4. Conclusions
Using wavelet coherence analysis, we found that Amazon-scale averaged
evapotranspiration (ET ) has strong correlations with
precipitation (P ) and the total water storage anomaly
(TWSA ) at the annual and multi-year (~2 to
~ 4 year) periodicities. But the strong annual
periodicity was no longer significant due to irregular short-term
pattern of ET after 2010 (drought event). Moreover, at multi-year
scale the correlation from 2006 to 2012, possibly indicating that
external climate drivers (drought and El Nino events) have enhanced the
impact of groundwater on ET . The spatial distributions of\(\phi_{ET-P}^{1km}\) and \(\phi_{ET-TWSA}^{1km}\) across the Amazon
basin have clear large south-north and small east-to-west patterns, and
our Budyko framework analysis demonstrates that the water and energy
limitation conditions vary significantly between the northern and
southern sub-basins of the Amazon. Although the Amazon basin is
generally energy limited, some southern sub-basins are water limited in
some years.\(\ \phi_{ET-P}^{\text{subbas}}\) and\(\phi_{ET-TWSA}^{\text{subbas}}\) are also well correlated in the
southern Amazon. The strength and significance of their correlations are
affected by the aridity index (PET / P ) of each sub-basin.
The spatial heterogeneity of \(\phi_{ET-P}^{1\ km}\) is negatively
correlated with the spatial variation of annual ET , which implies
that the spatial variation of ET is the primary cause for the
changing pattern of \(\phi_{ET-P}^{1\ km}\) inter-annually.\(\phi_{ET-P}^{1\ km}\) and \(\phi_{ET-TWSA}^{1km}\) show the
dynamic changes of the spatiotemporal correlation among ET ,P , and TWSA . The water limited area gradually decreases
due to the frequent deforestation in the southern Amazon basin.
The results of the three zones analysis also confirm that the effect of
drought on ET has the south-north patterns. The drought events
have weak impact on ET with sufficient annual P in the
northern Amazon basin, and in the southwest Amazon basin as well, where
the main source of ET in the mountain areas is snow and ice
cover. But the variation of phase in the southeast Amazon basin is
closely related to the drought events. During the drought year (2010),
the decreased phases (\(\phi_{ET-P}^{\text{Amazon}}\) and\(\phi_{ET-TWSA}^{\text{Amazon}}\)) likely indicate that ET was
supported by both rainfall and groundwater to maintain the same yield
compared to the years with sufficient P . After the drought year,
when the watershed was no longer water limited,\(\phi_{ET-TWSA}^{\text{Amazon}}\) increased rapidly, possibly
implying the groundwater system had recovered and ET was not
immediately supported by groundwater but by P , since\(\phi_{ET-P}^{\text{Amazon}}\) is much smaller than\(\phi_{ET-TWSA}^{\text{Amazon}}\) during this recovery period. Thus,
Amazon-wide annual ET is possibly not limited by rainfall
availability since groundwater plays an important role during dry years.
The energy and water limitations are switched in some regions over time.
Whether ET is driven by light-limited or water-limited needs to
be studied separately in specific areas. This work expounds a deeper
understanding of the control of ET in different regions in the
Amazon basin by studying the phase lag between two variables at
different scales. However, in most cases, the seasonality of ETis driven by the balance between radiation, rainfall, and vegetation
regulations, rather than being completely limited by any one of these
factors. Vegetation phenology further increases the complexity of
studying the relative importance of controlling ET factors in the
Amazon basin. Future, we expect to add datasets about radiation and
vegetation to further analyze the factors of controlling ET in
the Amazon basin.