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Buzzards Bay National Estuary Program

Parcel Loading Analysis: Step-by-step example for the Wareham River Estuary (PART 4)

<< Go Back to Wareham River Watershed Loading Analysis Demo, PART 3

Step 9: Putting together a loading spreadsheet

A watershed nitrogen loading spreadsheet is a mathematical computational model that expresses inputs and losses of nitrogen into a watershed or estuary, based on certain assumptions. In many respects, a nitrogen loading model is merely a count of houses, and a summary of acreages of certain land types, such as agricultural lands, golf courses, etc., that accounts for relevant to nitrogen inputs. Generally the data is organized into subwatersheds so that the appropriate assumed watershed attenuation how loading coefficients can be applied, or how data may be organized to fit the inputs needed in a linked water quality model.

These loading models can be very simple, or complex in terms of how they handle various sources, or how finely a watershed is divided into subwatersheds. Irrespective of how a model is constructed, loading estimates should be robust among models if they use the same loading assumptions and attenuation coefficients. In many southeastern Massachusetts estuaries, septic systems may account for 50% to 80% of total attenuated watershed loading, therefore correctly counting the number of houses or residential units (and accounting for any sewering) is to a large degree the core data assessment task.

To approximate nitrogen loading in any watershed, the key elements you need to know for each of its subwatersheds is the net nitrogen attenuation coefficient for that subwatershed, the number of septic systems, the number of buildings (to estimate lawns, driveways, and roof area), acres of agricultural land, golf courses road, and surface waters, and if applicable, data on point sources (like wastewater facilities), and number of farm animals.


Because septic systems are often the largest source of nitrogen, it is also important to characterize the amount of waste those systems in any particular watershed. The two basic approaches are to used to characterize septic loadings: census data (federal or local adjusted for summer seasonality where appropriate), or water use (from municipal records). Both approaches have limitations and confounding factors, but the water use data can better cumulatively characterize seasonal and weekend occupancy patterns in watersheds where such occupancy patterns are important. The water use approach has been adopted by the Massachusetts Estuaries Project because it helped characterize septic loadings on Cape Cod where many coastal village areas have highly seasonal shifts in occupancy.

On our subwatershed loading pages we include the nitrogen loading spreadsheets and GIS data that are the basis for our estimates of estuary loading. These spreadsheets use loading assumptions adopted by the Massachusetts Estuaries Project. A good example of how a loading spreadsheet can be constructed around a complex estuary watershed, go to our Wareham River watershed nitrogen loading page and open the loading spreadsheet at the bottom of that page.