Citation:

Dettmann, E., L. Charlestra, AND M. Abdelrhman. Time-scale dependence of response of an estuarine water-quality model to nutrient loading. New England Estuarine Research Society, New Bedford, MA, October 25 - 27, 2018.

Impact/Purpose:

This is a review of a U.S. Environmental Protection Agency model that is often used to describe and predict trends in water quality, for instance, oxygen and algae concentrations, in water bodies that receive discharges of nutrients from sources such as sewage treatment plants and runoff from streets and agricultural fields. In the past, it has been used to predict these trends over time periods of months, seasons and years. This review investigates the accuracy of the model over daily time periods. It is important to understand the ability of the model to predict water quality changes over this shorter time period if it is to be used in situations for which changes over a daily time period are important.

Description:

We describe calibration and evaluation of a water quality model being implemented for Narragansett Bay to quantify the response of concentrations of nutrients, phytoplankton chlorophyll a and dissolved oxygen in the Bay to loading rates of nutrients and other boundary conditions. We evaluate model capabilities in the context of suitability as a management tool in Narragansett Bay and similar estuaries. The model, the U.S. Environmental Protection Agency’s Water Quality Analysis Simulation Program (WASP), simulates concentrations of nutrients, chlorophyll a, and dissolved oxygen. It uses hydrodynamic transport simulated by the Environmental Fluid Dynamics Code (EFDC), observed loads of nutrients, biochemical oxygen demand, and freshwater, as well as boundary conditions for benthic nutrient and oxygen fluxes. The model employs a terrain-following vertical sigma grid with 5288 segments (661 segments in each of 8 layers). Model simulations of dissolved oxygen and phytoplankton chlorophyll a are compared with data from a network of fixed-site sensors in the Bay that provide near-surface and bottom oxygen and chlorophyll a concentrations at 15-minute intervals, generally from late May through late October. Nutrient results are compared with measurements on monthly surface samples throughout the Bay. WASP represents longer-term (monthly and seasonal) dynamics of dissolved oxygen and phytoplankton chlorophyll a well, but it underestimates diurnal excursions of dissolved oxygen concentrations. Sensitivity analyses show that while near-surface chlorophyll a concentrations react as expected to modifications of incident light and nitrogen loading rates, dissolved oxygen concentrations are relatively insensitive to these manipulations, suggesting that physical processes are the dominant factor influencing modeled oxygen concentrations. We present factors that may contribute to the weak coupling between phytoplankton and dissolved oxygen concentrations in this model.

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