Pressure affects evaporation
By opening the stomata, the plants control transpiration (stomata resistance). They react to fluctuating water potentials in the guard cells of the stomal openings and also influence the plant's photosynthesis capacity. When the osmotic and hydraulic pressures in the guard cells fall below certain limits, the stomata close completely. These limits depend on the type of plant. The same process takes place in the dark, since photosynthesis cannot take place. The absorption of water from the soil takes place with the stomata closed until the equipotential bonding to the soil water is established (Fig. 4). With the growth of the roots and the above-ground parts of the plant, the water potentials of the soil, roots and leaves initially increase during the vegetation period.
The transpiration of different plant species shows significant differences. Wild plants are adapted to the prevailing water and evaporation conditions in their natural locations. Through agricultural-technical measures (water tillage, irrigation, drainage) more favorable water conditions can be created for the cultivated plants. The maximum transpiration occurs when there is sufficient water available (low water tension). Under defined conditions it is called potential perspiration. These conditions are usually present in winter and early spring. With low water tension in the soil (high soil moisture after heavy rainfall) or with low evaporation requirements of the air, such conditions can also occur in summer.
If the plant-based dehydration in the main root space is not compensated by the access of precipitation or capillary rising groundwater, the soil water tension increases in the immediate vicinity of the roots. As a result, there is a reduction in the matrix potential. This reduces the potential gradient towards the roots. At the same time, the water conductivity of the soil decreases. Both of these result in a reduction in the supply of water to the roots. The amount of water released into the air by the plant population can no longer reach the amount that would correspond to the evaporation requirements of the air. The relative transpiration rate with increasing soil water tension (i.e. decreasing soil moisture) decreases the more the higher the potential transpiration. In very humid, mostly waterlogged locations, transpiration can also be restricted if the gas exchange between the roots is impeded in the case of crops that have not adapted. On the other hand, plants that are physiologically adapted to moist locations, such as reeds, only achieve their potential transpiration there. In the case of vegetated areas, in addition to transpiration, there is also evaporation from the top soil layer (evaporation) and, if necessary, the parts of the plant that are moist from precipitation (interception evaporation).
It is thus clear that the physical evaporation process in the system consisting of soil, plants and atmosphere depends on a number of external parameters which influence one another. As a result, the determination of the real evaporation from land surfaces becomes a complex problem, the exact solution of which is not possible with simple means. [HJL]
Evaporation process 1: Schematic representation of the flow and movement of water molecules over a water surface with TO= Temperature of the water surface, TL.= Temperature of the overlying air layer, eL.= Water dam pressure of the air, es= Water vapor pressure on the water surface (saturation vapor pressure). Evaporation process 1:
Evaporation process 2: Schematic representation of the daily cycle of the water potentials of the soil (ψm), Roots (ψW.), Scroll (ψB.) and air (ψL.) for five consecutive days without precipitation (day: TMax= 22 ° C, relative humidity = 36%; Night: Tmin= 10ºC, relative humidity = 80%). Evaporation process 2:
Evaporation process 3: Water absorption and transport path of water through the root system of a plant. Evaporation process 3:
Evaporation process 4: Schematic representation of the water release of a plant from the leaves with a detailed representation of the stomata apparatus. Evaporation process 4:
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