Assessment of Irrigation and Rainfall Effects on Soil Salinity in Selected Drip-Irrigated Orchards in San Joaquin Valley, CA

Preventing an accumulation of harmful salts in the root zone (RZ) of crops is necessary for sustaining irrigated agriculture in arid and semi-arid regions. Climate change may impact future maintenance of the RZ salt balance due to changing temperatures and evapotranspiration, altered precipitation patterns, and related shifts to alternative, lower quality irrigation waters. In this study, experimental field data, including soil water contents, electrical conductivities, spatial root distributions, and soil physical and chemical properties, were collected to assess salinity trends under different environmental and management conditions in almond and pistachio orchards at five locations in San Joaquin Valley (SJV), California, in 2017-2019. Diverse seasonal soil salinity trends were obtained for the eastern and western geo-hydrological regions of SJV and almond and pistachio orchards. The experimental data were then used to carry out numerical analyses with the one-dimensional HYDRUS-1D model to quantify water and solute transport in almond and pistachio trees' RZs. The model was successfully calibrated and validated using soil water content and electrical conductivity measurements at two experimental sites during 2017-2018 and 2019. Simulated soil water contents and evapotranspiration at these two sites showed good agreement with the measured data. Salinity patterns in the RZ were also described successfully by the HYDRUS-1D simulations. There were uncertainties in simulated trends at the other locations due to, for example, the presence of a duripan, solution chemistry effects on soil hydraulic properties, or functional errors with the GS3 soil sensors in some depths. Finally, additional simulations were carried out to predict future soil salinity accumulation and leaching trends for selected future climate (rainfall) and irrigation (the use of surface (less saline) and ground (more saline) waters) scenarios. Future rainfall trends were predicted using two climate models (CSM4_mid-range precipitations and CNRM_CM5_wetter precipitations) (CalAdapt, 2018). The simulated results of future salinity trends under different regional, geological, irrigation, and climate conditions in SJV can be used to design different salinity management options in the future