Tracking water pathways in steep hillslopes by δ18O depth profiles of soil water

123

Abstract. 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 finiteelement 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 t...

Section: Parameter Adequacymentioning

confidence: 99%

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

Sprenger

Volkmann

Blume

et al. 2015

123

“…The soil water isotope response seems to follow a replacement of the pore waters by a mixture of newly infiltrated precipitation and older water, which could probably be described by the advection dispersion equation as frequently applied for isotope modeling in the unsaturated zone (Adomako et al, 2010;Stumpp et al, 2012;Mueller et al, 2014;Sprenger et al, 2015b). However, during intense rainfall, the precipitation input seems to partly bypass the matrix leading to rapid changes in the tracer signal in the subsoil and a generally high variability of the soil water isotopes within the entire soil profile.…”

Section: Mixing Of Precipitation Input and The Critical Zone Water Stmentioning

confidence: 99%

“…However, recent experimental studies indicated potential for fractionation during the cryogenic extraction (Orlowski et al, 2016), probably induced by watermineral interactions (Oerter et al, 2014;Gaj et al, 2017a, b). The direct-equilibration method -as a potential alternative analysis method -we applied in our study was so far usually limited to study percolation processes (e.g., Garvelmann et al, 2012;Mueller et al, 2014;Sprenger et al, 2016a). To our knowledge, only Bertrand et al (2012) used the directequilibration method to study the water use by plant water use and the influence of evaporation fractionation, but they bulked the soil data of the upper 20 cm.…”

Section: Evaporation Dynamics Within the Soil-plant-atmosphere Interfacementioning

confidence: 99%

“…Nevertheless, detailed estimates of the evaporative fluxes require transient modeling of all water fluxes (i.e., precipitation, transpiration, evaporation, recharge) and their isotopic composition, which will be subject of future work. Potential transient numerical modeling approaches that account for isotopic fractionation of the soil water isotopes are available (Braud et al, 2005;Rothfuss et al, 2012;Mueller et al, 2014).…”

Section: Vegetation Affects Critical Zone Evaporation Lossesmentioning

confidence: 99%

See 1 more Smart Citation

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

Sprenger

Tetzlaff

Soulsby

2017

171

165

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn longterm experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ 2 H and δ 18 O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15-20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate traceraided hydrological models either at the plot to the catchment scale.

“…Soil water stable isotopes provide information on flow pathways and mixing within the unsaturated zone (e.g., Gazis and Feng, 2004;Stumpp and Maloszewski, 2010;Garvelmann et al, 2012;Mueller et al, 2014). Soil water stable isotope studies have also been used to reduce parameter uncertainty in unsaturated zone model approaches (Sprenger et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

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

Gaj

Beyer

Koeniger

et al. 2016

106