UC Santa Cruz

Interannual shifts between drought and abundant precipitation are projected to increase globally in both frequency and severity due to climate change, which will have consequences for plant water use, streamflow dynamics, and groundwater recharge. All of these topics have implications for water management. This dissertation presents research results focused on two major components of water storage in hydrologic reservoirs: the vadose zone in hillslopes and groundwater in aquifers. Water stored in the vadose zone plays a crucial role in the terrestrial water cycle by driving hydrologic partitioning; understanding subsurface moisture dynamics and its interactions with slope-aspect and vegetation is critical for accurate modeling and management of coupled hydrologic, geochemical, and biological systems. While past studies on the impact of aspect-driven differences in insolation quantify moisture in shallow soils, moisture dynamics in deeper weathered bedrock have largely been ignored. Furthermore, since winter precipitation in water-limited mediterranean systems sets the baseline for water availability later in the dry growing season, there is a need to understand how the structure of the vadose zone mediates how year-to-year variability in precipitation contributes to the available water used by plants. Building our mechanistic understanding of how the vadose zone regulates connectivity between hydrologic stores and fluxes clarifies the impacts of climate change. Managed aquifer recharge (MAR) is a set of methods and tools for augmenting natural hydrologic flows, and is increasingly considered as a strategy to augment groundwater supply and improve water system. However, choosing appropriate MAR options requires consideration of numerous hydrogeologic, water quality, and infrastructure factors, often involving participants with diverse technical backgrounds and priorities. As a result, MAR implementation has arguably lagged the growing need to augment groundwater supplies, despite having been shown to be effective in many basins. There is a need to support decision makers, technical personnel, and community groups and members collaborate to explore options, considerations, and perceptions surrounding MAR. This kind of decision support can build knowledge, capacity, and trust during the early stages of MAR project consideration and development, establishing a foundation for more successful implementation.In chapter 1, I quantify moisture dynamics using geophysical surveys and borehole measurements in two opposing hillslopes in the seasonally dry central California Coast Range. Despite greater insolation, the grassy equator-facing slope experienced less and shallower moisture withdrawal during the dry season relative to the pole-facing slope with oak trees, which experienced greater and deeper moisture withdrawal. Consequently, following the dry season, water content within the grassy equator-facing slope was higher, which may contribute to aspect-dependent differences in runoff generation, landslide susceptibility, and drought resilience.In chapter 2, I build on the datasets presented in chapter 1 by monitoring rock and soil moisture in conjunction with oak tree water potential and transpiration across three water years that spanned dry (drought) to wet (above average precipitation) conditions. Following an extremely dry year with relatively low evapotranspiration, we observed similar cumulative evapotranspiration across the below average and extremely wet water years, despite a doubling of annual precipitation. Although subsurface water storage started the growing season considerably more elevated in the wetter year, storage conditions quickly reached similar levels to those observed in drought years. As excess precipitation was not utilized for transpiration, we suggest precipitation during wet years mostly drained below the root zone, contributing to groundwater recharge or streamflow, highlighting the importance of precipitation timing and subsurface water routing relative to when plants use stored water.In chapter 3, I present a decision support framework (DSF) that can aid in the evaluation of numerous hydrogeologic, water quality, and infrastructure factors that are critical to MAR feasibility and planning. By integrating quantitative, qualitative, and categorical information relevant to key phases of design, development, and system operations, the DSF is intended to help decision makers, technical personnel, and community groups and members collaborate to explore options, considerations, and perceptions surrounding managed aquifer recharge. This will help to build knowledge, capacity, and trust during the early stages of MAR feasibility assessment and planning, as the DSF also makes apparent priorities, concerns, and potential trade-offs. I demonstrate application of the DSF using a representative case example and discuss how the approach can guide subsequent technical evaluation and project development, building from a foundation of mutual understanding, community-based knowledge, and cooperation.The first two dissertation chapters contribute to the growing body of research around subsurface moisture dynamics and how the frequency and timing of precipitation, coupled with slope-aspect, vegetation, and vadose zone storage mediate hydrologic partitioning. As climate change continues to alter water supply, this mechanistic understanding of how water moves through the environment can contribute to Earth system models to better predict hillslope-scale hydrologic partitioning and inform water resource planning. Highlighting the need to store water in times of abundant precipitation, the implementation of MAR is a critical water resource strategy to improve water supply during times of drought. By developing a DSF, the third dissertation chapter supports technical evaluation and project development for MAR, increasing institutional and public capacity to help address future water resource needs.