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Evapotranspiration represents the part of precipitation that is both
evaporated from the soil and transpired by vegetation. Depending on
conditions, this can vary from about 10% to about 75% of rainfall.
Evaporation of rain falling on the soil depends on the rate that the
water is absorbed into the ground, the temperature of the soil and of the
rain, the humidity of the air etc. To take an extreme example, the
evaporation of rain falling onto a hot road surface without good drainage
may reach almost 100%. At the other extreme, rain falling on coarse sand
under cool conditions would hardly evaporate to any significant extent. The
presence of vegetation can have a double effect. It can prevent some of the
rain from reaching the soil, in which case the evaporation may be greater;
on the other hand, it can also inhibit the evaporation of rain having
reached the soil. Much depends on the nature of the vegetation.
Transpiration of vegetation is also a two way process. Some rain is
absorbed into the plants directly by the leaves and some is absorbed from
the soil by means of the roots. The amount that evaporates through
transpiration is very variable according to the type of plant and the
condition of its water balance. Many plants have means to reduce the
transpiration to a minimum, especially those growing in desert regions. On
the other hand, plants which grow under wet conditions may transpire
abundantly.
In Cyprus, we have two general sets of conditions. In summer, there
is little rain but, when a storm breaks, the ground is often too hard to
absorb water, which runs or puddles until it is largely evaporated.
Except where there is ground water or cultivation there is almost no
transpiration from plants, despite low relative humidities and high
temperatures. Pine forests transpire little, compared to deciduous
forests.
In winter, the soil becomes absorbent after the first rains because
of bacterial activity and the water is quickly absorbed before it has a
chance to evaporate. Much of this will be absorbed by plants and they
start to grow. Transpiration will increase as the plants grow, and will
become important towards the end of the rainy season.
In order to be useful, the most rainwater and groundwater can be
collected in the dams from precipitation from the second or third major
rainfall in the autumn through towards the end of February, not counting
melt waters from snow on the Troodos massif, which are obviously delayed
from earlier falls. March and April rains are less effective due to the
increased transpiration losses.
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