Mechanistic models include equations that represent physical, chemical, and biological relationships; however, data are required to define the parameters for these equations, setup model spatial boundaries, describe boundary conditions, and calibrate model predictions to observed conditions. Modeling requires a variety of reliable information about factors such as the following:

• Geometry of the water body
• Forcing functions, which include boundary water surface elevations (in estuary and lake models), flows and constituent concentrations, and facility wastewater discharges
• Kinetic rates and constants for each biological process included in the model
• Weather information (e.g., precipitation, evapotranspiration)
• Land use and soils
• Sediment/nutrient loading from land
• Ground water/interflow concentrations

Geometric data such as river reach length, cross section profile, and lake and estuary bathymetry can be gathered from water management entities such as state agencies, the U.S. Army Corps of Engineers (USACE), the U.S. Geological Survey (USGS), and the National Oceanic and Atmospheric Administration (NOAA). Depending on the waterbody size or project concerns, geometric data can be gathered specifically for the project and mechanist model. Forcing function data describe environmental conditions that are external to the model and are unaffected by internal conditions of the modeled system, including meteorological conditions, water surface elevations, and solar radiation. A variety of sources can provide that information, including state agencies, NOAA, USACE, and the U.S. Bureau of Reclamation. Data on boundary conditions, such as inflows, and outflows ground water/interflow concentrations, surface water constituent concentrations, can be obtained from local and state geologic survey agencies, USGS, and EPA. Other critical model inputs, including facility wastewater discharges and withdrawals, can often be obtained from the same sources. Land use and soils data are available from the Multi-Resolution Land Characteristics consortium. Much of this information also is available through EPA BASINS, a geographic information system (GIS) environment in which users can access and download data and configure a selection of models.

Nutrient cycling, algal growth, light mechanics, and DO dynamics require kinetic rates and constants, including phytoplankton growth rate, death rate, settling rate, and light attenuation factors. Many of the parameters can be adequately estimated from literature values and/or previous modeling studies. For example, EPA’s rates, constants, and kinetics formulations in surface water quality modeling by Bowie et al. (1985) EXIT, is a helpful guide for parameter selection. You can adjust these values during calibration or after additional monitoring.