Evapotranspiration Explained

Evapotranspiration is the combined loss of water from the land to the atmosphere through evaporation from soil and water surfaces and transpiration from the leaves of plants, and in India it quietly returns a large share of every monsoon’s rain to the sky. It is the invisible half of the farm and river story: while we measure rain, reservoirs and borewells with care, a huge volume of water leaves every day without a sound, drawn up through roots and out of leaf pores or lifted off wet ground by sun and wind.

This explainer, last reviewed 4 November 2026, covers what evapotranspiration is, the difference between potential, reference and actual values, what controls it, how it is measured, and why it matters for India’s water budget, irrigation, crops and changing climate.

Quick Facts

Key Takeaways

• Evapotranspiration is evaporation plus plant transpiration, and it is one of the largest outflows in India’s water balance, often returning most of the rain that falls on land back to the air.
• Potential evapotranspiration is what the atmosphere could take if water were unlimited, while actual evapotranspiration is what is really lost given the water available.
• Farmers and irrigation engineers estimate a crop’s water need by multiplying reference evapotranspiration by a crop coefficient that changes with growth stage.
• Hot, dry, sunny, windy conditions raise evapotranspiration, while humid, cloudy, calm weather lowers it, so Rajasthan and Gujarat lose far more water to the sky than the northeast.
• Irrigation, mulching, drip systems and choosing less thirsty crops can cut wasteful evaporation without reducing yield.
• A warming climate raises the atmosphere’s demand for water, which can deepen drought stress even where rainfall stays similar.

What exactly is evapotranspiration?

Evapotranspiration is the sum of two processes that move liquid water into vapour: evaporation and transpiration. Evaporation happens when water on a wet soil surface, a lake, a river, a reservoir or the wet leaves of a plant turns to vapour and mixes into the air. Transpiration is the loss of water through tiny leaf pores called stomata, after the plant has drawn it up from the soil through its roots.

The two are hard to separate in a field and are usually treated together. A young crop shortly after sowing loses water mostly by evaporation from bare soil between seedlings. As the canopy grows and shades the ground, transpiration takes over and can account for most of the loss. Transpiration is not wasted entirely: the stream of water carries nutrients, cools the leaf in the heat, and keeps stomata open so carbon dioxide can enter for photosynthesis. That is why plants that are short of water also grow less.

Evapotranspiration is the main way the land surface returns water to the atmosphere, completing a key leg of the water cycle. It is also an energy process. Turning a litre of water into vapour takes a large amount of heat, so evapotranspiration cools the surface; irrigated fields and forests are noticeably cooler on hot afternoons than bare, dry ground.

What is the difference between potential, reference and actual evapotranspiration?

Potential evapotranspiration is the amount of water that would be lost if the soil were always wet and the vegetation always healthy, while actual evapotranspiration is the smaller amount really lost when water runs short. Potential evapotranspiration is therefore a measure of atmospheric demand, set by sunshine, temperature, humidity and wind. Actual evapotranspiration is what the land can supply.

In arid western India the gap between the two is wide. On a hot May day the demand might exceed six or seven millimetres, yet dry soil can supply almost nothing, so actual loss is small. In the wet Western Ghats during the monsoon, soil is saturated and the cloudy, humid air limits demand, so actual and potential values are both modest and nearly equal.

Reference evapotranspiration, usually written ET0, is a standardised version of potential evapotranspiration. It describes the loss from a hypothetical, well-watered, short green grass surface of uniform height. Fixing the reference crop lets meteorologists in different places calculate comparable numbers from standard weather observations. The Food and Agriculture Organization recommends the Penman-Monteith equation for this, which combines radiation, temperature, humidity and wind speed. A crop’s actual requirement is then derived from ET0 by applying a crop coefficient.

What controls how much water is lost?

The amount of water lost is controlled by four atmospheric factors, plus the water in the soil and the nature of the plants. The first is solar energy: sunshine supplies the heat needed to vaporise water, which is why evapotranspiration peaks around midday and in the long clear days of April and May. The second is air temperature, because warmer air can hold more vapour.

Humidity matters because the gap between how much vapour the air holds and how much it could hold, known as the vapour pressure deficit, is the pull that draws water out of leaves and soil. Dry air has a large deficit and drives high loss, whereas humid monsoon air has a small one. Wind removes the moist layer next to a leaf or a water surface and replaces it with drier air, which is why hot dry winds such as the loo over the northern plains are so desiccating.

On the ground, available soil moisture, root depth, leaf area, crop stage and stomatal behaviour all play a part. Plants close stomata when stressed to save water, which cuts transpiration but also growth. Surface cover also changes evaporation: a mulch of straw or plastic film, a thick canopy, or a layer of dry topsoil each reduce the loss from the soil. Taken together, these controls explain why the same rain can support a flourishing farm in one place and vanish quickly in another.

How much of India’s rain goes back to the air?

A large share of India’s rainfall returns to the atmosphere through evapotranspiration, and estimates suggest that over most of the country it is the single biggest outflow in the water balance. Of the rain that reaches the ground, only a portion becomes streamflow or recharges aquifers; the rest is held in the soil and then lifted back into the air by sun and plants. National assessments of water resources, which count India’s average annual precipitation at roughly 4,000 billion cubic metres, show that a majority is lost to evapotranspiration, with the remainder flowing in rivers or seeping underground.

The proportion varies enormously by region. In arid Rajasthan and Kutch, almost all rain that falls is evaporated or transpired, leaving little runoff. In the very wet Western Ghats and northeast, rainfall far exceeds the sun’s capacity to evaporate it, so surplus water runs off to feed rivers such as the Brahmaputra and the west-flowing streams. Forests lose water through their large leaf area but also hold soil moisture and moderate floods.

This shapes the nature of India’s water scarcity. A district can receive fair rainfall and still be dry if the atmosphere’s demand is high and the water table is low. Water planners therefore look at the balance between precipitation and potential evapotranspiration, a ratio called the aridity index, to classify regions as humid, semi-arid or arid.

How is evapotranspiration used to schedule irrigation?

Evapotranspiration is used to schedule irrigation by estimating the crop’s daily water use and replacing it before the soil dries too far. The standard approach multiplies reference evapotranspiration by a crop coefficient, written Kc, to obtain the crop’s own water use, expressed ETc = Kc x ET0. The coefficient is low at sowing, rises to a peak during the mid-season when the canopy is full, and falls as the crop ripens.

Take wheat grown in the Indo-Gangetic Plain in the cool months. Daily reference values might be only two or three millimetres in December but climb to four or five by March, so water need rises toward grain filling. A farmer or irrigation department can add up the water the crop is expected to use, subtract effective rainfall and soil storage, and arrive at how much irrigation is required and when.

Agricultural advisories drawn from weather stations increasingly use these calculations, and many state irrigation departments and agricultural universities publish crop water requirements. For reference, seasonal water needs differ widely: cereals like wheat and millets typically need several hundred millimetres, paddy rice needs considerably more once puddling and seepage are counted, and sugarcane, which grows for most of a year, can need well over a thousand millimetres. Matching water to the actual demand avoids both stress and waste.

How is evapotranspiration measured?

Evapotranspiration is measured directly with instruments and estimated indirectly from weather and satellite data. The oldest tool is the evaporation pan, an open tank of water whose daily drop in level shows how much the atmosphere pulls from a free water surface; India’s weather department has long used standard pans at observatories. A pan coefficient, usually less than one, converts pan loss into reference evapotranspiration.

More precise are lysimeters, large soil-filled containers buried in the field that are weighed to detect tiny changes in mass, revealing the water lost by crop and soil. Eddy covariance flux towers measure the upward movement of water vapour above a crop or forest canopy by sampling wind and humidity many times a second; a network of such towers operates over Indian croplands and forests for research.

For wider areas, scientists use weather-station equations such as Penman-Monteith and remote sensing. Satellites measure surface temperature and vegetation greenness, and models use an energy balance to estimate how much heat is going into evaporating water. These products now give district- and basin-scale maps of water use. Each method has errors, so researchers usually compare several, especially when assessing groundwater depletion or the water footprint of crops.

Which Indian crops and regions lose the most water?

The crops and regions that lose the most water are those with long seasons, dense canopies and hot dry air, such as sugarcane, banana and paddy in summer, and the semi-arid belts of the peninsula and northwest. The table below sets out indicative contrasts.

These patterns matter because they link directly to groundwater. In Punjab, Haryana and parts of the peninsula, irrigated rice and sugarcane draw heavily on aquifers, and much of what is pumped is ultimately lost to the air as evapotranspiration rather than returning to the ground. This is a key reason crop diversification toward millets, pulses and oilseeds is promoted in water-stressed districts.

Is climate change altering evapotranspiration in India?

Climate change is altering evapotranspiration mainly by raising the atmosphere’s thirst, although the observed picture in India is mixed. As temperatures rise, warmer air can hold more water vapour and the demand for moisture from soil and plants tends to increase. This means that in a warming world, a given amount of rain supports less soil moisture and drought can set in faster, a pattern researchers call flash drought when it develops over a few weeks.

Yet measurements have shown a puzzle. In the decades from around the 1960s, many Indian pan-evaporation records declined even as temperatures rose, an “evaporation paradox”. Explanations include reduced sunlight reaching the ground because of air pollution and haze, weaker surface winds, and rising humidity in irrigated regions. More recent analyses suggest the trend is changing as warming accelerates and air pollution controls begin to bite.

Land use adds another twist. Large-scale irrigation in the Indo-Gangetic Plain has increased evapotranspiration in the dry season, cooling the land and adding moisture to the air, which some studies suggest could even influence local rainfall and monsoon humidity. Forest clearing, in contrast, tends to reduce transpiration and local moisture recycling. Because these effects interact, projecting the future water budget calls for models that treat vegetation, irrigation and climate together.

How can farmers reduce wasteful evaporation?

Farmers can reduce wasteful evaporation by keeping water where plants can use it and by cutting open-surface losses. Drip and sprinkler irrigation deliver water near the roots rather than flooding the whole field, so far less is lost from bare wet soil. Mulching with crop residue, straw or plastic film shades the soil and slows evaporation, while also suppressing weeds that would otherwise transpire water.

Practices in rice are especially important. Alternate wetting and drying, where fields are allowed to dry briefly between irrigations, and direct-seeded rice reduce standing water and seepage while keeping yield, though results vary with soil and management. Laser land levelling spreads water evenly, minimising over-watering of low spots. Timing sowing to match rainfall, planting windbreaks, and conserving soil moisture through minimum tillage all lower the loss.

At the landscape scale, rainwater harvesting, farm ponds lined to reduce seepage, covered storage and groundwater recharge structures help, since open water surfaces in hot regions lose a lot to evaporation. Crop choice matters most of all: shifting from thirsty crops to those suited to the local climate cuts total consumption. Success depends on pairing such measures with accurate local evapotranspiration data, which advisory services increasingly provide.

Bottom Line

Evapotranspiration is the great hidden flow of India’s water cycle, quietly returning most of the rain to the air. Understanding it explains why dry regions stay dry, why irrigation needs differ by crop and season, and why a warming climate can intensify drought even without big changes in rainfall. For farmers and planners alike, managing evapotranspiration is as important as managing rainfall.

Frequently Asked Questions

What is evapotranspiration in simple words?

It is the total water that goes from land into the air, combining evaporation from soil and water surfaces with transpiration from plant leaves. It is measured in millimetres, like rainfall.

What is the difference between evaporation and transpiration?

Evaporation is water turning to vapour from soil, lakes or wet surfaces. Transpiration is water that a plant draws up through its roots and releases as vapour through the pores in its leaves.

What is reference evapotranspiration (ET0)?

ET0 is the water loss from a standard, well-watered short green grass surface, calculated from weather data using the Penman-Monteith equation. Crop water needs are found by multiplying it by a crop coefficient.

Why does evapotranspiration matter for Indian farmers?

It tells them how much water a crop uses each day, so they can irrigate at the right time and in the right quantity. This saves water and avoids stressing the crop.

Which conditions increase evapotranspiration?

Strong sunshine, high temperature, low humidity and strong wind raise it, as do well-watered soil and dense, leafy crops. Humid, cloudy, calm weather lowers it.

Does global warming increase evapotranspiration?

Warmer air demands more water from the land, which tends to increase evapotranspiration where moisture is available and can worsen drought. Actual trends in India are mixed because haze, wind and irrigation also play a part.

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