Introduction

Welcome to the technical documentation for the Hydrologic Engineering Center's Water Quality Engine (HEC-WQ Engine, or just WQ Engine). This software integrates with flagship HEC software programs to provide functionality for modeling water quality. The HEC-WQ Engine takes hydrologic input data from a driving program and uses it to calculate the fate and transport of water quality through a watershed. It provides a bridge to a set of water quality transformation libraries, developed by the USACE Engineering Research and Development Center's Environmental Lab (ERDC-EL), for calculating heat fluxes, nutrient transformations and algal growth.

HEC-ResSim

This manual describes the theoretical basis, numerical methods and implementation details for the water quality transport calculations. The first, and at present only, software to fully integrate the HEC-WQ Engine into its simulation setup and compute is HEC's Reservoir System Simulation (HEC-ResSim, or just ResSim); for this reason, the application to HEC-ResSim will be the focus of the manual. Documentation for setting up water quality model runs in ResSim, detailed descriptions of the software components, and viewing graphical output of the model results is provided in the HEC-ResSim Water Quality User's Manual - Working Draft for Review.

Units

HEC-ResSim and the HEC-WQ Engine have the ability to perform calculations with either English or SI unit system inputs for many of the hydrological and water quality input variables. However, the equations detailed in this document will only use the SI system, for consistency. The main reason for this is the widespread usage of SI units for the concentration of material in water (mg/L). The ERDC-EL libraries for calculating heat fluxes and nutrient transformations also accept only SI unit inputs and do all calculations natively in SI.

Water Quality Nomenclature

Many of the terms denoting components of water quality modeling in HEC-ResSim and the WQ Engine will be familiar to water quality modelers (e.g., initial conditions, boundary conditions). However, some of the terms used in this manual (and in HEC-ResSim) are less commonly used, and have been chosen to provide clarity and eliminate overlap with pre-existing ResSim computational components. Specifically, what are commonly referred to as water quality state variables (e.g., water temperature, dissolved oxygen, nutrient concentrations) are referred to here as water quality constituents. The name given to a calibration coefficient that can be varied within certain bounds to calibrate the model (e.g., a rate coefficient for nutrient transformation) is a water quality parameter. The definitions of these less commonly used terms are emphasized when first mentioned in the text, and the appendix Glossary of Water Quality Terms provides definitions for quick reference.

Technical Manual Setup

The remainder of the Introduction provides information about the general capabilities and limitations of the model. The second section provides information about the required input datasets. The third section describes the equations and algorithms used for transport of water quality within reservoirs, and the fourth section describes transport within stream reaches. These are followed by a description of methods used in reservoir operations for water quality. The final sections document the heat budget and nutrient transformation equations used in the ERDC-EL libraries, and the results of some verification and validation simulations.