What is meant by perspiration

Evaporation. Physical basics and regional to global distribution


1 Introduction

2. General information on evaporation

3. Physical basics
3.1 Evaporation
3.2 Perspiration

4. Regional evaporation distribution

5. Global evaporation distribution

6. Conclusion

List of figures


List of figures:

Figure 1: The water cycle

Figure 2: Scheme of the relation between evaporation and transpiration

Figure 3: Actual evaporation

Figure 4: Potential evaporation

Figure 5: The land surface process model

Figure 6: Mean annual evaporation in Switzerland

Figure 7: Isolation of mean annual evaporation heights

Figure 8: Relief map of Switzerland

Figure 9: Spatial distribution of the annual mean temperature in Switzerland

Figure 10: Mean annual evaporation worldwide in cm

1 Introduction

Evaporation plays a crucial and important role in the water cycle. In addition to precipitation and runoff, their determination is one of the most important variables in the water balance and has been one of the preferred topics in basic hydrological research for several years (KELLER 1980, 27).

Using the following figure, I would like to briefly go into more detail about the importance that evaporation has in the water cycle.

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Fig. 1: The water cycle, http://www.geolinde.musin.de/glossar/themen/wasserkreislauf.htm

It can be clearly seen that evaporation takes place over the sea as a large free water surface as well as over lakes and rivers. There is also evaporation from areas covered with vegetation and snow. Generally speaking, evaporation occurs on the entire surface of the earth and is therefore extremely important for the balance of the water balance. How evaporation behaves in different areas and which physical fundamentals belong to this process, I will discuss in the present work.

2. General information on evaporation

Evaporation is understood as the transition of the water from the liquid or solid to the gaseous state, whereby the absorption of latent heat is necessary for this. Latent heat means the value of 2453.4 J / g at a temperature of 20 ° C and a constant pressure of 1013 hPa. (WILHELM 1997, 145) The water vapor then passes through the evaporation process from the surface of the water or from the mainland into the atmosphere. (LAUER 1993, 66) In addition to precipitation, the process of evaporation establishes the connection between the separate water resources on earth, which means the presence of water in the sea, the atmosphere and on the mainland. (WECHMANN 1964, 381) Furthermore, it is the amount of evaporation that decides "what remains of the water intake from the precipitation for the runoff and thus for further water use." (KELLER 1980, 27) Overall, it is so, that 64% evaporates on land, on the oceans 103-116% and on the whole earth 100% of the precipitation volume of 973 l / m3. However, most of the evaporated water cannot be managed. However, evaporation is not only an important part of the water balance, but also of the earth's heat balance. Due to the energetic interconnection, it is an extremely important factor for the atmospheric circulation as well as for the climate. (BAUMGARTNER 1990, 327)

3. Physical basics

From a physical point of view, evaporation is defined as “the slow transition of a liquid to a gaseous state below the boiling point”. With regard to water, this means its transition from the liquid or solid state to the form of water vapor. (WECHMANN 1964, 379) In order for evaporation to take place, thermal energy is required, which is obtained from the surrounding air. Because of this, evaporation is a process associated with cooling. When one speaks of sublimation, what is meant is evaporation from surfaces of ice and snow. This type of direct evaporation requires around 10% more energy than that required for evaporation of water, which is 2374 joules per gram. (LAUER 1993, 67)

If one speaks of evaporation, this process can generally be divided into two different types: On the one hand there is evaporation. This means the passive evaporation of the vegetation-free land surface (BENDIX 2004, 110) or the evaporation that takes place wherever water is available in any form. This means ice and water surfaces of all kinds. Evaporation is most important over the large expanses of water in the oceans. (WECHMANN 1964, 381) On the other hand, there is the process of so-called transpiration. This term refers to the active evaporation of the vegetation. (BENDIX 2004, 110) Here, the water "that was needed to transport nutrients out of the soil and in the plant is released through the stomata, the stomata on the underside of the leaves." (WILHELM 1997, 144)

The sum of evaporation and transpiration is called evapotranspiration. It describes the total or land evaporation from overgrown and ungrown surfaces: ET = E + T

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Fig. 2: Scheme of the relation between evaporation and transpiration. LAUER 1993, 67

Figure 2 shows the scheme of the relation between evaporation and transpiration. It becomes clear that the evaporation, after it has reached an evaporation maximum with increasing water supply, drops slightly and then remains steadily the same, whereas the rate of transpiration, although it reaches its maximum later, maintains it longer, and then constantly shows a higher evaporation height than that Evaporation. The evapotranspiration is shown here as the sum of the two processes mentioned above. That the perspiration later her

If the maximum evaporation is reached than the evaporation, the water first has to reach the surface again through the plant from the soil and of course water is also retained in the plant in order to supply its vital functions. In this way, plants only release water to evaporate when there is a greater supply of water than is the case with ungrown areas, since here the water "only" has to be drawn from the ground.

If one also takes into account that part of the precipitation is held back by the foliage in closed plant stands and thus does not reach the ground, but evaporates immediately (= interception), the total evaporation (V) of a vegetated surface is made up of the components of evaporation (E), perspiration (T) and interception (I) together: V = E + T + I.

In general, evaporation and precipitation are measured in mm / water height (= Litemje m[1] ) specified per time unit. (LAUER 1993, 67) A distinction is also made between current and potential evaporation. Current or effective evaporation is understood as the amount of water that actually evaporates from a land area. This depends on the available energy and the available water. The potential evapotranspiration means the climatic evaporation power. This describes the amount of water that would evaporate with a given energy if the land surface were completely covered with plants with sufficient water supply. (WILHELM 1997, 144) "In the evaporation of water surfaces, both quantities generally reach the same value." (LAUER 1993, 68)

Figures 4 and 5 show the real and the potential evaporation as a daily value. It becomes clear here that the potential evaporation represents the maximum possible evaporation under the meteorological boundary conditions, whereas the actual evaporation reflects the value given by the soil Water supplies is possible. The current evaporation is always less than or equal to the potential evaporation. The growth of the plants is always reduced when the actual evaporation is smaller than the potential, as plants have to evaporate water in order to be able to grow. If the evaporation rate is low, the air is heated more and this results in a higher potential evaporation rate. (DWD 2008)

During the day, evaporation is based on the radiation balance, the saturation deficit of the air and the control options of the vegetation. Just like the radiation balance, precipitation and the resulting availability of water in the soil also show significant effects on evaporation. The soil storage, which is filled up by every precipitation, is also emptied in the subsequent dry phases due to continuous evaporation. If dry phases last longer, the vegetation comes under drought stress and adapts its transpiration to the water availability, whereby the evapotranspiration generally decreases. (BENDIX 2004, 110 f.)

3.1. Evaporation

From a purely physical point of view, evaporation is understood to mean "the evaporation that takes place from the bare earth surface (soil, snow, ice evaporation), the precipitation retained on plant surfaces (interception evaporation) and from free water surfaces (water evaporation)." (DYCK 1995, 180) The heat balance of the body of water determines the kinetic process of evaporation of water from a free water surface. The saturation deficit is an important influencing factor and as long as the air is not completely saturated, evaporation from open bodies of water can take place. (DYCK 1995, 180f.) The evaporation of water occurs more strongly, the lower the vapor pressure is. At low pressure and the resulting low density of the air, more water molecules will emerge through the water surface than at higher pressure, as they face greater resistance due to the denser packing of the air molecules. (WECHMANN 1964, 383/387) The removal of the water vapor is regulated by diffusion [1], Convection[2] and by advective air movements [3]. (DYCK 1995, 180f.) The movement of the air causes a direct air transport and thus an exchange of the moist and dry air.


[1] vertically rising air movement of air masses that have been heated by radiation

[2] Dispersion and mixing processes of air components

[3] wind

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