UC Davis

Salinity and nitrate leaching in California's Central Valley significantly impair agricultural productivity and groundwater sustainability. Through extensive biophysical and agrohydrological modeling and empirical field data, the research quantifies the impacts of these environmental stressors and offers evidence-based strategies for mitigating their effects. A biophysical model that incorporates soil, weather, crop data, water cost, and production costs predicted that salinity levels exceeding 5 dS/m can reduce crop yields by up to 45% for highly sensitive crops like almonds and table grapes. On the other hand, these crops could profit growers more because of their market values. The economic analysis shows that the profitability of agricultural production under different salinity scenarios varies significantly, with potential annual economic losses amounting to hundreds of millions of dollars, underscoring the critical need for tailored salinity management strategies. A salinity decision-support web tool was developed. This tool enables users to input data and predict crop yields and economic outcomes. The tool's application in real-world scenarios has shown that it can effectively assist farmers and policymakers in making informed decisions that optimize economic returns and environmental sustainability.Concurrently, the dissertation evaluates the effectiveness of various conservation practices, such as cover crops, irrigation nitrogen credits, and high-frequency low fertigation, in reducing nitrate leaching. Utilizing the APEX model, the results demonstrate that these practices can reduce nitrate leaching of the root zone by up to 90%, thereby substantially decreasing the risk of groundwater contamination. This study also evaluated an integrated AMRS model to assess and manage nitrate nitrogen (NO3-N) leaching into groundwater at a field scale. The model was evaluated at the deep vadose zone and shallow groundwater and showed that conservation practices such as HFLC can mitigate groundwater NO3-N contamination. This part of the research highlights the practical benefits of integrating agronomic and environmental conservation practices into farming operations to enhance the sustainability of water resources. Overall, this dissertation provides a comprehensive framework for understanding and addressing the dual challenges of salinity and nitrate leaching in irrigated agriculture. By offering detailed insights into the mechanisms underlying these challenges and developing practical tools for stakeholders, the research contributes to the sustainable management of natural resources in agricultural regions facing similar environmental pressures.