Geoscience Reference
In-Depth Information
To give an approximate figure, the mean annual temperature drops by
one Celsius degree for every 100-km movement northward or climbing
180 metres in altitude (0.5 to 0.6 degree or so for 100 m).
On these bases, and after having mapped millions of hectares with
his students, Dokuchaev constructed his classification of soils. Since
then, the world over, observations have been made that confirm his
ideas.
2.1.2 Mode of Intervention of Climate
Several climatic factors intervene in the genesis of soils, in particular
rainfall, evapotranspiration and temperature.
The quantity 'P - PET', where P is the annual rainfall and PET the
potential evapotranspiration
is termed climatic water balance. Under this
crude term is hidden the quantity of water that a grass-covered soil
permits to return to the atmosphere directly, mostly through transpiration
by plants. As a first approximation there are two cases:
If P - PET > 0, water is available for drainage to the bottom of
the soil; this water could evacuate salts of calcium, sodium, etc.
The soils will tend to be depleted and acidified.
If P - PET < 0, salts cannot easily be evacuated; if the water
deficit is large, the very soluble salts will not move; there is
risk of 'salinization' by mechanisms that will be discussed in
Chapter 13.
Actually, the quantity P - PET does not give precise indications.
In the Mediterranean region, in southern France for example, P - PET
is negative on the annual scale, but, as the rainfall is irregular, the
soil finds itself 'washed' in a few winter showers and salts do not
accumulate.
To set up a more precise argument, it is necessary to calculate a true
water balance by taking into account the additions of water, the transfers
within the soil and losses by evaporation, by root absorption and deep
drainage (beginning of Chap. 3). Recourse to computer simulation is
called for in order to work at hourly or daily lets.
Climatic water balance
Weathering of minerals will be a function of the soil microclimate
(temperature, water content). These two parameters are related to general
Temperature and water content
