Hydrologic and water quality (H/WQ) models consist of interrelated assemblages of mathematical equations that represent the processes governing the movement, fate, and transport of water, sediments, nutrients, pesticides, bacteria, and other constituents in and on the surface of the soil and through groundwater, stream, river, and reservoir systems. Reliable models provide information for making sound management, policy, and regulatory decisions. While calibration, validation, and evaluation (CV&E) are essential for ensuring model accuracy and precision, there is little consistency in how model practitioners conduct and document this process and communicate the results. Therefore, this Engineering Practice was designed to contribute to good modeling practice recommendations to improve modeling methodology and application of H/WQ models through improved model performance as well as enhanced communication of the processes utilized and results to modelers, decision-makers, and other modeling stakeholders.

It builds on recommended protocols recognizing the need for formal model development, application, and communication guidelines rather than ad hoc approaches because of the often-serious human health, regulatory, ecological, socio-economic, and policy implications of modeling applications. This Engineering Practice is not meant to serve as a comprehensive guide but rather to compile and enhance guidance from foundational manuscripts such as Beck (1987), ASCE (1993), Haan et al. (1995), Refsgaard et al. (2005), Jakeman et al. (2006), Engel et al. (2007), Moriasi et al. (2007), Biondi et al. (2012), Bennett et al. (2013), Ritter and Mu?oz- Carpena (2013) and Black et al. (2014), and advances described in recent compilations (e.g., Mu?oz-Carpena et al., 2006; Moriasi et al., 2012, 2015a).