4 Conclusions
The results of the present study have the potential to be used in
hydrological modelling and for a better understanding of
evapotranspiration patterns in both investigated areas. They reveal that
the annual amplitude of ET0 is very similar in both
semiarid regions under study, even if values are higher in the Caatinga
forest. They, moreover, demonstrate that actual evapotranspiration
(ETa) is low in the Caatinga forest when
ET0 presents its highest values throughout the year, and
vice versa, due to the annual distribution of precipitation. In the
Pinares forest, on the other hand, ETa follows the
ET0 up and down dynamics during the year, though with a
lesser degree during summer.
ETa in denser vegetation areas is more dependent on
meteorological factors (related to atmospheric demand for humidity) than
on vegetation. This behaviour is common to both forests. The irregular
distribution of vegetation density, the presence of reservoir water or
rivers and pixels of exposed soil from roads and badlands influenced the
values of maximum and minimum ETa. The Caatinga forest
showed a greater spatial variation of ETa than the
Pinares forest. The zones of a lower and higher Temporal Stability Index
(TSI) of ETa are always the same in both areas
throughout the year, what changes from season to season is the
intensity. The areas with the highest TSI are those that respond most to
differences in soil moisture. In the Caatinga forest these are the
highest areas of denser vegetation with their huge capacity to expand
leaf mass and increase evapotranspiration, when soil moisture rises
after precipitation. The Caatinga forest presents a greater extension
with lower temporal stability than the Pinares forest.
It is important to mention that semiarid forests may have their water
balance greatly affected by climate changes, due to temporal trends that
could also be observed in the present work: in the Caatinga forest,
potential evapotranspiration (ET0) shows a trend to
annually increase by 3.5 mm. Likewise, ETa demonstrated
a significant positive annual trend of 2.2 mm. The Pinares forest also
presents positive annual trends for ET0 and
ETa: 7.0 mm and 3.9 mm, respectively. Finally, we found
out that annual ET0 and ETa in Caatinga
were almost twice than in Pinares forest, which could be related with
the latitude of these regions.