| This dissertation examines hydrological fluxes in urban and agricultural ecosystems and ecohydrological feedbacks between water and vegetation productivity. This research has two distinct but related foci: Chapters 2 and 3 focus on agricultural ecohydrology, specifically evapotranspiration and yield patterns, while Chapters 4 and 5 focus on urban heat island impacts on plant phenology and ET. Chapter 2 introduces a new surface energy balance model, High Resolution Mapping of EvapoTranspiration (HRMET), which is used to estimate evapotranspiration rates at meter-resolution over two cornfields during the 2012 growing season. Results indicate that persistent patterns of evapotranspiration are present across the growing season and can be used to map drought sensitivity at subfield scales. Chapter 3 uses yield data from the same field site in conjunction with biophysical modeling using AgroIBIS-VSF to systematically assess the effects of shallow groundwater, growing season weather conditions, and soil texture on corn yield. Results indicate that shallow groundwater can either help or hurt corn yield, with substantial variability at a subfield scale. The optimum water table depth for corn production is deeper in both finer soils and wetter growing seasons. Chapters 4 focuses on the impacts of the urban heat island on plant phenology and finds that the potential growing season is ~5% longer in urban areas relative to surrounding rural areas, though land cover may impact the observed response of vegetation to changes in growing season length. Chapter 5 demonstrates that the urban heat island also causes an increase in evapotranspirative demand for urban vegetation via changes to air temperature, with negligible impacts of altered air moisture content. Furthermore, we find that changes in both growing season length and evapotranspirative demand are correlated with impervious cover in the area surrounding the sensor, indicating that fine-scale patterns of urbanization can have significant impacts on urban water, energy, and carbon cycles. Overall, the four studies contained within this dissertation shed light on the drivers of ecosystem productivity and hydrological fluxes in both urban and agricultural settings. |
