TITLE:
SWAT Model Prediction of Phosphorus Loading in a South Carolina Karst Watershed with a Downstream Embayment
AUTHORS:
Devendra M. Amatya, Manoj K. Jha, Thomas M. Williams, Amy E. Edwards, Daniel R. Hitchcock
KEYWORDS:
Water Quality Models; Lake Marion; Runoff; Groundwater (Baseflow); Losing Streams; Deep Percolation; Settling Rate
JOURNAL NAME:
Journal of Environmental Protection,
Vol.4 No.7A,
July
17,
2013
ABSTRACT: The SWAT model was used to
predict total phosphorus (TP) loadings for a 1555-ha karst watershed—Chapel Branch Creek (CBC)—which
drains to a lake via a reservoir-like embayment (R-E). The model was first
tested for monthly streamflow predictions from tributaries draining three
potential source areas as well as the downstream R-E, followed by TP loadings
using data collected March 2007-October 2009.
Source areas included 1) a golf course that received applied wastewater, 2)
urban areas, highway, and some agricultural lands, and 3) a cave spring
draining a second golf course along with agricultural and forested areas,
including a substantial contribution of subsurface water via karst
connectivity. SWAT predictions of mean monthly TP loadings at the first two
source outlets were deemed reasonable. However, the predictions at the cave
spring outlet were somewhat poorer, likely due to diffuse variable groundwater
flow from an unknown drainage area larger than the actual surface watershed,
for which monthly subsurface flow was represented as a point source during simulations.
Further testing of the SWAT model to predict monthly TP loadings at the R-E,
modeled as a completely mixed system, resulted in their over-predictions most
of the months, except when high lake water levels occurred. The mean monthly
and annual flows were calibrated to acceptable limits with the exception of
flow over-prediction when lake levels were low and surface water from
tributaries disappeared into karst connections. The discrepancy in TP load
predictions was attributed primarily to the use of limited monthly TP data collected
during baseflow in the embayment. However, for the 22-month period,
over-prediction of mean monthly TP load (34.6 kg/mo) by 13% compared to
measured load (30.6 kg/mo) in the embayment was deemed acceptable. Simulated
results showed a 42% reduction in TP load due to settling in the embayment.