As the most agriculturally productive state in the United States, California faces increasing pressure on its dwindling groundwater resources, which are significantly relied upon during periods of surface water shortages resulting from multi-year droughts. This situation emphasizes the importance of sustainable irrigation water management in water-intensive agricultural systems in California. Here, through a multi-year and multi-site experimental research, we investigated the orchard-level factors, i.e., actual evapotranspiration (ETa), crop coefficient (Kc), soil moisture availability, and plant-level factor, i.e., plant water status as midday stem water potential (Ψstem), their interactions and plant’s response to post-harvest deficit irrigation (PDI) to provide valuable insights for improving irrigation in three mature sweet cherry orchards of sparse, medium, and higher density under similar management practices in the Central Valley of California. While ETa and Kc offer an understanding of the changes in water use as influenced by the time of the season, irrigation events, and soil moisture availability, Ψstem as a direct physiological indicator offers insights into how water deficit impacts plant productivity. The combination of these approaches aids in a holistic understanding of the feedback between the soil, plant, and atmosphere, which is essential in developing efficient irrigation strategies.This thesis is constructed in two chapters, each exploring different perspectives of water management in sweet cherry orchards. The primary objective of the first chapter was to determine a general Kc curve under local growth conditions of California and evaluate the estimated sweet cherry crop evapotranspiration (ETc) using the derived Kc values with respect to the ETa and applied irrigation water. This chapter shows how ETa, Ψstem, irrigation events, and soil moisture availability interplay and contribute to the variability of Kc across orchards and years. The derived Kc-mid in our study is lower than the reported values in FAO-56, 66, and other available studies in different regions and conditions, reflecting the influence of the local management practices on Kc. Comparison of ETc, derived using the locally developed Kc curve, and ETa measured via the eddy covariance technique with growers' irrigation amounts showed substantial discrepancies based on the density, hydrological years, and pre-harvest, harvest, and post-harvest period.
The complex interplay of ETa, Ψstem, irrigation events, soil water availability, and the variability of Kc explored in the first chapter motivated the second chapter. Here we focused on the plant-based responses to regulated deficit irrigation in the post-harvest period (PDI) for a deeper understanding of efficient irrigation water management strategies. We evaluated the effects of PDI on the Ψstem, yield quantity and quality of sweet cherries, and water savings during the post-deficit period throughout two growing seasons. PDI was implemented in 2022 after careful observation of ETa and Ψstem from 2019-2021. The study included two irrigation treatments: Control (full irrigation by the growers) and PDI (reduced irrigation). With ~ 26 to 51% water savings during the post-deficit period, PDI significantly affected Ψstem (p≤0.05), with values of less than or equal to -1.5 MPa associated with potential reductions in fruit set and yield, although these differences in productivity were not statistically significant. Our results indicated that by keeping the post-deficit Ψstem above moderate stress of greater than -1.5 MPa, PDI has the potential to save ~ 31% water without reducing yield and with no significant difference in most of the quality parameters.
The findings of the thesis provide a robust baseline dataset incorporating plant-based variables such as Ψstem, yield quantity, and quality and orchard-based variables such as ETa, irrigation water application, and soil water availability useful in refining careful irrigation strategies in mature sweet cherry orchards in California. While ETa offered critical insights into the influence of the orchard-specific conditions, Ψstem served as a valuable tool to detect potential negative effects of reduced irrigation application on the orchard’s productivity. This integrated approach underscores the importance of combining plant- and orchard-level perspectives to develop adaptive, efficient, and sustainable irrigation practices maintaining yield and quality in California's mature sweet cherry orchards.
