Simulating soil-plant-atmosphere interactions for sub-daily in situ observations of stable isotopes in soil and xylem water to assess two-pore domain model hypothesis

2020 ◽  
Author(s):  
Fabian Bernhard ◽  
Stefan Seeger ◽  
Markus Weiler ◽  
Arthur Gessler ◽  
Katrin Meusburger
Keyword(s):  
Stable Isotopes ◽  
Soil Water ◽  
Vadose Zone ◽  
Model Performance ◽  
Root Water Uptake ◽  
In Situ Observations ◽  
Pore Domain ◽  
Model Structures ◽  
Isotope Measurements

<p>Recent advances in stable isotope measurements within the soil-plant-atmosphere continuum have paved the way to high-resolution sub-daily observations of plant water supply (Stumpp et al. 2018, Volkmann et al. 2016a, 2016b). It seems time is ripe for in-depth assessments of long-standing yet much-debated assumptions such as complete, homogenous mixing of water in the vadose zone (“one water world” versus "two water world") or absence of fractionation during root water uptake and vascular transport in plants.</p><p>Information on the nature of these processes contained in high-resolution data sets needs to be exploited. One way to test hypotheses and thereby advance our understanding of soil-plant water interactions is by analysing observations with numerical simulations of the system dynamics – a method also known as inverse modelling. By evaluating the model performance and parameter identifiability of different model structures, conclusions can be drawn regarding the relevance of the modelled processes for reproduction of the observations. Testing two different models allows thus to assess the impact of the difference.</p><p>We develop a framework for numerical simulation and model-based analysis of observations from soil-plant-atmosphere systems with a focus on isotopic fractionation. A central objective is to facilitate the evaluation of different model structures and thus test model hypotheses. This can assist development of models specifically tailored to the intended purpose and available data. The framework will first be tested with the "SWIS" model presented by Sprenger et al. (2018).</p><p>As an illustration of the framework, we will test the model performance on a dataset of continuous, in situ observations of stable isotopes in xylem water of beech trees and soil water in four depths combined with observations of soil water content. The model assumes one-dimensional soil water flow taking place in one or two separate flow domains for tightly and weakly bound pore water. These two water pools are separated by a matrix potential threshold and isotopic exchange is modelled only through the vapour phase. Root water uptake is parametrised using the Feddes-Jarvis model. First results allow to assess the relevance of the two-pore domain hypothesis for the different soil depths and xylem water.</p><p> </p><p>Sprenger, M., D. Tetzlaff, J. Buttle, H. Laudon, H. Leistert, C.P.J. Mitchell, J. Snelgrove, M. Weiler, and C. Soulsby. 2018. Measuring and modeling stable isotopes of mobile and bulk soil water. <em>Vadose Zone J.</em> 17:170149. doi:10.2136/vzj2017.08.0149</p><p>Stumpp, C., N. Brüggemann, and L. Wingate. 2018. Stable isotope approaches in vadose zone research. <em>Vadose Zone J.</em> 17:180096. Doi: 10.2136/vzj2018.05.0096</p><p>Volkmann, T.H., K. Haberer, A. Gessler, and M. Weiler. 2016a. High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. <em>New Phytologist</em>, 210(3), 839-849.</p><p>Volkmann, T.H., K. Haberer, A. Gessler, and M. Weiler. 2016a. High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. <em>New Phytologist</em>, 210(3), 839-849.</p>

Isotope labeling experiment to infer ecohydrological travel times

2020 ◽  
Author(s):  
David Mennekes ◽  
Michael Rinderer ◽  
Stefan Seeger ◽  
Hugo de Boer ◽  
Natalie Orlowski ◽  
...  
Keyword(s):  
Soil Water ◽  
Water Uptake ◽  
Temporal Resolution ◽  
Water Vapor Pressure ◽  
Root Water Uptake ◽  
Field Conditions ◽  
High Temporal Resolution ◽  
Travel Times ◽  
Isotope Measurements

<p>Stable water isotopes are promising tracers to study soil-tree interactions and root water uptake. Traditionally, destructive sampling techniques are applied to measure the isotopic signature in soils and plant tissues but these methods are limited in their temporal resolution. For calculating ecohydrological travel times from soil water to transpiration, high frequent isotope measurements are required. Recently, in-situ water isotope probes have been successfully applied in beech trees to yield high-frequent isotope measurements under field conditions but the complexity and heterogeneity of natural field conditions can make a systematical method testing difficult. Here, we test whether the new probes are capable of capturing tree species-specific differences in root water uptake and associated travel times.<br>We test this in a controlled experiment using large pots with three 4-6 meter high and 20 year old coniferous and deciduous trees: <em>Pinus pinea</em>, <em>Alnus</em> <em>x spaethii</em> and <em>Quercus</em> <em>suber</em> that are expected to have different water uptake strategies. We applied deuterated irrigation water to the homogeneous soils in the pots and traced the water flux from the soils through the trees with in-situ isotope probes in high temporal resolution.<br>This contribution presents preliminary results on ecohydrological travel times in relation to environmental parameters such as sap flow, photosynthetic activity, matrix potential, soil water content, water vapor pressure deficit and solar radiation.<br>Our in-situ isotope probes were capable to capture the breakthrough of the isotope tracer in all trees. The calculated travel times were shorter for the Pinus and Alnus compared to the Quercus which suggests differences in root water uptake. Detailed results from such controlled experiments are fundamental for testing new measurement techniques such as the in-situ isotope probes. Such results are important to better interpret results measured under natural and therefore more complex and heterogeneous field conditions.</p>

scholarly journals From near-surface to root-zone soil moisture using an exponential filter: an assessment of the method based on in-situ observations and model simulations

2008 ◽  
Vol 12 (6) ◽  
pp. 1323-1337 ◽  
Author(s):  
C. Albergel ◽  
C. Rüdiger ◽  
T. Pellarin ◽  
J.-C. Calvet ◽  
N. Fritz ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Soil Water ◽  
Surface Soil ◽  
Root Zone ◽  
Synthetic Data ◽  
Surface Soil Moisture ◽  
Data Set ◽  
Water Index ◽  
In Situ Observations

Abstract. A long term data acquisition effort of profile soil moisture is under way in southwestern France at 13 automated weather stations. This ground network was developed in order to validate remote sensing and model soil moisture estimates. In this paper, both those in situ observations and a synthetic data set covering continental France are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter equation using a time constant, T, is used to compute a soil water index. The Nash and Sutcliff coefficient is used as a criterion to optimise the T parameter for each ground station and for each model pixel of the synthetic data set. In general, the soil water indices derived from the surface soil moisture observations and simulations agree well with the reference root-zone soil moisture. Overall, the results show the potential of the exponential filter equation and of its recursive formulation to derive a soil water index from surface soil moisture estimates. This paper further investigates the correlation of the time scale parameter T with soil properties and climate conditions. While no significant relationship could be determined between T and the main soil properties (clay and sand fractions, bulk density and organic matter content), the modelled spatial variability and the observed inter-annual variability of T suggest that a weak climate effect may exist.

Characterization of hydraulic properties in the upper vadose zone for two aquifers in Slovenia

2021 ◽  
Author(s):  
Vesna Zupanc ◽  
Matjaž Glavan ◽  
Miha Curk ◽  
Urša Pečan ◽  
Michael Stockinger ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Soil Water ◽  
Water Flow ◽  
Vadose Zone ◽  
Hydraulic Properties ◽  
Isotope Composition ◽  
Transport Processes ◽  
Precipitation Event ◽  
Water Fluxes ◽  
Flow And Transport

<p>Environmental tracers, present in the environment and provided by nature, provide integrative information about both water flow and transport. For studying water flow and solute transport, the hydrogen and oxygen isotopes are of special interest, as their ratios provide a tracer signal with every precipitation event and are seasonally distributed. In order to follow the seasonal distribution of stable isotopes in the soil water and use this information for identifying hydrological processes and hydraulic properties, soil was sampled three times in three profiles, two on Krško polje aquifer in SE Slovenia and one on Ljubljansko polje in central Slovenia. Isotope composition of soil water was measured with the water-vapor-equilibration method. Based on the isotope composition of soil water integrative information about water flow and transport processes with time and depth below ground were assessed. Porewater isotopes were in similar range as precipitation for all three profiles.  Variable isotope ratios in the upper 60 cm for the different sampling times indicated dynamic water fluxes in this upper part of the vadose zone. Results also showed more evaporation at one sampling location, Brege. The information from stable isotopes will be of importance for further analyzing the water fluxes in the vadose zone of the study sties. <br>This research was financed by the ARRS BIAT 20-21-32 and IAEA CRP 1.50.18 Multiple isotope fingerprints to identify sources and transport of agro-contaminants.  </p>

scholarly journals Regional climate-model performance in Greenland firn derived from in situ observations

1969 ◽  
Vol 35 ◽  
pp. 75-78 ◽  
Author(s):  
Charalampos Charalampidis ◽  
Dirk Van As ◽  
Peter L. Langen ◽  
Robert S. Fausto ◽  
Baptiste Vandecrux ◽  
...  
Keyword(s):  
Regional Climate Model ◽  
Climate Model ◽  
Regional Climate ◽  
Ice Sheet ◽  
Model Performance ◽  
Mass Conservation ◽  
Mass Budget ◽  
Run Off ◽  
In Situ Observations

Recent record-warm summers in Greenland (Khan et al. 2015) have started affecting the higher regions of the ice sheet (i.e. the accumulation area), where increased melt has altered the properties of firn (i.e. multi-year snow). At high altitudes, meltwater percolates in the porous snow and firn, where it refreezes. The result is mass conservation, as the refrozen meltwater is essentially stored (Harper et al. 2012). However, in some regions increased meltwater refreezing in shallow firn has created thick ice layers. These ice layers act as a lid, and can inhibit meltwater percolation to greater depths, causing it to run off instead (Machguth et al. 2016). Meltwater at the surface also results in more absorbed sunlight, and hence increased melt in the accumulation area (Charalampidis et al. 2015). These relatively poorly understood processes are important for ice-sheet mass-budget projections.

scholarly journals Bayesian inverse modelling of in situ soil water dynamics: using prior information about the soil hydraulic properties

2011 ◽  
Vol 8 (1) ◽  
pp. 2019-2063 ◽  
Author(s):  
B. Scharnagl ◽  
J. A. Vrugt ◽  
H. Vereecken ◽  
M. Herbst
Keyword(s):  
Water Content ◽  
Soil Water ◽  
Vadose Zone ◽  
Hydraulic Properties ◽  
Prior Information ◽  
Hydraulic Parameters ◽  
Soil Hydraulic Properties ◽  
Soil Hydraulic Parameters ◽  
Soil Hydraulic

Abstract. In situ observations of soil water state variables under natural boundary conditions are often used to estimate field-scale soil hydraulic properties. However, many contributions to the soil hydrological literature have demonstrated that the information content of such data is insufficient to reliably estimate all the soil hydraulic parameters. In this case study, we tested whether prior information about the soil hydraulic properties could help improve the identifiability of the van Genuchten-Mualem (VGM) parameters. Three different prior distributions with increasing complexity were formulated using the ROSETTA pedotransfer function (PTF) with input data that constitutes basic soil information and is readily available in most vadose zone studies. The inverse problem was posed in a formal Bayesian framework and solved using Markov chain Monte Carlo (MCMC) simulation with the DiffeRential Evolution Adaptive Metropolis (DREAM) algorithm. Synthetic and real-world soil water content data were used to illustrate our approach. The results of this study corroborate and explicate findings previously reported in the literature. Indeed, soil water content data alone contained insufficient information to reasonably constrain all VGM parameters. The identifiability of these soil hydraulic parameters was substantially improved when an informative prior distribution was used with detailed knowledge of the correlation structure among the respective VGM parameters. A biased prior did not distort the results, which inspires confidence in the robustness and effectiveness of the presented method. The Bayesian framework presented in this study can be applied to a wide range of vadose zone studies and provides a blueprint for the use of prior information in inverse modelling of soil hydraulic properties at various spatial scales.

Inferring plant physiologic parameters for root water uptake modelling from high frequency in-situ isotope measurements

2020 ◽  
Author(s):  
Stefan Seeger ◽  
Michael Rinderer ◽  
Markus Weiler
Keyword(s):  
Soil Moisture ◽  
Water Uptake ◽  
Root Distribution ◽  
Fine Root ◽  
Root Water Uptake ◽  
Plant Water ◽  
Plant Water Uptake ◽  
Fine Root Distribution ◽  
Isotope Measurements

<p>In the face of global climate change, a well-informed knowledge of plant physiologic key parameters is essential to predict the behavior of ecosystems in a changing environment. Many of these parameters may be determined with lab or pot experiments, but it could prove problematic to transfer results obtained in a such experiments with small trees to fully grown trees. Therefore, new approaches to determine relevant parameters for mature trees are still required. Regarding plant water uptake, parameters related to fine root distribution (maximum depth, depth distribution and rhizosphere radius) and parameters describing the physiological limits of root water uptake are important, but usually hard or costly to assess for fully grown trees.  In-situ isotope probes (Volkmann et al. 2016a  & 2016b) are a promising recent development that offer new possibilities for the investigation of plant water uptake and associated physiological parameters.</p><p>In this study we used in-situ stable water isotope probes in soil (six depths from 10 to 100 cm) and in tree xylem of mature (140 years) European beech trees (three heights between 0 and 8 m). With those probes, we monitored soil and xylem isotope signatures after an isotopically labeled (Deutrium-Excess = 100 ‰) irrigation pulse equivalent to 150 mm of precipitation and foursubsequent natural precipitation events over a period of twelve weeks with a high temporal resolution (six or more measurements per probe per day). Those measurements were complemented with measurements of soil moisture and sap flow dynamics. We interpolated our measured soil isotope and soil moisture data in order to obtain spatially and temporally continuous data for those soil parameters. Then we used this data as an input to the Feddes-Jarvis plant water uptake model, in order to predict the isotopic signature of plant water uptake at daily time steps. With the help of our observed isotopic signatures, we were able to directly constrain the critical water potential parameter of the Feddes model as well as the underlying fine root distribution. Furthermore, the observed dampening of the breakthrough curve of our Deuterium-labeling pulse allowed us to infer information on the rhizosphere  radius and water transport velocities in the fine roots and stem between the points of root water uptake and the eight meter stem height.</p><p>With our field experiment we showed that in-situ isotope measurements in soil profiles and in tree xylem sap can help to constrain plant water uptake modelling parameters. Future experiments might use this approach to scrutinize lab-scale derived hypothesizes regarding tree water uptake and to investigate the temporal and spatial dynamics of root water uptake in the field.</p><p> </p><p><em>Volkmann, T. H., Haberer, K., Gessler, A., & Weiler, M. (2016a). High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. New Phytologist, 210(3), 839-849. </em></p><p><em>Volkmann, T. H., Kühnhammer, K., Herbstritt, B., Gessler, A., & Weiler, M. (2016b). A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, cell & environment, 39(9), 2055-2063. </em></p><p><em>Jarvis, N. J. (1989). A simple empirical model of root water uptake. Journal of Hydrology, 107(1-4), 57-72. </em></p>

scholarly journals In situ unsaturated zone water stable isotope (<sup>2</sup>H and <sup>18</sup>O) measurements in semi-arid environments: a soil water balance

2016 ◽  
Vol 20 (2) ◽  
pp. 715-731 ◽  
Author(s):  
Marcel Gaj ◽  
Matthias Beyer ◽  
Paul Koeniger ◽  
Heike Wanke ◽  
Josefina Hamutoko ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Water Balance ◽  
Soil Water ◽  
Root Zone ◽  
Soil Water Balance ◽  
Vacuum Extraction ◽  
Rain Event ◽  
Arid Environments ◽  
Semi Arid

Abstract. Stable isotopes (deuterium, 2H, and oxygen-18, 18O) of soil water were measured in the field using a liquid water isotope analyzer (tunable off-axis integrated cavity output spectroscope, OA-ICOS, LGR) and commercially available soil gas probes (BGL-30, UMS, Munich) in the semi-arid Cuvelai–Etosha Basin (CEB), Namibia. Results support the applicability of an in situ measurement system for the determination of stable isotopes in soil pore water. High spatial and temporal resolution was achieved in the study area with reasonable accuracy and measurements were in agreement with laboratory-based cryogenic vacuum extraction and subsequent cavity ring-down laser spectroscopic isotope analysis (CRDS, L2120-i, Picarro Inc.). After drift and span correction of the in situ isotope data, precision for over 140 measurements taken during two consecutive field campaigns (June and November 2014) was 1.8 and 0.48 ‰ for δ2H and δ18O, respectively. Mean measurement trueness is determined using quality check standards and was 5 and 0.3 ‰ for δ2H and δ18O, respectively. The isotope depth profiles are used quantitatively to calculate a soil water balance. The contribution of transpiration to total evapotranspiration ranged between 72 and 92 %. Shortly after a rain event, the contribution of transpiration was much lower, at 35 to 50 %. Potential limitations of such an in situ system are related to environmental conditions which could be minimized by using a temperature-controlled chamber for the laser spectrometer. Further, the applicability of the system using previously oven-dried soil material might be limited by physicochemical soil properties (i.e., clay minerals). Uncertainty in the in situ system is suggested to be reduced by improving the calibration procedure and further studying fractionation effects influencing the isotope ratios in the soil water, especially at low water contents. Furthermore, the influence of soil-respired CO2 on isotope values within the root zone could not be deduced from the data.

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