GEOGRAPHY & CARTOGRAPHY CALCULATOR
Indexpotential Evapotranspiration
A precise tool.
What is the Indexpotential Evapotranspiration & How does it work?
The Thornthwaite method estimates potential evapotranspiration (PET) from air‑temperature data and geographic location. It is widely used in hydrology because it requires only readily available climate information, making it practical for large‑scale water‑balance studies. First, the monthly mean temperatures are transformed into a heat‑index (I) that captures the cumulative thermal energy of a year. An empirical exponent (a) is then derived from I, reflecting how temperature sensitivity changes with climate. Finally, PET for each month is calculated with a simple power‑law expression that scales with temperature and the heat‑index. The result is an estimate of the amount of water (in mm day⁻¹) that could be evaporated and transpired if water were not limiting.
PET = 16 left(frac{10,T}{I}right)^{a}
PET = potential evapotranspiration (mm day⁻¹)
T = mean monthly temperature (°C)
I = heat index = sum_{m=1}^{12}left(frac{T_m}{5}right)^{1.514}
a = (6.75times10^{-7})I^{3} – (7.71times10^{-5})I^{2} + (1.792times10^{-2})I + 0.49239
T = mean monthly temperature (°C)
I = heat index = sum_{m=1}^{12}left(frac{T_m}{5}right)^{1.514}
a = (6.75times10^{-7})I^{3} – (7.71times10^{-5})I^{2} + (1.792times10^{-2})I + 0.49239
Parameters
Result
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Frequently Asked Questions
What is potential evapotranspiration?
Potential evapotranspiration (PET) is the amount of water that would evaporate from a reference crop if there were no limitations such as lack of water or wind.
How does the Thornthwaite method estimate PET?
The Thornthwaite method estimates PET by transforming monthly mean temperatures into a heat-index and then using an empirical exponent to reflect temperature sensitivity.
What data is needed for this calculation?
You need monthly mean air temperatures and geographic location information for the Thornthwaite method.
Why is the Thornthwaite method widely used in hydrology?
It is widely used because it requires only readily available climate information, making it practical for large-scale water-balance studies.
What does the heat-index (I) represent in this method?
The heat-index represents the cumulative thermal energy of a year, derived from monthly mean temperatures.
How does temperature sensitivity change according to the Thornthwaite method?
Temperature sensitivity changes as reflected by an empirical exponent (a), which is derived from the heat-index (I).
Can this method be used for any location?
Yes, the Thornthwaite method can be applied to any location where monthly mean air temperatures and geographic data are available.
Results are for informational purposes only and do not constitute professional advice.