Targeted irrigation expands scope for winter cereal production in water-limited areas of California's San Joaquin Valley

CONTEXT: Efforts to achieve groundwater sustainability in California's San Joaquin Valley will entail substantial pumping reductions, and much irrigated cropland may become newly fallowed. Winter crops grown under water -limited conditions could offer an alternative to fallowing, but their viability may be limited by variable rainfall and high crop failure rates.OBJECTIVE: The first objective of this study was to evaluate how small (100-200 mm), targeted irrigation events impact crop establishment, forage and grain yields, and the resulting economic and agronomic water produc-tivity of winter wheat at 4 sites in the San Joaquin Valley. Additionally, we assessed the probability of producing economically viable forage yields across the region based on historical precipitation totals combined with 0, 100, or 200 mm of supplemental irrigation.METHODS: We used APSIM to run 20-yr simulations of winter wheat establishment and productivity under historical weather conditions at four groundwater dependent sites in California's San Joaquin Valley. For each site, we simulated wheat productivity (biomass and grain) under three irrigation scenarios (no irrigation, 100 mm, or 200 mm) and three sowing dates (mid-October, mid-November, and mid-December). We applied model outputs to calculations of economic and agronomic water productivity, and used the modeled relationship be-tween soft dough biomass yield and total water input (precipitation plus irrigation) to determine the likelihood of a successful wheat crop in any given year across the region.RESULTS AND CONCLUSIONS: We found that two 100 mm applications of irrigation applied at times of critical soil water depletion decreased crop failure rates from 45% of years to 0% of years at the driest site. Economic water productivity was highest when wheat was harvested for forage at the soft dough stage, and agronomic water productivity of soft dough forage was highest with supplemental irrigation and early planting. Given the precipitation requirements to achieve economically viable soft dough-stage forage yields, the addition of 100 mm of irrigation in a single event expanded the potential cropping area to 11% (126,356 ha) of groundwater dependent cropland in the region, while 200 mm of irrigation expanded the potential cropping area to 100% (1,109,888 ha) of cropland.SIGNIFICANCE: These results suggest that small, targeted supplemental irrigation events could greatly expand the scope for water-limited winter forage production in the San Joaquin Valley. Such an approach could serve as an alternative to land fallowing in areas transitioning away from irrigated summer crops where appropriate technological and policy mechanisms are in place.