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Desertification Indicator System for Mediterranean Europe


1. Definition

Name POTENTIAL EVAPOTRANSPIRATION
Brief definition Potential evapotranspiration is the potential evaporation from soils plus transpiration by plants. It only occurs at the potential rate when the water available for this process is non-limiting. The rate of evaporation depends on climatic conditions, specifically the radiative energy of the sun, wind, the vapour deficit of the air, and temperature. Potential evaporation is often calculated from these measurements using the Penman Monteith equation. It can also be estimated from readily available rainfall and temperature data using simple equations such as that of Thornthwaite, and this has in fact been done for more than sixty years. Open pan evaporation measurements are also a source of information. Actual evapo-transpiration is the evaporation that actually takes place and this is strongly influenced by the available soil moisture. The ratio of actual and potential evaporation is an important indicator of aridity.
Unit of measure mm, cm
Spatial scale Local
Temporal scale weeks, months or years

2. Position within the logical framework DPSIR

Type of Indicator Pressure, impact and response

3. Target and political pertinence

Objective This indicator describes the capacity of the prevailing climate to evaporate water from soils, plants, open water or other surfaces. It is a suitable headline indicator as data are readily available and have been used in other contexts for many decades.
Importance with respect to desertification

Potential evapotranspiration is used with precipitation data to calculate water balances. Changes in values clearly enable the impact of land use change to be seen. Potential evaporation transpiration calculations are useful in calculating plant water requirements and also the losses of water to evaporation from open areas such as irrigated fields and reservoirs.

Potential evapotranspiration also depends on the topography and aspect, and these effects can be calculated so that maps of potential evaporation can be linked to a digital elevation model.

Although evaporation is a loss, it is also a necessary requirement for plant growth. Many plants perform better when evaporation occurs at near the potential rate.

Evaporative cooling is very positive and results in much cooler soil conditions. In Spain it was found that the soil temperature in the upper few cms reached about 65 °C during a dry early summer but was only 40 °C during moist years. Evaporation means that there is more plant growth, more organic matter and more shade and these can be positive with respect to desertification

International Conventions and agreements This indicator has already been used in different ways by organisations such as WMO and FAO as it plays a role in climate change and food security.
Secondary objectives of the indicator The indicator can and has been used for many purposes. These include characterising the climate regime of soils and ecosystems, calculating water requirements for irrigation, to calculate the water balance and to estimate water losses from dams.

4. Methodological description and basic definitions

Definitions and basic concepts The basic concepts go back to scientists in Europe and the USA who were investigating the evaporative demand of crops. A group of researchers led by Thornthwaite and Mather developed and promoted the methodology during the 1950s. Global data bases are available and calculations are not difficult.
Benchmarks Indication of the values/ranges of value The potential evaporation in the Mediterranean varies monthly according to the climate. The values should be averaged for ten day periods. Values probably reach up to 12 mm day in the summer. Recently, it has been demonstrated that evaporative losses are reduced by atmospheric pollution. Potential evaporation rates today maybe ten per cent lower than fifty years ago because of pollution.
Methods of measurement Calculations should be made according to the procedure described by Thornthwaite. Published values can be used. There is a vast literature available to the user.
Limits of the indicator Many hydrologists have shown that there may be better ways of estimating potential evaporation. However, because only simple temperature and precipitation data are needed this is a great advantage for an indicator. For scientific research other more precise methods should be used.
Linkages with other indicators Infiltration capacity, Parent material, Soil crusting, Soil erosion (USLE), Soil quality index, Soil texture, Water storage capacity, Runoff water storage, Water availability.

5. Evaluation of data needs and availability

Data required to calculate the indicator

Daily or weekly measurements of temperature and rainfall. An equation to calculate the model can be downloaded from:

http://www.simulistics.com/examples/catalogue/modeldescription.php?Id=Thornthwaite

Potential evapo-transpiration (mm/day),

PET = if Ta > 0 then dl*16*(10*Ta/I)^a
else 0

where:

Ta is mean monthly temperature (Celsius)
a = 0.49+0.079*I-7.71*10^ -5*I^2+6.75*10^ -7*I^3
dl = daylength in hours / 12
I = sum(i) i is a monthly heat index given by

i = if Ta>0 then (Ta/5)^1.5

else 0

Data sources Meteorological services
Availability of data from national and international sources Data could be provided by many organisations using it for other purposes (e.g. WMO, FAO)

6. Institutions that have participated in developing the indicator

Main institutions responsible Developed by Thornthwaite (USA), described as a desertification indicator by Foundation for Sustainable Development, Netherlands
Other contributing organizations  

7. Additional information

Bibliography

Thornthwaite CW. The water balance." (with J. R. Mather) Publ. in Climatology, 8:1-104, 1955.

"Instructions and tables for computing potential evapotranspiration and the water balance." (with J. R. Mather and D. B. Carter) Publ. in Cli The determination of evaporation from land and water surfaces." (with Benjamin Holzman) Monthly Weather Review, 67:4-11, 1939.

http://www.simulistics.com/examples/catalogue/modeldescription.php?Id=Thornthwaite

Other references

Amongst the papers by Thornthwaite are:

"The role of evaporation in the hydrologic cycle." Transactions of the American Geophysical Union, 20:680-86, 1939.

"Atmospheric moisture in relation to ecological problems." Ecology, 21:17-28, 1940.

"Atlas of climatic types in the United States 1900-1939." U. S. Dept. of Agri. Miscellaneous Publications No. 421, 9 pp. text, 48 pp. maps, 1941.

Contacts Name and address

A.C. Imeson

Foundation for Sustainable Development (3D-EC)

3de@hetnet.nl