♦ DESCRIPTION ♦
The program ONLINE_PENMAN
calculates potential evapotranspiration [ET]o by the Penman method,
given specified climatic conditions.
The input is: (a) the chosen month,
(b) mean monthly air temperature Ta (oC),
(c) mean monthly net radiation Qn (cal/cm2/day),
(d) relative humidity RH (%), and (e) wind velocity at 2-m depth v2 (km/day).
The output is daily potential evapotranspiration, in cm/day, and monthly potential evapotranspiration, in cm.