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Plain-Language Summary: Volcanic landscapes begin with high permeability, but with time develop a weathered surface that reduces permeability and diverts increasing amounts of water to stream runoff. On the island of Hawai’i the young volcanoes have no permanent streams; stream incision becomes important the older surfaces (more than about 20,000 years). By treating the weathered surface as a porous-plastic medium we Wnd that weathering can induce compaction of the soil that reduces permeability. The reduction in inWltration and initiation of stream incision fundamentally changes the hydrologic and geomorphic evolution of the landscape. Weathering affects both the chemistry and material properties of the surface and strongly inNuences landscape development, in ways that can be predicted with reactive transport and mechanical modeling. Geochemical tracers can be used to identify and quantify weathering processes and constrain these models. 

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.