Note: Content may be edited for style and length. Science News. Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2. ScienceDaily, 2 April Vegetation controls the future of the water cycle: Engineers demonstrate the key role of vegetation in responding to rising CO2 levels and show how plants will regulate future dryness.

Retrieved November 10, from www. You should remove them'. This notion has for many years determined how streams were managed to prevent flooding during high rainfall 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. 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. The water eventually is released to the atmosphere as vapor via the plant's stomata — tiny, closeable, pore-like structures on the surfaces of leaves. Overall, this uptake of water at the roots, transport of water through plant tissues, and release of vapor by leaves is known as transpiration. Water also evaporates directly into the atmosphere from soil in the vicinity of the plant.

Any dew or droplets of water present on stems and leaves of the plant eventually evaporates as well. Scientists refer to the combination of evaporation and transpiration as evapotranspiration, abbreviated ET. If you search for the definition of evapotranspiration, you will find that it varies. In general, evapotranspiration is the sum of evaporation and transpiration.

Some definitions include evaporation from surface-water bodies , even the oceans. But, since we have a Web page just about evaporation, our definition of evapotranspiration will not include evaporation from surface water. Here, evapotranspiration is defined as the water lost to the atmosphere from the ground surface, evaporation from the capillary fringe of the groundwater table , and the transpiration of groundwater by plants whose roots tap the capillary fringe of the groundwater table.

The banner at the top of this page offers an even more simple definition. The transpiration aspect of evapotranspiration is essentially evaporation of water from plant leaves. Studies have revealed that transpiration accounts for about 10 percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water lakes , rivers, streams providing nearly 90 percent, and a tiny amount coming from sublimation ice changing into water vapor without first becoming liquid.

Just as you release water vapor when you breathe, plants do, too — although the term "transpire" is more appropriate than "breathe. If the bag had been wrapped around the soil below it, too, then even more water vapor would have been released, as water also evaporates from the soil.

Plants put down roots into the soil to draw water and nutrients up into the stems and leaves. Some of this water is returned to the air by transpiration. Transpiration rates vary widely depending on weather conditions, such as temperature, humidity, sunlight availability and intensity, precipitation, soil type and saturation, wind, and land slope.

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. After a plastic bag is wrapped around part of a plant, the inside of the bag becomes misty with transpired water vapor. 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. It is an important part of many cells and life processes.

As part of these life processes, nitrogen is transformed from one chemical form to another. For example, it is needed to make proteins, DNA, RNA and chlorophyll, which plants need to be able to carry out photosynthesis to make food. The transformations that nitrogen undergoes as it moves between the atmosphere, water, the land and living organisms make up the nitrogen cycle.

Plants take up nitrogen compounds through their roots. Animals obtain these compounds when they eat the plants. When plants and animals die or when animals excrete wastes, the nitrogen compounds in the organic matter re-enter the soil.

A significant problem facing New Zealand is the leaching of nitrogen compounds from the land into our waterways. Because nitrogen supports plant and algal growth, the increased levels of nitrogen in the waterways can cause unexpected — and even harmful — effects in the ecosystem such as algal blooms. Riparian planting of trees and other plants can have a positive effect on mitigating nitrogen run-off.

This mitigation is achieved both through the increased uptake of nitrogen compounds by tree roots and the physical barrier that plants create — preventing the nitrogen from getting washed into the stream or river with sediments.

There are several activities related to the concept of cycles. These include Carbon cycle quiz , Building a water cycle and Nitrification and denitrification. A large number of articles, interactive images and PLD sessions provide further information on natural cycles.