Graduate Thesis Or Dissertation

Ecohydrological mediation of water budget partitioning and time scales of subsurface flow in a seasonally semi-arid grassland

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  • Understanding how the interactions and feedbacks between plant function, climate, and soils ultimately affects the terrestrial water balance and subsurface flow processes is major challenge in scientific hydrology. This dissertation summarizes the findings of a manipulative climate warming experiment, an observational field study that utilized stable-isotope tracers, and associated modeling analyses that I used to examine the physiological and physical mechanisms by which grassland ecosystems mediate water-balance partitioning and transit times of water flowing through the subsurface. Utilizing the climate-controlled mesocosm experiment, I examined the responses of evapotranspiration, soil moisture, and potential groundwater recharge to a 3.5˚C temperature increase in a grassland ecosystem experiencing a Mediterranean climate. I hypothesized that a warmer climate would cause a shift in the soil-water balance toward greater evapotranspiration, and less recharge. The results showed that warming treatments enhanced evapotranspiration during the spring. However, this reduced soil moisture more rapidly, resulting in less evapotranspiration during the summer than occurred under ambient temperatures, and no difference when considered over the entire year. Groundwater recharge was reduced during late-spring storms relative to the ambient temperature treatment, but these reductions were a small fraction of the annual total, and were offset by slightly greater recharge in the fall under warming treatments. The results highlighted the potential for interactions between climate, vegetation, and soils to moderate the hydrological response to climate warming, particularly in environments where precipitation is seasonal and out of phase with the vegetation growing season. I conducted additional field studies that utilized three lysimeters with surface conditions ranging from bare soil through two stages of aggrading grassland vegetation. Using hydrometric data, stable-isotopes as conservative tracer, and two hydrograph-separation techniques I evaluated whether aggrading grassland vegetation and root systems alter time scales of subsurface flow, and how this alteration may influence potential groundwater recharge. I tested the hypothesis that soil structural change under aggrading vegetation would enhance the rapid infiltration of precipitation-event water, resulting in greater potential recharge during individual storms. Contrary to this expectation, results from both hydrograph-separation techniques showed that precipitation-event water comprised 0 - 6% of potential recharge among all the storm events I analyzed, being greatest under bare soil, and always zero in grasslands ranging in age from 3.8 - 5.9 years. These results contradicted my original hypothesis, and were attributed to the low intensity of local precipitation, large soil-water storage potential, and the predominantly shallow rooting tendency of the grassland vegetation. In a final analysis I used stable-isotope measurements and a linear-time-invariant convolution approach to model mean-transit times and transit-time distributions of subsurface flow under each surface condition, and over the entire water year. The results showed that mean-transit times were not significantly different in the presence of aggrading vegetation. From these analyses I concluded that physical alteration of the soil by aggrading plant root systems was not an ecohydrologically significant mechanism in this system. Ex post facto analyses showed that, at the time scale of individual storm events, the reduction of effective precipitation by interception and evaporative loss from the grassland canopies was of much greater importance--even with very low leaf area indices. At the annual time scale, root expansion enabled much greater exploitation of soil water during the summer drought, causing a shift from a recharge-dominated to an evapotranspiration-dominated soil-water balance within the first year of growth.
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