A capillary barrier cover (CBC) is a geotechnical structure which a coarse-grained soil layer covered by a fine-grained soil layer. A CBC can retain downward water infiltration, increase water storage capacity and lateral diversion, and prevent capillary rise. Geotextiles are usually set up as isolation layers between fine-grained and coarse-grained layers to prevent fine particles entering the coarse-grained layer, resulting in a decrease in downward water infiltration and water storage capacity. However, crustal stress, farming, animal, plant activities, and other factors may cause damage to the isolation layer. At present, there is no reliable and accurate method to determine the location and degree of damage to the isolation layer. The existing methods search for the damage location by excavating the whole fine layer, which incurs high maintenance costs. If the damaged position of the CBC isolation layer can be accurately obtained, it can reduce maintenance costs. Therefore, this study investigated the influence of a coarse-grained layer mixed with different particle sizes and proportions of fine particles on water storage capacity through laboratory soil column experiments. The results are as follows: (1) Fine particle mixing into the coarse-grained layer will reduce water storage capacity, and there is a worse admixture ratio that minimizes water storage capacity. (2) The CBC enhances the fine-grained layer volumetric water content (VWC), but the enhancement degree decreases as the distance from the fine-coarse interface increases. (3) A method has been proposed to determine the location and degree of damage to the isolation layer. When the VWC at the fine-coarse interface reaches a stable level during breakthrough, the CBC effect exists, the higher the VWC at the fine-coarse interface, the stronger the CBC; when the VWC at the fine-coarse interface is unstable during breakthrough, the CBC effect disappears, and the median diameter of the fine particles mixed into the coarse-grained layer is finer than or equal to the fine-grained particles' median diameter.

The Yangtze River and the Huang River are the two largest rivers in China. Annual runoff ratios (runoff/precipitation, denoted as RR) of the head regions of these two basins (HYR and HHR, respectively) have significantly decreased over the past several decades, closely related to changes in water storage capacity (WSC) and terrestrial water storage (TWS). However, such effects have rarely been quantified due to limitations associated with complicated arctic hydrological processes and the absence of long-term reliable TWS data. In this study, a TWS reconstruction dataset (TWSrec) was validated, and demonstrated good performance in capturing TWS variations derived from the Gravity Recovery and Climate Experiment (GRACE) and in the terrestrial water budget for these two head regions. Long-term (1980-2015) changes in TWS and WSC were then detected and their effects on RR were quantified through trend detection, change point analysis, and path analysis. Results showed that TWS increased significantly with a rate of 27.6 mm/10 yr and 19.8 mm/10 yr at HYR and HHR, respectively. These increases were mainly caused by wetting (increases in precipitation) or soil moisture increases from the TWS component perspective. WSC (represented as the ratio of TWS to precipitation) gradually enlarged in response to continuous climate warming. RR decreased significantly with rates of 2.0%/10 yr at HYR and 3.6%/10 yr at HHR, attributed to the increased evaporation ratio (similar to 80%) and increased WSC (similar to 20%) in both head regions. Further analysis suggested that permafrost degradation under climate warming could increase WSC. These results demonstrate that climate change has resulted in unstable terrestrial water storage at HYR and HHR, and that increases in WSC due to permafrost degradation play an important role in accurately simulating runoff in the Tibetan Plateau and other permafrost-degradation regions.