A field experiment was conducted between 1992 and 1997 in a commercial orchard of mature ‘Hass’ avocados on Mexican seedling rootstock ( Persea americana Mill.) to determine how yield was influenced by the amount of irrigation water applied and the frequency of application. Three amounts of water (targeted at 90%, 110%, and 130% of estimated crop evapotranspiration) were applied at three frequencies (one, twice, and seven times per week) with microsprinklers located beneath the tree canopy. The site was set up as a randomized complete block design with six blocks, each including one replicate of all irrigation treatments. One or two trees located at the center of the replicates were used to measure yields and tree size, and as the locations where samples of soil and soil water were obtained for analysis from beneath the tree canopy. The average electrical conductivity and chloride concentration of the irrigation water, corrected for rain, were 0.7 dS·m ⁻¹ and 1.8 mmol·L ⁻¹ , respectively. From May 1994 to Nov. 1996, salinity of the saturated-paste extracts of soil samples obtained in the 0- to 120-cm depth interval averaged ≈2 dS·m ⁻¹ for all irrigation treatments. Irrigation treatments also had little influence on the maximum soil-water salinity, ≈4 dS·m ⁻¹ , in and below the lower portion of the root zone. Consequently, irrigation treatments had little influence on the fraction of applied water that was not used by the crop, the leaching fraction. Chloride concentrations in leaves were affected by applied water but did not attain levels that are associated with leaf injury. Trees irrigated seven times per week had lower yields than trees that received less frequent irrigation. During the last 2 years of the experiment, when yields no longer increased with time, the yields for treatments irrigated once and twice per week increased with increasing amounts of applied water. We were able to explain the influence of both amount of applied water and soil salinity on avocado yields and leaching fraction using production function concepts. Yields increased with increasing amounts of applied water because of increased water availability for crop use before a soil-water salinity of ≈4 dS·m ⁻¹ restricted water uptake. The threshold salinity above which yield decline occurred was determined to be 0.57 dS·m ⁻¹ and yield declined by 65% per unit of salinity above the threshold. Our results suggest that maximum yields of ‘Hass’ avocado on Mexican seedling rootstock are not achievable when the average annual salinity of irrigation water, including rainfall, is greater than ≈0.6 dS·m ⁻¹ .

The Sustainable Groundwater Management Act (SGMA) of 2014 and the Central Valley Salinity Alternatives for Long-Term Sustainability (CVSALTS) initiative were conceived to reverse years of inaction on the over-pumping of groundwater and salination of rivers that both threaten agricultural sustainability in the State of California. These largely stakeholder-led, innovative policy actions were supported by modern tools of remote sensing and Geographic Information System technology that allowed stakeholders to make adjustments to existing resource management and jurisdictional boundaries to form policy-mandated Groundwater Sustainability Agencies (GSAs) and Salinity Management Areas (SMAs) to address future management responsibilities. Additional resources mobilized by the California Department of Water Resources (CDWR) and other water resource and water quality management agencies have been effective in encouraging the use of spreadsheet accounting and numerical simulation models to develop robust and coherent quantitative understanding of the current state and likely problems that will be encountered to achieve resource sustainability. This activity has revealed flaws and inconsistencies in the conceptual models underpinning this activity. Two case studies are described that illustrate the disparity in the challenges faced by GSAs in subregions charged with developing consensus-based Groundwater Sustainability Plans (GSPs). These case studies also illustrate the unique aspect of SGMA: that alongside mandates and guidelines being imposed statewide, local leadership and advocacy can play an important role in achieving long-term SGMA and CVSALTS goals.

Salinity and trace mineral accumulation threaten the sustainability of crop production in many semi-arid parts of the world, including California’s western San Joaquin Valley (WSJV). We used data from a multi-year field-scale trial in Kings County and related container trials to simulate a forage-grazing system under saline conditions. The model uses rainfall and irrigation water amounts, irrigation water quality, soil, plant, and atmospheric variables to predict Bermuda grass (Cynodon dactylon (L.) Pers.) growth, quality, and use by cattle. Simulations based on field measurements and a related container study indicate that although soil chemical composition is affected by irrigation water quality, irrigation timing and frequency can be used to mitigate salt and trace mineral accumulation. Bermuda grass yields of up to 12 Mg dry matter (DM)·ha⁻¹ were observed at the field site and predicted by the model. Forage yield and quality supports un-supplemented cattle stocking rates of 1.0 to 1.2 animal units (AU)·ha⁻¹. However, a balance must be achieved between stocking rate, desired average daily gain, accumulation of salts in the soil profile, and potential pollution of ground water from drainage and leaching. Using available weather data, crop-specific parameter values and field scale measurements of soil salinity and nitrogen levels, the model can be used by farmers growing forages on saline soils elsewhere, to sustain forage and livestock production under similarly marginal conditions.