The Penman equation for estimating potential evapotranspiration is a combination of which two approaches?
- AWater budget and heat budget
- BEnergy balance and mass transferCorrect
- CRadiation balance and soil moisture budget
- DAerodynamic and temperature gradient methods
Solution
Concept
Penman equation combining energy balance and mass transfer
Formula
PET = (A·Hn + Ea·γ) / (A + γ)
Step-by-step solution
1. Penman's method estimates potential evapotranspiration (PET) by synthesizing two governing physical transport mechanisms. 2. The first is the energy balance method, calculating the net solar and terrestrial radiation available to vaporize water. 3. The second is the aerodynamic / mass transfer method, measuring vapor transport via wind speed and vapor pressure deficit. 4. Hence, Penman combines the energy balance and mass transfer approaches. Answer: B
Explanation
Penman’s equation (1948) for potential evapotranspiration is based on a sound theoretical formulation that combines two distinct physical approaches: the energy balance method (net radiation required to evaporate water) and the aerodynamic or mass transfer method (wind and vapor pressure gradient transporting water vapor away from the surface).
Why the other options are wrong
A) Water budget is a volumetric catchment water accounting method. C) Soil moisture budget tracks root zone storage, not atmospheric evaporative demand. D) Aerodynamic method requires vapor pressure gradient, not merely temperature gradient.
Shortcut
Penman equation = Energy balance (radiation) + Mass transfer (aerodynamic).
Common mistake
Mistaking water balance accounting or temperature gradient methods for the micrometeorological aerodynamic approach.
Correct answer: B) Energy balance and mass transfer