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

Abstract

Abstract. Storage and release of water in the soils is critical for sustaining plant transpiration and groundwater recharge. However, the subsurface mixing of water available for plants or quickly flowing to streams and groundwater is not yet understood. Moreover, while water infiltrating into soils was shown to bypass older pore water, the mechanisms leading to a separation between water routed to the streams and water held tightly in smaller pores are unclear. Here we present an extensive data set, for which we sampled fortnightly the isotopic composition (2H and 18O) of mobile and bulk soil water in parallel with groundwater, stream water and rainfall in the Mediterranean long-term research catchment, Vallcebre, in Spain. The data revealed that mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix, but they constitute two separate subsurface water pools; despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentation results from rewetting of small soil pores with isotopically depleted winter/spring rain. Thus, stable isotopes, and therefore water residence times too, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing water uptake by plants or process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Recently, McDonnell () highlighted a compartmentalization within the terrestrial water cycle beyond the traditional compartments that represent the stores. For example, within the soil water storage, stable isotopic compositions of mobile and bulk soil water are often different (e.g., Brooks et al, ; Goldsmith et al, ; Sprenger, Tetzlaff, Buttle, Laudon, Leistert, et al, ; Sprenger et al, ), which indicates that some part of the infiltrated precipitation recharges to groundwater and streamflow more quickly than others. The slower component of the subsurface flow (studied with bulk water isotopic compositions) was found based on stable isotope data to be transpired preferably over the fast flow (studied with suction lysimeters), resulting in so‐called “ecohydrological separation” (Brooks et al, ; Evaristo et al, ; sections and ).…”

“…Recently, McDonnell (2017) highlighted a compartmentalization within the terrestrial water cycle beyond the traditional compartments that represent the stores. For example, within the soil water storage, stable isotopic compositions of mobile and bulk soil water are often different (e.g., Brooks et al, 2010;Goldsmith et al, 2012;Sprenger, Tetzlaff, Buttle, Laudon, Leistert, et al, 2018;Sprenger et al, 2019), which indicates that some part of the infiltrated precipitation recharges to groundwater and streamflow more quickly than Dashed lines indicate the mixing of water with different ages at the interfaces. The central demographics reflect the unsaturated (yellow) and saturated (blue) zone of the critical zone; the demographics of the vegetation (green) and additional stores on the surface (e.g., snow or glaciers) are shown separately.…”

“…The water ages of the subsurface fluxes are therefore largely controlled by the connectivity of and flow resistance in these pores. As mobile and matrix soil water can be of distinct isotopic composition over long periods spanning varying wetness conditions (Sprenger et al, ), one of the central questions is how much water can exchange (or mix) among the different pore spaces (conceptually shown in Figure b).…”

Persistence and Plasticity in Conifer Water‐Use Strategies

“…In regions such as the western United States where recharge occurs from winter snowpack, a strong isotopic gradient emerges during the growing season because the surface and deep soil waters originate from summer and winter season precipitation, respectively (Hu et al, ; Martin et al, ). However, the idealized case where isotopically enriched summer precipitation is layered atop isotopically depleted winter precipitation is often disrupted by processes such as rapid penetration of rain through preferential flow paths, hydraulic redistribution, lateral flow, and waters of distinct seasonal origins being held selectively according to pore size (Berry et al, ; Brooks et al, ; Dubbert & Werner, ; Sprenger et al, ; Thomas et al, ). Furthermore, the presumably simple transfer of the isotopic ratio of precipitation to the surface soil water can be affected by evaporative enrichment of the surface water and precipitation throughflow effects as precipitation interacts with the canopy (Goldsmith et al, ).…”

Technical note: Evaporating water is different from bulk soil water in δ<sup>2</sup>H and δ<sup>18</sup>O

“…between large pores and small pores (Brooks et al, 2010;Goldsmith et al, 2012;Good et al, 2015;Sprenger et al, 2019a). The irrigation water would first enter large pores, because small pores are 370 occupied by bound water and large pores are empty (Beven and Germann, 1982;Gerke and Van Genuchten, 1993;Sprenger et al, 2019a). Water flow is much faster in large pore than in small pores (Van Genuchten, 1980).…”