Estimates of the amount of nitrogen and phosphorus entering the Chesapeake are made by the Bay Program’s Watershed Model. It is a computer model that can be thought of as a giant accounting program: It tracks nutrient reduction efforts taking place throughout the watershed and estimates cleanup progress.

The model divides the Bay’s 64,000-square-mile watershed into more than 94 river basin segments. Using information about land use as well as human and agricultural animal populations in each segment, it calculates the amount of nutrients and sediments likely to enter rivers or groundwater. Information about discharges from wastewater treatment plants and industries is also incorporated. The model simulates the transport of nutrients and sediments downstream toward the Bay. The closer a segment is to the Chesapeake, the more its nutrients will be “delivered” to tidal waters. The farther away, the greater chance that some nutrients, mainly nitrogen, will be removed by biological or chemical processes.

Finally, the model estimates the total amount of nutrients reaching the Bay from nine major drainage basins. For accuracy, model data are compared with water quality monitoring information at points throughout the watershed.

Over time, data about land use changes and nutrient control practices—such as the construction of runoff control devices or the adoption of nutrient management plans or upgrades at wastewater treatment plants—are fed into the computer to estimate progress toward nutrient reduction goals under average rainfall conditions.

In the real world, nutrient inputs vary based on weather conditions—the more rain, the more nutrients are washed off the land. The model is a tool that uses simulated weather for a “normal” year, allowing year-to-year progress to be estimated under identical conditions.

The model is not reality—it is a simplified representation of the natural world, and the information it generates is only as good as what is fed into it. In some cases, nutrient control practices are adopted but not recorded by government agencies and are not reflected in the model. Other practices are not as effective as credited by the model.

The model offers a general picture of how nutrient reduction efforts under way are faring. One can get a sense of where most of the nutrients are coming from, where progress is likely being made and where it lags.