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Development and Calibration of the Falls Lake Watershed Model to Estimate Nutrient Loading from Point and Nonpoint Source

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A watershed model for the Falls Lake watershed, North Carolina was developed to aid in the establishment of a nutrient management strategy for Falls Lake. The North Carolina General Assembly Session Law 2005-190 requires development and implementation of a nutrient management strategy for water supply reservoirs that were impaired due to excessive nutrient loading. Falls Lake, a multi-purpose man-made reservoir, was listed on North Carolina's 2008 303(d) list as impaired for chlorophyll a, an indicator of nutrient enrichment. The upper portion of the lake was also listed for turbidity. The 770 square-mile watershed is within the northeastern Piedmont of North Carolina and spans portions of six counties including parts of the cities of Durham and Raleigh. Nine water supply reservoirs in the watershed, including Falls Lake, serve 500,000 people.

With guidance from a Technical Advisory Committee composed of members of state and local governments and other professionals, the Watershed Analysis Risk Management Framework (WARMF) was used to develop a model of the upper portion of the Falls Lake watershed to estimate nutrient loading from point and nonpoint sources. WARMF was selected for this study because it is a physically-based model and has the capability to assess the impact of point and nonpoint sources in a large watershed with varying land cover and management conditions. It is a watershed decision support system designed to support the watershed approach, Total Maximum Daily Load (TMDL) calculations, and the stakeholder process. In addition, WARMF has the capability to represent on-site wastewater systems separately, includes reservoirs and lakes in the watershed, and estimates loading from various sources.

WARMF was calibrated for flow, total suspended solids (TSS), total nitrogen (TN), total Kjeldahl nitrogen (TKN), nitrate, ammonia, and total phosphorus (TP) for the five major subwatersheds in the system: Knap of Reeds Creek, Flat River, Little River, Eno River, and Ellerbe Creek watersheds. It was calibrated using the best available data including: USGS streamflow data, estimates of atmospheric deposition, meteorological information, managed flow, land cover, soil and topographic properties, point sources, sanitary sewer overflows, sand filter systems, and land application of nutrients for several land cover types. The model calibration covered the 2004 through 2006 timeframe. The model was validated using data collected in 2007. WARMF simulated the hydrology and water quality in the calibrated subwatersheds reasonably well. The error values associated with model predictions were generally low, suggesting a reasonable agreement between the simulated and observed values. This paper provides information on the development, calibration, and validation of the WARMF model and presents nutrient load estimates from various sources in the five major subwatersheds of the Falls Lake Watershed.
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Keywords: Falls Lake; Management Strategy; Nutrient Loading; Nutrients; WARMF Model

Document Type: Research Article

Publication date: 2011-01-01

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