These two pages present the estimated amount of nitrogen and phosphorus reaching the Bay (also called “loads”) from various land uses by major river basin and by political jurisdiction within each basin.

The figures also show the amount of change from 1985, the baseline for measuring nutrient reductions. Negative percentages represent decreases, positive percentages represent increases.

These estimates come from the Bay Program’s Watershed Model, the tool used to measure progress toward meeting nutrient goals. Like a giant accounting program, the model tracks changes in land use, which may increase or decrease nutrient loads, and credits implementation of actions which reduce nitrogen and phosphorus runoff and discharges.

Taking those factors into account, the model estimates the amount of nitrogen and phosphorus that would be expected to reach the Bay under “normal” flow conditions.

In reality, the actual loads to the Bay in 2002, a drought year, were likely less than those shown here. That’s because low rainfall years result in fewer nutrients being washed off the land than in higher rainfall years. (For that reason, actual loads for 2003—which had the third highest flows into the Bay on record—would be much higher.)

In addition, the model does a poor job of estimating “lag times” for nutrients that move through groundwater. Half of the nonpoint source nitrogen reaches streams through groundwater rather than runoff. On average, it takes about 10 years for those nutrients to reach the stream. As a result, roughly half of the nitrogen control actions taken today won’t be felt in the Bay until years in the future.

It is best, therefore, to think of these numbers as estimates of nutrient reductions that will occur in the Bay at some time in the future under “average” rainfall conditions.

What is more important than the absolute numbers is the relative size and direction of nutrient changes for various land uses in various places—are they going up or down, and by a lot or a little?