Vapour Pressure Deficit (kPa) May

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，日尺度水汽压亏缺（vapour pressure deficit, VPD）的需求频率要高得多，尤其是在利用彭曼-蒙特思方程（Penman-Monteith equation）开展作物潜在蒸发量模拟时（Penman, 1948）。本节首先概述用于估算水汽压亏缺的方法，随后对该方法进行验证，并最终给出水汽压亏缺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水汽压亏缺估算，其核心假设为：实际水汽压（actual vapour pressure, VP）作为一类保守性气候要素，可通过月尺度回归方程求得，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公式（Tetens formula, 1930），计算得到日尺度饱和水汽压。基于实际水汽压与饱和水汽压的日尺度数据，即可计算得到日尺度水汽压亏缺；若有需求，也可同步求得相对湿度（Relative Humidity, RH）。该方法被称为舒尔茨-查普曼模型（Schulze-Chapman model）。