What does A represent?

Slope of saturation vapour pressure-temperature curve at air temperature

Ea in the Penman equation is

Drying power of air (aerodynamic term)

Extraterrestrial solar radiation at the top of atmosphere

0.05 for water, 0.15-0.25 for green crop

2072 Chaitra · 4 marks

Penman equation and its variables

Short answer

PET = (A·Hn + Ea·γ) / (A + γ). Hn is net radiation (mm/day), Ea is aerodynamic term (mm/day), A is slope of saturation vapor pressure curve (mm Hg/°C), γ is psychrometric constant (0.49 mm Hg/°C). Hn depends on extraterrestrial radiation Ha, albedo r, sunshine ratio n/N, air temperature Ta (K), and actual vapor pressure ea; Ea depends on wind speed u₂ (km/day) and vapor pressure deficit (ew − ea).

Write down the Penman equation and explain all variables and constants involved in it.

The Penman equation for potential evapotranspiration (or open-water evaporation) is

PET = \frac{A\,H_n + E_a\,\gamma}{A + \gamma}

H_n = H_a(1-r)\left(0.29\cos\varphi + 0.52\frac{n}{N}\right) - \sigma T_a^4(0.56 - 0.092\sqrt{e_a})\left(0.1 + 0.9\frac{n}{N}\right)

E_a = 0.35\left(1 + \frac{u_2}{160}\right)(e_w - e_a)

Symbol	Meaning	Unit
$PET$	Potential evapotranspiration	mm/day
$A$	Slope of the saturation vapour pressure-temperature curve at the air temperature	mm Hg/°C
$\gamma$	Psychrometric constant (0.49)	mm Hg/°C
$H_n$	Net radiation in evaporation units	mm of water/day
$E_a$	Drying power of air (aerodynamic term)	mm/day
$H_a$	Extraterrestrial (top of atmosphere) solar radiation	mm/day
$r$	Reflection coefficient (albedo): 0.05 water, 0.15-0.25 green crop	-
$a$ ( $=0.29\cos\varphi$ ), $b$ (0.52)	Constants of the radiation relation; $\varphi$ = latitude	-
$n, N$	Actual and possible sunshine hours	h
$\sigma$	Stefan-Boltzmann constant ( $2.01\times10^{-9}$ )	mm/day/K⁴
$T_a$	Mean air temperature	K
$e_w, e_a$	Saturation and actual vapour pressure ( $e_a = RH\times e_w$ )	mm Hg
$u_2$	Wind speed at 2 m above ground	km/day

✓ Answer checked by Claude Sonnet 5.5

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