Bound Soil Water Research Articles

Assessing the isotopic biases of soil water from cryogenic water extraction in different soil types in China

AbstractThe utilization of deuterium (δ2H) and oxygen (δ18O) isotope ratios in cryogenically extracted water from soil samples is a widely employed method in hydrological and ecological research. Nevertheless, an increasing body of research indicates that cryogenic water extraction (CWE) leads to δ2H depletion in soil water. To investigate the widespread existence of this phenomenon, samples from eight physicochemically distinct soils in China underwent rehydration with a reference water at five different water contents and were subsequently extracted using CWE. In comparison to the reference water, significant and inconsistent δ2H depletion was observed in all eight soil samples. The δ18O bias also exhibited variation, ranging from enrichment to depletion. Generally, Z score assessments indicated unacceptable results for all soils. Water content emerged as the most influential variable affecting both δ2H and δ18O biases, while soil properties had different impacts on these biases. Source water, as calculated by a linear regression model, revealed that the isotopic composition of extracted soil water differed from that of the reference water. The cryogenic extraction error in soil water could not solely attributed to fractionation processes during the extraction but resulted from the release of tightly bound soil water into the reference water. Using the influencing factors, correction models for δ2H and δ18O biases by CWE were developed. By these models, the δ2H and δ18O biases were mostly successful corrected. High soil water extraction efficiency (e.g., 99%) was recommended to minimize isotopic biases. These efforts necessitate further testing, particularly in ecohydrological studies involving isotope measurements of soil water through CWE.

A δ18O and δ2H stable water isotope analysis of subalpine forest water sources under seasonal and hydrological stress in the Canadian Rocky Mountains

AbstractSubalpine forests are hydrologically important to the function and health of mountain basins. Identifying the specific water sources and the proportions used by subalpine forests is necessary to understand potential impacts to these forests under a changing climate. The recent “Two Water Worlds” hypothesis suggests that trees can favour tightly bound soil water instead of readily available free‐flowing soil water. Little is known about the specific sources of water used by subalpine trees Abies lasiocarpa (Subalpine fir) and Picea engelmannii (Engelmann spruce) in the Canadian Rocky Mountains. In this study, stable water isotope (δ18O and δ2H) samples were obtained from S. fir and Engelmann spruce trees at three points of the growing season in combination with water sources available at time of sampling (snow, vadose zone water, saturated zone water, precipitation). Using the Bayesian Mixing Model, MixSIAR, relative source water proportions were calculated. In the drought summer examined, there was a net loss of water via evapotranspiration from the system. Results highlighted the importance of tightly vadose zone, or bound soil water, to subalpine forests, providing insights of future health under sustained years of drought and net loss in summer growing seasons. This work builds upon concepts from the “Two Water Worlds” hypothesis, showing that subalpine trees can draw from different water sources depending on season and availability. In our case, water use was largely driven by a tension gradient within the soil allowing trees to utilize vadose zone water and saturated zone water at differing points of the growing season.

Hydrological functioning of thawing soil water in a permafrost‐influenced alpine meadow hillslope

AbstractThe complex variation of the hydrological functioning of soil water has been widely studied using measurements of the stable isotopic composition of either the bulk soil water (BSW) collected by the cryogenic vacuum extraction method or the mobile soil water (MSW) collected by the zero‐tension soil lysimeter method. We collected samples of precipitation, BSW, MSW, and hillslope runoff in an alpine meadow hillslope influenced by permafrost to investigate the hydrological functioning of thawing soil water. The results showed that the δ2H values in BSW were generally more negative than those of the MSW, which may be a result of the depleted spring snowmelt that formed the tightly bound soil water. The relatively stable characteristics of δ2H in the MSW were related to the dynamic mixing of newly infiltrated precipitation with slightly enriched mobile water held by the low soil matrix potential. The δ2H values in the BSW showed higher temporal variations than the values in the MSW, which were mainly related to the mixing of depleted tightly bound soil water and enriched MSW in varying fractions. The MSW better represented the lateral subsurface flow (SSF, which further generated streamflow) than the BSW samples considering the relatively close stable isotopic values between the lateral SSF and the MSW in the same soil layer. The results of this study show the influences of the varying water level maintained by the underlying permafrost on the water and isotopic dynamics of both the soil water pools and streamflow.

Vegetation source water identification using isotopic and hydrometric observations from a subhumid mountain catchment

AbstractThis study coupled long‐term hydrometric and stable water isotope data to identify links between subsurface water storage and vegetation in a subhumid mountain catchment in Arizona, USA. Specific observations included catchment‐scale hydrologic fluxes and soil water storage and stable water isotopes from stream water, soil water, groundwater, and sap water from Arizona pine (Pinus arizonica) and Douglas fir (Pseudotsuga menziesii) individuals. Here, we find that tightly bound soil water was sufficient to meet dry period vegetation water demand when the former was defined in terms of field capacity as opposed to a matric tension threshold. This water was a mixture of summer and winter precipitation that predominates in both shallow and deep soil waters, and contributed significantly to streamflow. We also identified a less common mobile water type that did not contribute significantly to streamflow and was related to infiltration during isotopically depleted precipitation events. Although each water type was used by both Arizona pine and Douglas fir vegetation, the second water type was dominant in Douglas fir sap water. Therefore, we conclude that Arizona pine and Douglas fir can occupy different ecohydrological niches at this subhumid mountain location. Further, a lack of isotopic distinction between tightly bound and inferred mobile soil water signals that the ecohydrological water source separation hypothesis is not entirely applicable at this site. The results of this study broadly highlight how alternative definitions of tightly bound water can influence interpretation of data, and contribute to a more thorough understanding of interactions between subsurface storage and plant water dynamics.

A pool-weighted perspective on the two-water-worlds hypothesis.

The 'two-water-worlds' hypothesis is based on stable isotope differences in stream, soil and xylem waters in dual isotope space. It postulates no connectivity between bound and mobile soil waters, and preferential plant water uptake of bound soil water sources. We tested the pool-weighted impact of isotopically distinct water pools for hydrological cycling, the influence of species-specific water use and the degree of ecohydrological separation. We combined stable isotope analysis (δ18 O and δ2 H) of ecosystem water pools of precipitation, groundwater, soil and xylem water of two distinct species (Quercus suber, Cistus ladanifer) with observations of soil water contents and sap flow. Shallow soil water was evaporatively enriched during dry-down periods, but enrichment faded strongly with depth and upon precipitation events. Despite clearly distinct water sources and water-use strategies, both species displayed a highly opportunistic pattern of root water uptake. Here we offer an alternative explanation, showing that the isotopic differences between soil and plant water vs groundwater can be fully explained by spatio-temporal dynamics. Pool weighting the contribution of evaporatively enriched soil water reveals only minor annual impacts of these sources to ecosystem water cycling (c.11% of bulk soil water and c.14% of transpired water).

An Improved Microwave Semiempirical Model for the Dielectric Behavior of Moist Soils

Soil semiempirical dielectric models (SEMs) are powerful, and they are generally considered a useful hybrid of both empirical and physical models. In this paper, the Wang–Schmugge dielectric model is improved to more accurately estimate the relative complex dielectric constants (CDCs) of moist soils. Instead of the Debye relaxation spectrum of liquid water located outside of the soil (i.e., free out-of-soil water) adopted in the Wang–Schmugge model, the Debye relaxation formula related to the free-water component inside the soil [i.e., free soil water (FSW)], which is correlated with the soil texture, is employed in the improved SEM. In addition, the effective conductivity loss term related to both soil texture and soil moisture is introduced to explain the ionic conductivity losses of FSW. Since the soil moisture influence is reduced at high frequencies, the effective conductivity loss term related to only the soil texture is also analyzed for 14–18 GHz. As in the Wang–Schmugge model, the relative CDC of bound soil water varies with the soil volumetric moisture content when the soil moisture is lower than the maximum bound water fraction in the new model, which takes a different approach than the Mironov mineralogy-based SEM. The proposed model obtains better fitting results than the three most widely employed SEMs. The improved model exhibits a significantly improved accuracy with a higher correlation coefficient ( $R^{2}$ ), a closer 1:1 relationship, and a lower root-mean-square error, including in the L-band, and especially in the imaginary part of the L-band.

Tightly bound soil water introduces isotopic memory effects on mobile and extractable soil water pools

ABSTRACTCryogenic vacuum extraction is the well-established method of extracting water from soil for isotopic analyses of waters moving through the soil–plant–atmosphere continuum. We investigate if soils can alter the isotopic composition of water through isotope memory effects, and determined which mechanisms are responsible for it. Soils with differing physicochemical properties were re-wetted with reference water and subsequently extracted by cryogenic water distillation. Results suggest some reference waters bind tightly to the soil and not all of this tightly bound water is removed during cryogenic vacuum extraction. Kinetic isotopic fractionation occurring when reference water binds to the soil is likely responsible for the 18O-depletion of re-extracted reference water, suggesting an enrichment of the tightly bound soil water pool. Further re-wetting of cryogenically extracted soils indicates an isotopic memory effect of tightly bound soil water on water added to the soil. The data suggest tightly bound soil water can influence the isotopic composition of mobile soil water. Findings show that soils influence the isotope composition of soil water by (i) kinetic fractionation when water is bound to the soil and (ii) equilibrium fractionation between different soil water pools. These findings could be relevant for plant water uptake investigations and complicate ecohydrological and paleohydrological studies.

The two water worlds hypothesis: Addressing multiple working hypotheses and proposing a way forward

AbstractRecent studies using water isotopes have shown that trees and streams appear to return distinct water pools to the hydrosphere. Cryogenically extracted plant and soil water isotopic signatures diverge from the meteoric water lines, suggesting that plants would preferentially use bound soil water, while mobile soil water that infiltrates the soil recharges groundwater and feeds streamflow all plots on meteoric water lines. These findings have been described under the “two water worlds” (TWW) hypothesis. In spite of growing evidence for the TWW hypothesis, several questions remain unsolved within the scope of this framework. Here, we address the TWW as a null hypothesis and further assess the following: (a) the theoretical biophysical feasibility for two distinct water pools to exist, (b) plant and soil processes that could explain the different isotopic composition between the two water pools, and (c) methodological issues that could explain the divergent isotopic signatures. Moreover, we propose a way forward under the framework of the TWW hypothesis, proposing alternative perspectives and explanations, experiments to further test them, and methodological advances that could help illuminate this quest. We further highlight the need to improve our sampling resolution of plants and soils across time and space. We ultimately propose a set of key priorities for future research to improve our understanding of the ecohydrological processes controlling water flows through the soil–plant‐atmosphere continuum.

The two water worlds hypothesis: ecohydrological separation of water between streams and trees?

Recent work in ecohydrology has shown that in some forested watershed systems, streams and trees appear to return different pools of water to the hydrosphere. Thus far, evidence for this has come exclusively from wet Mediterranean climates. This short opinion article outlines the hypothesis and a research agenda for future work. The most pressing issue is the need to gather more data points whereby dual isotope‐based studies in forested catchments compare samples of plant water and tightly bound soil water as well as mobile waters (soil, groundwater, and streamflow) in the catchment. New work is needed to test the hypothesis across different climates and vegetation regimes, especially places that contrast with the Mediterranean climates and forest types where two water worlds have been found. These include, but are not restricted to humid areas where plant water use and precipitation input are in phase, wetter zones where seasonality of precipitation is low, and drier zones where water stress is higher. Of equal importance to these basic research issues are the practical issues surrounding the sampling methods of plant and soil waters. Studies are needed to compare extraction techniques for low and high mobility soil waters and to understand the effect of sampling protocol on water isotope composition. Once these issues are resolved, high frequency sampling of soil and xylem waters will be especially instructive in development of mechanistic models of ecohydrological interaction—and an explanation for the hypothesis that is still wanting. WIREs Water 2014, 1:323–329. doi: 10.1002/wat2.1027This article is categorized under: Science of Water > Hydrological Processes

Generalized refractive mixing dielectric model of moist soils considering ionic relaxation of soil water

A generalized multi-relaxation refractive mixing dielectric model of moist soils is suggested for frequencies in the range from 0.2 to 14.8 GHz. The model is based on dielectric measurements in this frequency range at a temperature of 20°С for relative moisture content (by weight) changing from 2 to 43%. The model expands the range of applicability of the single-relaxation model suggested previously to the megahertz frequency range with allowance for the Maxwell–Wagner ionic relaxation which is clearly manifested in this frequency range for bound soil water. It is demonstrated that the error in calculating the complex dielectric permittivity spectra of the moist soil has the same order of magnitude as the error of experimental data used for model construction.

A tightly bound soil–water scheme within an atmosphere–land–surface model

Summary The concept of tightly bound water, in which a reservoir of soil water is bound tightly within small soil pores but is still available for evapotranspiration, is parameterised for the first time within the land surface scheme of a fully-coupled regional-scale atmosphere-land surface model. The Weather Research and Forecasting (WRF) regional climate model and the NOAH land surface scheme are selected and a case study is performed on the Olifants River Basin in the Limpopo region of South Africa. Accurate knowledge of water availability in this water-stressed region is of great importance for adaptation and future water policy. Results of a simulation forced by ERA40 re-analysis show that the standard land surface scheme is unable to reproduce the observed runoff despite rainfall and atmospheric conditions similar to observed. This version of the model over-estimates mean annual runoff by 120%. The tightly bound water scheme shows a significant improvement, reducing the bias to 22%. The inclusion of the tightly bound water scheme has little effect on the basin average annual rainfall despite increasing annual evapotranspiration. The tightly bound water physics dampens the response of runoff to precipitation and provides additional de-coupling between precipitation and runoff, increasing the variability in this relationship. Simulations with the WRF model forced with both 1980s and 2040s CCSM3 data show that the tightly bound water scheme significantly reduces runoff in different climates and projects a greater relative future decrease in runoff, from 4% to 10% for the same precipitation decrease of 2.5%. The scheme also affects the projected changes in spatially averaged 100-year return precipitation and runoff with significance at the 0.9 confidence level.

Generalized refractive mixing dielectric model for moist soils

This paper introduces a new soil dielectric model in the microwave band offering variation over a wide range of soil moisture, texture, mineral content, and wave frequency. The soil dielectric models commonly used in microwave remote sensing algorithms rarely apply intrinsic bound soil water (BSW) dielectric properties. However, models incorporating these properties could offer additional applicability and accuracy. This paper describes a technique developed for estimating the complex dielectric constant (CDC) for both the BSW and the free soil water (FSW) as separate water constituents present in moist soil. For this purpose, it has been shown that there exist two specific soil moisture regions where alteration in soil dielectric property is generated, exclusively, by either BSW or FSW increments, with their CDCs being constant values. The CDCs of the BSW and FSW are derived through straight line fitting with the help of a measured soil complex refractive index (CRI) as a function of moisture. The term CRI is understood here as a square root of the CDC. In order to distinguish between the two moisture regions, the maximum bound water fraction (MBWF) parameter was introduced, which also can be obtained through straight line fitting, using measured CRIs as a function of moisture over both specific moisture regions. Thus, obtained fittings are known in the literature as the refractive mixing dielectric model (RMDM). Finally, a method for deducing the Debye relaxation parameters for both types of soil water was proposed, with their CDCs being input parameters. It was shown that the soil water CDCs measured only at two frequencies are sufficient for deriving the Debye relaxation parameters. A set of equations representing the RMDM, the Debye formula, and the formulas proposed for deriving the Debye relaxation parameters is designated as the generalized refractive mixing dielectric model (GRMDM). In contrast to all previously known soil dielectric models, the GRMDM allows for CDC prediction beyond the frequency range where the initial soil CDC data are acquired.

Thermodielectric effects on radar backscattering from wet soil

Thermodielectric interactions between free and bound soil water at different soil textures and frequencies affect radar backscatter. Simulations with a semi-empirical backscatter model show a potential for temperature corrections in inference of water content. A new thermodielectric response backscatter difference index was derived for remote mapping of surface soil texture.