Methods and Results
|CORRECTING CLOUDY IMAGES
The main factor
determining evapotranspiration is the net amount of radiation, which
is modulated by the presence or absence of clouds.
Meteostats visible and infrared spectral ranges can only provide
information about the earth surface where there is no cloud cover
(otherwise, the measured radiances are those reflected from the
top of the clouds, not the earths surface). Therefore, the
first step is detecting clouds. An algorithm is used to separate
the pixels of the images into clear sky and clouds. The cloudy pixels
are further classified as representing low, intermediate or high
amounts of cloud cover.
Where there is no
cloud cover, the raw reflectance data of the earths surface
must be corrected for the influence of the atmosphere using radiative
transfer models, with both radiosonde and synoptic data furnishing
supporting meteorological information. For cloudy pixels, an indirect
method has been developed which exploits the results of the cloud
classification. Net radiation is assessed for both clear and cloudy
a reference micro-meteorological station are first assessed to determine
the relation between the net surface radiation and the evapotranspiration.
It is assumed that this relation, which is linked to the available
soil humidity, is representative for the entire Belgian territory.
Finally, the evapotranspiration is then computed for each pixel.
Because evapotranspiration depends on the
annual and daily cycles of the solar energy and is modulated by
the presence of clouds, it is important to take all available high-frequency
meteorological data into account.
|THE SATELLITE WATCHES ALWAYS AND
The data obtained
by this method can be used in several ways:
- to analyse the temporal variability of evapotranspiration (how
does it change with time);
- to investigate the spatial distribution of evapotranspiration
(how much in different places);
- to validate the algorithms used in meteorological and climate
- to study the water budget at different spatial scales.
Analysis of the hourly
evapotranspiration rates shows that successive images may be almost
uniform or, on the contrary, differ widely depending on the position
and evolution of cloudy areas. This spatial information could never
be obtained by a simple spatial interpolation of evapotranspiration
values computed in a few meteorological stations, but can only be
achieved using geostationary satellite data. Temporal mean results
of the evapotranspiration over the daylight period or over entire
months may then be calculated by integrating the hourly evapotranspiration
Daily total of ETR (mm d-1)
Figures 1 to 5 show the daily total evapotranspiration values for
four successive days in August and September 1997. Cloud cover was
high on August 30th (2) and thus the evapotranspiration values remain
very low all over the country. By contrast, the next day was sunnier
and higher values (around 2.5 to 3.5 mm d-1) were attained. Only
the eastern part of the country had good weather on September 1st
(4), which explains the greater evapotranspiration values for this
region. The following day (5), evapotranspiration was more intense
in the western part, with the highest values obtained at the seacoast.
over Belgium. Monthly mean daylight totals of the hourly mean values
The figure shows the result of the space and time integration of hourly
images over entire months from 1994 to 1997. The monthly means of
daily evapotranspiration totals are obviously marked by the annual
cycle. Fair weather in spring 1994 and 1995 accounted for the mean
values ranging from 1 to 1.5 mm d-1. The highest mean value, nearly
3.5 mm d-1, was obtained in July 1995.
|WHY MEASURE EVAPOTRANSPIRATION
Since July 1995,
this fast method of evapotranspiration assessment over Belgium has
been performed daily by means of an automatic computer procedure
using data from the previous day.
The data thus obtained can be used in a number of fields:
On a local scale,
evapotranspiration data can be used to calculate the irrigation
needs for crops in dry periods. Matching irrigation and rainfall
amounts to crop evapotranspiration can be compared to transactions
performed on a bank account. The soil is the "bank" for
holding water. Rainfall and irrigation are deposits into the account,
while evapotranspiration represents withdrawals from the account.
This approach has even been called "checkbook" irrigation
On a larger scale, evapotranspiration integrating
the effects of several meteorological variables, is a complex key
variable of the hydrological and meteorological models (i.e. important
for weather forecasting and climate studies). Furthermore, possible
climate changes are likely to result in evapotranspiration changes
and feedback between the atmosphere, the soil and the vegetation.
Such changes may very well forebode major shifts in the distribution
and abundance of vegetation communities.