Evapotranspiration
Evapotranspiration called in meteorology the sum of transpiration and evaporation , so the evaporation of water from animal - and plant life as well as soil and water surfaces. The evapotranspiration value plays an important role in hydrology, as well as in agriculture and horticulture, and in calculating global water requirements .
Until recently, this demand was significantly underestimated: according to new analyzes and data collections, the global freshwater demand is estimated at 4,370 km³ annually (2015), with the limit of sustainable use being given at 4,000 km³ ( see also World Exhaustion Day ); the proportion of evapotranspiration is now assumed to be around 20%.
The amount of heat required for evaporation (transition from a liquid to a gaseous state of aggregation ) is 2257 kJ / kg (20 ° C); it is released again in the reverse process, the condensation .
Factors
- Water content of the soil
- Intensity (biomass, production) and species structure (some plants evaporate more than others) of the vegetation
- Covering the ground and exposure to sunlight
- humidity
- Temperature of the earth or water surface
- Temperature of the air layers close to the ground
- Wind speed on the earth's surface
Instrumental measurement options
Evaporimeter
Evaporation measurements are carried out with atmometers . One way of measuring evaporation is the so-called class-a-pan ( evaporimeter or evaporation boiler ), a round, water-filled measuring boiler for measuring the potential evaporation from land areas (land evaporation boiler).
This evaporation measurement method used by the US Meteorological Service, the US Weather Bureau, consists of a flat, water-filled, cylindrical shape attached to a wooden lattice frame. The water level is at least 15 cm above the surface of the earth, so that the water surface is about 50 cm high. The evaporimeter has an area of 1 m². The water is exposed to evaporation in the free atmosphere; the loss of water is recorded every 24 hours. B. measured by pressure sensors.
With this measurement method, the so-called potential evapotranspiration (ETp) can be measured. It indicates how much water would evaporate under given circumstances if there was enough water. Correction factors have been developed for real applications, for example in irrigated agriculture . However, this measuring method is not very suitable for a statement about real evaporation. The water surface is a kind of "island" on the land: due to the fact that unsaturated air is constantly being brought in and the water in the basin is more willing to evaporate (compared to ground water), no conclusions can be drawn about the real evaporation (ETR) from the soil become. The water surface is not representative enough of land evaporation. However, due to its standardized size, the class-A-pan is good for comparing the ETP of different areas. Parallel to the evaporation measurement with the class-A-pan, the precipitation must also be measured in order to register the amount of water entered into the tub.
Piche atmometers or Czeratzki atmometers are used for evaporation measurements that take wind influence into account.
Lysimeter
A lysimeter can also be used for the measurement.
FAO-56
A widely used method for measuring evapotranspiration is the calculation from the above-mentioned climatic factors using a method recommended by the FAO (“modified Penman- Monteith” formula, FAO-56). The necessary parameters - air temperature and relative humidity, wind speed and solar radiation - are recorded by a weather station and processed in a computer model. This so-called reference evaporation (ETo) corresponds to the evaporation as it would occur on a flat surface evenly overgrown with grass. Since this value is also of little significance for the respective culture, culture-specific correction values are used. With their help, one arrives at the culture-specific evapotranspiration (ETc), which, taking into account the respective phenological phase of the plants, delivers a quite reliable value. The calculation of evaporation is of practical importance, especially for the adequate control of irrigation systems, which are supposed to re-supply the correct amount of water to the soil and the plant depending on evaporation and growth requirements.
foil
It is particularly time-consuming, but somewhat more precise, to completely seal a plant or a location with a film. The water evaporating into the film is collected and measured. It should be noted here that the film naturally forms a not inconsiderable influencing factor.
Potential evapotranspiration
As potential evapotranspiration (ET P ) or also potential landscape evaporation (pLV), the amount of water that can evaporate or evapotranspire under certain climatic conditions is given in information on the climate . The difference between precipitation and potential evapotranspiration results in the climatic water balance .
Individual evidence
- ↑ a b Deutschlandfunk.de , Research News , December 3, 2015, Dagmar Röhrlich : Resources scarcer than expected (last accessed: December 3, 2015) According to: Science , December 4, 2015, 1248–1251, Fernando Jaramillo, Georgia Destouni: Local flow regulation and irrigation raise global human water consumption and footprint , sciencemag.org: Abstract ( Memento of the original from December 20, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (Last accessed: December 3, 2015)
- ↑ Harald Schrödter: Evaporation: Application-oriented measuring methods and determination methods. Springer, 2013. ISBN 978-3-642-70434-5 . Cape. 3.8.2 on p. 48.
