Mechanisms of consistently disjunct soil water pools over (pore) space and time

Summary

This field study in a Mediterranean Scots pine forest reveals that soil water exists as two isotopically distinct pools—mobile water in larger pores and tightly bound water in smaller pores—with minimal mixing even during intense rainfall. The compartmentalisation arises from rewetting of small pores by isotopically depleted winter and spring precipitation, such that residence times and isotopic signatures vary across both soil depth and pore size. The findings suggest that stable isotope applications in ecohydrology and hydrological models require explicit representation of pore-scale water partitioning to accurately simulate evapotranspiration and groundwater recharge.

Regional applicability

This Spanish Mediterranean study is not directly representative of United Kingdom soil and climate conditions, which are cooler and more temperate with different precipitation seasonality. However, the mechanistic insights regarding pore-scale water compartmentalisation and stable isotope fractionation are relevant to UK hydrological and ecohydrological research, particularly for refining critical zone models and plant water-uptake interpretations in any soil type where pore-size heterogeneity affects water availability.

Key measures

Stable isotope ratios (²H and ¹⁸O) in mobile soil water, bulk soil water, groundwater, stream water, and rainfall; temporal resolution fortnightly for soil and groundwater, high-frequency for stream and rainfall; soil texture and forest type characterisation

Outcomes reported

The study measured isotopic composition (²H and ¹⁸O) of mobile water, bulk soil water, groundwater, stream water and rainfall to characterise subsurface water mixing and residence times. Findings revealed that mobile and tightly bound soil water form two distinct, largely non-mixing pools despite high soil wetness from rainfall events.

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