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
|
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