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On the Temporal Resolution of Mass Balance Models for Soluble Chemicals in Soils

A daily step model of chemical mass balance in the topsoil is presented and validated at three experimental sites in Europe, and subsequently applied to perform two distinct numerical experiments. Firstly, an experiment was run using hypothetic soluble chemicals with half-lives ranging from 10-1 to 104, with a range of representative European climate and soil properties, assuming uniform constant emissions of the chemicals throughout the year. Chemical mass in soil from the daily step model calculations can be surrogated by the monthly step model consistently parameterized in terms of absolute values, patterns and inter-monthly variability with decreasing accuracy at higher chemical half-lives. Leaching fluxes can be also surrogated by the monthly step calculation, although with higher errors. Runoff is correct in orders of magnitude, but it shows only a weak correlation with the monthly mean of the daily model output. For leaching and runoff, the accuracy depends mainly on soil properties. Variability is well reproduced for both leaching and runoff. The second experiment represented a pulse emission of chemicals discharged on a single day in a 12 month period. Results from the annual average mass of chemicals in the soil, annual runoff and leaching fluxes from the daily step model were compared to results obtained from the experiment assuming constant removal rates for the year. The two values are within a factor of 10 for half-lives longer than 10 days: therefore it is possible to emulate the daily step model with a simple constant removal rate model for screening level assessment. The experiments suggest that simpler schemes may be a practical screening-level approximation of detailed daily step models for both continuous and pulse emissions, two cases providing extreme bounds of variation to real world emissions.