Figure 3
3.2 Temporal variations of precipitation, AET, and TWSA in
the
EIB
The monthly series from the average of five GRACE TWSA products and the
GPM precipitation data were used to simulate the monthly AET, utilizing
the hydrologic budget method in the EIB and its 16 closed basins. The
difference between precipitation and AET (described as (P – ET)) was
also analyzed. There is a direct physical relationship between the TWS
and (P – ET); according to the water balance within a closed basin, the
change in TWS is equivalent to the accumulated (P – ET). Figure 4
presents the temporal variations in precipitation, AET, (P – ET), and
TWSA in the EIB. There was little difference in the mean precipitation
of the EIB between months (Figure 4(b)). In terms of the intra-annual
variation of precipitation at the basin scale, there were three major
findings. First, there was little difference in the mean precipitation
for each month in the CSB and BLB, particularly the CSB, in which the
coefficient of variation was 0.24 (Table 1). This was also the major
cause of the marginal difference in the monthly precipitation of the
EIB, as this precipitation accounted for a large proportion of the EIB.
Second, the monthly precipitation from June to October was lower
November to May of the following year in the HRB, IIRB, ASB, ISB, and
TuRB. Precipitation from June to October accounted for less than 20% of
the annual precipitation, in which the former two basins accounted for
less than 10%. Third, under the impacts of the East Asian summer
monsoon, the monthly precipitation in nine basins (i.e., GHCB, IMPB, JB,
MPLB, MPIRB, QB, QPB, TaRB, and TB) in East Asia varied from month to
month. The coefficient of variation exceeded 0.90. Precipitation in
these basins was mainly concentrated from May to October, and this
precipitation accounted for more than 80% of the annual precipitation,
while it was >90% for the QB and QPB.
The mean annual precipitation and AET of the EIB for 2002–2020 was
353.4 and 358.7 mm, respectively. This indicates that water consumed by
the AET was greater than the precipitation that occurred in the EIB. The
number of years where P < AET and P > AET was
similar, although the mean (P – ET) of the former (21.4 mm) was greater
than the latter (10.1 mm). During November to March of the following
year, precipitation in the EIB was able to meet the AET demand (i.e., P
> AET); however, monthly precipitation was less than the
AET from April to October. In other words, precipitation in the EIB was
insufficient to meet the demand from the AET during this period.
The annual and monthly TWSA of the EIB was generally decreasing over the
2002–2020 period, particularly for 2005–2015 (Figures 8(c) and 8(d)).
Overall, the results were consistent among the five TWSA products for
the annual and monthly series. The accumulated (P – ET) was highly
correlated with the TWSA series; the R2 between the
monthly series of accumulated (P – ET) and the mean of the five TWSA
products was 0.97, and reached up to 1.00 between the two annual series.
According to the standardized monthly series, the TWSA was lagging by
approximately one month compared with the (P – ET).