Midsummer Deficit Irrigation of Alfalfa for Water Conservation in the San Joaquin Valley of California

A four-year research experiment was conducted on sandy loam soil at the University of California Kearney Agricultural Research and Extension Center in Parlier, California, to investigate the effect of midsummer deficit irrigation on alfalfa yield, irrigation water productivity (IWP), and crop water productivity (CWP). The experiment was a randomized block design with two treatments: full and deficit irrigations with three replications. Applied irrigation water was measured using flow meters and soil matric potentials were monitored using watermark soil moisture sensors. Actual evapotranspiration (ETa) values were estimated from Tule Technologies stations. The deficit irrigation treatments resulted in 454, 706, 625, and 815 mm of irrigation water savings as compared to the full irrigation treatments in 2019, 2020, 2021, and 2022, respectively. These values represent 30.3%, 40.9%, 37.0%, and 49.1% of the applied water savings. Alfalfa yield in the deficit treatments was reduced by 3.94, 2.04, 1.25, and 0.40 Mg ha-1; the equivalent of 18.1%, 11.1%, 7.1%, and 3.0% of the yield for the full irrigation treatment for the four years: with an average reduction of 10.7%. IWP was higher when deficit irrigation was implemented and resulted in 17.09, 16.11, 15.40, and 15.54 kg ha-1 mm-1, in 2019, 2020, 2021, and 2022, respectively. The production function using applied irrigation water (IW, mm) was: Y(yield in Mg ha-1)=0.50x(IW)2-1,633.75x(IW)+1,338,472 and Y=-0.1137x(IW)2+233.55x(IW)-103,036 for the full and deficit irrigation treatments, respectively. CWP was 18.6, 16.4, 14.9, and 12.3 kg ha-1 mm-1 for fully irrigated treatments, and 15.2, 14.9, 14.3, and 12.6 kg ha-1 mm-1 for the deficit irrigation treatments, for 2019, 2020, 2021, and 2022, respectively. Results from this work provide growers with viable deficit irrigation practices that could be implemented during drought periods.