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.