Figure 4
3.3 The main attribution of AET changes in the
EIB
Figure 5 presents the annual and monthly AET trends over the 2002–2020
period in the EIB and its 16 closed basins. There was a non-significant
increasing trend in the annual AET of the EIB for 2002–2020, with a
magnitude of 12.4 mm/10a; this was higher than the corresponding
precipitation increase. By contrast, there were non-significant
decreasing trends in annual AET for the BLB, IIRB, QPB, and TuRB,
alongside the annual precipitation for these basins; the greatest rate
of decrease occurred in the IIRB, reaching -9.7 mm/10a, which was
consistent with the precipitation trends in this basin. In the remaining
12 basins, the AET was increasing, and this increase was significant in
the HRB. The highest rate of increase was 56.2 mm/10a in the HRB; this
is also consistent with the precipitation trends in these basins. These
results indicate that precipitation is one of the important factors
impacting changes in the AET. In addition, the increase in AET in the
IMPB reached 34.6 mm/10a. The AET increase in the ASB, CSB, GHCB, ISB,
MPLB, MPIRB, and TB was between 10 and 20 mm/10a, while this increase
other basins was <10 mm/10a. Although the AET trends were
consistent with those of precipitation in most basins, they showed
opposite trends in the CSB and JB. In these two basins, AET and
precipitation was increasing and decreasing, respectively, indicating
that in addition to precipitation, the consumption of other water
sources by AET is also an important factor affecting changes in AET.
Based on the main attribution identification method described in the
Section 2.3.2, the main factors affecting changes in AET changes were
analyzed. Additionally, the contribution of precipitation and other
water sources to changes in the AET was also semi-quantified by
comparing the trend magnitude of AET and precipitation. The increase in
precipitation in the EIB accounted for 30% of the AET, indicating that
the increasing consumption of other water sources was the dominant
factor increasing the AET in the EIB. This increase in consumption may
be due to a range of mechanisms: (1) increased evaporation rates from
rising temperature; and (2) increased irrigation diversion and glacial
melt runoff, producing an increase in AET.
At the basin scale, the increase in precipitation accounted for more
than 60% of the increase in AET in the HRB, QB, GHCB, and TB,
particularly the two former basins, where this proportion exceeded 90%.
This indicates that the main factor affecting the increase in AET in
these basins was higher precipitation.
In the ASB, TaRB, MPLB, and MPIRB, the proportion of precipitation
increase that contributed to the AET increase was <30%, and
the proportion was <10% for the former two basins. This
indicates that the higher precipitation only made a relatively small
contribution to the AET increase. The increase in the consumption of
other water sources caused by enhanced evaporation rates played a
dominant role in the increasing AET. In the CSB and JB, precipitation
was decreasing, potentially inducing a decrease in the AET as there are
reduced water sources for evaporation consumption; however, the AET in
these basins was increasing, indicating that increased evaporation rates
consumed other AET water sources. This is the main reason underpinning
the increasing AET in these two basins. In the QPB, the AET decrease
only accounted for 4% of the decrease in precipitation. This means that
as precipitation decreased by -9.6 mm/10a, the increasing ET consumption
of other water sources from enhanced evaporation was the main
contributor to the restricted AET of only -0.4 mm /10a in the basin.
In the BLB, IIRB, and TuRB, the decrease in AET accounted for 50% ± 2%
of the decrease in precipitation; this means enhanced evaporation rates
led to the consumption of other water sources, resulting in an
increasing AET. This partially compensated for the AET decrease in the
basins, and indicates that the increases consumption of other water
sources and the precipitation decrease accounted for approximately half
of the AET decrease. In the IMPB and ISB, the precipitation increase
accounted for 50% ± 4% of the AET increase, indicating that the
increase in precipitation and the consumption of other water resources
as a result of enhanced evaporation rates contributed to approximately
half of the change in the AET.