Potomac Highlands Watershed School

WV Potomac Tributary Strategy: Chapter 5

Excerpted with Permission from the WVPTS Draft – April 24, 2004


CHAPTER 5 - at a glance…

  • The Chesapeake Bay Program uses mathematical models to simulate changes in the Bay ecosystem due to changes in population, land use, or pollution management.

  • The Chesapeake Bay Watershed Model (CBWM) simulations are not the same as actual conditions. They are the best scientific estimate of what average loadings are likely to be.

  • The CBWM simulates one acre of each land use within each of the 94 separate model segments to represent all similar land use within the segment.

  • Each of the Bay jurisdictions face different challenges in reducing their nutrient and sediment loads.

  • Based on current model estimates, between 1985 and 2002 nitrogen loads dropped 5%, phosphorus increased about 1%, and sediment decreased 17% in West Virginia,

A number of the WVPTS stakeholders consider model estimates to be inaccurate, and reject the use of these estimates in the Tributary Strategy process.

Table of Contents:


What is the Chesapeake Bay Watershed Model?

The Chesapeake Bay Program uses various mathematical models to simulate processes in the 64,000 square mile Chesapeake Bay drainage basin, which is much too large and complex to isolate for experiments in the real world. These models allow Bay scientists to simulate changes in the Bay ecosystem due to changes in population, land use, or pollution management. There are three main models used by the CBP: the Estuary Model, the Airshed Model, and the Watershed Model. The Estuary Model, commonly referred to as the water quality model, examines the effects of the loads generated by the Airshed and Watershed Models on Bay water quality. The Airshed Model tracks nitrogen emissions from all sources in the airshed, and covers the eastern United States from Texas and North Dakota eastward to Maine and Florida.

The CBP model of particular concern in developing West Virginia’s tributary strategy is known as the Chesapeake Bay Watershed Model (CBWM). The current version of the Watershed model divides the watershed into 94 model segments; a version currently in development will utilize more than 500 segments and work on a much finer scale. The model uses rainfall, evaporation, and meteorological data to calculate runoff and subsurface flow for all the basin land uses including forest, agriculture, and urban lands. The surface and subsurface flows simulate soil erosion and the pollutant loads from the land to the rivers. The model also routes flow and associated pollutant loads from the land through lakes, rivers, and reservoirs to the Bay.

The CBWM uses mathematical representations based on the best available science to create its simulations of the real world. These simulations allow Bay scientists to predict changes to the Bay ecosystem, both positive and negative, due to changes in management, such as reducing the quantity of fertilizer applied to agricultural lands, installing new pollution controls at sewage treatment plants, and controlling urban sprawl.

As with all models, the CBWM simulations, or scenarios, are not the same as actual conditions. They are, however, the best scientific estimate of what average conditions are likely to be in a complex system where reality is enormously difficult to measure. The CBWM uses knowledge of cause and effect relationships gained through monitoring programs and research to produce estimates of what might happen in the Bay watershed in the future, and to predict probable conditions in areas that lack adequate monitoring data. In addition, as with all other models, the quality of the information "input" to the model will have a significant impact on the quality of the simulations. One of the goals of the WV PTS stakeholders and their agency partners is to make certain that the information input to the model from WV is as accurate as possible. This is critical, because the model will be used to estimate the results of the pollution reduction strategies developed by the WVPTS stakeholder group.

Overall, as with all predictive tools, the CBWM has both strengths and weaknesses. Some of the things the model does well are:

  • Equitably accounts for all major load sources based on the best available science.
  • Reasonably represents the relative impact of management actions at the model segment, major tributary and basin levels.
  • Reasonably predicts the likely relative impact of one set of actions compared to another; for example, nutrient and sediment reductions that might be expected from planting a cover crop versus installing a riverside grassy buffer.

On the other hand, the CBWM:

  • Can’t predict what is not known. For example, the model simply doesn’t include contributions of sediment from dirt roads because the predictive tools to estimate those loads do not currently exist. On the plus side, the CBP is constantly seeking to fill in model gaps as scientifically defensible information becomes available.
  • Is limited by the quality of input data. This has been a real concern for West Virginia, particularly in the areas of actual water quality data to calibrate the model for our state and in accurately accounting for environmental practices in agriculture (see Chapter 6).
  • Is not currently set up to be a local TMDL model for West Virginia (it could be with the proper input information). For better or worse, WV has had to use it in that manner due to the lack of the correct kind of real world data.
  • Is not a crystal ball, and can’t be expected to tell us precisely what will happen at a specific time. The CBWM uses professional estimates of future land uses, population, animals, and air to evaluate the relative impact of "what if" management scenarios.

A recent "white paper" by the CBP’s scientific and technical advisory committee indicates that, based on water quality monitoring results, the CBWM is likely to overestimate progress made by the states towards achieving their cap load reductions. This happens because the CBWM generally uses best management practice "efficiency" assumptions based on idealized research studies, rather than from field studies on these practices as they are actually installed. This paper also considers critical the need for long-term small watershed studies to better determine BMP efficiencies. 24 For more information on the CBP’s models, visit http://www.chesapeakebay.net/model.htm.

How the Watershed Model works.

At its core, the current Watershed Model (version 4.3) operates at the level of 94 "segments" in the nine major Chesapeake Bay tributary watersheds. Model calibration also takes place at the segment level (see box). A new version of the model (v.5) that will have 500 segments and, therefore, allow much greater precision, is due in 2006.

Each segment is divided into Forest, Mixed Open, Agriculture, and Urban land use categories based on the best available data (for example, agricultural acreage is based on the agricultural census in each state.) The Urban land use is further broken down into Urban Pervious (land area where water can soak into the ground) and Urban Impervious (where water cannot soak into the ground). The Agriculture land use is further broken down into Cropland (conventional /conservation till), Hayland, and Pasture.

What It Means: Model Calibration

The CBWM uses mathematical relationships (such as between water quality, land use, hydrology and soil type) derived from many small scientific studies and applies them more generally on a large scale. For example, the scientific literature indicates that a forested riparian buffer will likely prevent the transport of at least 30% of the nitrogen that would otherwise flow from cropland into streams. In order to determine how closely the model approximates reality, the modeling team conducts "calibration runs" where the model is tested against real world monitoring data. The WVDA is working to produce WV data to help better calibrate the model.

Within each model segment, each land use subcategory is uniquely defined for that area based on: hydrologic parameters; sedimentation rates; nutrient inputs; plant cover and uptake rates; nutrient cycling / export rates. The CBWM simulates one acre of each land use to represent all similar land use within the segment. In other words, one soil, cover type, nutrient application rate, slope, infiltration rate, and particle size distribution represents one land use type within any given segment. The effectiveness of BMPs may differ from segment to segment based on local conditions.

Using the above information, the CBWM is used to estimate past and current conditions, and to predict how changes in land management will affect future conditions. For example, the model can be used to estimate the sources of nutrients and sediment to the Bay in any given year. Figure 5 presents model estimates of the sources of nitrogen, phosphorus and sediment in the entire Bay watershed in 2002.

How West Virginia compares to other Bay states

Pennsylvania and Virginia contain the largest percentage of the Chesapeake Bay watershed, followed by Maryland, New York, West Virginia, Delaware, and Washington DC (figure 6). As the watershed areas of each jurisdiction differ greatly, it is not surprising that their relative contributions to the Bay’s sediment and nutrient problems differ as well. Figure 7 compares nutrient and sediment loads from the seven political jurisdictions, as estimated by CBWM, for 1985 baseline, 2002 progress, and 2010 Cap Load Allocations. The jurisdictions with the largest land area (Pennsylvania, Virginia and Maryland) also contribute the largest nutrient and sediment loads. Each jurisdiction has a different mix of land uses that produce their nutrient and sediment loads and require a different mix of remedies. For example, nitrogen from the highly urbanized Washington DC area comes almost entirely from point sources, in particular the mammoth wastewater treatment plant at Blue Plains, while nitrogen from rural Delaware comes mostly from highly concentrated agriculture. Thus far, CBP signatories Maryland, Virginia, and Washington DC have made the most progress in reducing their baseline (1985) nutrient loadings – but all jurisdictions still have a long way to go to meet the Cap Load Allocations.

Load estimates by Land Use for West Virginia

Figure 8 provides West Virginia nutrient/sediment loads as estimated by the CBWM for the baseline year (1985) and indicates progress made in reducing these loads as of 2002. These estimates indicate an overall five percent reduction in Total Nitrogen (TN), an increase in Total Phosphorus (TP) of less than one percent, and a seventeen percent decrease in sediment. By land use, agriculture was identified as contributing the largest loads for TN (48%), TP (60%) and sediment (70%). Reductions in TN loads from point sources and agriculture were notable (16% and 14%, respectively), while nitrogen from septic fields increased by 96%. Reductions in TP loads were notable for agriculture (6%) and from urban non point sources (18%), while TP from point sources increased by 29%. The agricultural sector was solely responsible for substantial reductions (24%) in sediment loads.

The agricultural sector’s reductions in TN (14%) and TP (6 %) occurred during a period of rapid change in the region’s agricultural industry, as noted in Chapter 2. Between 1985 and 1997, the dairy and swine industries declined dramatically (43% and 48% respectively), beef increased slightly (6%), and the poultry industry boomed (layers increased by 198%, broilers by 159%, and turkeys by 43%). Overall, the CBP estimates that TN generation from animal manure increased by 39% during this period, and TP by 41%.   Despite these increases, estimated load reductions noted for the industry occurred because of aggressive implementation of Best Management Practices in the region (see Chapter 6 in WVPTS).