FAO56 crop and water stress coefficients for cotton using subsurface drip irrigation in an arid US climate

Water shortages are a continuing reality for today’s irrigated agriculture especially in the arid US southwest where cotton (Gossypium hirsutum L.) is an important crop. Decreasing water availability region coupled with high requirements inducing conservation practices, including adoption of subsurface drip irrigation (SDI) and deficit management. A 3-year study (2016, 2017, 2018) was conducted on sandy loam soil using SDI at Maricopa, Arizona, USA. The objectives were to determine actual single (Kc act) basal (Kcb crop coefficients, corresponding growth stage lengths, stress adjustment criterion estimating evapotranspiration (ETc) by widely used FAO56 coefficient procedures. Treatments evaluated full (100%) (70%), both replicated three times managed optimum N rate. Irrigation scheduling based Kcb curve locally developed earlier studies Maricopa location furrow irrigation. Actual ETc (ETc determined balance 0–1.9-m profile content measurements made every 6–14-days. also quantified effects act yield due treatment. rates 100% treatment, often exceeding 12.0 mm d−1 during mid-season all years. Cumulative treatment 961, 1039, 1008 mm; lint 1470, 1780, 2200 kg ha−1 2016, 2018, respectively. Treatment significant 70% 21–23% less than that each year. Lint yields treatments reduced 15–16% two years, 36% 2017 when occurred growing season. average values Kc 1.31 1.26, Though higher those previously reported this location, these considered appropriate climate consistent standard after local adjustment. observed initial ≈ 0.21 they 0.37 end season, closely agreeing tabulated stages. lengths 33, 42, 51, 28 initial, development, mid-season, late season stages, (Ks) modeled five different baseline depletion fraction no (p) varied from 0.60 0.70 0.025 increments. Best correlation coefficients (r) between ratio Ks obtained p 0.60, 0.675, 0.675 value 0.65, which same FAO56, recommended reduction stress.