The first author was funded by the DFG Research Group: From Catchments as Organised Systems to Models based on Functional Units (FOR 1598). The second author was funded by the DFG project Coupled soil-plant water dynamics – Environmental drivers and species effects (contract numbers: GE 1090/10-1 and WE 4598/2-1). The isotope data in the precipitation for Roodt were provided by FNR/CORE/SOWAT, project of the Luxembourg Institute of Science and Technology – LIST. Sampling of the isotope profiles was made possible by the support of the CAOS Team and Begona Lorente Sistiaga, Benjamin Gralher, Andre Böker, Marvin Reich and Andrea Popp. Special thanks to Britta Kattenstroth and Jean Francois Iffly for their technical support in the field and Barbara Herbstritt for her support in the laboratory. For Roodt, soil texture and hydraulic parameter information were provided by Conrad Jackisch and Christoph Messer (KIT, Karlsruhe, Germany) and hydraulic conductivity data were provided by Christophe Hissler and Jérôme Juilleret (LIST). Pore water isotope and soil moisture data for Hartheim were provided by Steffen Holzkämper and Paul Königer. Temperature and precipitation data for Hartheim were provided by the Chair of Meteorology and Climatology, University of Freiburg.Peer reviewedPublisher PD

Blume, Theresa

Sprenger, Matthias

Volkmann, Till H.M.

Weiler, Markus

English

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Estimating flow and transport parameters in the unsaturated zone with pore water stable isotopes

Determining the soil hydraulic properties is a prerequisite to physically
model transient water flow and solute transport in the vadose zone.
Estimating these properties by inverse modelling techniques has become more
common within the last 2 decades. While these inverse approaches usually
fit simulations to hydrometric data, we expanded the methodology by using
independent information about the stable isotope composition of the soil
pore water depth profile as a single or additional optimization target. To
demonstrate the potential and limits of this approach, we compared the
results of three inverse modelling strategies where the fitting targets were
(a) pore water isotope concentrations, (b) a combination of pore water isotope
concentrations and soil moisture time series, and (c) a two-step approach
using first soil moisture data to determine water flow parameters and then
the pore water stable isotope concentrations to estimate the solute
transport parameters. The analyses were conducted at three study sites with
different soil properties and vegetation. The transient unsaturated water
flow was simulated by solving the Richards equation numerically with the
finite-element code of HYDRUS-1D. The transport of deuterium was simulated
with the advection-dispersion equation, and a modified version of HYDRUS was
used, allowing deuterium loss during evaporation. The Mualem–van Genuchten
and the longitudinal dispersivity parameters were determined for two major
soil horizons at each site. The results show that approach (a), using only the
pore water isotope content, cannot substitute hydrometric information to
derive parameter sets that reflect the observed soil moisture dynamics  but
gives comparable results when the parameter space is constrained by
pedotransfer functions. Approaches (b) and (c), using both  the isotope
profiles and the soil moisture time series, resulted in good simulation
results with regard to the Kling–Gupta efficiency and good parameter
identifiability. However, approach (b) has the advantage that it considers
the isotope data not only for the solute transport parameters  but also for
water flow and root water uptake, and thus increases parameter realism.
Approaches (b) and (c) both outcompeted simulations run with parameters
derived from pedotransfer functions, which did not result in an acceptable
representation of the soil moisture dynamics and pore water stable isotope
composition. Overall, parameters based on this new approach that includes
isotope data lead to similar model performances regarding the water balance
and soil moisture dynamics and better parameter identifiability than the
conventional inverse model approaches limited to hydrometric fitting
targets. If only data from isotope profiles in combination with textural
information is available, the results are still satisfactory. This method
has the additional advantage that it will not only allow us to estimate
water balance and response times  but also site-specific time variant
transit times or solute breakthrough within the soil profile

Estimating flow and transport parameters in the unsaturated zone with pore water stable isotopes

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