Vapour Pressure Deficit (kPa) December

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，每日水汽压亏缺（vapour pressure deficit, VPD）的需求频率远高于月度数据，尤其在利用彭曼-蒙蒂思方程（Penman-Monteith equation）模拟作物潜在蒸发量时。本节首先概述了一种用于估算VPD的方法，随后对该方法进行验证，最后展示了VPD的空间分布图与统计特征。

针对南非任意指定站点的日尺度VPD估算，本文提出的假设为：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出，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给出的特滕斯（Tetens, 1930）公式，计算得到逐日饱和水汽压（saturated vapour pressure）。从实际水汽压与饱和水汽压的逐日数据中，即可计算得到VPD（若有需求，还可同步得到相对湿度relative humidity, RH）。该方法被称为舒尔茨-查普曼模型（Schulze-Chapman model）。