3.2 Phase lag for sub-basins of the Amazon
To further analyze the relationship between phase lags and other
climatic indicators, sub-basins within the Amazon basin are examined
(Figure 4). Sub-basin #1 is narrow, crosses almost the entire Amazon
basin horizontally, and closely conforms to the Amazon River (cross
hatched in Figure 4). The sub-basin delineation from Mayorga et al.
(2005) is not accurate for sub-basin #1 since it only approximates the
floodplain of the main stem of Amazon River and includes minor
catchments bordering the floodplain. Thus, the results for this
sub-basin will not be discussed.
Inter-annual variability in \(\phi_{ET-P}^{\text{subbas}}\) (blue
lines) and \(\phi_{ET-TWSA}^{\text{subbas}}\) (red lines) differs
among the sub-basins, although coherent patterns are evident (Figure 4).\(\phi_{ET-P}^{\text{subbas}}\) ranges from 5 to 8 months, generally.
However, some sub-basins have either larger (10 – 12 months) (#5, #9,
#16, #20, and #25) or much smaller (#32 and #33) values of phase
lag. Since the periodicity for all data sets is ~ 12
months, a phase lag close to 12 months is equivalent to a small lag.
Thus, \(\phi_{ET-P}^{\text{subbas}}\) for those sub-basins with larger
values (10 – 12 months) are equivalent to -2 – 0 months, about the
same as those sub-basins with smaller lags, and they are all smaller
than those of other sub-basins with normal 5 – 8 months lag. Only the
sub-basins mentioned above with smaller\(\phi_{ET-P}^{\text{subbas}}\) (or 10 – 12 months ones) have
positive \(\phi_{ET-TWSA}^{\text{subbas}}\). All these sub-basins are
located in the south of the Amazon basin, which are generally water
limited comparing to other sub-basins (Figure 5). The remaining
sub-basins, with 5 ~ 8 months\(\phi_{ET-P}^{\text{subbas}}\), all have negative\(\phi_{ET-TWSA}^{\text{subbas}}\), ranging from approximate -5 to -3
months. These sub-basins are located in the north of the Amazon basin,
which are energy limited (Figure 5). For these sub-basins ET may
be suppressed by excessive rainfall and low radiation. In the southern
basins, the linear correlations between\(\phi_{ET-P}^{\text{subbas}}\) and\(\phi_{ET-TWSA}^{\text{subbas}}\) are higher (Figure 4) than those of
the northern basins (red is more obvious for southern basins, meaning
the correlation coefficients are larger in those basins), and the
correlation is also more significant (p is significantly less
than 0.05 ). It indicates that rainfall and water storage have
mutual constraints in affecting ET . ET decreases as
rainfall decreases. Nonetheless, the trend of ET has resumed
before the peak of the dry season and increased with the increase of
solar radiation, showing that trees can obtain soil water / groundwater
even during the peak of the dry season.
Qualitatively, north-to-south patterns of phase lag are obvious due to
these sub-basins vary considerably in topography and rainfall patterns.
For example, the elevation and the surface slope of the Andes mountain
area and southern basins are obviously higher than those of the Amazon
River basin. P (Figure S2) and ET (Figure S3) present
different seasonality in each subbasin. And the intensity of rainfall in
each subbasin also varies (Figure S4), which also present north-south
pattern. We explore the light and water limitations in the sub-basins by
using Budyko analysis in the following Section.