SiSPAT-Isotope, a coupled heat, water and stable isotope (HDO and (H2O)-O-18) transport model for bare soil. Part II. Evaluation and sensitivity tests using two laboratory data sets

摘要

Stable water isotopes are tracers of water movement within the soil-vegetation-atmosphere system. They have the potential for a better understanding of water vapour transport within soils, evaporation and transpiration processes. To better understand those potentialities and possible lack of knowledge, a coupled heat-water and stable isotope transport model, called SiSPAT-Isotope was developed for bare soil. We presented the theoretical basis of the model in the first part of the paper, including a first validation of the likelihood of model results and a comparison with existing analytical solutions. In this companion paper, we go a step further by comparing the model results with two data sets collected on laboratory columns. In both cases, five soil columns were saturated and let drying during 173 and 253 days, respectively. At selected dates, one of the column was cut into slices and analysed to determine the volumetric water content, the deuterium and oxygen 18 concentrations profiles. The first data set was acquired on disturbed soil columns. The second one was collected on non-disturbed soil columns and it included a complete monitoring of atmospheric variables. It was not the case for the first one and a sensitivity analysis of model results to the air humidity was performed, showing its large influence on surface isotope concentrations. For both data sets, we also conducted a sensitivity analysis to the formulation of the kinetic fractionation factor, conditioning the resistance to isotope transport between the soil surface and the atmosphere, and to the value of soil tortuosity. The results showed that the model was able to reproduce the behaviour of the observed concentration profiles. A fair agreement between measured and calculated values was obtained for all profiles for the disturbed soil. Near surface concentrations were in general overestimated for the undisturbed soil, raising the question of possible influence of immobile water on concentrations values. We showed that soil tortuosity was mostly influential on the depth of the peak isotope concentration, which opens perspectives for its retrieval from the measurement of isotope concentration profiles. When only molecular diffusion was considered, the model was not able to reproduce the liquid slopes of the deuterium/oxygen 18 relationships at the beginning of the drying process. Results were more in agreement with the data when molecular diffusion combined with turbulent transfer were considered. Further laboratory and field experiments are, however, still required to derive a formulation of the kinetic fractionation factor adapted to drying soils and non-saturated conditions. (c) 2005 Elsevier B.V. All rights reserved.