Hydrological drought projection in the Lower Songhua River basin under climate and land-use change

Climate change and land-use change exacerbate risks of hydrological drought. This study integrates SWAT, five CMIP6 models, and PLUS to assess impacts on runoff and hydrological drought in the lower Songhua River basin (2020–2100) under SSP1-2.6, SSP2-4.5, and SSP5-8.5, using the Standard Runoff Index (SRI). Both models performed satisfactorily (SWAT: R2 and NSE > 0.7; PLUS: accuracy 0.91, Kappa 0.83). Results indicate: (1) Annual precipitation increased by 9.57%, 9.78%, and 11.28%, and temperature by 1.24°C, 1.89°C, and 2.67°C relative to the baseline (1985–2014); (2) Under SSP1-2.6 and SSP2-4.5, arable land decreased (−12.3%, −8.7%) and forest increased (+14.2%, +10.6%); urban land expanded by 156.2% under SSP5-8.5; (3) Land-use change reduced annual runoff by 9.61%, 9.97%, and 10.58%; (4) SRI showed an upward trend under SSP1-2.6 (+0.008/yr) but declined under SSP2-4.5 (−0.0057/yr) and SSP5-8.5 (−0.0203/yr). This study reveals composite climate–land-use impacts on basin hydrology, providing a scientific basis for water resource management and drought risk prevention.