Spatial Model

  for more details about the model.

Animation of surface water flow

Interpolated precipitation patterns

Surface water flow was calibrated against the USGS gaging station located at downstream side of bridge on MD Rte.263, 200 ft east of intersection of MD Rte.4, 2.4 mi south of Huntingtown. Data are available for the period starting 1988.10.01 and until 1997.09.30. The model is able to reproduce runoff in a spatially realistic manner based on topography, land use and soil type. The general hydrologic trends seem to be well captured by the model. The animation shown on the right presents the spatial pattern of surface water flow over a characteristic period of three weeks at the end of 1990.

The closest climatic data are measured at MECHANICSVILLE 5 NE, and OWINGS FERRY LANDING. The model uses interpolated data from these stations to cover the Hunting Creek area. Data are available for 1980-96.

Calibration results for the gaging station in the Hunting Creek subwatershed.

Sources of error in this calibration: We are using daily total precipitation in the model. Therefore we cannot distinguish between a downpour and a drizzle if the total amount of rainfall delivered over one day is the same. The runoff associated with these events can in fact be quite different. We do not have a climatic station located directly on the watershed. Therefore we use interpolated data from two stations nearby. However some rainfall events can be very local and therefore will not be properly simulated. We also cannot exclude a chance of some error in the data.

Animation of dissolved inorganic nitrogen flows

There are four major sources of nutrient loading in the watershed:

1. Atmospheric deposition (data - mg/L - were downloaded from National Atmospheric Deposition Program , (NRSP-3)/National Trends Network. (1999). NADP Program Office, Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820);
2. Discharge from sewage treatment plants (this input may be considered negligible, since in this watershed all sewage undergoes tertiary treatment);
3. Discharge from septic tanks (calculated as a function of discharge per individual multiplied by number of dwelling units multiplied by 2.9 - average number of people per dwelling unit in Maryland);
4. Application of fertilizers in agricultural and residential habitats (was estimated based on the yield and soils map available from MOP).

The atmospheric deposition is one of the major sources of nitrogen in this watershed. For Maryland there are two stations, one in White Rock, Carol County, the other one is in Wye, Queen Anne County. There are no direct measurements available for Hunting Creek, however, both stations show quite consistent patterns of annaul average dynamics. Since there is not a very considerable spatial heterogeneity in N deposition ( see the isopleths at the NADP site) we used the Wye data, since this station seemed to be closer to the Hunting Creek conditions in terms of its distance to large industrial discharges and sources of pollution in the Metro area.

Annual average concentration of NO3+NH4 deposited from the atmosphere

Annual total atmospheric N deposition (concentration x precipitation)

Dynamics of loading with the 2-year crop rotation introduced

Annual dynamics of nitrogen loading (kg/ha)

Total annual nitrogen loading in Hunting Creek (kg/ha)

Currently it appears that the fertilizers and the atmospheric deposition present the major sources of nitrogen pollution on the watershed delivering almost 80% of nitrogen to the area. However the fate of nitrogen from different sources may be different and one of the main uses for the simulation model is to track the pathways of nutrients from different sources to the estuary.

The water quality data are available from USGS, measured at the same gaging station on MD Rte.263. There are records for specific conductance, suspended sediment, total suspended solids, total organic carbon, dissolved nitrite-plus nitrate, total nitrogen, and total phosphorus. The period covered is 1990-1996.

Annual average concentration of TN in Hunting Creek

Annual total precipitation in Hunting Creek (inches)

The variations in the average concentration of total nitrogen in Hunting Creek seem to be mostly explained by the atmospheric loading and the rainfall patterns.

The model was calibrated against the USGS data measured at the gaging station.

Calibration results for the Total N in Hunting Creek (1990-1995).
Solid line - model output, dots - experimental data.

The model was able to reproduce the trends of nitrogen concentration at the gaging station. It should be noted that the water quality data are quite patchy and considerable time periods remain unaccounted for by the observations. Besides it may be fairly easy to miss a peak of water flow while getting the samples, the nutrient concentrations tending to be the highest during the peak flow. Therefore the water quality data are likely to represent the baseflow concentration and consequently they usually underestimate the true long term nutrient dynamics.

In addition to the daily nitrogen dynamics we obtained a fairly good fit for the annual average concentration. This certainly increases our confidence in the model performance, since it shows that the model does a good job predicting the integral fluxes of nutrients over the watershed. This type of analysis is especially important when comparing the various scenarios of development in the region.

Comparison of annual average concentrations
measured at the USGS station and simulated.

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