Consistency of Long-Term Trends and Closure of the Surface Water Balance
in the Amazon River Basin
Abstract
We study the consistency of long-term trends in the surface water
balance of 63 sub-catchments of the Amazon River basin. Monthly time
series of precipitation, evaporation, runoff and soil water storage are
obtained from remote sensors and flow stations (CHIRPS, ETR-Amazon,
ANA-Brazil and JPL-GLDAS). Missing data during the period 1995-2015 are
reconstructed applying an adaptation of the methodology proposed by
Kondrashov & Ghil (2006). Empirical mode decomposition (Huang et al.,
1998) is applied to filter out different modes of natural variability,
with the aim to isolate the long-term trend of time series. The
Mann-Kendall and Sen tests are applied to the residue and the sign and
magnitude of the trends was obtained. No generalized unidirectional
trends were found for the Amazon basin for any of the variables studied
(Table 1), although some unidirectional trends were found in groups of
sub-catchments that belong to the same stream. The consistency of the
general water balance equation [dS/dt=P(t)-E(t)-R(t)], and its
long-term approximation [\overline{R} =
\overline{P} - \overline{E}] were
evaluated. The general water balance does not close for 37
sub-catchments, while in the long-term the error in the balance tends
asymptotically to a constant value, different from zero, which indicates
that in the period of 20 years studied the long-term condition is
fulfilled, but there is no closure for the long-term water balance
either. The consistency of the surface water balance equation was also
studied regarding the signs of the trends, finding that in 32 (51%) of
sub-catchments studied the trend signs are not consistent with the water
balance equation. Finally, for the remaining 31 sub-catchments (with
trends consistent in signs), the consistency of the water balance
equation was evaluated regarding the magnitude of trends, finding
closure errors on water balance of trends up to 281% of the average of
the magnitudes. We discuss several reasons for the lack of closure of
the water balance, including the very existence of trends that violate
the stationary hypothesis underlying the long-term approximation of the
surface water balance. The used data and some of the results are
available on the supplemental files. The Raw Data CSV file contains the
spatial average timeseries at the basin that drain to the flow stations
for the water balance equation variables. The Filled Data CSV file
contains those timeseries after appliying the reconstruction. MKtrends
and SenTrendMagnitude CSV files contains the results of the appliying
the Mann-Kendall and Sen trend tests to the filled data.