Ideas for Easy Calculation of Penman Equation

Abstract

We propose some ideas for easy calculation of the Penman equation.
(1) Wind velocity at 2m height is necessary in the Penman equation. However, wind velocity measurement heights at meteorological stations vary considerably. So we come up with a simple equation to convert wind velocity at the various heights to 2m height values.
u₂=u_H · log (200)/log (100 · H)
u₂, u_H: wind velocity at 2m and Hm height (m·s^-1)
(2) To save manhours for data processing, we have provided approximate equations to account for declination δ and the distance from the earth to the sun, as the number of days from 1 st. January, J.
δ=23.45 cos {0.966 (J-173)}
d/d=1.0000+0.01676 cos {0.977 (J-186)}
d, d: real and average distance to the sun These equations are provided to reduce errors occurring during summer time, or during irrigation periods. The approximate error is zero during the summer solstice (J=173), or aphelion (the farthest day to the sun, J=186).
(3) Different kinds of sunshine recorders are used. Old solar cell-type sunshine recorder indicates the long duration of sunshine. To modify the observed hours recorded by it and bring it into par with the values observed using a standard sunshine recorder, or rotary-type, we have come up with the following next conversion equation.
Y=-0.00302 X³+0.101X²+0.207X
X, Y: Duration of sunshine by old solar cell-type, and rotary-type sunshine recorder value.