Effects of climatic seasonality on the isotopic composition of evaporating soil waters

Abstract

Abstract. Stable water isotopes are widely used in ecohydrology as tracers of the transport, storage, and mixing of water on its journey through landscapes and ecosystems. Evaporation leaves a characteristic signature on the isotopic composition of the water that is left behind, such that in dual-isotope space, evaporated waters plot below the Local Meteoric Water Line (LMWL) that characterizes precipitation. Soil and xylem water samples can often plot below the LMWL as well, suggesting that they have also been influenced by evaporation. These soil and xylem water samples frequently plot along linear trends 5 in dual-isotope space. These trendlines are sometimes termed "evaporation lines" and their intersection with the LMWL is sometimes interpreted as the isotopic composition of the precipitation source water. Here we use numerical experiments based on established isotope fractionation theory to show that these trendlines are often by-products of the seasonality in evaporative fractionation and in the isotopic composition of precipitation. Thus, they are often not true evaporation lines, and, if interpreted as such, can yield highly biased estimates of the isotopic composition of the source water.

‘Teflon Basin’ or Not? A High-Elevation Catchment Transit Time Modeling Approach

“…We also used a time-invariant TTD modeling approach due to data limitations, since time-variant TTD modeling approaches typically require longer tracer records and/or higher sampling frequencies [68][69][70][71][72][73]. Although headwater catchments-often characterized by high responsiveness and high seasonal variability in the climatic conditions-are known to be non-steady systems [74,75], this approach is still feasible [32,68], but time-variant approaches are becoming popular, e.g., [63].…”

Tracing ecosystem water fluxes using hydrogen and oxygen stable isotopes: challenges and opportunities from an interdisciplinary perspective

“…The third main aspect highlighted during the workshop's discussions is the need to incorporate knowledge regarding 5 fractionation effects (e.g., Dawson and Ehleringer, 1993) into the models that are used to interpret isotope data. For example, process-based models may help interpreting observations and experimental data (Benettin et al, 2018). Clarifications are needed on which parameters to include in a model and on when it might be possible to ignore their influence.…”

“…Indeed, evaporation, which is largely controlled by surface energy variations, is a major contributor to isotopic fractionation, especially at shallow soil depths. In soils and groundwater, isotopic heterogeneity results from differences in the inputs (precipitation, throughfall, snowmelt), differences in the temporal integration of previous precipitation events, and differences in the subsequent fractionation from evaporation and transpiration (Benettin et al, 2018). As a general 15 rule, smaller storage-to-output ratios with short residence times generally lead to higher temporal variability.…”

Estimating water flux and evaporation losses using stable isotopes of soil water from irrigated agricultural crops in tropical humid regions

“…where steady state conditions can be expected in rice fields (Wei et al, 2015). Variables δ * and m were calculated following Equations 2 and 3 (as described in Benettin et al (2018) and Gibson (2016)):…”

“…where δA is the isotopic composition of atmospheric vapor [‰] (calculated according to Benettin et al (2018) (Benettin et al, 2018). Furthermore, εk was calculated according to Benettin et al (2018), presuming diffusive transport in soil pore spaces (Barnes and Allison, 1983). The equilibrium isotopic separation between liquid and vapor was computed as + = ( + − 1)10 3 [‰] (Gat, 1996;Horita et al, 2008.…”