Scenarios

A scenario is a certain plan or vision of the future, that is expressed in terms which are compatible with the model. Plans and visions may be described verbally or as diagrams or tables and may incorporate numerous processes and factors, which may not be directly included into the modeling tool. If we intend to use the model for analysis of the possible outcomes of these plans or visions for the particular system that is modeled, we need to translate the verbal or numerical description into inputs that are assumed in the model.

Generating the right scenarios may turn out to be a fairly complex job. Some scenarios are inappropriate for the model at hand. Or rather the model is inappropriate for such scenarios and needs to be modified or completely rebuilt. In some cases we can modify the scenario is such a way that it can be still fed into the existing model, maybe at the expense of some of the detail and precision. For such scenarios the model can be used for some estimates of ranges, thresholds, it may be still useful for approximate evaluations.

The scenarios that the model is intended to be used for are concerned with various projected zoning options for Calvert County. The Hunting Creek is quite representative for the County as a whole. It well represents the land use types and the development projections that are currently debated. The most ambigous part of the project is to interpret the available figures on zoning restrictions and the numbers of dwelling units to be built in terms of maps that can be readily processed by the model.

The alternative zoning plans for the Calvert County, developed by the County Department of Planning an Zoning, belong to the category of scenarios that cannot be precisely translated into the HCM input parameters and therefore need to be somewhat adjusted and aggregated to fit into the model. The Calvert County zoning plans can be summarized in the following way:

OPTION 1 EXPECTED UNDER CURRENT ZONING
  COUNTY HUNTING CREEK
  NEW LOTS NEW COM NEW+COM EXIST TOTAL Ncells DENS
RURAL 11000 1704 661 2365 1502 3867    
RCD 7337 1429 464 1893 1089 2982 931 0.320
FCD/RPD 3663 275 197 472 413 885 859 0.103
TC - HUNTINGTOWN 8500 134 0 134 11 145    
TC - PRINCE FREDERICK   880 127 1007 10 1017 169 0.602
R-1 "4,000" 116 12 128 198 326 28 1.164
TOTAL 23500 2834 800 3634 1721 5355    
EXISTING LOTS "23,500"              
LOTS COMMITTED "7,600"              
TOTAL LOTS AT BUILDOUT "54,600"              
OPTION 2 "REDUCE BUILDOUT TO 44,000"
  COUNTY HUNTING CREEK
  NEW LOTS NEW COM NEW+COM EXIST TOTAL Ncells DENS
RURAL 5000 775 661 1436 1502 2938    
RCD 3335 650 464 1114 1089 2203 931 0.237
FCD/RPD 1665 125 197 322 413 735 859 0.086
TC - HUNTINGTOWN 4900 77 0 77 11 88    
TC - PRINCE FREDERICK   507 127 634 10 644 169 0.381
R-1 "3,000" 87 12 99 198 297 28 1.061
TOTAL 12900 1447 800 2247 1721 3968    
EXISTING LOTS "23,500"              
LOTS COMMITTED "7,600"              
TOTAL LOTS AT BUILDOUT "44,000"              
OPTION 3 "REDUCE BUILDOUT TO 36,000"
  COUNTY HUNTING CREEK
  NEW LOTS NEW COM NEW+COM EXIST TOTAL Ncells DENS
RURAL "1,899" 295 661 956 1502 2458    
RCD 1267 247 464 711 1089 1800 931 0.193
FCD/RPD 632 48 197 245 413 658 859 0.077
TC - HUNTINGTOWN "1,861" 29 0 29 11 40    
TC - PRINCE FREDERICK 0 193 127 320 10 330 169 0.195
R-1 "1,140" 33 12 45 198 243 28 0.868
TOTAL 4900 550 800 1350 1721 3071    
EXISTING LOTS "23,500"              
LOTS COMMITTED "7,600"              
TOTAL LOTS AT BUILDOUT "36,000"              

The corresponding input parameters that can be used in the model are the land use map and the dwelling units density map. As approximate scenarios of development we have generated three maps of dwelling unit densities, assuming that within the particular zoning subdivisions the assumed total number of dwelling units will be uniformly distributed across all the available areas. These maps can be used for a general estimate of the impact of development on water quality in the estuary, however they would not be accounting for the specific patters of distribution of population densities within the zones.


Option 1
Option 2
Option 3


Landuse map altered by buildout.

Both of these maps can be only approximately estimated based on the data in the zoning plans for two main reasons:

  1. There is no way to define the exact distribution of densities within the zoning subdivisions. This is further complicated by the additional restrictions on clustering that are adopted. For example it is known that building lots are allowed only on 20% of parcels in the Farm community and Resource preservation districts. However there are no immediate rules to define the location of this development within the parcels.
  2. In many cases the existing patterns of development do not match the projected zoning rules (so called "grandfathering" of building permits). There are no spatial data available that would take these inconsistencies into account.

Running the model for these scenarios (1-3) we have concluded that the difference between the 3 buildout plans is quite visible in terms of the impact on the nitrogen concentration in the Hunting Creek estuary. Build out under current zoning plans will result in —14.66% increase in the amount of nitrogen delivered to the estuary. The two alternative plans will result in 11.11% and 7.52% increase, respectively. With most of the nitrogen from the septic tanks channeled directly into the groundwater, as it is the case under current septic design, the increase in the population density results in increased amounts of nitrogen in the estuary.


% change of annual average concentration of TN in Hunting Creek
in the scenarios relative to the concentration under current development.

The scenarios analyzed include:

However if the septic design is altered in such a way that the nutrients leached are made available for the uptake and retention in plants, then the picture changes quite dramatically. In scenarios 4-6 we have evaluated the response of the estuary to the same buildout alternatives if the septic discharge is channeled into the root zone and further used by plants. In this case the difference between the buildout alternatives becomes quite subtle. It changes from 7% in the maximum buildout case, to 6.75% and 6.7% in the two other buildout alternatives, respectively. Now the effect of increased population density is almost totally wiped out by the strong signals coming from the alternative sources of nitrogen load, that is the atmospheric deposition and fertilizer application. These two loadings significantly dampen the effect of the population increase. The interaction between population growth and the atmospheric deposition is not quite understood, though obviously it should be expected that higher development and denser transportation networks will only further contribute to higher concentrations of nutrients and toxics in the atmosphere.


No development in riparian buffer
No development in R&APD

No development in wetlands
All the above

Development scenarios modified by "visions"

OPTION RCD FCD/RPD TC - PRINCE FRED R-1
  NN Density NN Density NN Density NN Density
Zoning 931 0.3203 859 0.1030 169 0.6018 28 1.1643
Z--WL 931 0.3203 785 0.1127 169 0.6018 28 1.1643
Z--RPD 931 0.3203 708 0.1250 169 0.6018 28 1.1643
Z-WL-RPD-APD 931 0.3203 549 0.1612 169 0.6018 28 1.1643
Z-River Buf 750 0.3976 609 0.1453 149 0.6826 26 1.2538
TOTAL 2982   885   1017   326  

Two other scenarios were considered to find out what is the effect of actual spatial location of the development. In Scenario 7 all river buffer and critical area cells were made unavailable for development. As a result the increase in nitrogen loading fell over 2% to 4.37%. When a similar reduction of the number of buildable cells was allocated on a random basis (Scenario 8), the difference was larger and resulted in 5.87% increase in nitrogen concentration. This only proves that the spatial distribution of dwelling units is an important factor that may even have a larger influence on the system than the total number of units in a particular zone.

For reference purposes we also ran a scenario that assumed maximum build out but on a totally forested watershed (Scenario 9). Even though this is quite unrealistic with the environmental practices of today, yet it was interesting to find out that for this development pattern we have actually decreased the pollution of the estuary relatively to the current conditions, even for the maximum projected population growth in the watershed. Of course it is hardly possible to implement this development strategy, however it is important to stress that environmental and management practices associated with development may be even more important than only the target numbers for density growth.

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