Water and Nitrate‐Nitrogen Losses From a Small, Upland, Coastal Plain Watershed

Abstract

AbstractSurface runoff (SRO) and shallow‐subsurface flow (SSF) from a small, upland, Coastal Plain watershed underlain by plinthite were monitored for a 10‐y period. Samples collected during surface runoff and subsurface‐flow events were analyzed for NO3‐N. The major water runoff loss from the system was found to be subsurface flow, accounting for 79% of total runoff loss, which occurred primarily from December through May. Nitrate‐N concentrations in surface runoff and subsurface flow were relatively uniform over the period, averaging 0.47 and 8.75 mg/L, respectively. The combination of high volume of subsurface flow and its relatively high NO3‐N content, resulted in 99% of total NO3‐N loss via subsurface flow. Predictive equations for monthly surface and subsurface water runoff losses from this watershed were developed using multiple linear regression. The equations contain seasonal and climatic parameters, including pan evaporation. Tests of the equations with observed results gave significant (P ≤ 0.01) correlation coefficients, r, of 0.82 and 0.85 for surface runoff and subsurface flow, respectively. Mean surface and subsurface NO3‐N concentrations were multiplied by predicted monthly flows to compute predicted monthly NO3‐N losses. Comparison of predicted and actual monthly NO3‐N losses gave significant (P ≤ 0.01) correlation coefficients, r, of 0.59 and 0.86 for surface and subsurface flows, respectively. The good fit of predicted and observed results can be partially attributed to the seasonal and climatic data base collected over a relatively long period of 10 y.