Evapotranspiration

The evapotranspiration corresponds to the quantity of total Eau transferred from the ground towards the atmosphere by the evaporation to the level of the ground and by the Transpiration of the Plante S. It corresponds to the flow of Latent heat in the assessment of energy:

RN (Radiation Net) + H ( $\ sensitive Phi$ of heat) + IT ( $ETR$ ) + G ( $\ Phi$ of heat in the ground) + Δ CO ² + Δ Mr. = 0 (the delta being very weak taking into consideration term and precision other of the measurements made by the sensors).

the symbols used are American.

The Net radiation is measured by a Pyrradiomètre. The heat flow in the ground is measured by a Fluxmètre. The significant and latent heat flows are calculated starting from differential measurements of room temperature and wet of Psychromètre S placed at two level above vegetable cover.

It is commonly represented by the initials $ET$ .
It can be potential ( $ETP$ ), or real ( $ETR$ ).
It can be maximum $ETM$ = $ETm$ = $ETP$ X Kc (Kc being a farming coefficient)

The $ETP$ either is measured, or calculated starting from various data weather (speed of the wind, hygroscopy, temperature, etc). There exist several methods of calculating ( $ETP$ Turkish, $ETP$ Penman-Monteith).

The $ETR$ corresponds to the latent heat flow of the assessment of energy calculated above a vegetable cover. The apparatuses of calculation of the $ETR$ were the subject of the patent: U.S. Obvious n° 4.599.889. The apparatuses which were industrialized in France starting from this patent are primarily the BEARN and the SAMER S. Mr. used in HAPEX MOBILHY (1986) and with the the United States the AquaSaver station. Prize winner of the Ministry for Research in 2001, Christian de Pescara proposed a new apparatus allowing to answer the questions: How much water (ETR cumulated every 24:00)? When is it necessary to irrigate (temperature of surface)? The knowledge of the ETR is paramount because it translates the interaction of the hydrous complex: Ground-Plant-climate

Various expressions of the evapotranspiration

ETp: It is usually defined as the sum of evaporation by the surface of the ground and perspiration by the foliage of a culture whose stomata are entirely opened, when the ground provides all required water. It is a theoretical value, calculated by formulas starting from measurements on a park weather.

This concept of potential water consumption was introduced by Howard Latimer Penman in 1948. In France the potential evapotranspiration is calculated on a fescue 7 cm height herd (grass), completely covering the ground, fed well out of water, active phase where are carried out weather measurements. A comparative study of growth and within a sufficiently large piece.

ETp depends only on one particular culture where are carried out weather measurements. A comparative study of the various formulas was carried out within the F.A.O. ( Food and Agriculture Organization off the United Nations ) by the I.NI.A in Portugal. The calculations carried out over several years show a strong divergence.

ETo: Evapotranspiration of reference. It is a limit of ETp used for practical reasons. As a reference, it is measured and calculated on vegetable cover considered. ETo thus corresponds to a potential evaporation under real hydrous conditions. Certain climates, less moderate than Great Britain, make it possible to maintain this fescue of reference, in particular in California for network CIMIS.

ETM: Maximum evapotranspiration. It is the maximum value of the evapotranspiration of a culture given, at a vegetative stage, under climatic conditions given, taken into account by ETp. It is a correction of ETp according to vegetable cover. ETM = Kc X ETP, Kc being the farming coefficient. To determine the farming coefficient, Christian de Pescara proposes the following method: it is necessary to lead the culture to the ETM that one can determine by an apparatus calculating above the piece the ETR or by a Lysimètre. Then we have ETRmax = ETM and we calculate: Kc = ETRmax/ETp. Thus we can gauge the farming coefficients Kc.

Measuring units and orders of magnitude of ETp

As for the measurement of precipitations, the unit is the mm height of water. 1 mm correpond to 1 liter per square meter or 10 cubic meter per hectare. ETp can reach 4-6 mm/jour in full summer in European moderate zone and 6-8 mm/jour in zone méditéranéenne.

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