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Understanding how subsurface water storage—created and structured by the geology and geomorphology of the critical zone—governs hydrologic connectivity between landscapes and streams is essential for explaining spatial and temporal variation in stream water chemistry. Most headwater studies have focused on high‐resolution stream water chemistry at the catchment outlet, rarely examining the spatial variability among tributaries and the main channel, or how these patterns relate to the underlying geology and geomorphology. Linking upstream spatial and temporal variability with chemical dynamics at the outlet over time is even less common. We conducted weekly synoptic sampling along Lookout Creek, located within the HJ Andrews Experimental Forest Long Term Ecological Research programme. Lookout Creek is in the volcanic terrain of the western Cascades, Oregon. The catchment spans multiple geologic units (e.g., lava flows) and geomorphic features (e.g., earthflows). We measured stream chemistry along the main stem and five tributaries to assess how varying degrees of hydrologic connectivity influence solute concentrations and transport across this geologic and geomorphologic template. To identify the timing and magnitude of hydrologic connectivity between tributaries, the main stem, and the catchment outlet, we analysed spatiotemporal patterns in stream chemistry using concentration‐discharge relationships, principal component analysis, and a metric of subcatchment synchrony. We found that in previously glaciated catchments with active earthflows, solute concentrations and base‐cation‐to‐silica ratios were higher, and more solutes had a chemostatic or mobilising behaviour, indicating high subsurface storage. This variability in subsurface storage, and its influence on hydrologic connectivity, ultimately determined the degree of chemical synchrony with the catchment outlet. Our findings suggest that, under future climate scenarios with shifts in precipitation phase and timing, headwater systems with substantial subsurface storage are likely to be more chemically resilient. 

Understanding how diverse headwater streams contribute water downstream is critical for accurate modelling of seasonal flow dynamics in larger systems. This study investigated how headwater catchments, with diverse subsurface storage, influence downstream flows within Lookout Creek—a 62 km², 5th‐order catchment in the rain‐snow transition zone in western Oregon, USA. We analysed one year of hydrometric and water stable isotope data collected at 10 stream locations, complemented by a decade of precipitation isotopic data. As expected, isotopic data revealed that most of the streamflow was sourced from large fall and winter storms. Generally, stream isotope ratios decrease with elevation. However, some streams had higher isotopic values than expected, reflecting the influence of isotopically heavy storms and relatively low storage. Other streams that tended to have low flow variability in response to precipitation inputs had lower isotopic values, indicating higher elevation water sources than their topographic watershed boundaries. Both hydrometric data and water isotope‐based end‐member mixing models suggest storage differences among headwater catchments influenced the seasonal water contributions from tributaries. Most notably, the contributions of Cold and Longer Creeks, which occupy less than 10% of the Lookout Creek drainage area, sustain up to 50% of the streamflow in the summer. These catchments have high storage and high groundwater contributions, as evidenced by flat flow duration curves. Finally, our data suggest that geologic variability and geomorphic complexity (presence of earthflows and landslides) can be indicators of storage that dramatically influence water movement through the critical zone, the variation in streamflow, and the response of streams to precipitation events. Heterogeneity in headwater catchment storage is key to understanding flow dynamics in mountainous regions and the response of streams to changes in climate and other disturbances. 

The soil mantle of the tropical karst landscapes of southern Mexico was a key resource for ancient Maya agriculture and experienced deep transformation due to long-term human impacts under changing environmental conditions. We conducted a comparative analysis of three compound soil toposequences in mountainous (Sierra de Chiapas/Middle Usumacinta Valley, Busiljá, and Chinikihá archaeological sites) and platform (NE Yucatán Peninsula, Yalahau region) karst landscapes to reconstruct general tendencies and regional variations in pedodiversity development and soil–human interactions since the Early Preclassic Period. Toposequence characterization is based on macro- and micromorphological observations, accompanied by a suite of laboratory data. Calcareous upland geoforms of all toposequences have similar soil combinations consisting of shallow Rendzina and deep red clayey Terra Rossa types of profiles. We argue that Rendzinas, now dominant in the upland soil cover, in most cases, are not a product of incipient pedogenesis on limestone; they have developed from the residues of Terra Rossa soils after their advanced erosion. Pedosediments generated by ancient soil erosion have been found in the piedmont and depression positions in the mountainous landscapes of Chiapas, as a result of lateral downslope soil removal, and in the subsurface karstic cavities in the platform of NE Yucatán, indicating vertical “soil piping.” The soils of the lowland domains show contrasting differences between the toposequences: gleyic clay–rich soils and humic alluvial soils prevail in Chinikihá and Busiljá, whereas hydromorphic carbonate soils have formed in Yalahau karstic depressions. These differences in the lowland soil properties led to divergent ancient Maya land use strategies; in Chinikihá and Busiljá, the major agricultural domain was developed in the lowlands, implying largescale artificial drainage. On the contrary, in Yalahau, mostly upland Rendzinas were cultivated, implying “precision agriculture” and “container gardening.”