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.