Miller, G. R. (2009). Measuring and modeling interactions between groundwater, soil moisture, and plant transpiration in natural and agricultural ecosystems. Ph.D. Dissertation. University of California at Berkeley. https://nature.berkeley.edu/biometlab/pdf/miller_2009_dissertation.pdf
Plant transpiration serves a critical function in the terrestrial hydrologic cycle, acting as the primary link between the atmosphere and subsurface stores of water. To properly manage our water resources under changing and uncertain climate conditions, we will first need to understand the complex interactions and feedbacks between vegetation, soil moisture, groundwater, and the atmosphere. This dissertation focuses on measuring and modeling the flow of water through these connections.
The primary study site is a semi-arid oak savanna in California, located in the foothills of the Sierra Nevada. Here, a suite of tree and stand scale ecohydrological measurements are collected. The measurements, taken at half-hourly to biweekly intervals over the 2007 and 2008 growing seasons, include individual tree transpiration (from sap flow), stand evapotranspiration (using the eddy-covariance method), soil moisture content, soil and leaf water potential, tree diameter, stable isotope ratios, and depth to groundwater. This work develops and tests a novel method for locating the sap fow and soil moisture sensors—based on a geostatistical analysis and an artificial intelligence algorithm. It uses the resulting data to quantify the proportion of evapotranspiration due to groundwater uptake by woody vegetation, finding that the blue oaks at the site are heavily dependent on deep sources of water during the dry summer months.
Two modeling studies explore the dynamic relationships between soil moisture, vadose zone processes, evapotranspiration, and groundwater recharge. The first tests the applicability of an analytical, stochastic soil moisture model to the data from the oak savanna and several other micrometeorological sites. It illustrates the importance of understanding the relationship between soil moisture and the onset of plant stress and notes the benefits and drawbacks to using simple, point models of the water budget. The second uses a numerical, reactive flow and transport code to describe the application of food-processing wastewater to agricultural lands in California’s Central Valley. It indicates that the biosphere and its control over the nitrogen-carbon-oxygen system may highly influence salinity attenuation, demonstrating the necessity of including multiple plant, soil, and microbial processes in order to capture the complexity of their interactions.