Figure 7
Across the entire EIB, it was expected that the increase in precipitation would cause an increase in the TWSA; however, the TWSA showed a significant decreasing trend. This indicates that the increase in water consumption by the AET was the main factor driving the significant decrease in the TWSA. Figure 8 shows the trends in the annual precipitation, AET and TWSA, mean annual precipitation and AET, and the accumulated (P – ET) values in each closed basin. In general, precipitation, AET, and changes in TWSA were in balance at the closed basin scale; however, the changes in P, AET and TWSA were not in balance. As such, the contribution from changes in precipitation and AET to changes in TWSA at the closed basin scale was qualitatively analyzed.
Similar to the EIB, there were significant decreasing trends in the TWSA when precipitation exhibited increasing trends in most basins, including the ASB, GHCB, HRB, IMPB, ISB, MPLB, MPIRB, TaRB, and TB. This indicates that the increase in AET was the main cause of the significant decrease in the TWSA in these basins. In the CSB and JB, the decrease in precipitation and the increase in AET led to a decrease in the TWSA. The increasing magnitude of AET was three and 1.5 times the decreasing magnitude of precipitation in the CSB and JB, respectively. This indicates that the increase in AET contributed to the decrease in the TWSA to a greater extent than did the decrease in precipitation. The results for the BLB, IIRB, and TuRB were the opposite to those of the aforementioned basins. It was expected that the decreasing AET would induce an increase in the TWSA; however, the TWSA exhibited significant or non-significant decreasing trends in these basins. This indicates that the decrease in precipitation was the main factor affecting the decrease in the TWSA of these basins. The decreasing precipitation in the QPB may cause a decrease in the TWSA; however, there was a slightly increasing trend in the TWSA of the basin, largely due to the decrease in the AET. The increasing in the AET for the QB may cause a decrease in TWSA; however, there was a significant increasing TWSA in the basin largely caused by the increasing precipitation.
According to the physical influence mechanism, the change in the TWSA in a closed basin is directly correlated to the accumulated (P – ET), and this study was able to verify this. For the depth results, the accumulated (P – ET) for 2002–2020 was -5.3 cm/10a; this is consistent with the decreasing magnitude of TWSA in the EIB. At the basin scale, the magnitude of trends of TWSA in each basin ranged from -9.2 to 5.7 cm/10a, with an average of -3.0 cm/10a. The accumulated (P – ET) in each basin varied from -9.6 to 6.2 cm/10a, with a mean of -3.1 cm/10a. There was a high consistency between the TWSA changes and accumulated (P – ET). The results were similar to the depth results for the EIB and each basin. The TWSA decreased by -5615.6×108m3/10a during 2002–2020 in the EIB. This annual reduction in the TWSA in the EIB was approximately equivalent to the mean annual runoff of the Yellow River. At the basin scale, the decreasing magnitude of the TWSA was relatively higher in the western and southwestern basins of the EIB, including the CSB, IIRB, ASB, and HRB; the magnitude of the decrease in the CSB exceeded than -3300×108 m3/10a. The amount of accumulated (P – ET) was consistent changes in the TWSA; the R2 between the two series composed from each basin value reached 0.99.