Assessing the ability of mechanistic volatilization models to simulate soil surface conditions: A study with the VoltʹAir model Original Research Article

Ammonia and pesticide volatilization in the field is a surface phenomenon involving physical and chemical processes that depend on the soil surface temperature and water content. The water transfer, heat transfer and energy budget sub models of volatilization models are adapted from the most commonly accepted formalisms and parameterizations. They are less detailed than the dedicated models describing water and heat transfers and surface status. The aim of this work was to assess the ability of one of the available mechanistic volatilization models, VoltʹAir, to accurately describe the pedo-climatic conditions of a soil surface at the required time and space resolution. The assessment involves: (i) a sensitivity analysis, (ii) an evaluation of VoltʹAir outputs in the light of outputs from a reference Soil–Vegetation–Atmosphere Transfer model (SiSPAT) and three experimental datasets, and (iii) the study of three tests based on modifications of SiSPAT to establish the potential impact of the simplifying assumptions used in VoltʹAir. The analysis confirmed that a 5 mm surface layer was well suited, and that VoltʹAir surface temperature correlated well with the experimental measurements as well as with SiSPAT outputs. In terms of liquid water transfers, VoltʹAir was overall consistent with SiSPAT, with discrepancies only during major rainfall events and dry weather conditions. The tests enabled us to identify the main source of the discrepancies between VoltʹAir and SiSPAT: the lack of gaseous water transfer description in VoltʹAir. They also helped to explain why neither VoltʹAir nor SiSPAT was able to represent lower values of surface water content: current classical water retention and hydraulic conductivity models are not yet adapted to cases of very dry conditions. Given the outcomes of this study, we discuss to what extent the volatilization models can be improved and the questions they pose for current research in water transfer modeling and parameterization.