Testing the Riparian Ecosystem Management Model (REMM) on a Riparian Buffer with Dilution from Deep Groundwater

Abstract

Abstract. The Riparian Ecosystem Management Model (REMM), developed to quantify water quality benefits of riparian buffers, was field-tested using five years (2005-2009) of measured hydrologic and water quality data on a site in the upper coastal plain of North Carolina. This buffer site received nitrate-nitrogen (NO 3 -N) loading from a pasture fertilized with poultry litter. Field results showed reductions in groundwater NO 3 -N concentrations moving through the buffer to the stream for the five-year period; however, further analysis of the groundwater data indicated that dilution was a one of the contributing factors to those observations. Previous testing of REMM was carried out at riparian sites having minimal groundwater dilution. This modeling study is the first attempt to calibrate and validate REMM on a riparian buffer site showing reductions in NO 3 -N concentrations due to groundwater dilution. REMM was calibrated using daily measured water table depths (WTDs) and monthly groundwater NO 3 -N concentrations. Results of model testing showed simulated WTDs and NO 3 -N concentrations in good agreement with measured values. The mean absolute error (MAE) and Willmott‘s index of agreement (d) between simulated and measured WTDs ranged from 18 to 41 cm and from 0.59 to 0.95, respectively. The root mean square error (RSME) and Willmott‘s index of agreement (d) between simulated and measured NO 3 -N concentrations ranged from 1.20 to 8.41 mg L -1 and from 0.11 to 0.73, respectively, for the five-year simulation period. REMM predictions of annual plant nitrogen (N) uptake and denitrification were within the ranges reported in other riparian buffer field studies. A 33-year simulation was carried out using the calibrated and validated model to assess the long-term NO 3 -N removal performance of the buffer. Results showed that the buffer on average reduced groundwater NO 3 -N concentrations from 13Â mg L -1 at the field edge to 1 mg L -1 at the stream edge. In terms of loadings, the NO 3 -N and total N reductions from the field edge to the stream were 81% and 89%, respectively. Results of this modeling study showed that REMM reasonably simulated short-term (five years) and long-term (30+ years) groundwater hydrology and NO 3 -N dynamics for a riparian ecosystem, despite the absence of a groundwater dilution component in the model.