Evaluation of a functional model for simulating boron transport in soil

There has been renewed interest in the application of functional models to the transport of nonpoint source pollutants at polypedon (i.e., farm) and w atershed scales because of the ease of their coupling to a geographic infor mation system and to the accepted organizational hierarchy of pedogenetic m odeling approaches. However, very little work has been done to evaluate the performance of a functional transient-state model for the transport of a r eactive solute over an extensive study period. Subsequently, the functional model TETrans (Trace Element Transport) was evaluated for model performanc e with boron (B) transport data collected from a meso-scale soil lysimeter column over a 1000-day study period (i.e., 40 irrigations). Because the abi lity to simulate water now has been evaluated previously for TETrans, the f ocus of this evaluation centered around the performance of various function al models of B adsorption used as subroutines within the TETrans model, inc luding the (1) Freundlich, (2) kinetic Freundlich, (3) Langmuir, (4) temper ature-dependent Langmuir, and (5) pH-dependent Keren adsorption isotherm eq uations. Model performance was evaluated with statistical functions, specif ically the Average Absolute Prediction Error, the Root Mean Square Error, t he Reduced Error Estimate and the Coefficient of Residual Mass, and graphic displays of observed and predicted B concentration profiles. Even though n o single adsorption isotherm equation, when coupled to TETrans, could be co nsidered poor in its performance, results indicated that the order of model performance was the pH-dependent Keren equation first, followed by the tem perature-dependent Langmuir and kinetic Freundlich equations, the Freundlic h equation, and, finally, the Langmuir equation. Overall, the TETrans model was able to simulate the transport of B with deviations because no functio nal adsorption equation incorporated all the influences of pH, ionic streng th, temperature, and kinetic effects into a single equation. The inability to correctly predict one of the measured peaks in B concentration near the soil surface suggests that problems with the timing of the sample collectio n may have occurred for the shallowest sampling depth.