Abstract The FAO-56 crop coefficient (Kc) approach is widely used for making irrigation decisions. Allen and Pereira (2009) extended this approach by developing a method for estimating Kc using a density coefficient (Kd), which is estimated from the fraction of ground covered by vegetation and plant height. In this study we evaluated this method using detailed measurements of transpiration (T), evapotranspiration (ET), soil attributes, weather, and tree physiological variables in 12 apple (Malus domestica Borkh.) orchards in the Western Cape Province of South Africa. Mid-summer canopy cover of the orchards was less than 20 % in young non-bearing and exceeded 60 % in mature full-bearing orchards. Data were collected over three growing seasons (October 2014 to May 2017) in orchards planted to the Golden Delicious/Reinders®, Cripps Pink, Cripps Red, and Rosy Glow apple cultivars. The original Allen and Pereira (A&P) method significantly overestimated the basal crop coefficients (Kcb) by on average 47 % in mature and 103 % in young orchards, respectively. However, improved Kcb estimates were obtained by adjusting the ratio of the resistances (i.e. rl /100) in the A&P method, where rl is the mean leaf resistance and 100 s/m is the typical resistance for annual crops. We defined a resistance parameter “α” for apple orchards which is equivalent to the bulk canopy resistance of a well-watered tree. Replacing rl /100 with rl /α, and using the measured mean rl and other biophysical measurements to solve the A&P equation for α gave a value ∼ 37 s/m. The improved Kcb values were used to derive the orchard Kc taking into account the contribution of cover crops whose transpiration was measured using miniature stem heat balance sap flow gauges. The seasonal total transpiration (T) estimated as T = Kcb x ETo, where ETo is the reference ET closely matched the measured values with a RMSE (root of the mean square error) of ∼ ±16 mm. Therefore, using the mean canopy resistance which is representative of apple trees in the A&P method has the potential to accurately predict both the crop coefficients and water use of apple orchards from planting until full bearing age.

中文翻译：

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使用南非西开普省 12 个果园的实测数据估算不同冠层覆盖的苹果园的作物系数

摘要 FAO-56 作物系数 (Kc) 方法被广泛用于制定灌溉决策。Allen 和 Pereira (2009) 通过开发一种使用密度系数 (Kd) 来估算 Kc 的方法扩展了这种方法，密度系数 (Kd) 是根据植被覆盖的地面比例和植物高度估算的。在这项研究中，我们使用对南非西开普省 12 个苹果 (Malusdomestica Borkh.) 果园的蒸腾 (T)、蒸散 (ET)、土壤属性、天气和树木生理变量的详细测量来评估该方法。果园的仲夏树冠盖度在年轻的非结实果园中不到 20%，在成熟的满载果园中超过 60%。在种植 Golden Delicious/Reinders®、Cripps Pink、Cripps Red、和 Rosy Glow 苹果品种。最初的 Allen 和 Pereira (A&P) 方法显着高估了成熟果园的基础作物系数 (Kcb)，平均分别高出 47% 和 103%。然而，通过调整 A&P 方法中的抗性比（即 rl /100）获得改进的 Kcb 估计值，其中 rl 是平均叶抗性，100 s/m 是一年生作物的典型抗性。我们为苹果园定义了一个阻力参数“α”，它相当于一棵浇水充足的树的大面积树冠阻力。用 rl /α 代替 rl /100，并使用测得的平均 rl 和其他生物物理测量值来求解 α 的 A&P 方程，得出值 ~ 37 s/m。改进的 Kcb 值用于推算果园 Kc，同时考虑到使用微型茎热平衡液流计测量蒸腾作用的覆盖作物的贡献。季节性总蒸腾量 (T) 估计为 T = Kcb x ETo，其中 ETo 是参考 ET，与测量值密切匹配，RMSE（均方误差根）约为 ±16 mm。因此，在 A&P 方法中使用代表苹果树的平均冠层阻力有可能准确预测苹果园从种植到满龄期的作物系数和用水量。其中 ETo 是参考 ET 与测量值密切匹配，RMSE（均方误差根）为 ±16 mm。因此，在 A&P 方法中使用代表苹果树的平均冠层阻力有可能准确预测苹果园从种植到满龄期的作物系数和用水量。其中 ETo 是参考 ET 与测量值密切匹配，RMSE（均方误差根）为 ±16 mm。因此，在 A&P 方法中使用代表苹果树的平均冠层阻力有可能准确预测苹果园从种植到满龄期的作物系数和用水量。