3.4 Phase for kilometer scale variables
By downscaling P and TWSA to the same resolution asET , we calculated 1 km spatial scale phase relationships betweenET and P (\(\phi_{ET-P}^{1km}\); Figure 6) and ETand TWSA \((\phi_{ET-TWSA}^{1km}\); Figure 7). Large
south-to-north and small east-to-west gradients occur for both phase
lags across all years analyzed (2002 – 2013). There is a band of the
positive \(\phi_{ET-P}^{1km}\) (~6 month) in the
middle latitudes of the Amazon basin (-5° ~ -10°) that
varies to the negative \(\phi_{ET-P}^{1km}\) (~-6
month) in north-south extent across years (Figure 6), indicating that
the leaf area in this area is reduced due to deforestation activities
and that cloudy conditions limit the available energy driving ET(Zhang et al., 2001; Spracklen et al., 2012). Meanwhile, the location of
this band of positive \(\phi_{ET-P}^{1km}\) generally corresponds to
negative \(\phi_{ET-TWSA}^{1km}\) values (~-6 month).
The importance of plant controlling should be considered in the water
balance accounting of forests in the Amazon basin, as these evergreen
trees have deep root systems and can extract water from the aquifer to
meet the atmospheric water demand (Maeda et al., 2017). However,
deforestation interrupts the water absorption path of roots from
groundwater (Zemp et al., 2017). \(\phi_{ET-P}^{1km}\) and\(\phi_{ET-TWSA}^{1km}\) have zero-to-positive and zero-to-negative
variation patterns from 2002 to 2013, which indicate that the
correlation between ET and P (TWSA ) has decreased
in the central of Amazon region due to deforestation during the
observation period (Figure 7). The increase in deforestation has
weakened the carbon sink of terrestrial ecosystems. Meanwhile, Humphrey
et al. (2018) proved that the inter-annual variability of the
CO2 growth rate is closely related to TWSA .
Considering that plant transpiration is related to CO2absorption through leaf stomata, the variability of ET is also
closely associated with TWSA . A consistently positive\(\phi_{ET-P}^{1km}\) and \(\ \phi_{ET-TWSA}^{1km}\) occur in the
southwest Amazon basin with high elevation. Here, in the Andes with low
vegetation cover, the only source for ET is snow and ice cover,
and ET should mostly be driven by solar radiation. Thus, in these
areas ET is unaffected by groundwater depth and the timing forET and groundwater variation may remain relatively constant
inter-annually. The drastic transition could be artificial and regarded
as the unchanged absolute value of the phase since the periodicity for
all data sets is ~12 months. For example, a negative
6-month phase lag is the same as a positive 6-month lag. Overall, the
water-limiting areas (green) of the Amazon basin continue to shift to
energy-limiting areas (blue). It could also suggest a system that
switches between water and energy limitation with a growth cycle that
coincidentally appears to lead rainfall, among other possibilities.
Across all 1 km grid cells, \(\phi_{ET-P}^{1km}\) and\(\phi_{ET-TWSA}^{1km}\) are not well correlated to the values ofET , P , or TWSA alone. Across the basin, the spatial
standard deviation of annual ET is negatively correlated to that
of \(\phi_{ET-P}^{1km}\) (Figure 8; r = -0.92 andR2 = 0.85 ). Since the inter-annual variation ofET is small (Figure 3c), in some areas ET and the aridity
index may remain about the same regardless of annual precipitation. When
the areas with low ET in dry years have recovered from water
limitation, indicating ET possibly increased in these areas and
the values become close to those in the unaffected areas, the overall
spatial standard deviation of ET decreases, but the spatial
variation of \(\phi_{ET-P}^{1km}\) becomes larger due to the generally
small variation in ET (Figure 8). This relationship implies that
the spatial variation of ET is the primary cause for the changing
spatial pattern of \(\phi_{ET-P}^{1km}\) inter-annually, but this
relationship does not hold for \(\phi_{ET-TWSA}^{1km}\) (data not
shown). Thus, the variation of the spatial standard deviation ofET means the change of ET spatial heterogeneity.