Evaluation of a Modeling Approach to Assess Nitrogen Assimilative Capacity due to River Restoration

Abstract

To improve water quality and enhance habitat for fish and wildlife, a restoration project was initiated on the Truckee River at McCarran Ranch, Nevada. A water quality model was parameterized using Hydrologic Simulation Program–Fortran to represent the river’s restored characteristics to test the hypothesis that restoration of the Truckee River improves the river’s nitrogen (N) assimilative capacity. This model was calibrated to data collected postrestoration, and modified to represent prerestoration conditions and additional potential restoration scenarios. Restoration scenarios involved changes in benthic algae habitat that affect in-stream N and dissolved oxygen (DO) concentrations. Assimilative capacity was assessed by comparing N loads for restoration scenarios that would result in meeting or exceeding prerestoration minimum DO concentration simulated under 2004 conditions. Modeled algal dynamics affected model results, which indicated that there is a certain amount of increased habitat that is beneficial to N uptake while maintaining acceptable DO concentrations, and that the amount and species of N assimilated depends on the type and extent of restoration. However, evaluation of the modeling approach highlights some critical model limitations. Thus, interpretation of model results should be used with caution.