A multiscale analysis using young water fractions and transit time distributions in the Yellow River Source Area

Estimating water yield is a crucial aspect of evaluating water conservation strategies and ensuring sustainable development in watersheds. The widespread application of isotopes to quantify the temporal dynamics of precipitation transforming into runoff has helped to identify the influence of watershed runoff and mixing processes on nutrient transport and biogeochemistry. Nevertheless, in permafrost regions characterized by strong landscape heterogeneity, sparse and discontinuous data collection poses challenges in obtaining isotope data of permafrost thaw meltwater for studying its influence on catchment hydrology. The primary objective of this study is to assess the accuracy and reliability of the convolution integral model in simulating the transit time distribution in permafrost regions, considering the introduced parameters. Additionally, the study aims to evaluate the water retention capacity of the permafrost watershed and explore the key physical control factors influencing the impact of permafrost thaw on mean transit time (MTT). The northeastern part of the Qinghai-Tibet Plateau, situated in the source region of the Yellow River (SAYR), is at the boundary between discontinuous permafrost and seasonal frozen ground. Permafrost degradation is evident, leading to a complex runoff generation mechanism. Within five nested sub-catchments of the SAYR region (20,000~120,000 km²), we collected high-resolution water stable isotope data for both rainfall and runoff, and we quantified the contribution of permafrost thaw meltwater during the melting period. The influence of permafrost meltwater from the active layer on water transit times was accounted for in the convolution integral method by introducing an additional source contributing to runoff (Q₀, x% contribution with isotope ratio δ₀). The study finds that additional sources of soil melt water runoff contribution are crucial to solving the problem of non-convergence of the convolution integral model in permafrost areas, and the MTTs of the watersheds are mainly influenced by river channel topography, the water retention capacity of the watersheds depended on the topographical and morphological characteristics of the watershed, and is secondarily affected by land use type, soil type, and frozen soil thermal stability.