The impact of evaporation fractionation on the inverse estimation of soil hydraulic and isotope transport parameters

Choosing a suitable process-oriented eco-hydrological model is essential for obtaining reliable simulations of hydrological processes. Determining soil hydraulic and solute transport parameters is another fundamental prerequisite. Research discussing the impact of considering evaporation fractionation on parameter estimation and practical applications of isotope transport models is limited. In this study, we analyzed parameter estimation results for two datasets for humid and arid conditions using the isotope transport model in HYDRUS-1D, in which we either did or did not consider fractionation. The global sensitivity analysis using the Morris and Sobol' methods and the parameter estimation using the Particle Swarm Optimization algorithm highlight the significant impact of considering evaporation fractionation on inverse modeling. The Kling-Gupta efficiency (KGE) index for isotope data can increase by 0.09 and 1.49 for the humid and arid datasets, respectively, when selecting suitable fractionation scenarios. Differences in estimated parameters propagate into the results of two practical applications of stable isotope tracing: i) the assessment of root water uptake (RWU) and drainage travel times (i.e., the time elapsed between water entering the soil profile as precipitation and leaving it as transpiration or drainage) in the lysimeter (humid conditions) and ii) evaporation estimation in a controlled experimental soil column (arid conditions). The peak displacement method with optimized longitudinal dispersivity provides much lower travel times than those obtained using the particle tracking algorithm in HYDRUS-1D. Considering evaporation fractionation using the Craig-Gordon (CG) and Gonfiantini models is likely to result in estimates of older water ages for RWU than the no fractionation scenario. The isotope mass balance method that uses the isotopic composition profile simulated by HYDRUS-1D while considering fractionation using the CG and Gonfiantini models, or the measured evaporation isotope flux, provides comparable results in evaporation estimation as the HYDRUS-1D water mass balance method and direct laboratory measurements. In contrast, the no fractionation scenario reasonably estimates evaporation only when using the HYDRUS-1D water mass balance method. The direct use of simulated isotopic compositions in the no fractionation scenario may result in large biases in practical applications in the arid zone where evaporation fractionation is more extensive than in humid areas.