SWAT Model Prediction of Phosphorus Loading in a South Carolina Karst Watershed with a Downstream Embayment

Devendra M Amatya,Daniel R Hitchcock,Manoj K Jha,Thomas M Williams,Amy E Edwards

doi:10.4236/jep.2013.47a010

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.

Highlights

In recent years there has been an increasing demand for the sustainable management of water quantity and water quality of various water bodies such as streams, lakes, reservoirs, and groundwater throughout the nation
This study provided an opportunity to test the SWAT model’s simple approach of completely mixed system and simple settling rate to predict total phosphorus (TP) loadings at the reservoir-like embayment (R-E) embayment downstream at the lake edge instead of using other complex eutrophication model like WASP [28] applied by [14] for simulating the water quality of Lake Marion
When months with very high measured TP values (April, May in 2009) and large discrepancies in flow measurements with overpredictions during low lake level (e.g. April and October in 2008) and underpredictions during higher lake levels (e.g. June 2008 and March and August in 2009) as reported by [27] were omitted in the analysis, the correlation substantially increased to adjusted R2 = 0.57 for a zero intercept, with a biased slope of 0.49 significant at p = 0.0005 (Figure 6(b)). These results indicate that the model may be able to predict the monthly TP loading, except for conditions with very low and high lake levels and very high TP levels, consistent with the findings by [16], and that such prediction may be double, on average, of the observed monthly TP loads calculated with one monthly TP value

Summary

Introduction

In recent years there has been an increasing demand for the sustainable management of water quantity and water quality of various water bodies such as streams, lakes, reservoirs, and groundwater throughout the nation. The 2000 National Water Quality Inventory (NWQI) reported that nutrients were the leading pollutants in lakes and reservoirs, the fifth in rivers and streams and the eleventh in estuaries [2]. The nutrients nitrogen (N) and phosphorus (P) stimulate the growth of a variety of types of aquatic plants resulting in eutrophication and impairment in the beneficial uses of water bodies [3].

Objectives

Methods

Results

Conclusion