Watershed Modeling in Support of Water Quality Management of the Currituck Sound

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Watershed Modeling in Support of Water Quality
Management of the Currituck Sound

• Jerad Bales, Ana María García
• U.S. Geological Survey
• North Carolina Water Science Center

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Background

• Water quality in Currituck Sound is affected by
• Inputs
• Transport and transformation in the sound
• Potential sources of inputs
• Tributary streams
• Connections to Albemarle Sound
• Connections to Chesapeake Bay
• Ground water
• Atmosphere

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Watershed Modeling Purpose and Scope

• SWAT was applied to simulate flow, sediment and
  nutrient transport in two watersheds.
• Tull Creek watershed
• West Neck Creek watershed
• Water quality parameters simulated
• total suspended solids (TSS),
• total phosphorus (P)
• total nitrogen (N).
• Load estimates for
• existing conditions and an alternative scenario.

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Location of Tull Creek and West Neck Creek
Watersheds
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Modeling Procedure

• Model Setup
• Calibration
• Hydrology
• Water Quality
• Simulation of current (06-07) conditions
• Simulation of alternative scenarios

Validation of watershed models require a longer
period of record (gt 5 years) than was available
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Soil Water Assessment Tool (SWAT)

• Physical, process-based representation of
  hydrologic cycle and pollutant transport

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Hydrologic and Terrain Data

• Land use/cover
• USGS National Land Cover Dataset (NLCD)
• Soil Map and Database
• NRCS State Soil Geographic Dataset (STATSGO)
• Weather
• NC State Climate, NCDC
• Topography
• LIDAR
• Flow and Water Quality
• USGS Flow Gauges, Water Quality Data

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Tull Creek SWAT Model

• 116 sub-basins (0.45-476 ha)
• 150 km2 drainage area
• 55 in agriculture
• 43 marshland and forest
• Predominant soils have poor drainage capacity

Outlet to Tull Bay
2001 National Land Cover Dataset
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Tull Creek Hydrologic Calibration

• Considerations
• tillage
• cover crop
• crop rotation
• artificial drainage
• Limitations
• extent of the ditch drainage network
• tidal-flow gage.

R2 0.83 NSE 0.87
Simulated and observed flows 01-2006 07-2007
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Tull Creek Total N and Dissolved P Calibration
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West Neck Creek SWAT Model

• 93 sub-basins ranging in area from 0.88 ha. to
  318 ha
• Total delineated drainage area of 88 square
  kilometers
• Significantly urban (36 and 37 agricultural)
• Wetlands and forest (27 ) buffer the southern
  portion

Outlet to North Landing River
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West Neck Creek Hydrologic Calibration

• Calibrated parameters for overlapping HRUs were
  applied
• Agricultural drainage not simulated
• Limitations
• Bi-directional flow
• Wind influences flow, but precipitation alone has
  little effect on flow (Caldwell, 2001)

R2 0.58 NSE 0.76
Simulated and observed flows 02-1998 06-1990
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West Neck Creek Total N and Dissolved P
Calibration

• Water quality samples were collected once a
  month.
• Urban areas contributed to high Org-N loads.
• Other potential sources.

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Water, total suspended solids, TN and TP yield
estimates for 06-07
From Tull Creek (agricultural watershed) to
West Neck Creek (urban watershed)
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Summary and Conclusions

• Tool for estimating terrestrial inputs to
  Currituck Sound was developed for typical
  agricultural and urban basins.
• Nutrient inputs are rather low, likely due to
  marshlands buffering streams
• Sediment inputs are higher from agricultural
  basin.
• Model was used to simulated effects of an
  agricultural BMP, resulting in about 50 lower
  sediment and nutrient inputs.
• Model can be used to evaluate effects of other
  land use and management changes.
• Terrestrial inputs from remainder of basin
  required.