Hydrologic models are widely used for projecting influences of changing climate on water resources. In this study, we compared the original Soil and Water Assessment Tool (SWAT) model and an enhanced version of SWAT model with physically based Freeze-Thaw cycle representation (SWAT-FT) for simulating future annual ET, stream flow, water yield, surface runoff, and subsurface runoff in the Upper Mississippi River Basin (UMRB). SWAT-FT projected fewer frozen days than the original SWAT model due to its better representation of snow cover insulation effects. Both models derived declining trends in annual streamflow and terrestrial water yield in the late 21st century due to increased ET under warmer climate. However, these two models exhibited contrasting mechanisms underlying the streamflow decline. For original SWAT model, the decrease in surface runoff was the major driver, while for SWAT-FT, reduced subsurface runoff was the main cause. In general, the original SWAT model predicted more surface runoff and less subsurface runoff than SWAT-FT. Further geospatial inspection shows large discrepancies between these two models, particularly in the northern colder parts of the UMRB, where the maximum differences in annual surface and subsurface runoff reached 130 mm yr⁻¹ and 140 mm yr⁻¹, respectively. Collectively, the results demonstrate the importance of accounting for Freeze-Thaw cycles for reliable projection of future water resources.

水文模型被广泛用于预测气候变化对水资源的影响。在这项研究中，我们将原始的土壤和水评估工具（SWAT）模型和SWAT模型的增强版本与基于物理的冻结-融化循环表示（SWAT-FT）进行了比较，以模拟未来的年ET，河流量，水产量，地表密西西比河上游流域（UMRB）的径流和地下径流。与原始SWAT模型相比，SWAT-FT预测的冻结天数要少，因为它能更好地表示积雪的隔热效果。由于在温暖的气候下ET的增加，两种模型都得出了21世纪后期年流量和地面水产量的下降趋势。但是，这两个模型显示出流量下降背后的对比机制。对于原始SWAT模型，地表径流减少是主要原因，而对于SWAT-FT，地下径流减少是主要原因。通常，原始的SWAT模型比SWAT-FT预测更多的地表径流和更少的地下径流。进一步的地理空间检查显示，这两个模型之间存在较大差异，特别是在UMRB北部较冷的地区，那里年地面和地下径流的最大差异达到了130 mm yr^-1和140 mm yr ^-1。总的来说，结果表明考虑冻融循环对于可靠预测未来水资源的重要性。