Towards the Estimation of Biospheric Contributions to the Regional Carbon Cycle in the Eastern United States Using a Dynamic Global Vegetation Model

To obtain accurate greenhouse gas emissions estimates, it is important to understand the influence of the biosphere on atmospheric carbon dioxide. High spatial and temporal resolution assessments of the biospheric contribution to atmospheric carbon dioxide are needed due to the strong diurnal dependence on biospheric activity and large spatial variation in vegetation within a given region. To study this, the Vegetation-Global-Atmosphere-Soil (VEGAS) Model was run for the Eastern United States for November 2016 through October 2017 at a spatial resolution of 9 km. The VEGAS model is a Dynamic Global Vegetation model that simulates dynamic vegetation growth and competition among five plant functional types (PFTs): broadleaf trees, needle leaf trees, cold grass, warm grass, and cropland. The model has an hourly time step and outputs flux between the atmosphere and biosphere as well as carbon pool size. To build up the carbon pools, a 300-year spinup simulation is performed using 6-hourly Climate Forecast System Reanalysis (CFSR) climatology data. After the spinup simulation, VEGAS is then driven with 3-hourly North American Regional Reanalysis (NARR) data for the simulation year. Results from the 300-year simulation show that Net Primary Productivity (NPP) is maximum in July in the Midwest with values around 1.5 GtC/m²yr. During the summer, the terrestrial biosphere in the Midwest acts as a sink with a seasonal average carbon flux of 0.7 GtC/m²yr. The same region acts as a carbon source during the winter with a seasonal average carbon flux to the atmosphere of 0.5 GtC/m²yr. The Eastern United States is of importance from a modeling perspective because of the range of vegetation types in the region. For example, NPP values can vary as much as 0.25 GtC/m²yr over 30 miles in Southern New York. Over the Baltimore/Washington area, biospheric CO₂ uptake can exceed fossil fuel emissions in the summer. Properly simulating these contributions from the biosphere on the regional scale can reduce the error in estimations of diurnal fluxes of carbon dioxide. Results from this analysis will be used in a larger project to estimate greenhouse gas emissions over the Baltimore/Washington D.C. area.