Evaporation and evapotranspiration
The return half of the water cycle, and the reason a rainfall deficit becomes a drought faster in a hot year than a cool one.
Over land, evapotranspiration returns roughly two thirds of precipitation to the atmosphere before it ever reaches a river. It is the largest term in the land water balance after rainfall itself, and the one most people never think about.
The distinction that matters most
| Row | Potential (PET) | Actual (AET) |
|---|---|---|
| What it measures | How much water the atmosphere could remove if water were freely available | How much water is actually removed |
| Limited by | Energy, temperature, humidity and wind — atmospheric demand | Water availability in soil and plants — supply |
| In a wet period | PET and AET are close | AET approaches PET |
| In a dry period | PET rises with heat | AET falls as soil dries — the gap widens |
| Used for | Irrigation scheduling; drought indices such as SPEI | Water balance modelling; catchment yield |
The gap between them is the essence of agricultural drought. A hot dry summer raises PET sharply while AET is capped by what is left in the soil, and crops experience the difference as stress.
How it is calculated
Evapotranspiration is almost never measured directly outside research flux-tower sites. It is calculated from meteorological variables, and the method matters: different formulations give materially different numbers for the same conditions.
| Method | Inputs required | Where it is used |
|---|---|---|
| FAO-56 Penman-Monteith | Temperature, humidity, wind speed, solar radiation | The international reference method for crop water requirements |
| Hargreaves | Temperature and extraterrestrial radiation only | Where only temperature data exists; less accurate but far more widely applicable |
| Thornthwaite | Temperature and day length | Older method, still common in climatological drought indices |
| Priestley-Taylor | Radiation and temperature | Where advection is limited, such as large well-watered surfaces |
A PET figure without its method stated is not comparable with another. FAO-56 Penman-Monteith is the reference against which the others are calibrated.
Calculate an irrigation requirement →
Why it is rising
Warmer air holds more moisture and therefore has a greater capacity to draw water from soil, plants and open water. Atmospheric evaporative demand has increased measurably across much of the world, which means the same rainfall now supports less. This is one of the clearest and least contested mechanisms by which warming intensifies drought, independently of any change in rainfall.
Reservoir evaporation
Open water evaporation is a genuine consumptive loss, and it is proportional to surface area rather than volume. A shallow reservoir with a large surface in a hot climate can lose a very large share of its storage this way — in some arid-region schemes, more than the demand it was built to supply.
Sources
- European Centre for Medium-Range Weather Forecasts / Copernicus Climate Change Service — ERA5 global reanalysis. Open — attribution required · Copernicus licence
- Food and Agriculture Organization of the United Nations — AQUASTAT — Global Information System on Water and Agriculture. Public but restricted · CC BY-NC-SA 3.0 IGO
- UK Centre for Ecology and Hydrology — UK Hydrological Outlook and Monthly Hydrological Summary. Open — attribution required · Open Government Licence v3.0