September 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Penman-Monteith标准参考作物蒸散量（Erpm）详细空间分布图，是基于174个站点的观测数据，通过Erpm与A型蒸发皿（A-pan）的月比值关系推导得到（Crosby与Crosby, 1996）。基于上述比值关系，Schulze（1997）划分出14个比值模式相似的区域，并据此利用网格化A型蒸发皿数据绘制了Erpm空间分布图。本图集采用的分布图则基于Schulze与Maharaj（2004）的研究成果，采用更具物理确定性与基础性的方法：以南非地区50年时长、1′×1′（1.7×1.7公里）分辨率的网格化逐日最高、最低气温数据为基础，进一步生成逐日饱和水汽压数据。本数据集还包含经经验方法确定的南非地区逐月网格化实际水汽压数据。FAO（Food and Agriculture Organization）1992年对Penman-Monteith标准参考作物蒸散量（Erpm）的定义为：假想作物的蒸散速率——该假想作物设定株高0.12米、冠层阻力固定为70 s·m⁻¹、反照率为0.23，其蒸散过程与大片高度均匀、长势旺盛、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。