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作物系数多在文献中定义为实际作物腾发量ET_o与参考作物腾发量ET_O的比值。影响它的有作物本身生物学性状、土壤水分、栽培因素等。它具有全生育期值年际间的稳定性与不同生育期值年际间的不稳定性。作物系数值的分布还带有地域性。近期研究趋向于把实际作物系数分解为三部份,即基本作物系数K_b,因土壤水分变化引起的变值K_a及因灌水方式引起的变值K_s。
Crop coefficients are mostly defined in the literature as the ratio of actual crop evapotranspiration ET_o to reference crop evapotranspiration ET_O. Affect its biological properties of the crop itself, soil moisture, cultivation factors. It has the inter-annual stability of the whole growth period value and instability between different growth period values. Distribution of crop coefficient values also with regional. Recent researches tend to decompose the actual crop coefficient into three parts: the basic crop coefficient K_b, the change K_a due to the change of soil moisture and the change K_s due to the irrigation method.