Modeling the Influence of Riparian Soil Air Co2 Concentrations on Stream Water Alkalinity

Welsch, Daniel Lewis, Department of Environmental Sciences, University of Virginia
Hornberger, George, Department of Environmental Sciences, University of Virginia

Stream water chemical composition is tightly coupled to the concentration of CO2 in soil air. However, little is known about the inorganic C cycle in soils. I describe a series of simple physically - based models that simulate soil temperature, soil tension, soil CO2 processes, and soil chemistry. I apply these models to simulate the spatial and temporal dynamics of soil CO2 concentrations throughout a small catchment in the Virginia Blue Ridge. When output from the simulations is compared with field measurements, I find that despite some model deficiencies, I am able to reasonably simulate the gross overall patterns through space and time of soil air CO; concentration. During the growing season when soil temperature is high, I find that soil water status is the limiting control on soil respiration and CO; concentration. I also find that soil CO; concentration can be high despite low respiration values because of changing diffusivity of the soils with moisture content. The ability to predict stream alkalinity values over scales shorter than monthly or annually is needed to understand the response of stream chemistry to acidic inputs which occur across short time scales (days). I apply the models described above to a nine year record of discharge and stream chemistry from a small catchment in the Shenandoah National park of Virginia. I find that I am able to reasonably predict the minimum stream alkalinity values for all years and I am able to predict the entire annual cycle for six of the nine years. The three years for which I overpredict summer stream alkalinity had summer precipitation which was much greater than normal and greater than the period for which the model was calibrated. Models of soil and stream water and catchment acidification have typically been applied without consideration of climate change. Soil air CO2 concentrations have potential to increase as climate warms and becomes wetter. I simulate this increase by applying the coupled models described above to predicting daily stream water alkalinity values for SFBR for 60 years into the future given stochastically generated daily climate values. This is done for nine different scenarios of climate change and atmospheric deposition change. I find that stream water alkalinity continues to decline for all scenarios except when climate is gradually warming and becoming more moist. In all other scenarios, base cation removal from catchment soils is responsible for limited alkalinity change resulting from climate change. This has strong implications given the extent that models such as MAGIC are used to establish policy and legislation concerning deposition and emissions.

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PHD (Doctor of Philosophy)
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