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Appendix K

Allocation Methodology to Relate Relative Impact to Needed Controls

Introduction

The nutrient allocation procedures agreed to by 5 of the 7 Bay watershed partners and followed 

by EPA are described in section 6.9 of the main document. The reader should be familiar with 

section 6.9 before reading this appendix. The goal of this appendix is to expand the options that 

were considered before selection of the final procedures and to provide rationale for the final 

decisions. The information presented in this appendix is the same information that was used to 

inform the decisions in the spring and summer of 2009. The watershed model was version 5.2 

which had known limitations. Many of the values given in this appendix will be different from 

the final version as these decisions were not revisited with version 5.3 of the watershed model.

Relative Effectiveness Options

Section 6.9.2.1 of the main document is a discussion of the relative effectiveness of major basins 

in improving dissolved oxygen in the critical areas of the tidal waters of the Chesapeake. 

Relative effectiveness is a combination of riverine effectiveness, also known as a delivery factor, 

which is expressed as 

pounds of reduction reaching tidal waters

pounds of reduction to the local river 

and estuarine effectiveness, which is expressed as

improvement in dissolved oxygen .

pounds of reduction reaching tidal waters

Multiplying the two together gives

improvement in dissolved oxygen .

pounds of reduction to the local river

Riverine Effectiveness Options

No options were considered in calculating riverine delivery factors. The principles of calculating 

delivery factors in the Chesapeake Bay Program watershed models are long-standing and have 

been approved several times in workgroups and subcommittees and have been reviewed in 

independent STAC review of the watershed model in 2007 and 2009. Nitrogen delivery factors 

are calculated for each river segment. Nitrogen levels are lowered naturally in river systems 

through denitrification, providing a long-run removal of nitrogen. Phosphorus and sediment do 

not undergo a similar process to denitrification and do not have long-run removal mechanisms 

other than delivery through the river system and burial. Burial is offset by scour, both of which 

are episodic in nature. This does not hold true in reservoir systems, where burial is much more 

significant and is not offset by scour to a great degree. Due to the lack of spatially and 

temporally detailed phosphorus and sediment data that would be needed to precisely calibrate 
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scour and burial on the segment scale, the calculation of delivery factors for phosphorus and 

sediment is closed around reservoirs rather than segment by segment. That is, all segments 

upstream of a reservoir or an entrance to the tidal system and downstream of other reservoirs 

receive the same riverine delivery factor.

Geographic Grouping of Estuarine Effectiveness

The estuarine effectiveness is calculated by comparing the dissolved oxygen simulated in the 

Bay of the calibration run to the dissolved oxygen simulated in the Bay when a given watershed 

area has reduced loads relative to the calibration loads. The effectiveness for that given area is 

then calculated by dividing the improvement in dissolved oxygen by the reduction in loads. A 

choice has to be made regarding the geographic areas to test.

Each area along the estuary would theoretically have a different estuarine effectiveness, but there 

are limitations to what can be effectively calculated. If the area tested has a low total load and 

therefore a small change in going to a reduced load, the estuarine model may not be able to 

resolve the change in dissolved oxygen. Tested areas must be aggregated up to a reasonably 

large load to be able to record the change. Also the estuarine model takes a few days to complete 

a run and it would be time prohibitive to make more 100 or so runs.

There is no difference in estuarine effectiveness between loads in the same non-tidal watershed. 

Loads from areas just west of Washington DC would have the same estuarine effectiveness as 

loads from West Virginia since then enter the tidal waters at the same point, although they would 

have different overall effectiveness scores because of the riverine effectiveness. Therefore the 

head of tide of large river systems is a natural place to define a discrete watershed. The estuarine 

portion of major river systems like the Potomac and Rappahannock would have significantly 

different effects on the critical area for dissolved oxygen and also have large enough loads to 

resolve these differences so these are another reasonable place to lump geographically. The 

Eastern Shore is not amenable to simple rules like this as there are no large non-tidal river 

systems with large estuarine river systems. There is, however significant differences in estuarine 

effectiveness between the northernmost and southernmost portions of the Eastern Shore. The 

Eastern Shore was therefore divided in to four sections.

The final geographic breakout, as a balance between the desire to calculate a different 

effectiveness where a distinction exists and the limiting factors of computer run time and the 

ability of the estuarine model to resolve the oxygen effect of small differences in loading are:

Susquehanna

West Shore

Patuxent Above Fall Line

Patuxent Below Fall Line

Potomac Above Fall Line

Potomac Below Fall Line

Rapp Above Fall Line

Rapp Below Fall Line

York Above Fall Line

York Below Fall Line

James Above Fall Line

James Below Fall Line

Upper East Shore

Middle East Shore

Lower East Shore

East Shore VA

To be clear, the allocation calculations are split between these geographic areas within 

jurisdictions, resulting in 30 different spatial units. The allocations, however, are expressed on 
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the jurisdiction and basin scale. That is, there is a calculation of Maryland Potomac above and 

below the fall line, but the allocation is expressed only as Maryland Potomac.

Choice of the Critical Designated Uses and Segments for Calculating 

Relative Effectiveness

To estimate the estuarine effectiveness, the change in dissolved oxygen must be calculated for a 

relevant area of the Chesapeake. The most persistent areas of dissolved oxygen violations are in 

the main stem of the Chesapeake from roughly the route 50 bay bridge south to the mouth of the 

Potomac and also the lower Potomac region. The deep water and deep channel designated uses 

are impaired at a higher rate than the open water designates use and also better integrators of 

bay-wide rather than local loads.

The deep water and deep channel designated uses of CB3MH, CB4MH, and CB5MH and the 

deep water designated use of POTMH_MD were selected as the most appropriate grouping to 

use in calculating estuarine relative effectiveness for the following reasons

1. These segments and designated uses had high levels of impairment

2. They are centrally located

3. The represent a large group of segments and a large volume of the bay.

4. Deep water and deep channel designated uses are good geographic integrators

Further tests of other combinations showed that the estuarine effectiveness was not particularly 

sensitive to the addition or subtraction of any given designated use.

Metric for Relative Effectiveness

To estimate the change in dissolved oxygen an appropriate metric of dissolved oxygen must be 

calculated that is sensitive to load changes across a wide array of segments, designated uses, and 

impairment levels and is relevant to the assessment of dissolved oxygen criteria. Three metrics

were investigated:

1. Percent non-attainment. 

2. Average dissolved oxygen during the summer assessment period

3. 25th percentile (quartile) of dissolved oxygen during the summer assessment period

Three criteria were applied in determining which of these make the best metric

1. Relevance to attainment of dissolved oxygen standards

2. Broad applicability to designated uses and water quality segments

3. Linearity of response – does the first pound have the same effect as the last?

Percent non-attainment is clearly the most relevant metric to standards attainment although other 

measures of dissolved oxygen are certainly also relevant. The quartile is more relevant than the 

average in that we are estimating increases in the lower values of oxygen.

Percent non-attainment is applicable only to areas that are not in attainment in the calibration and 

do not come into attainment when simulating a reduction in any single basin, which is a 

considerable limitation. Average dissolved oxygen is not an appropriate measure for many open 

water segments. An impaired open water segment may have average dissolved oxygen near 

saturation, but experience large swings between super saturation and low oxygen. A load 
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reduction may not change the average, but improve the water quality by reducing the variability 

of oxygen levels and the frequency of low values. The quartile is applicable to all segments and 

all designated uses.

Linearity of response is a crucial component. If a metric responded much more to the first pound 

of reduction than to the last, then smaller basins would be estimated to have a greater pound for 

pound influence than larger basins. To determine the linearity of the response to the three 

candidate metrics a run was made with the Susquehanna at half the level of reduction normally 

used to calculate estuarine effectiveness. In general, across multiple segments and designated 

uses, the response for all three metrics was mostly linear. There was not a significant difference 

between the metrics on this count.

The average was judged to be not suitable due to its limited applicability. The percent nonattainment was judged to be slightly more relevant than the quartile, but the quartile was selected 

as the appropriate metric due to its universal applicability.

Level of Effort Options

Section 6.9.2.2 of the main document describes the expression of level of effort as between the 

two extreme scenarios of no action scenario and E3. Selection of these two scenarios is an 

expression of the third principle under section 6.9.1 that all previous reductions are credited 

toward achieving the allocations. 

Atmospheric deposition

The atmospheric deposition options and rationale for choosing a level for federal targeting in 

developed in section 6.10.1. The method of incorporation is to hold atmospheric deposition 

constant through the bookend scenarios of No action and E3, and through all of the prospective 

management scenarios unless specific actions are called for in state plans that go beyond the 

federal levels. One example of states going beyond the federal level is that the E3 has

atmospheric deposition set to a level that incorporates reduced agricultural emissions and other 

possible state actions. This allows the jurisdictions to be responsible strictly for the reductions 

that they can control and not for federal actions on atmospheric deposition.

Scenario Options

The E3 scenario was selected as the appropriate lower end of loading rather than other candidate 

scenarios such as the ‘current programs’, ‘maximum feasible’, or ‘all forest’. Current programs 

could be used as the lower end and an assessment made of how far efforts had to increase beyond 

current programs, but this would violate the expression of equity above in that jurisdictions that 

had already achieved significant reductions would have to do proportionately more than

jurisdictions that have not. Maximum Feasible would be a similar expression as E3 and would 

meet the equity provision, but it was judged to be much more subjective and therefore inferior to 

E3 as a metric. The all forest scenario would be an expression of anthropogenic, rather than 

controllable loads. The all forest was used in the 2003 goal setting. Basing the allocation 

method on E3 recognizes that various sources have different possible levels of reduction. An 

allocation based on anthropogenic load could require levels of reduction beyond E3. For 

example, if an allocation required all loads to go 60% of the way toward all forest, certain 

theoretical land uses that can only achieve a 50% reduction at E3 would not be able to achieve 
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the allocations, while waste water treatment plants would be able to achieve much a much larger 

than 60% reduction from no action. These distortions would increase at smaller scales as sources 

become more dominant locally.

No Action was chosen as the upper end to follow the principle of accounting for previous 

reductions. Using a starting point that incorporated management practices or higher levels of 

treatment would give a disadvantage to jurisdictions that had implemented these actions.

Allocation Method Options

Section 6.9.2.3 describes the method used to relate relative effectiveness to reduction effort. 

With this basic outline there an infinite number of ways to define the allocation and still meet 

water quality standards. The major decisions to be made are the number of lines that represent 

different source categories and the shapes of those lines. The options were discussed in the 

WQGIT and the PSC and agreement was reached between the EPA and 5 of the 7 jurisdictional 

partners.

Number of Allocation Lines

During the allocation process, the WQGIT recognized that different source categories had 

different abilities to make progress toward an E3 level of implementation. Figure K-1 is a plot of 

implementation progress through 2009 plotted on the same vertical axis as the allocation charts, 

percent of E3 from no action.

0%

20%

40%

60%

80%

100%

Agriculture Developed WWTP

Percent reduction from No Action 

toward E3

2009 implementation as a percentage between 

2010 No Action and E3

TN TP

Figure K-1: 2009 implementation by sector

The 2009 implementation represents the choices that the jurisdictions have made to date, 

presumably taking into account the same types of criteria that will be used to make decisions on 

restoration spending in the future.

Note that this does not necessarily mean that more money or effort went to any particular sector, 

just that using the allocation metric of no action and E3 with the current definitions the sectors 

have varying current levels. Figure K-1 was created with phase 5.3 output, while the EPA and 

the WQGIT saw similar information using phase 5.1.
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There is a clear separation between the sources in that jurisdictions have chosen to set waste 

water treatment at a level closer to E3, relative to no action, than either agriculture or developed 

land. There is also a separation between agriculture and developed land, but it is not as large as 

the separation between WWTP and all other sources. With this information, the decision was 

made to use 2 lines, one for WWTP and one or all other sources.

Shape of the Allocation Lines

Several allocation line shapes were discussed with the three main shapes being:

1. Straight: This is the most straightforward expression of the allocation principle stated 

under section 6.9.1 that areas with a greater pound for pound effect on water quality 

should do more

2. Hockey Stick: It was recognized that a natural maximum existed for some sources, 

particularly with waste water treatment where a given technology could reach a 

concentration that could be expressed as a percentage from no action to E3. A hockey 

stick line has a maximum for watershed areas in the range of relative effectiveness and 

slopes down for lower levels of relative effectiveness.

3. Z-curve: similar to the hockey stick, but also recognizing that a natural minimum also 

may exist. Again related to waste water treatment plants, a given technology producing a 

known concentration may be seen as a minimum technology that should be implemented.

As reported in more detail in section 6.9.2.3 the wastewater line was set first in a ‘hockey stick’ 

shape such that the upper 50% of the relative effectiveness values were at a maximum attainment 

percentage, based on a given concentration and the rest sloped off to a minimum value also 

based on a concentration. The straight line for all other sources was set such that a zero relative 

effectiveness would have a 20% lower value on the percent controllable axis than the area with 

the maximum relative effectiveness value. The intercept for the all other sources was set such 

that the water quality standards were attained. Figure K-2, which is also figure 6-7 in the main 

document is the implementation of this method for Nitrogen.
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Nitrogen -- Phase 5.3 -- Goal=190

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 10

Relative Effectiveness Percent reduction from 2010 noBMPs to E3 All Other

WWTP

Figure K-2. Allocation methodology example showing the “hockey stick” and straight line reductions 

approaches, respectively, to wastewater (red line) and all other sources (blue line).

To make the above decision, the partnership was presented with several options for constructing 

the lines. Basin-jurisdiction loads were calculated for each option.

Table K-1: Initial options presented to the WQGIT on 9/30/2009

Lines 2 2 2 2 2

WWTP rule 3-8 mg/l 3-8 mg/l 3-8 mg/l HS 3-8 mg/l HS 3-8 mg/l Z

Other Load Rule 20% 10% 20% 10% 20% Largest

DO goal 200 200 200 200 200 Difference 2010 Noact E3 load TS load

DC Potm 2.82 2.82 2.37 2.37 2.37 16% 9.68 1.53 2.12

DE Esh 5.12 5.21 5.25 5.34 5.21 4% 9.28 3.45 6.43

MD Esh 12.54 12.76 12.81 13.03 12.70 4% 23.94 8.25 13.84

MD Patux 3.26 3.25 3.15 3.13 3.27 4% 6.57 2.15 3.17

MD Potm 14.73 14.52 14.10 13.89 14.29 6% 30.31 9.65 14.66

MD Susq 0.81 0.83 0.83 0.85 0.82 4% 1.35 0.61 0.97

MD Wsh 10.18 10.15 10.15 10.11 10.12 1% 36.50 6.15 9.49

NY Susq 10.54 10.41 10.54 10.41 10.55 1% 16.36 7.78 8.68

PA Potm 4.76 4.58 4.83 4.65 4.83 5% 7.08 3.12 4.31

PA Susq 67.96 68.59 68.81 69.44 68.37 2% 121.19 49.23 68.86

VA Esh 1.60 1.59 1.61 1.61 1.60 1% 3.25 0.88 1.67

VA James 28.84 28.14 28.49 27.78 29.58 6% 52.63 15.80 28.85

VA Potm 16.85 16.47 16.09 15.72 16.50 7% 33.05 10.72 15.81

VA Rap 6.54 6.37 6.49 6.32 6.60 4% 10.61 4.33 6.49

VA York 6.55 6.32 6.53 6.30 6.72 6% 10.54 4.05 6.48

WV Potm 5.65 5.44 5.71 5.50 5.73 5% 8.32 3.76 5.69

Total 198.77 197.46 197.76 196.45 199.27 1% 380.66 131.45 197.53
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Table K-1 is a sample of options that were explored. Several more options were generated 

before the final decision was made.

Calculation of Equivalent Allocation Options

For any given level of water quality there is an infinite number of lines that can be drawn on the 

allocation plots like figure K-2. To calculate an equivalent line to an existing line, it is necessary 

to meet the condition of:

or the sum of all delivered loads for each state/basin/fall-line combination times its estuarine 

delivery factor must equal a constant for the family of lines that meets the same water quality.

Expanding the delivered load term to create an equation between relative effectiveness and 

delivered load gives:

where:

Xi is the relative effectiveness

E3i and NoBMPi are the loads for that state/basin/fall-line/sector for the two scenarios

m and b are the slope and intercept of the line and the only unknowns

Given a slope or an intercept the above equation can be solved numerically for the other 

parameter of the line. This equation was implemented in excel for multiple lines with enforced 

maximum and minimum to accommodate the decisions above.

DeliveredLoad EstuarineDelivery C

E3i NoBMPi E3i 1 mXi b EstuarineDeliveryi C