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We investigated how runoff-to-groundwater partitioning changes as a function of substrate age and degree of regolith development in the Island of Hawai’i, by modeling watershed-scale hydrodynamic properties for a series of volcanic catchments of different substrate age developed under different climates. In the younger catchments, rainfall infiltrates directly into the groundwater system and surface runoff is minimal, consisting of ephemeral streams flowing on the scale of hours to days. The older catchments show increasing surface runoff, with deeper incision and perennial discharge. We hypothesize that watershed-scale hydrodynamic properties change as a function of their weathering history—the convolution of time and climate: as surfaces age and become increasingly weathered, hydraulic conductivity is reduced, leading to increased runoff-to-recharge ratios. To test this relationship, we calculated both saturated hydraulic conductivity (k) and aquifer thickness (D) using recession flow analysis. We show that the average k in the younger catchments can be between 3 to 6 orders of magnitude larger than in older catchments, whereas modeled D increases with age. Ephemeral streams with zero baseflow at daily timescales cannot be evaluated using the same method. Instead, we calculated the recession constant for two contiguous catchments developed on young ash or lava deposits of different ages. Increasing bedrock age results in slower recession response in these ephemeral streams, which is consistent with decreasing hydraulic conductivity. Our results highlight the role of the weathering history in determining the evolution of watershed-scale hydrologic properties in volcanic catchments. 

Stream networks can retain or remove nutrient pollution, including nitrate from agricultural and urban runoff. However, assessing the location and timing of nutrient uptake remains challenging because of the hydrological and biogeochemical complexity of dynamic stream ecosystems. We used a novel approach to continuously characterize the biological activity in a stream with in situ measurement of dissolved gases by membrane inlet mass spectrometry (MIMS). In a headwater stream in western France, we compared in situ measurements of O2, CO2, N2, and N2O (the main gases associated with respiration, including denitrification) with more traditional laboratory incubations of collected sediment. The in situ measurements showed near-zero denitrification in the stream and the hyporheic zone. However, the laboratory incubations showed a low but present denitrification potential. This demonstrates how denitrification potential is not necessarily expressed in field hydrological and geochemical conditions. In situ measurements are thus crucial to quantify expressed rates of nutrient removal. Broader application of in situ gas measurement based on technologies such as MIMS could enhance our understanding of the spatiotemporal distribution of stream and hyporheic processes and overall nutrient retention at stream network scales. 

Abstract Weathering and erosion processes are crucial to Critical Zone (CZ) evolution, landscape formation and availability of natural resources. Although many of these processes take place in the deep CZ (∼10–100 m), direct information about its architecture remain scarce. Near‐surface geophysics offers a cost‐effective and minimally intrusive alternative to drilling that provides information about the physical properties of the CZ. We propose a novel workflow combining seismic measurements, petrophysical modelling and geostatistical analysis to characterize the architecture of the deep CZ at the catchment scale, on the volcanic tropical island of Basse‐Terre (Guadeloupe, France). With this original workflow, we are for the first time able to jointly produce maps of the water table and the weathering front across an entire catchment, by means of a single geophysical method. This integrated view of the CZ highlights complex weathering patterns that call for going beyond “simple” hillslope CZ models.