Hunting Creek watershed
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Hunting Creek watershed |
A landscape model was used to make these estimates. Landscape modeling is an innovative decision making tool based on system analysis and computer simulation. Models serve as repositories of current understanding of ecosystem processes and the various data sets available regionally. They ensure that the data is internally consistent and sufficient. They promote decisions based on the entire data set and allow scenarios of development to be analyzed and compared.
Due to the time constraints, the greater availability of digital mapping data for Hunting Creek, and as Hunting Creek is representative of the County as a whole, we have focused our studies on the Hunting Creek Watershed in order to:
We have designed a modeling approach that can estimate the impact of economic development on environmental conditions in a watershed. The model takes into account information about economic indicators (such as land use patterns, amount of nutrient pollution from various sources, etc.), physical characteristics of the area (elevation, soil types, etc.), and biological characteristics (habitat types, vegetation types, etc.). It then produces a spatial simulation of ecological dynamics and generates output for a variety of environmental indicators such as water quality in streams and estuaries. We can then modify input data to reflect existing options of economic change and compare the different alternatives in terms of their environmental effects.
We have applied our modeling approach to the Hunting Creek, as a representative of the Calvert County as a whole. Our goal was to compare the different buildout alternatives in terms of water quality in Hunting Creek. For this study we have focused on nitrogen and were comparing the concentration of nitrogen in the estuary zone of Hunting Creek under the 3 alternative buildout projections (buildout, number of dwelling units = n1,n2,n3). There are five major sources of nutrient loading in the watershed:
The atmospheric deposition is related to regional development in the airshed, that is much larger than the watershed. This loading is mostly defined outside of the County boundaries and the contribution of local versus remote factors is poorly understood. However quantitatively the atmospheric deposition accounts for more than 50% of the nitrogen loading.
There was no direct data on fertilizer application by the farmers and residents. The amount of nitrogen delivered with fertilizers can be estimated based on the expected yield for particular soils. Farmers are assumed to apply the amounts of fertilizers that would guarantee that their yield will be no less than the expected. Assuming that crop rotation practices (corn, winter wheat, soy beans) are applied throughout the County, the fertilizers were still the second largest source of nitrogen (~30%).
The loading from septic tanks can be calculated as discharge per each individual multiplied by 2.9 (average number of people per dwelling unit in Maryland) multiplied by number of dwelling units. Under current population densities in Hunting Creek watershed, the septic systems are the third largest source of nitrogen that contributes ~20% of total load.
Discharge from sewage treatment plants has been considered negligible, since in this watershed all sewage undergoes tertiary treatment (land application). However the indirect flows of nitrogen from these sources are worth further consideration in the future. The amount of nitrogen that comes from decomposition of dead organic material is significant but since it is delivered directly into the root zone it is rapidly recaptured by the vegetation and may be disregarded in the overall balance.
The fate of nitrogen from the various sources in the watershed is different. Different forms of nitrogen are retained in soil and plants at different rates, depending upon the landscape characteristics, types of vegetation and management practices. One of the main purposes of the simulation model is to track these pathways of nutrients from various sources to the estuary.
We have analyzed a number of growth scenarios with the model:
The results in terms of percent change in the amount of nitrogen in the Hunting Creek estuary are as follows:
The nitrogen content in the estuary is sensitive to the increase in the amount of dwelling units on the watershed. The existing design of septic drainage fields effectively removes nitrogen from the active root zone and allows it to accumulate in the ground water that eventually finds its way into the river system. Scenarios 4-6 employ septic systems that discharge in the near surface vegetative layer. In that case the signal from the septics is considerably lower and there is not a big difference between the different zoning strategies. However preservation of an extended river buffer zone (200 m) further diminishes the amount of nutrients entering the estuary. If similar total preserved area is chosen randomly, the gain is significantly less (Scenario 8). Furthermore, if we assume maximum build out but on a totally forested watershed (Scenario 9) we can actually decrease 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 buildout.
Go to Patuxent model description
E-mail to Alexey Voinov