## 5 Conclusions  
SPEI03 was selected as the optimal time scale, and VHI was optimized by using the maximum correlation coefficient method. The optimal contributions of VCI and TCI to VHI were evaluated. Based on SMA and SPEI, the enhancement performance of VHIopt was verified. Furthermore, the spatiotemporal evolution of drought in the YRB from 2001 to 2021 was further analyzed by using VHIopt. The main results are summarized below:(1) In the YRB, the 3-month time scale SPEI drought index shows the best correlation with VHI and is most suitable for analyzing the optimal weights for VHI. The contribution of VCI to VHIopt is lower than that of TCI. Validation based on SPEI03 and SMA standards demonstrates that VHIopt offers superior accuracy drought in monitoring across various land cover types, including forests, grasslands, and croplands, compared to VHIori. Specifically, VHIopt demonstrates higher precision and reliability in monitoring agricultural drought compared to VHIori, and it shows a higher consistency with drought conditions observed using SMA and SPEI03. (2) Temporally, VHIopt in the growing season generally exhibits an upward trend, while the drought area shows a decreasing trend in variability, indicating a weakening of drought conditions in the YRB. However, there are notable differences in the changes in VHIopt and drought area across various regions. In the downstream, the slope value of VHIopt is \(-0.0031\), indicating an intensifying trend in drought conditions. In contrast, forested areas primarily experience moderate to mild drought conditions, demonstrating a lower sensitivity to drought. (3) In 2001, the YRB experienced its most severe drought, with the affected area nearly the entire area of the basin. Over time, the drought area during the growing season generally decreased, with 2012 marking the year with the smallest drought area. In other years, drought conditions were primarily concentrated in the middle, western, and downstream of the basin. The Theil-Sen median trend analysis and Mann-Kendall test indicate that drought conditions in most of the YRB alleviated from 2001 to 2021. However, significant intensification of drought was observed in parts of the Hetao Plain, Ningxia Plain, Guanzhong Plain, and much of the downstream. In this study, we have improved the calculation method for VHI and introduced a new drought index, VHIopt. This index serves as a simple and effective tool for monitoring agricultural drought, thereby improving operational drought management. Future research will concentrate on validating and applying VHIopt across a wider range of applications to further advance studies in agricultural drought. Conceptualization, H.G. and P.D.M. ; methodology, Q.H. ; software, H.G., X.M., W.W. and Q.H. ; validation, Q.H. ; formal analysis, Q.H. ; resources, X.M. and H.G. ; data curation, Q.H., Y.W., Y.C. and R.L. writing--original draft preparation, Q.H. ; writing--review and editing H.G., Y.W. and X.M. ; visualization, Q.H. ; supervision, X.M. and H.G. ; project administration, H.G. and X.M. ; funding acquisition, X.M. All authors have read and agreed to the published version of the manuscript. This research was funded by the Youth Innovation Teams in Colleges and Universities of Shandong Province (Grant No. 2022 KJ178), the Shandong Provincial Natural Science Foundation (grant number ZR2021 QD055), the Third Xinjiang Scientific Expedition Program (grant number 2021 XJKK140304), and Natural Science Foundation of Rizhao City (grant number RZ2021 ZR41). The authors declare no conflicts of interest. Figure 12: Scatter plots of SMA (CLEAM RSM) versus VHIori and SMA (CLEAM RSM) versus VHIopt under different vegetation types (forest (**a,b**), grassland (**c,d**), cropland (**e,f**), changing vegetation types (**g,h**), and all vegetation types (**i,j**)) and the red dashed lines in all subfigures represent the lines of linear fit.