The Oklahoma Dispersion Model: An operational management tool for determining near-surface dispersion conditions for agricultural practices across Oklahoma

Dramatic increases in the number of swine operations across Oklahoma since 1994, and the growing odor concerns associated with them, served as a major motivator for the development of the Oklahoma Dispersion Model (ODM), a Web-based management tool for the assessment of current and future dispersion conditions. Besides odors associated with animal waste products, other agricultural applications of the model include operations such as prescribed burning (smoke) and pesticide application. The ODM (http://agweather.mesonet.org/models/dispersion) generates both graphical and text output that depict current and future conditions for atmospheric dispersion (dilution of plume) and transport direction (direction of plume movement). With such knowledge, producers and land managers can better assess appropriate times for the near-surface release of gases and particulates. The Model has been operational on the Web since April 1998 (one unanticipated application was its use in debris burning after the Oklahoma City F5 tornado of May 3, 1999).

Statewide maps showing current dispersion conditions and transport direction are generated every 15 minutes using weather data from the Oklahoma Mesonet, the state's automated weather station network of 116 stations. These maps are updated every 15 minutes in near-real-time. The latest 60-hour NGM (Nested Grid Model) MOS (Multiple Output Statistics) forecasts for specific sites within and surrounding Oklahoma are used to generate similar maps valid at 3-hour increments throughout the duration of the 60-hour period; these maps are updated every 12 hours. In addition to graphical output, tabular output for specific MOS sites is available.

The Oklahoma Dispersion Model utilizes the traditional Gaussian plume model to estimate horizontal and vertical dispersion. Pasquill stability class is calculated in two ways: for current conditions, Mesonet weather data is used in conjunction with EPA recommended algorithms; for forecast conditions, the Turner (1964) method for calculating stability classes is used. Rural sigma-y and sigma-z values are calculated using the Briggs equations (1973). A scheme is used (to be described in detail in the paper) which breaks the atmosphere into five dispersion categories. The resulting dispersion maps feature a five-color classification scheme, ranging from excellent (dark green) to very poor (red). Corresponding maps for transport direction utilize a station plot of temperature, relative humidity, wind speed, and wind direction.

In summary, the Oklahoma Dispersion Model is an example of how the traditional Gaussian plume model can be used in an operational setting to aid in management decisions, in this case for agricultural and natural resource applications. This paper will discuss details of the model itself, present examples of graphical and text output from the Web site, and show how the model can be used as a management tool in agricultural operations.