Geoscience Reference
In-Depth Information
- Remote sensing can be used to work out the emission and reflection
capabilities of leaved surfaces. This is made possible by using different wave
lengths or combinations of different wave lengths, which lead to the creation of
indexes such as the Normalized Difference Vegetation Index (NDVI), which is the
most common index that exists. The emission and reflection capabilities of leaved
surfaces are linked to the chlorophyll that exists in the leaves. The photosynthesis
activity is associated with the presence of water and for this chemical reaction to
take place, water is needed:
CO
2
+ H
2
O + energy -> C
6
H
12
O
6
This method has not, however, become very widespread for several different
reasons, including the fact that the orbiting satellites used to take the measurements
do not pass very frequently, a clear sky is required so that the layers of vegetation
on the ground can be seen, and the correlation that exists between the signal that is
received by the satellite's sensor and the dehydration rate of the leaves is not very
strong. As a matter of fact, other factors can also influence the dehydration rate of
the leaves, such as insect attacks.
- If we now take a closer look at the water reserves of the soil the following can
be noted: if we use the principle that there is a strong relationship that exists
between the water content of the soil and the water content of the plant, the idea is
then to evaluate water reserves, on a regional level in order to avoid any difficulties
that might be linked to spatial variability. The soil is considered as being made up of
one or several water reservoirs that are filled up by rainfall, which are emptied
owing to the process of evapotranspiration. The term water reserve is used to refer
to the plants useful water reserve, the one that belongs to the soils and the ground
and does not refer to phreatic water.
If we take the situation of when the soil's water reserve is full (generally in
winter time), it is possible to calculate how the water reserve is emptied by using a
decreasing exponential function. This process is much more progressive compared
with the one which occurs concerning the filling of soil water reserve by rainfalls.
The water from rainfall is considered as being entirely seeped into the soil. The
removal of run-off water is not validated for a specific given moment, however,
different studies have shown that the water budget, which is calculated monthly or
over a period of 10 days, is validated. The daily needs of people have influenced the
results of the water budget, initially not scheduled for a daily time scale. But other
factors also influence these results as the way in which potential evapotranspiration
is calculated, or the spatial distribution of real soil water content. On an area of
several kilometers, this approach leads to satisfactory results during the growing
season, in other words from spring time to the beginning of autumn. From the
month of September onwards the nights become a lot colder and, therefore, the
temperature of the air becomes much closer to the dew point temperature, at least on
the surface of the ground. The resulting condensation covers the grass and the other
materials on the ground with dew, which, in turn, reduces the risk of a fire
