1. Introduction
The continuous increase in water demand is subject of concern (Cosgrove
& Loucks, 2015; Boretti & Rosa, 2019) and demands efficient management
systems, particularly in semiarid regions (Sun et al., 2019), which
account for 15% of the global land area and feed 14% of the global
population. Besides, semiarid ecosystems are very fragile and sensitive
to climate change (Safriel & Adeel, 2005).
The Brazilian semiarid zone is characterized by an extremely negative
atmospheric water balance (annual precipitation of 549 mm against
potential evaporation of 2,500 mm) and a considerable spatial and
temporal variability in water supply. This area presents only two annual
seasons: dry and rainy (de Araújo & Piedra, 2009; de Figueiredo, de
Araújo, Medeiros, & Costa, 2016; Pinheiro, Metselaar, Lier, & Araújo,
2016). These features contrast with other semiarid areas, such as the
Spanish Northern Plateau, where precipitation distribution is relatively
uniform throughout the year and presents well-defined seasons (Calama et
al., 2019). These differences may lead to a distinct spatial and
temporal behaviour of the actual evapotranspiration
(ETa) in those semiarid areas.
Evapotranspiration (ET) is one of the most challenging components of the
water balance equation to be measured (Castelli et al., 2018). It can be
quantified using lysimeters or aerodynamic methods (e.g., Bowen ratio
and Eddy Covariance), but such methods only represent the specific area
around the experimental setup. The spatial variation of
evapotranspiration on a large scale can be better captured by models
based on remote sensing (Jaafar & Ahmad, 2020). The high spatial
resolution and long registration period of a USGS Landsat permits to
assess a spatiotemporal ET series of vegetated and non-vegetated
surfaces (Chen & Liu, 2020; Jaafar & Ahmad, 2020).
Thermal-energy balance models that use remote sensing, such as SEBAL
(Surface Energy Balance Algorithm for Land), have been successfully
applied to assess ETa at field scale in many areas of
the world. Teixeira, Bastiaanssen, Ahmad, and Bos (2009) applied SEBAL
to estimate evapotranspiration in irrigated agriculture and Caatinga
vegetation in the semiarid region of the São Francisco River basin,
north-eastern Brazil; Losgedaragh and Rahimzadegan (2018) used it to
compute evaporation from the freshwater bodies; the same authors (2018),
as well as Rahimzadegan and Janani (2019), applied SEBAL to agricultural
lands in a semiarid climate in Iran; Senkondo, Munishi, Tumbo, Nobert,
and Lyon (2019) used it in a sub-tropical region in Tanzania’s Kilombero
Valley; whereas Gobbo et al. (2019) and Grosso et al. (2018) applied it
to assess ET at irrigated and non-irrigated maize fields in Italy.
In the years to come, a significant reduction of precipitation and a
temperature increase can be expected over global typical semiarid
regions as a result of climate change (Yang, Zhang, Hao, & Yue, 2019).
This trend may influence both the atmospheric and vegetation water
demand, that is, the ETa. Therefore, it is crucial and
challenging to research for a better hydrological understanding of
water-balance changes that may occur in semiarid ecosystems, such as
Caatinga (Brazil) and Tierra de Pinares (Spain). The aim of this study
is, hence, to compare spatial and temporal patterns of actual
evapotranspiration, as well as to identify temporal evapotranspiration
trends in both semiarid forests: Caatinga (Brazil) and Tierra de Pinares
(Spain).