How are plants involved in the water cycle

Earth is called the "Blue Planet" when viewed from space because almost 70% of its crust consists of water bodies. Water is essential for the survival of life on earth, as is evident from the fact that the first life form originated in the seas more than 3.5 billion years ago.

Water evaporates from bodies of water and plants due to the sun’s heat, and rises higher in the atmosphere in the form of water vapor. At cloud level, the cold temperature condenses water vapor into microscopic water droplets. These water droplets accumulate to form rainclouds that cause precipitation on earth, thus, returning water to the earth. This cyclic process of evaporation, transpiration, precipitation and absorption of water is called water cycle.

The water cycle is an important ecological process that maintains the proportion of water in earth’s atmosphere and ecosystems. The water cycle involves cyclic movement of water from water bodies and groundwater into the atmosphere through plants, which play a role in this cycle by photosynthesis and transpiration.
Read more about the water cycle.

What is Transpiration?

Transpiration is the process by which moisture leaves the green plants through small openings in their leaves called stomata. Stomata are present on leaves and stems of plants and are outlets for plants to exchange water and gases.

Root hair present at the terminal ends of roots absorb moisture from surrounding soil and transport it to the leaves through the stem. Leaves release this absorbed moisture into the atmosphere by transpiration.

In dry weather conditions, the stomata expand and open wide to release water vapor during transpiration to keep the plant cool and also pulls up groundwater through their roots to the leaves. During wet and cool weather conditions, the openings of stomata shrink to prevent transpiration, reducing absorption of groundwater through roots.
Read more about two environmental factors that affect transpiration.

Role of Plants in Water Cycle

Plants need water to grow and maintain their structure. They absorb groundwater, that is, water collected below ground level due to percolation of rain water, through their root system. During precipitation, water that falls on the ground is absorbed deep into the soil by plant roots.

Vegetation plays an important role in the water cycle by preventing soil erosion and increasing groundwater levels. In areas with thick vegetation cover, the foliage cover breaks the force of precipitation falling on the ground, which may otherwise cause erosion. Green plants also release water vapor in the air as a by-product of photosynthesis, thus involving groundwater in the water cycle.

Effect of Plants on Water Cycle

The role of plants in the water cycle is easily demonstrated by comparing tropical rainforests and places that have been heavily deforested. Rainforests are densely packed with different types of vegetation, from tall trees to ground level grasses. Plants in such areas have high transpiration rates, and water vapor released from plants is visible in the form of steam rising from the vegetation. This expelled water vapor helps to cool the area and retain moisture.

On the other hand, many urban areas have cut down vast forest land for constructing buildings and procuring raw materials for various manufacturing industries. Lack of forest cover results in soil erosion and depleted groundwater reserves as there are not plant roots to absorb water deep into the soil.

Over time, deforested areas become arid and often get flooded or face droughts. Without plants, there is no way for surface runoff to percolate deep into the ground and therefore, the groundwater level keeps getting depleted. Moreover, no transpiration occurs in deforested area, eventually leading to low moisture content in the atmosphere and dry arid environmental conditions.

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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 

1. Water moves through plant tissues, serving critical metabolic and physiologic functions in the plant 

1. Leaves release water vapor into the air through their stomata 

How much water do plants transpire?

Credit: Ming Kei College

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, USGS, U.S. Global Change Research Program

How do plants help in the water cycle activity?

What role do plants play in the water cycle quizlet?