It's tough to compress a 64,000-square-mile watershed into a computer that calculates the amount of nutrients a drop of water landing in the headwaters of New York can carry to the Chesapeake.

The drop will gather differing amounts of nutrients depending on whether it fell in a thunderstorm or during a gentle drizzle. It'll gain few nutrients if it runs through a forest. But a drop will gather much more if it goes through a cornfield or feedlot.

But the drop, and the associated nutrients, could get absorbed by a streamside buffer and never complete its trip to the Bay. If it makes it to a stream, some of the nutrients could be absorbed by biological processes in the water.

Difficult as it may seem, those complexities, and much more, are simulated in the Chesapeake Bay Watershed Model, different versions of which have served as a cornerstone for management decisions over the last two decades.

Now, the Bay Program is preparing to roll out the latest version of the model. Known as Phase 5, (Actually, it's Phase 5.1 going on 5.2.) it provides the most detailed simulations yet of where nutrients originate, how they move through the landscape and the effectiveness of various control actions. So many calculations are involved that the model can take several days to complete a simulation.

Phase 5 will be a key decision-making tool over the next two years as the region develops a new Bay cleanup plan, known as a Total Maximum Daily Load, which will require major nutrient and sediment reductions throughout the watershed. It's no overstatement to suggest that it influences how billions of dollars will be spent.

In that regard, Phase 5-which has been in development for more than five years-will bring both good news and bad news.

The good news is that it will offer the most realistic and finely tuned data yet devised for estimating water and nutrient flow from distant headwater streams to the tidal waters of the Chesapeake.

The bad news is that the improved model has, in effect, discovered significantly more nutrients than were previously estimated. In addition, it indicates that tributary strategies-river-specific plans written to meet past cleanup goals-are less effective than originally thought.

The bottom line: The gap between where cleanup efforts are today, and where they need to get, is likely to grow.

"It's not that the job has gotten harder," said Lewis Linker, modeling coordinator at the EPA's Chesapeake Bay Program Office. "It's been that hard all along. We just didn't know it."

The exact size of the gap won't be known until late winter when final changes to the model and the information going into it, are completed. But preliminary results indicate that, at the current rate of progress, years could be added to the cleanup.

"Generally, it indicates that the job in front of us is going to continue to get tougher," said Rich Batiuk, associate director for science at the EPA's Bay Program Office. "That may sound like bad news. But the model is closer to reality, and that is what we should be basing our management decisions on."

The model dates to 1982, and each new version incorporates better science. "Every time we do it, we understand the system better, we have more confidence; we get closer to the 'truth'," Batiuk said. "We will never get to the truth, but we get closer to what we think is really happening out in the real world."

Bay watershed models work by taking information about nutrient inputs from fertilizer; animal manure; discharges from wastewater treatment plants and industries; atmospheric deposition; and septic systems, then applying those inputs to the appropriate land uses throughout the watershed.

The models divide the Bay's 64,000-square-mile watershed into segments and calculates the amount of nutrients and sediment that would reach the stream from various land uses in each segment. It also reduces nutrients to reflect the impact of various actions, known as best management practices, which are designed to control runoff.

The model uses a 10-year span of meteorological information, including a mix of wet, dry and average rainfall years, to estimate the amounts of nutrients washed off the landscape. Typically, increased rainfall and severe storms drive more nutrients into streams and the Bay. The model simulates nutrient movement at one-hour intervals throughout the 10-year period.

The output is then averaged over the 10 years to determine the amount of nutrients delivered to streams and the Bay under "normal" conditions.

The new model gives different results from earlier versions largely because it is able to handle far more detailed information. Gary Shenk, integrated analysis coordinator at the Bay Program Office and lead developer for Phase 5, put it bluntly: "Phase 5 is a better model."

The old model, Phase 4.3, divided the watershed into 94 segments; Phase 5 divides it into 851 land and river segments-every stream with a flow of 100 cubic feet per second is a separate segment.

Phase 4.3 had nine land uses; Phase 5 has 24. Significantly, Phase 5 also has far more calibration sites-locations where model output can be compared with monitored data. Phase 4.3 had only 15 calibration sites for the entire watershed; Phase 5 has 223 sites where either flows or nutrient concentrations can be compared with model estimates.

The old model used meteorology from 1985 through 1994, the most recent data available at the time. But a recent, longer-term analysis covering 30 years of data, found that 1985-94 was actually about 5 percent drier than normal.

A switch to using data from 1991 through 2000, which is more representative of long-term hydrology, increases estimates of nutrient runoff-wet conditions drive more nutrients into streams.

Other factors also contribute to the increase. Recent analyses, for instance, have found that nutrient loads from the coastal plain-which has historically been more difficult to assess-are higher than previously thought.

The way best management practices are handled has also changed. Previously, the model essentially incorporated BMPs as a filter between a land use and a stream: A cover crop would filter one percentage of nutrients from the runoff, while a grass waterway would filter a different percentage.

In Phase 5, the BMPs can be adjusted to better reflect reality. Under high flows, many become less effective, a change now captured in the model. Others may break altogether after particularly severe storms.

"In Phase 5, you can make the filter as smart as you want," Shenk said.

Some changes don't come from the model, but improved information going into the model.

Atmospheric deposition had been underestimated near the Bay, where heavy vehicle traffic produces more nitrogen oxide emissions.

In the past, only figures from wastewater dischargers deemed "significant" were included. Generally, those were facilities that were either close to the Bay, or had flows of more than 500,000 gallons a day. But discharges from the hundreds of smaller facilities around the watershed are now seen as important-a change that increases both nitrogen and phosphorus.

Not all improvements increased estimated nutrient levels, though. The model incorporates new best management practices that were not modeled before.

Reservoirs tend to reduce nutrient and sediment transport, and the new model includes better estimates of the impact from dozens of reservoirs throughout the watershed.

But overall, the net change will mean more nitrogen and phosphorus, although potentially less sediment, in the model.

The impact of those changes will play out over the next two years, as the Bay Region finalizes a Total Maximum Daily Load, or TMDL, for the Bay. A TMDL is a pollution budget that determines the maximum amount of pollution that a water body can receive and still meet its water quality standards. That "load" is then allocated to contributing sources.

In the case of the Bay, the maximum load is identified using a separate water quality model that simulates the impact of nutrients and sediment on the Chesapeake's water quality. (In 2003, that load was set at 175 million pounds of nitrogen and 12.8 million pounds of phosphorus. But that model is also getting an update and those numbers may change somewhat.)

When the maximum load is set, Phase 5 of the Chesapeake Bay Watershed Model will become the primary tool to allocate nitrogen, phosphorus and sediment limits to each of the Bay's major tributaries, and to each state.

States will use the model to further subdivide allocations to more local levels-some are considering county-level nutrient and sediment goals.

Because of its ability to assign nutrient loads to a more local level, Batiuk said Phase 5 will make cleanup efforts more "real" to communities. In turn, he said, that may result in "more shoulders against the wheel" when it comes to pushing for nutrient reductions.

In that regard, the model is an essential tool for managers. It is the only way to match nutrient and sediment sources to specific areas, and then estimate the effectiveness of various actions that could be taken to control runoff.

"A model is the only thing with which you can ask 'what if' type questions," Shenk said.

For example, the model can help predict what mix of actions are likely to help meet water quality standards: Are actions taken in one tributary more important than another for Bay water quality? What happens if riparian forest buffers were planted along all rivers and streams? How would planting 1 million acres of switchgrass for biofuels affect nutrient and sediment runoff?

The model can help identify the mix of nutrient reduction actions that would be the most effective-and cost-effective-ways to reach cleanup goals.

Still, while it is considered one of the most sophisticated hydrological models in the nation, the watershed model is not reality. Even with all of its complexities, Phase 5 represents a "significant oversimplification" of the actual Chesapeake Bay watershed, noted a review conducted by the Bay Program's Scientific and Technical Advisory Committee.

Nonetheless, the review team said, "we believe that the [watershed model] is appropriate given the scale, complexity and mechanical basis of the modeling and management frameworks that are feasible with the current state-of-the-science of watershed modeling, for management purposes."

So far, output from Phase 5 better aligns with estimates from the U.S. Geological Survey regarding nutrient loads being exported from rivers.

While models can help plan the path that leads to a clean Bay, the ultimate verdict of their success would come from a Bay that meets its water quality standards.

Those standards are based on the conditions needed to support various types of aquatic life in the Bay, from striped bass swimming near the surface to underwater grasses in shallow water to worms and clams that dwell in deeper water.

Meeting the standards has to happen in the real world-not in a model. "We plan with the models," Shenk said. "We use monitoring to gauge attainment of water quality criteria."

Chesapeake Bay Program Decision Support System

Environmental models are essential for simulating ecosystems that are either too large or too complex to isolate for experiments in the real world. Models use mathematical representations of the real world to estimate the effects of complex and varying environmental events and conditions.

They allow scientists to simulate changes in an ecosystem that are due to changes in population, land use or pollution management. These simulations, called scenarios, allow scientists to predict positive or negative changes within an ecosystem that are due to management actions such as improved sewage treatment, controlling urban sprawl and reduced fertilizer or manure application on agricultural lands.

Models produce estimates, not perfect forecasts. They reduce, but do not eliminate, uncertainty in environmental decision-making. Used properly, they are a tool that can assist in developing nutrient and sediment reductions that are the most protective of the environment, while being equitable, achievable and cost-effective.

The Bay Program uses three models to help inform decision making:

  • The Airshed Model uses information about emissions from power plants, vehicles and other sources within a 570,000-square-mile airshed-an airshed seven times the size of the Bay watershed-which contributes nitrogen deposition to the Bay region. The airshed model estimates how much of those emissions are deposited on the Chesapeake and its watershed, as well as where they land. That information is fed into Watershed Model.
  • The Watershed Model incorporates information about land use, fertilizer applications, wastewater plant discharges, septic systems, air deposition, farm animal populations, meteorology and other variables to estimate the amount of nutrients and sediment reaching the Chesapeake, as well as where they originate. It is the key tool used to develop pollution control strategies.
  • The Estuarine Model, also called the Bay Water Quality Model, simulates water movement horizontally and vertically throughout the Bay. It uses data from the Watershed Model (as well as data from the Airshed Model regarding air deposition directly on the Bay) to estimate nutrient and sediment impacts on the Bay, as well as what level of nutrient and sediment reductions are needed to achieve the Chesapeake's water quality standards.