New Precipitation Is Scarce in Deep Soils: Findings From 47 Forest Plots Spanning Switzerland

Abstract As precipitation infiltrates into soils, it can recharge them, displace previously stored waters, or bypass already‐filled pores. Using 3,697 δ 2 H and δ 18 O measurements of water collected nearly monthly over >3 years in 47 forest plots across Switzerland, we present a systematic investigation of the controls on mobile soil water transport. We quantified the lags and damping of water as it percolates downward using young water fraction analysis ( F yw ), and the fractions of soil water composed by precipitation that fell within the previous month (new water fractions, F new ). The F new of water sampled in surface soils ranged widely, from 0% to 50%, but those fractions typically decreased with depth and converged on values of 0%–20% at depths below 80 cm. Soil characteristics explained much of the variation in F yw and F new , as did climatological and root characteristics to a lesser, but still statistically significant, degree.