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Permafrost in High Mountain Asia (HMA) is becoming increasingly vulnerable to thaw due to climate change. However, the lack of either in situ ground surface or borehole temperature data beyond the Tibetan Plateau prevents comprehensive assessments of its impact on the regional hydrologic cycle and local cascading hazards. Although past studies have generated estimates of permafrost extent in Central Asia, many are limited to the Tibetan Plateau, excluding the more remote reaches of the Tien Shan, Pamirs, and Himalayas. By leveraging surface temperatures from both the Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric Infra-Red Sounder (AIRS), this study advances further understanding of remotely sensed permafrost occurrence at high altitudes, which are prone to error due to frequent cloud cover. We demonstrate that the fusion of MODIS and AIRS products can accurately estimate long-term thermal regimes of the subsurface, with reported correlation coefficients of 0.773 and 0.560, RMSEs of 0.890 °C and 0.680 °C, and biases of 0.003 °C and 0.462 °C, respectively, for the ground surface and the depth of zero annual amplitude, during a reference period of 2003–2016. Furthermore, we provide a range of possible permafrost extents based on established equations for calculating the temperature at the top of the permafrost to demonstrate temperature sensitivity to soil moisture and snow cover. The MODIS-AIRS product is recommended to be a robust source of ground temperature estimates, which may be sufficient for inferring mountain permafrost presence in HMA. Incorporating the influence of soil moisture and snow depth, although limited by biased estimates, also produces estimates of permafrost regional areas comparable to previously reported permafrost indices. A total permafrost area of 1.69 (± 0.32) million km2 is estimated for the entire HMA, across 15 mountain subregions. 

Abstract Solute transit or travel time distributions (TTDs) in catchments are relevant to both hydrochemical response and inference of hydrologic mechanisms. Long‐tailed TTDs and fractal scaling behavior of stream concentration power spectra (∼1/frequency, or 1/frequency to a power <2) are widely observed in catchment studies. In several catchments, a significant fraction of streamflow is derived from groundwater in shallow fractured bedrock, where matrix diffusion significantly influences solute transport. I present frequency and time domain theoretical analyses of solute transport to quantify the influence of matrix diffusion on fractal scaling and long‐tailed TTDs. The theoretical concentration power spectra exhibit fractal scaling, and the corresponding TTDs resemble a gamma distribution. The tails of the TTDs are influenced by accessible matrix width, exhibiting a sustained power‐law (rather than exponential) decline for large matrix widths. Application to an experimental catchment shows that theoretical spectra match previously reported power spectral estimates derived from concentration measurements. 

Abstract Considerable debate revolves around the relative importance of rock type, tectonics, and climate in creating the architecture of the critical zone. We demonstrate the importance of climate and in particular the rate of water recharge to the subsurface, using numerical models that incorporate hydrologic flowpaths, chemical weathering, and geomorphic rules for soil production and transport. We track alterations in both solid phase (plagioclase to clay) and water chemistry along hydrologic flowpaths that include lateral flow beneath the water table. To isolate the role of recharge, we simulate dry and wet cases and prescribe identical landscape evolution rules. The weathering patterns that develop differ dramatically beneath the resulting parabolic interfluves. In the dry case, incomplete weathering is shallow and surface parallel, whereas in the wet case, intense weathering occurs to depths approximating the base of the bounding channels, well below the water table. Exploration of intermediate cases reveals that the weathering state of the subsurface is strongly governed by the ratio of the rate of advance of the weathering front itself controlled by the water input rate, and the rate of erosion of the landscape. The system transitions between these end‐member behaviours rather abruptly at a weathering front speed ‐ erosion rate ratio of approximately 1. Although there are undoubtedly direct roles for tectonics and rock type in critical zone architecture, and yet more likely feedbacks between these and climate, we show here that differences in hillslope‐scale weathering patterns can be strongly controlled by climate.