Simulation of solute transport using a continuous time Markov process: 2. Application to transient field conditions

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Abstract
A technique for modeling the movement of solute in the unsaturated zone using a continuous time Markov process is presented and field‐tested under a transient water flow regime. Three treatments, a fallow and two cropped, were used to quantify the effect of crop uptake on water and solute movement and to test model performance under field conditions. Field measured NO3− and Br − concentrations were simulated quite well using three scenarios of water flux. Average water fluxes calculated from 40 observations within a treatment and used to estimate water flow resulted in the best simulations,while the average water flux plus one standard deviation (SD) represented a net downward movement of solute, and simulations using average flux minus one SD represented a net upward movement of solute. Under fallow conditions, solute which was unaccounted for by mass balance of measured data was simulated as leached under extreme flux conditions. Simulations of the cropped treatments were improved substantially in inclusion of either a continuous time crop uptake function or a constant crop uptake coefficient. Model predictions were found to be much more sensitive to crop uptake than to the flux of soil water.