Vapour Pressure Deficit (kPa) October

Daily values of vapour pressure deficit (VPD) are, however, required far more frequently, particularly when modelling crop potential evaporation using the Penman- Monteith equation (Penman, 1948). In this section a method used to estimate VPD is outlined, the method is verified and then maps and statistics of VPD are presented. The hypothesis in estimating daily values of VPD at any specified location in South Africa is that actual VP, actual vapour pressure, which is considered to be a conservative climate element, and which may be derived from the monthly regression equations be held constant at that location for a given month. The fluctuating day-to-day daily temperature values at that location are then used with the Tetens (1930) formula given above to calculate a daily saturated VP, saturated vapour pressure. From actual vapour pressure and saturated vapour pressure daily values of VPD (and RH, if desired) can then be computed. This approach is known as the Schulze-Chapman model.

然而，水汽压亏缺（vapour pressure deficit, VPD）的逐日数据使用频率要高得多，尤其在借助彭曼-蒙特斯（Penman-Monteith）方程（Penman, 1948）开展作物潜在蒸发量模拟研究时。本节将首先概述一种VPD估算方法，随后对该方法进行验证，并最终给出VPD的空间分布图与统计特征。针对南非任意指定站点的逐日VPD估算，本文提出如下假设：实际水汽压（actual vapour pressure, VP）作为保守性气候要素，可通过月度回归方程推导得到，且在给定月份内该站点的实际水汽压保持恒定。随后，利用该站点逐日波动的气温数据，结合前文提及的特滕斯（Tetens, 1930）公式，即可计算得到逐日饱和水汽压。基于实际水汽压与饱和水汽压的逐日数据，即可计算得到VPD（若有需求，还可同步计算相对湿度RH）。该方法被称为舒尔茨-查普曼（Schulze-Chapman）模型。