Here is an essay on ‘Evapotranspiration’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Evapotranspiration’ especially written for school and college students.
Essay # 1. Meaning of Evapotranspiration:
The total amount of water lost from the field by both soil evaporation and plant transpiration is called evapotranspiration (ET).
Evaporation is an energy-dependent process involving a change in state from a liquid to a vapor phase. The rate of transpiration is a function of the vapor pressure gradient, the resistance to flow, and the ability of the plant and soil to transport water to the transpiration sites.
Transpiration provides the major driving force for plant water absorption against the gravitational pull and frictional resistances in the water pathway through the plant. The uptake rate is controlled primarily by the transpiration rate; root pressure, the active absorption of water, plays a minor role in absorption and is only evident when transpiration is low or ceases.
Essay # 2. Potential Evapotranspiration:
Potential ET is a combination of evaporation and transpiration with the soil surface completely covered with vegetation and abundant moisture. It can be estimated by open-pan evaporation. Most crops do not stay at the potential ET throughout their life cycles because there are times when they do not have a full plant canopy and/or the soil is unable to supply the water to replace transpiration.
Annual crops start out with very little leaf area and increase it through the growing season. Since crop plants grow fastest under warm temperatures and high solar radiation and since atmospheric demand is greatest with these conditions, high leaf area occurs during the peak of potential ET. This usually causes the peak water demand to occur in mid-summer.
When potential evapotranspiration is compared with precipitation, it is apparent why moisture deficits often occur during the period of most rapid growth rate. For high yields, the crop must be supplied with water during this period.
This can be done either by sufficient storage of soil moisture to supply crops during the deficit period or by irrigation. In many farming areas, the most productive soils are those with a high water-storage capacity, which allows crops to keep producing during periods when precipitation is less than ET.
Essay # 3. Environmental Factors Affecting Evapotranspiration:
Water loss to the atmosphere is determined by both environmental and plant factors. The environmental effect on ET is called atmospheric demand or evaporatory demand. The greater the atmospheric demand, the faster water can be evaporated from a free water surface.
The following factors influence atmospheric demand:
1. Solar Radiation:
Of solar radiation absorbed by the leaf, 1 to 5% is used for photosynthesis and 75 to 85% is used to heat the leaf and for transpiration. Increased solar radiation increases atmospheric demand.
2. Temperature:
Increasing the temperature increases the capacity of air to hold water, which means a greater atmospheric demand.
3. Relative Humidity:
The greater the water content of air, the higher the ψair, which means that atmospheric demand decreases with increasing relative humidity.
4. Wind:
Transpiration occurs when water diffuses through the stomata. A diffusion gradient barrier builds up around stomata when the air is still. This means that water diffusing from the wet leaf interiors is almost matched by water buildup outside the leaf, which reduces the diffusion gradient and thus reduces transpiration. When turbulence (wind) removes the moisture next to the leaf, the difference in water potential inside and immediately outside the stomata) opening is increased and net water diffusion from the leaf is increased.
Climatologists measure atmospheric demand by determining the amount of water evaporation from an open pan. The highest atmospheric demand occurs at the time of year when solar radiation and temperatures are greatest.
Plant factors, as well as atmospheric demand, modify the ET rate by affecting the resistance to water movement from soil to air.
Essay # 4. Plant Factors Affecting Evapotranspiration:
1. Stomatal Closure:
Most transpiration occurs through stomata because of the relative impermeability of the cuticle, and little transpiration occurs when stomata are closed. As stomata open wider, more water is lost, but the loss increase is less for each unit increase in stomatal width.
Many factors influence stomatal opening and closing, the major ones under field conditions being light and moisture level. In most crop plants light causes stomata to open. A low moisture level in the leaf (low ψleaf) causes guard cells to lose their turgor, resulting in stomatal closure.
2. Stomatal Number and Size:
Most leaves of productive crops have many stomata on both sides of their leaves. Stomatal number and size, which are affected by both genotype and environment, have much less effect on total transpiration than stomatal opening and closing.
3. Leaf Amount:
The more leaf surface area, the greater the ET. The amount of ET, compared with open-pan evaporation, increases. However, there is less increase in water loss for each unit increase in the LAI. There are some indications that ET will not increase with increases in the LAI over that required to intercept 80% of solar radiation.
4. Leaf Rolling or Folding:
Many plants have mechanisms in leaves that favor reduced transpiration when water becomes limiting. Some grass species, like maize, reduce their exposed leaf area by leaf rolling, while many others, like bluegrass, reduce exposed leaf area by leaf folding. Broad leaves have other mechanisms to reduce water loss; for example, soybean has a tendency to roll the leaves over so the silvery pubescence (hair) on the exposed lower surface can reflect more light.
5. Root Depth and Proliferation:
The availability and extraction of soil moisture by the crop is highly dependent on rooting depth and proliferation. Deeper rooting increases water availability, and root proliferation (roots per unit soil volume) increases water extraction from a unit volume of soil before permanent wilting occurs.
Knowing how environment and the plant influence ET helps to explain the daily pattern of ET in the field. Stomata open in response to light, and ET increases with increases in solar radiation and air temperature. If atmospheric demand does not become greater than the ability of the plant to supply water to leaves, the greatest ET will occur during the afternoon when air temperatures are highest. Daily ET starts to decrease in the late afternoon, primarily because of less light energy and decreasing temperature.
Under high soil water, ET would usually increase with increased atmospheric demand. Limiting soil moisture, however, causes changes in the relationships among atmospheric demand, soil moisture, stomatal closure, and rate of water flow through the plant. As moisture level in the soil is reduced, the ET level of a high atmospheric demand day (clear, dry) is reduced to a level similar to that of a lower atmospheric demand day (partly cloudy, humid).
This is probably caused by stomatal closure or by increased resistance to translocation during the afternoon of high, but not low, atmospheric demand days. In other words, when moisture is limiting in the soil in a high atmospheric demand day, the leaves are losing water more rapidly than the roots or the translocation system can supply it.
This results in a ψleaf low enough to cause stomatal closure and/or results in slower water absorption and movement, due to increased resistance within the soil and plant.
On the lower demand day, water uptake by roots can keep up with the water lost by leaves and so water loss continues unencumbered until a low ψsoil is reached. This illustrates the interaction among atmospheric demand, soil factors, and plant factors that influences the ET rate in the field.
No comments yet.