高寒区冻融作用下流域时变蓄-泄关系及枯季径流模拟

In high altitude and cold region, the freezing and thawing processes of frozen ground result in the storage-discharge relationship of saturated zone presenting time-varying characteristics, which have remarkable impact on low flow hydrograph. This project selects two typical small catchments in Lhasa River basin in south Tibet Plateau for the study, and it is focus on the role of frozen ground on low flow. The freezing and thawing processes will be observed at multiple sites by different methods to explore their spatio-temporal pattern at catchment scale. Based on the numerical groundwater model coupled water and heat transport processes, the effects of freezing and thawing processes on the active layer, aquifer hydraulic properties, and the volume of soil water frozen and released will be studied. The mechanism of frozen ground working on time-varying characteristics of storage and discharge relationship will be revealed. A catchment scale storage-discharge relationship model will be developed to reflect the role of spatial heterogeneity of frozen ground. The impact of frozen ground on storage and discharge relationship and low flow processes will be evaluated quantitatively. Considering the climate change scenarios in the future, the change of low flow can be predicted by this model. It will provide helps for understanding hydrologic regimes in the high altitude and cold basins and will promote hydrologic predictions in the Tibet Plateau.

针对青藏高原高寒区冻土冻结和融化作用导致的流域含水层蓄-泄关系时变特性及其对枯季径流的影响，本项目拟选择具有多年和季节性冻土的青藏高原拉萨河典型小流域为研究对象。通过强化观测、实验及模拟，探求冻土发育时空演变规律。基于水热耦合地下水数值模型，研究冻融作用下冻土层厚度、含水层渗透性和土壤水分冻结、释放的变化规律，阐明冻融过程对蓄-泄关系时变特征的作用机理。建立反映冻土发育空间异质性的流域蓄-泄关系模型，定量分析冻融作用对流域蓄-泄关系和枯季径流的影响，预测升温背景下流域枯季径流的变化趋势，为揭示青藏高原地区枯季径流演变规律提供科学支撑。

青藏高原高寒区冻土广布，受全球气候变化影响，高寒区冻土呈退化趋势，改变了流域对水分的调节作用及径流产生的机制。本项目选择具有多年和季节性冻土的青藏高原典型流域，结合野外观测实验、数据分析、模型构建及模拟，研究了冻土对流域蓄泄关系和径流过程的影响。主要研究成果如下：（1）针对退水斜率曲线分析方法（-dQ⁄dt~Q）推求流域蓄泄关系时易受观测资料误影响的问题，提出了一种改进的变增步长方法，以减小观测误差对估算-dQ⁄dt的影响；改进了流域退水参数推求方法，克服传统方法常常低估退水斜率的问题；并提出了一种更加灵活的基于分位数的概率退水分析方法，以取代确定性方法构建流域主退水曲线。（2）分析了流域退水参数和温度的定量关系以及径流对蓄泄变化的敏感性，揭示气候变暖下背景下冻土退化对流域蓄泄参数的影响。研究表明多年冻土层退化增加流域的地下水储量，增强了流域径流对地下水储量响应的非线性以及径流稳定性。（3）基于温度及其控制的水文过程随海拔变化而改变的特征，构建了一种考虑冻融过程的垂向分带概念性水文模型，刻画冻土冻融对径流过程的影响及其随海拔的变化。研究表明冻融过程影响的水量随着海拔变化呈现先升高后减小的特征。随着气候变暖，冻土退化，流域多年冻土下线上移，冻融过程影响的水量呈降低趋势；同时，地下水补给量增加，使得流域基流和枯季径流显著增加。研究可为青藏高原地区径流演变规律和水资源管理提供科学支撑。