April Standard Deviations Of Daily Reference Crop Evaporation (mm) Fao Penman-Monteith Equivalent

The first detailed maps for South Africa of Penman-Monteith equivalent reference crop, Erpm, were derived from monthly ratios of Erpm: A-pan using data at 174 sites (Crosby and Crosby, 1996). From these ratios, Schulze (1997) identified 14 `regions` of similar ratio patterns which were then used to map Erpm from gridded A-pan values. The maps in the present Atlas are based on a much more deterministic and more fundamental approach with daily maximum and minimum temperatures, and hence daily saturated vapour pressures, generated over South Africa on a 1` x 1` (1.7 x 1.7 km) raster for 50 years, based on research by Schulze and Maharaj (2004). Empirically determined month-by-month gridded values of actual vapour pressure for South Africa. The FAO (1992) definition of Penman-Monteith derived reference crop evaporation (Erpm) is `The rate of evapotranspiration from a hypothetical crop with an assumed crop height of 0.12 m, a fixed canopy resistance of 70 s.m-1 and albedo of 0.23, which would closely resemble evapotranspiration from an extensive surface of green grass cover of uniform height, actively growing, completely shading the ground and not short of water`.

首份针对南非地区的彭曼-蒙特思（Penman-Monteith）等效参考作物蒸散量（Erpm）精细化分布图，系基于174个站点的逐月Erpm与A级蒸发皿（A-pan）蒸发量比值，由Crosby与Crosby于1996年推导得出。
基于上述比值，Schulze于1997年划分出14个比值特征相似的“区域”，并借此通过网格化A级蒸发皿蒸发量数据绘制了Erpm分布图。
本图集所采用的分布图，则基于Schulze与Maharaj于2004年的研究成果，采用更为严谨且基础的确定性方法：以南非全域50年的1角分×1角分（约1.7×1.7千米）栅格（raster）分辨率的逐日最高、最低气温数据为基础，进而计算得到逐日饱和水汽压。
同时配套了通过经验方法确定的南非逐月网格化实际水汽压数据。
联合国粮食及农业组织（Food and Agriculture Organization，FAO）1992年对彭曼-蒙特思型参考作物蒸散量（Erpm）的定义为：“某假想作物的蒸散速率，该假想作物设定株高0.12米、固定冠层阻力（canopy resistance）70 s·m⁻¹、反照率（albedo）0.23，其蒸散过程与大片均匀高度、活跃生长、完全覆盖地面且水分充足的绿色草地的蒸散过程高度相似。”