Illuminating hydrological processes at the soil‐vegetation‐atmosphere interface with water stable isotopes

Abstract

Water stable isotopes (18O and 2H) are widely used as ideal tracers to track water through the soil and to separate evaporation from transpiration. Due to the technical developments in the last two decades, soil water stable isotope data have become easier to collect. Thus, the application of isotope methods in soils is growing rapidly. Studies that make use of soil water stable isotopes often have a multidisciplinary character since an interplay of processes that take place in the vadose zone has to be considered. In this review, we provide an overview of the hydrological processes that alter the soil water stable isotopic composition and present studies utilizing pore water stable isotopes. The processes that are discussed include the water input as precipitation or throughfall, the output as evaporation, transpiration, or recharge, and specific flow and transport processes. Based on the review and supported by additional data and modeling results, we pose a different view on the recently proposed two water world hypothesis. As an alternative to two distinct pools of soil water, where one pool is enriched in heavy isotopes and used by the vegetation and the other pool does not undergo isotopic fractionation and becomes recharge, the water gets successively mixed with newly introduced rainwater during the percolation process. This way, water initially isotopically enriched in the topsoil loses the fractionation signal with increasing infiltration depth, leading to unfractionated isotopic signals in the groundwater.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…This "two-water-world" hypothesis (McDonnell, 2014) is controversial (Berry et al, 2017;Sprenger et al, 2016) and could be at odds with the existence of subsurface reservoirs such as layers of saprolite and fractured, partly weathered immobile material that hold water that is accessed by trees (Oshun et al, 2016). For example, in seasonally dry climates, trees may derive a significant portion of their moisture from immobile weathered material well below the soil (Zwieniecki and Newton, 1996;Graham et al, 2010;Nie et al, 2012).…”

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…The frequently measured soil water isotope data we have presented and the inherent evaporation signal can help to calibrate or benchmark the representation of soil-vegetationatmosphere interactions in tracer-aided hydrological modeling from the plot (Rothfuss et al, 2012;Sprenger et al, 2016b) to the catchment scale van Huijgevoort et al, 2016). So far, the isotopic composition of mobile water has been used to better constrain semi-distributed models van Huijgevoort et al, 2016).…”

“…Such enrichment from kinetic fractionation was found in soil water isotopes across various climatic regions, with more pronounced evaporative signals reaching deeper into the soils in arid and Mediterranean environments than in temperate regions (Sprenger et al, 2016b). However, the temporal variability of the isotopic fractionation in the field has not yet been studied, despite recent technical developments that enabled easier analysis of stable isotopes in soil water (see review by Sprenger et al, 2015a).…”

The test of the ecohydrological separation hypothesis in a dry zone of the northeastern Tibetan Plateau

“…McCutcheon, McNamara, Kohn, and Evans (2017) suggested that cannot relate soil water mobility to isotopic composition, and the isotopic distinction between root-absorbed and draining waters may not be an appropriate indicator of ecohydrological separation of soil waters. Sprenger, Leistert, Gimbel, and Weiler (2016) proposed that subsequent mixing of the evaporated soil water with nonfractionated precipitation water (i.e., subsequent fading of fraction effects) could explain the differences in the isotopic signal of water in the top soil and in the xylem of plants on the one hand and groundwater and streamwater on the other hand. Sprenger, Leistert, Gimbel, and Weiler (2016) proposed that subsequent mixing of the evaporated soil water with nonfractionated precipitation water (i.e., subsequent fading of fraction effects) could explain the differences in the isotopic signal of water in the top soil and in the xylem of plants on the one hand and groundwater and streamwater on the other hand.…”