UC Davis

California's irrigated agriculture faces tightening water constraints from climate variability, chronic groundwater overdraft, and the Sustainable Groundwater Management Act (SGMA), which requires high- and medium-priority basins to reach sustainability within 20 years. Pistachio (Pistacia vera) acreage grew from roughly 25,000 bearing acres in 1979 to over 450,000 in 2023 and is projected to expand by a further 47 percent by 2031, concentrating future water demand in basins now under mandatory groundwater allocations. Winter cover cropping is widely promoted as a climate-smart practice that improves soil structure, infiltration, and nutrient retention, yet California adoption remains below five percent, partly because its water use consequences in a Mediterranean climate are poorly characterized. This dissertation evaluates the soil water, evapotranspiration, and grower-perceived consequences of winter cover cropping in California pistachio orchards using field measurements, remote sensing, surveys, and process-based modeling.Chapter 1 quantifies the soil water budget of winter cover cropping in a young commercial pistachio orchard near Davis, California, using paired eddy covariance (EC) towers and 72 neutron probe access tubes (installed to a nominal 3.6 m, standardized to 3.0 m for measurement) across replicated cover crop (CC) and no cover (NC) treatments over three winter and two growing seasons. Cover crops increased actual evapotranspiration (ETₐ) by 15 to 30 mm and depleted soil water storage in the upper (0 to 1.2 m) profile, but those losses were largely offset by enhanced storage in the lower (1.2 to 3.0 m) profile, where cover crop soil water content was 8 to 17 percent higher than NC across all five seasons. Post-termination summer ETₐ was 32 to 76 mm lower under CC, consistent with mulch-driven suppression of soil evaporation. Interval-based deep percolation (DP) estimates were 1.3 to 7.7 times the conventional endpoint estimates. Rather than removing water from the orchard, CC redistributed it downward into the lower (1.2 to 3.0 m) profile, a pattern invisible at the conventional root zone (0 to 1.2 m) monitoring depth. Accurately characterizing CC water dynamics in deep-rooted perennial systems therefore requires full-profile (0 to 3.0 m) monitoring matched to functional rooting depth, with interval-based water balance accounting.Chapter 2 evaluates how well remote sensing evapotranspiration (RSET) products perform in young pistachio orchards and whether they can detect treatment-scale differences in actual evapotranspiration (ETₐ) between adjacent management systems, both questions directly consequential under SGMA. Daily through seasonal ETₐ estimates from six OpenET models and the OpenET Ensemble were compared against eddy covariance measurements from paired CC and NC treatments over three growing and three winter seasons spanning fifth- to seventh-leaf trees. SIMS produced the lowest monthly RMSE (19 mm month⁻¹), outperforming the Ensemble (23 mm month⁻¹), while DisALEXI recovered the CC ETₐ signal in six of six seasons (mean absolute deviation 20 mm) versus five of six for SIMS. Actual crop coefficients (Kₐ) derived from EC measurements rose from 0.57 in fifth leaf (NC) to 0.95 in seventh leaf (NC), filling a gap in the published literature for young California pistachios. No single product served both uses, because the two applications reward different model properties: volumetric water accounting favors the lowest total magnitude error (SIMS), whereas treatment-effect detection favors sensitivity to fine-scale differences between adjacent fields (DisALEXI). Agencies conducting SGMA water accounting should therefore prioritize SIMS, while growers and researchers evaluating cover crop or management effects should prioritize DisALEXI.Chapter 3 integrates grower perspectives with process-based simulation to project the long-term consequences of winter cover cropping in California pistachio orchards. An Institutional Review Board (IRB) approved online survey of pistachio growers, farm managers, and agronomic advisors provided structured, empirical input on current cover crop practices, perceived benefits, and primary concerns across the production region. The Agricultural Policy/Environmental eXtender (APEX) model was calibrated and validated against measured data from the paired CC and NC treatments of Chapters 1 and 2, including EC-derived ETₐ, neutron probe soil water content, leaf area index, cover crop biomass, and pistachio yield. The calibrated model then generated projections of cover crop effects under survey-informed climate scenarios.Together, these chapters show that winter cover cropping in California pistachio orchards has water consequences that are more nuanced, and more recoverable, than shallow-profile annual system studies suggest, that the RSET models underpinning SGMA accounting differ in their ability to support volumetric accounting versus treatment-effect detection, and that integrating grower perspectives into process-based modeling yields decision-relevant projections of long-term orchard floor management. The cross-chapter contribution is a measurement-to-management framework: field observation reveals where CC water goes, remote sensing determines which tools can track it at scale, and modeling extends both forward in time. This framework supports a more cautious interpretation of CC water costs in deep-rooted perennial systems and clearer guidance for selecting RSET models and designing field measurements for SGMA-era water accounting.