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

对于当今的灌溉农业来说，缺水是一个持续的现实，特别是在美国西南部干旱的地区，那里的棉花（Gossypium hirsutum L.）是重要的农作物。该地区水供应的减少，加上棉花对水的需求较高，正在引发节水做法，包括采用地下滴灌（SDI）和亏水灌溉管理。在美国亚利桑那州马里科帕（Maricopa），使用SDI在沙质壤土上进行了为期3年的棉花研究（2016、2017和2018）。目的是确定实际单季（K _{c act}）和基础（K _{cb act}）作物系数，相应的生长阶段长度以及估算作物蒸散量（ET _c）的水分胁迫调整标准。），采用广泛使用的FAO56作物系数程序。评估的处理方式为完全灌溉（100％）和亏缺灌溉（70％），均重复3次并以最佳氮肥处理。灌溉计划是根据早先研究在Maricopa地点使用沟灌进行局部开发的FAO56 K _cb曲线得出的。实际的ET _c（ET _{c act}）由土壤水分平衡确定，使用每6–14天进行的0–1.9-m的土壤剖面水分含量测量。该研究还量化了由于亏水灌溉处理对ET _c行为和产量的影响。对于100％的治疗，ET _c发生率很高，通常超过12.0 mm d ^-1在所有赛季的中期。100％处理的累积ET _c为961、1039和1008 mm；2016年，2017年和2018年的皮棉产量分别为1470、1780和2200 kg ha ^-1。在所有年份中，对ET _c行为和棉花产量的治疗效果均显着。每年治疗70％的累积ET _c比治疗100％的少21-23％。70％处理的皮棉产量在两年中降低了15-16％，而在2017年生长季节初期发生土壤水分胁迫时，皮棉产量降低了36％。K _{c act}和K _{cb act}的3年平均值赛季中期分别为1.31和1.26。尽管高于该地点先前报告的棉花值，但在使用SDI的干旱气候下，这些中期值被认为是合适的，并且与局部气候调整后的FAO56中期标准棉花值一致。在初始阶段观察到的平均K _{c act}和K _{cb act}为≈0.21，在季节结束时为0.37，与这些阶段的FAO56表格棉花值非常吻合。确定的SDI棉花在生长初期，发育中，中期和后期的平均生长期分别为33、42、51和28。FAO56的水分胁迫系数（K _s在没有应力（p）的情况下，使用五个不同的损耗分数基线值（以0.625至0.70的变化，以0.025的增量）对70％的处理进行了建模。在2016年，2017年和2018年获得的基线p值分别为0.60、0.675和0.675的情况下，获得70％和100％处理的ET _c作用比与70％处理的模型K _s之间的最佳相关系数（r）， 分别。建议使用三年3年平均基准p值0.65（与FAO56中棉花的基准p值相同）来估计由于水分胁迫而导致的ET _c减少。