Accurate measurements of actual evapotranspiration (ET_a) and crop coefficients (K_c) are essential to know crop water requirements and to improve irrigation scheduling. The eddy covariance (EC) technique is increasingly being used to do so. Precise information on K_c for lowland rice is essential for local- and regional-scale irrigation planning but it is lacking for tropical humid climates such as those found in eastern India. We used the EC technique to measure ET_a and K_c—the ratio of ET_a to reference potential evapotranspiration (ET₀)—of tropical lowland rice in eastern India over 2 years. ET₀ was estimated by four different approaches—the Food and Agriculture Organization-Penman–Monteith (FAO-PM) method, the Hargreaves and Samani (HS) method, the Mahringer (MG) method, and pan evaporation (E_pan) measurements. Measurements were taken when rice was grown in the dry season (January–May) and wet season (July–November) and in between growing seasons when the field was kept fallow. The magnitude of average ET_a during dry seasons (2.86 and 3.32 mm d⁻¹ in 2015 and 2016, respectively) was higher than that of the wet seasons (2.3 and 2.2 mm d⁻¹) in both the study years. Of the four methods tested for ET₀ estimation, the FAO-PM method best-represented ET₀ in this region of India. The energy balance was found to be more closed in the dry seasons (75–84%) and dry fallow periods (73–81%) as compared to the wet season (42–48%) and wet fallow (33–69%) periods of both years of study, suggesting that lateral heat transport was an important term in the energy balance. The estimated K_c values for lowland rice in dry seasons by the FAO-PM method at the four crop growth stages, namely, initial, crop development, reproductive, and late-season, were 0.23, 0.42, 0.64, and 0.90, respectively, in 2015 and 0.32, 0.52, 0.76, and 0.88, respectively, in 2016. The FAO-PM, HS, and MG methods produced reliable estimates of K_c values in the dry seasons, whereas E_pan performed better in wet seasons. The actual K_c values derived for tropical lowland rice in eastern India are different from those suggested by the FAO implying revision of K_c values for regional-scale irrigation scheduling.

准确测量实际蒸散量 (ET _a ) 和作物系数 (K _c ) 对于了解作物需水量和改进灌溉计划至关重要。涡流协方差 (EC) 技术越来越多地用于这样做。低地水稻K _{c 的}精确信息对于地方和区域规模的灌溉规划至关重要，但对于热带潮湿气候（如印度东部发现的那些气候）则缺乏。我们使用 EC 技术测量了 ET _a和 K _c - ET _a与参考潜在蒸散量 (ET ₀ )的比值- 印度东部热带低地水稻在 2 年内。ET ₀由四种不同的方法估计——粮食及农业组织-彭曼-蒙特斯 (FAO-PM) 方法、哈格里夫斯和萨马尼 (HS) 方法、马林格 (MG) 方法和锅蒸发 (E _pan ) 测量。当水稻在旱季（1 月至 5 月）和雨季（7 月至 11 月）种植时以及在田间休耕的生长季节之间进行测量。在两个研究年份中，旱季平均 ET _a的大小（2015 年和 2016 年分别为2.86 和 3.32 mm d ^-1）均高于雨季（2.3 和 2.2 mm d ^-1）。在测试 ET ₀估计的四种方法中，FAO-PM 方法最能代表 ET ₀在印度这个地区。与雨季 (42–48%) 和湿休耕 (33–69%) 相比，旱季 (75–84%) 和旱季休耕期 (73–81%) 的能量平衡更为封闭两年的研究期间，表明横向热传输是能量平衡中的一个重要术语。根据FAO-PM方法估算的旱季低地水稻在作物初期、作物发育、繁殖和晚季四个阶段的K _c值分别为0.23、0.42、0.64和0.90， 2015 年和 2016 年分别为 0.32、0.52、0.76 和 0.88。FAO-PM、HS 和 MG 方法在旱季产生了可靠的 K _c值估计，而 E _pan在雨季表现更好。实际ķ _Ç印度东部热带低地水稻得出的值与粮农组织建议的值不同，这意味着要针对区域规模的灌溉计划修改 K _c值。