October Means Of Reference Crop Evaporation (mm) Fao Penman-Monteith Equivalent

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`.

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.

联合国粮食及农业组织（FAO）1992年给出的彭曼-蒙特斯（Penman-Monteith）参考作物蒸发量（Erpm）定义为：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70 s·m⁻¹、反照率0.23，其蒸散过程可近似模拟大面积均匀高度的绿色草本覆盖地表的蒸散——这类草地长势旺盛、完全遮蔽地面且水分充足。针对南非地区的彭曼-蒙特斯等效参考作物蒸发量Erpm首张详细分布图，由Crosby与Crosby（1996）基于174个站点的观测数据，通过计算Erpm与A型蒸发皿（A-pan）的月比值推导得到。基于这些比值，Schulze（1997）划分出14个具有相似比值分布特征的“区域”，随后借助网格化A型蒸发皿数据绘制了Erpm空间分布图。本图集采用的方法更具确定性与基础性：基于Schulze与Maharaj（2004）的研究成果，以50年时长、南非全境1′×1′（即1.7×1.7公里）的栅格为基础单元，生成逐日最高、最低气温及对应饱和水汽压数据；并结合针对南非地区经经验确定的逐月网格化实际水汽压数据，完成相关制图工作。