『Abstract
　This paper presents an efficient and effective modeling approach to estimation of nitrogen retention in streams and rivers. The approach involves an extension of a newly developed longitudinal solute transport model, variable residence time (VART), by incorporating a first-order nitrogen reaction term. Parameters involved in the VART model are estimated using monthly mean flow and water quality data obtained through both field measurements and watershed modeling using the Hydrologic Simulation Program Fortran model. It is found that there is a strong correlation between nitrate-nitrogen removal rate and water temperature. In addition, low nitrate-nitrogen concentrations commonly occur when total organic carbon (TOC) and dissolved oxygen (DO) are also low, and high nitrogen concentrations correspond to high DO and TOC, indicating that denitrification is the primary biogeochemical process controlling nitrogen removal in natural rivers. The new approach is demonstrated through the computation of nitrogen removal in the Amite River, LA, USA. Functional relationships between the nitrate-nitrogen removal rate and water temperature are established for the Amite River. Monthly mean nitrate-nitrogen concentrations along the river are computed using the extended VART model, and computed nitrogen concentrations fit observed ones very well. The estimated annual nitrate-nitrogen removal in the Amite River is 27.4 tons or 15.5％ of total nitrate-nitrogen transported annually through the Amite River.

Keywords: Denitrification; Hyporheic exchange; Nitrogen retention; River; VART model』

1. Introduction
2. Variable residence time (VART) model for nitrogen retention
3. Parameter estimation
　3.1. Flow and water quality data
　3.2. HSPF model
　3.3. Estimation of physical parameters in VART model
　3.4. Estimation of nitrogen decay rate (k)
4. Computation of nitrogen retention in the Amite River
5. Summary and conclusions
Acknowledgment
References