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Abstract

Preferential flow (PF) refers to the rapid channeling of water through a small portion of the soil pore space. This process impacts groundwater recharge, water quality, and soil moisture storage. PF is not explicitly included in many hydrological models, but recent work has shown that it occurs after ~45% of rain events and is ubiquitous across ecosystem types. More frequent PF is linked to higher storm intensity, net primary productivity, and soil clay content across a wide range of sites in the USA. However, we have a poor understanding of the depth dependency of PF and the environmental and climate drivers influencing PF occurrence in shallow vs. deeper soil layers. PF occurrence may be influenced by horizon type (eluvial vs. illuvial), porosity, roots, and the amount and type of clay. As these variables often change with depth, they likely influence the propagation of PF through soil profiles. We leveraged a unique PF database created from high-frequency soil moisture and precipitation datasets from 40 NEON sites across the USA to determine how the probability of PF changes across sites and with soil depth. We asked (1) are there common patterns in depth distributions of PF occurrence across soil orders or ecosystem types? and (2) do the drivers of PF differ in surface vs. subsurface horizons?
Preliminary results show that depth distributions of PF occurrence broadly fall into two groups: exponential decline (majority of PF events occurred in surface soils) or a uniform distribution (PF occurrence was similar across soil layers). For Alfisols, Mollisols, and Spodosols there was a positive relationship between PF occurrence in surface horizons and PF occurrence in subsurface horizons, suggesting that PF tends to propagate to deeper layers in these soil types. PF occurrence in surface horizons was primarily impacted by storm intensity, but total rainfall amount was a stronger predictor of PF occurrence in subsurface horizons. These results suggest that the depth of PF occurrence is influenced both by precipitation dynamics and soil horizon characteristics. Understanding when and where PF occurs in soil profiles improves our ability to predict how changes in climate and land cover could influence future soil water fluxes, with implications for ecosystem productivity, groundwater recharge, solute fluxes, and water quality.