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.