Evapotranspiration and the Water Cycle

Evapotranspiration is the sum of all processes by which water moves from the land surface to the atmosphere via evaporation and transpiration.

Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff

What is evapotranspiration?

Evapo- for evaporation

-transpiration for transpiration

Evapotranspiration is the sum of all processes by which water moves from the land surface to the atmosphere via evaporation and transpiration. Evapotranspiration includes water evaporation into the atmosphere from the soil surface, evaporation from the capillary fringe of the groundwater table, and evaporation from water bodies on land. Evapotranspiration also includes transpiration, which is the water movement from the soil to the atmosphere via plants. Transpiration occurs when plants take up liquid water from the soil and release water vapor into the air from their leaves.

In order to understand evapotranspiration, let’s explore what transpiration is in more detail.

What is transpiration?

The release of water vapor (gas) from plant leaves.

Transpiration has three main steps

1. Roots uptake water from the soil
2. Water moves through plant tissues, serving critical metabolic and physiologic functions in the plant
3. Leaves release water vapor into the air through their stomata

How much water do plants transpire?

Plant transpiration is pretty much an invisible process. Since the water is evaporating from the leaf surfaces, you don't just go out and see the leaves "breathing". Just because you can't see the water doesn't mean it is not being put into the air, though. One way to visualize transpiration is to put a plastic bag around some plant leaves. As this picture shows, transpired water will condense on the inside of the bag (this photo shows transpiration after 1 hour). During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.

Since water vapor also evaporates from the soil, we would have seen even more water vapor captured if we had wrapped the plastic bag around the soil as well.

What affects transpiration?

Plants put down roots into the soil to draw up water and nutrients into its stems and leaves. Some of this water is returned to the air by transpiration. Transpiration rates vary widely depending on weather and other conditions, such as

• Type of plant: Plants transpire water at different rates. Some plants which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants.
• Soil type and saturation: Clay particles are small (smaller than 0.002 mm), holding onto water whereas sand particles which are large (0.05-2 mm) release water readily (think of how water disappears into the sand quickly at the beach). When moisture is lacking, plants can begin to senesce (premature aging, which can result in leaf loss) and transpire less water.
• Sunlight availability and intensity
• Precipitation: During dry periods, transpiration can contribute to the loss of moisture in the upper soil zone, which can have an effect on vegetation and food-crop fields.
• Humidity: As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.
• Temperature: Transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer due to stronger sunlight and warmer air masses. Higher temperatures cause the plant cells which control the openings (stoma) where water is released to the atmosphere to open, whereas colder temperatures cause the openings to close.
• Wind & air movement: Increased movement of the air around a plant will result in a higher transpiration rate. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.
• Land slope

How does transpiration affect groundwater?

In many places, plant roots are found in the top layer of soil, above the water table. The top layer of soil is often wet to some extent, but is not totally saturated. Soil below the water table is very wet.

The top layer of soil gets wet when it rains (a form of precipitation), but if there is no more precipitation, the soil will dry out. Therefore, the plants are dependent on water supplied by precipitation since the water table is usually below the depth of the plant roots.

As this diagram shows, in places where the water table is near the land surface, such as next to lakes and oceans, plant roots can penetrate into the saturated zone below the water table, allowing the plants to transpire water directly from the groundwater system. Here, transpiration of groundwater commonly results in a drawdown of the water table much like the effect of a pumped well (cone of depression—the dotted line surrounding the plant roots in the diagram).

Sources and more information

• Evapotranspiration and Droughts - Hanson, R.L., 1991, Evapotranspiration and Droughts, in Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., Compilers, National Water Summary 1988-89--Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 99-104.