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Table 1. Synthesis of meteorological data obtained for the Caatinga forest and the Pinares forest.
Table 2. Ratio ETa/ET0 (which equals Kc.Ks, Eq. 13) for the Caatinga (Teixeira, 2018) and Pinares (Liu et al., 2017) semiarid forests.
Table 3. Trend analysis of weather variables and potential and actual evapotranspiration (ET0 and ETa, respectively). S indicates the trend (negative or positive), Sen’s Slope represents the annual increase/decrease of the variable and p-value is the trend significance. *There is a significant positive temporal trend at the 5% level.
Figure 1. Geographical location of the study areas and meteorological stations: Caatinga forest in north-eastern of Brazil and Tierra de Pinares forest in Valladolid (Castile and Leon), Spain.
Figure 2. Satellite data of the period from 1995 to 2019, used for the assessment of actual evapotranspiration in the Caatinga forest and the Pinares forest.
Figure 3. Time series of daily rainfall and actual (ETa) and potential evapotranspiration (ET0) for (a) the Caatinga forest and (b) the Pinares forest. Daily values are interpolated from the satellite overpass time.
Figure 4. Monthly fraction of evapotranspiration (ETfrac= Kc.Ks) for both forests (a) Caatinga and (b) Pinares obtained by ETa_SEBAL/ET0_PM for each scene of Landsat used and (c) for each month by ETfrac average.
Figure 5. Box-plot of daily actual evapotranspiration (ETa) for the two forests (a) Caatinga (N ≈ 146,799 pixels) and (b) Pinares (N ≈ 168,956 pixels), elaborated for each of 111 Landsat overpasses and computed by using the Surface Energy Balance Algorithm for Land (SEBAL) model. The colours represent the different seasons of the year.
Figure 6. Validation of daily actual evapotranspiration (ETa) for the two forests: (a) Caatinga and (b) Pinares, using ET0 calculated with the Penman-Monteith FAO-56 equation and Kc.Ks. The statistical parameters are determination coefficient (R2), Nash–Sutcliffe coefficient (NSE), and Pearson’s correlation coefficient (r).
Figure 7. Pearson’s correlation (r) between Normalized Difference Vegetation Index (NDVI) and actual evapotranspiration (ETa) for each pixel of Caatiga forest (a) and Pinares forest (b) images.
Figure 8. Temporal Stability Index (TSI) of ETa for the (a) rainy season and (b) dry season of the Caatinga forest, and for the four seasons (c) spring, (d) summer, (e) autumn and (f) winter of the Pinares forest.
Figure 9. Yearly data of the variables used for trend analysis ((a, d) rainfall; (b, e) maximum and minimum temperature; and (c, f) potential and actual evapotranspiration, ET0 and ETa, respectively) for (a, b, c) Caatinga forest and (d, e, f) Pinares forest. Yearly ETa was calculated by multiplying monthly ETfrac(Kc.Ks) and daily Penman-Monteith ET0.