To provide a common background and platform for consensual development of calibration and validation guidelines, model developers and/or expert users of the commonly used hydrologic and water quality models globally were invited to write technical articles recommending calibration and validation procedures specific to their model. This article introduces a special collection of 22 research articles that present and discuss calibration and validation concepts in detail for 25 hydrologic and water quality models. The main objective of this introductory article is to introduce and summarize key aspects of the hydrologic and water quality models presented in this collection. The models range from field to watershed scales for simulating hydrology, sediment, nutrients, bacteria, and pesticides at temporal scales varying from hourly to annually. Individually, the articles provide model practitioners with detailed, model-specific guidance on model calibration, validation, and use. Collectively, the articles in this collection present a consistent framework of information that will facilitate development of a proposed set of ASABE model calibration and validation guidelines.

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Cover cropping practices are being researched to reduce artificial subsurface drainage NO3–N losses from agricultural lands in the Upper Mississippi watershed. This study was designed to investigate the influences of fall planting date and climate on cereal rye (Secale cereale L.) biomass and N uptake in the spring, and to assess subsurface drainage NO3–N loss reductions. A soil–plant–atmosphere simulation model, RyeGro, was developed and used to predict rye cover crop establishment and growth, soil water balance, N cycling, and drainage NO3–N losses from mid‐September through May in southwestern Minnesota. An imbedded stochastic weather generator provided model climate inputs. Inclusion of a rye cover crop sown on 15 September reduced N losses by 11.1 kg N ha−1 or 45% for a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] crop rotation. Fall sowing dates of 1, 15, and 30 October resulted in reductions of 7.8, 5.8, and 4.6 kg N ha−1, respectively, by the end of May. Desiccation of the rye on 1 May resulted in reductions of 4.5, 2.2, 1.2, and 0.7 kg N ha−1, for the 15 September and 1, 15, and 30 October sowing dates, respectively. Cover cropping practice provides promising opportunities for reductions in N losses for cropping rotations wherein the primary crops are harvested before mid‐September and planted after mid‐May. We predict that a winter rye crop can reduce drainage NO3–N losses on average 7.4 kg N ha−1 for southwestern Minnesota if planted on 15 September and desiccated on 15 May.

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This paper compares parameters that characterize vegetation flexibility effects on flow resistance and drag. Drag forces have been measured in a flume for simple cylindrical obstructions of the same shape and size but with different flexibility under several flow conditions. This data set is used to fit drag parameters and to relate their value to flexibility through the Cauchy Number. A formulation is presented where the drag coefficient is evaluated as a function of a new calibration velocity parameter which is related to the elastic modulus of the obstruction. While the use of a Vogel exponent and reference velocity provides a similar response, the reference velocity when used is somewhat nebulous and appears to have a critical impact on the parameter and the drag force calculated. The proposed formulation for drag reduction is more consistently estimated for the range of flexibilities in this study.

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Effective impervious area (EIA), or the portion of total impervious area (TIA) that is hydraulically connected to the storm sewer system, is an important parameter in determining actual urban runoff. EIA has implications in watershed hydrology, water quality, environment, and ecosystem services. The overall goal of this study is to evaluate the application of successive weighted least square (WLS) method to urban catchments with different sizes and various hydrologic conditions to determine EIA fraction. Other objectives are to develop insights on the data selection issues, EIA fraction, EIA/TIA ratio, and runoff source area patterns in urban catchments. The successive WLS method is applied to 50 urban catchments with different sizes from less than 1 ha to more than 2000 ha in Minnesota, Wisconsin, Texas, USA as well as Europe, Canada, and Australia. The average, median, and standard deviation of EIA fractions for the 42 catchments with residential land uses are found to be 0.222, 0.200, and 0.113, respectively. These values for the EIA/TIA ratio in the same 42 catchments are 0.50, 0.48, and 0.21, respectively. While the EIA/TIA results indicate the importance of EIA, 95% prediction interval of the mean EIA/TIA is found to be 0.07 to 0.93, which shows that using an average value for this ratio in each land use to determine EIA from TIA in ungauged urban watersheds can be misleading. The successive WLS method was robust and is recommended for determining EIA in gauged urban catchments. Copyright © 2016 John Wiley & Sons, Ltd.