 2006).H/WQ models are valuable because they represent processes operating at various spatial and temporal scales simultaneously in a complex and interrelated manner. For example, the hydrology component of the Root Zone Water Quality Model (RZWQM; Ahuja et al., 2000) consists of interrelated hydrologic processes such as precipitation (including rainfall, snow, or irrigation), evaporation and transpiration (ET), surface runoff, infiltration, percolation, lateral flow, streamflow, and groundwater. However, no matter how detailed a model, it remains a simplification of the natural system in which some processes are characterized by rates and thresholds whose values are unknown. Be- cause many model parameters are typically available for calibration, the mere fitting of modeling results to observed data may result in good statistics of model performance but poor correspondence to the actual processes that the model is intended to represent. In such a case, a good fit is obtained for the wrong reasons. Application of such results can lead to incorrect conclusions with negative consequences (Dressel, 2010). The use of proper C/V practices can help prevent this circumstance while also allowing for a common foundation for interpretation of results and their comparison with other modeling studies (Refsgaard and Henriksen, 2004;Douglas-Mankin et al., 2010;Tuppad et al., 2011).Resear...

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