Events & Media

The Bren School of Environmental Science & Management
at the University of California, Santa Barbara



"Ecohydrologic model uncertainty and application in an urban environment:
the RHESSys model in Mission Creek"

Catherine Shields
Bren School of Environmental Science & Management

Wednesday, Sept. 5, 2012
9 a.m.
Bren Hall 2436 (Dean's Conference Room)

Naomi Tague, faculty advisor
Arturo Keller and John Melack, committee members

As the home of a large and growing fraction of the world’s human population, urban environments present an important opportunity for research. In water-stressed semi-arid areas, the impact of urbanization on ecohydrologic processes, such as vegetation water use and net primary productivity, is of particular concern.  This dissertation seeks to increase understanding of ecohydrologic processes within an urban and semi-arid context through the application of an ecohydrological model (the Regional Ecohydrologic Simulation System, RHESSys).  The model’s performance is rigorously tested in the study site, the Mission Creek catchment in Santa Barbara County. This analysis quantifies the impact of uncertainties from both calibrated soil parameters and inputs (precipitation scaling, temperature, and outdoor water use) on model output, leading to an improved understanding of  the relative importance of “natural” and “anthropogenic” sources of uncertainty. We find that the relative impact of different sources of uncertainty varies throughout the water year and with the model output of interest. Motivated by results indicating that the calibration of soil parameters provided some of the greatest potential for reducing output uncertainty, I devised and tested a series of alternative calibration approaches aimed at reducing the potential for observation and input error to introduce bias in the calibration process. Finally, I use the calibrated model to conduct an analysis of the role of effective impervious surface area (EIA) on vegetation water use and productivity. I demonstrate that connectivity between impervious areas and the drainage network can greatly influence modeled vegetation water use and productivity. Reducing the EIA fraction of total impervious area potentially increases runoff infiltration and can mitigate or even cancel reduction in vegetation water use and NPP directly due to vegetation loss with increased impervious surface coverage. My results indicate that runoff from impervious surfaces into neighboring vegetated areas can increase transpiration and NPP in these vegetated areas well into the summer dry season. As an extended summer dry season is common to Mediterranean sites such as the study catchment, this finding is of particular interest with regards to maximizing vegetation productivity in water stressed urban areas while minimizing the need for additional water inputs from irrigation. A first order approximation of the the EIA fraction into  a catchment scale model indicates that the effect of accounting for the EIA fraction on model ET and NPP estimates will likely vary between wet and dry years, with an increasing effect during wet years and a minimal effect during dry years.