A-Pan Equivalent Reference Potential Evaporation (October)

Reasons for having selected the A-Pan as a reference for potential evaporation is that the US Weather Bureau Class A evaporimeter is, universally, the most common evaporation pan in usage, having been adopted as the standard evaporation pan since the International Geophysical Year 1957/8. Also when operated properly it is accepted as a reasonably reliable, simple and inexpensive local integrator of atmospheric evaporative demand, particularly for time periods longer than one day. Problems associated with the A-Pan as a reference for potential evaporation is that they are found generally near dams, existing irrigation projects or in areas of low relative relief where commercial agriculture is practiced. These sites are not always representative of higher altitudes where water is generated. The data from over 570 South African stations at which a minimum of three years` concurrent monthly temperature, rainfall and A-pan evaporation observations are available, were used to develop temperature based equations of A-pan equivalent evaporation for each of the 12 evaporation regions identified in South Africa. The equations, unique for each month and for each region, were used to develop the 12 rasterised images of mean monthly A-pan equivalent evaporation on a 1` x 1` of a degree latitude/longitude grid.

选择A型蒸发盘（A-Pan）作为潜在蒸发量参考标准的原因在于：美国国家气象局（US Weather Bureau）A级蒸发皿是当前全球应用最为广泛的蒸发观测盘，自1957/1958年国际地球物理年（International Geophysical Year）起便被确立为标准蒸发盘。
此外，若操作规范，该装置可作为可靠、简便且低成本的局地大气蒸发需求累积测算工具，尤其适用于单日以上的观测时段。
但A型蒸发盘作为潜在蒸发量参考标准也存在局限性：其布设点位通常集中于水库、现有灌溉工程周边，或是商业农业发达的低相对高差区域。此类站点并不总能代表产水功能突出的高海拔环境。
研究团队采用了南非境内570余个气象站点的观测数据，这些站点均具备至少三年的同步逐月气温、降水量及A型蒸发盘蒸发量观测序列，借此针对南非划定的12个蒸发分区分别构建了基于气温的A型等效蒸发量（equivalent evaporation）计算公式。
基于各分区及各月份专属的上述计算公式，研究人员生成了12幅分辨率为1′×1′经纬度网格的逐月平均A型等效蒸发量栅格图像（rasterised images）。