Numerical analysis of the effect of the lower boundary condition on solutetransport in lysimeters

Field lysimeters are often used to assess environmental behavior of agroche micals, Most lysimeters used to date have a free-draining lower boundary wh ere leaching out of the lysimeter occurs by gravity alone. In this ease, th e lower boundary of a lysimeter is open to the atmosphere, and consequently , leachate is collected only if the bottom of the lysimeter becomes water s aturated. In a field soil, such local water saturation does not occur. The objective of this study was to evaluate the effect of the lower boundary co ndition on chemical leaching. Numerical simulations were used to compare so lute transport in field soils and in lysimeters. Simulations were carried o ut in homogeneous sandy and loamy soils under steady-state, unsaturated wat er flow conditions. Water flow was described by the Richards equation and s olute transport by the advection-dispersion equation. The effect of linear and nonlinear and instantaneous and kinetic sorption was investigated. The results showed that for a conservative solute the differences between field soil and lysimeter increase as the coarseness of the soil increases. Decre asing water Bur increases the difference ht tween field soil and lysimeter, In general, solute transport in the lysimeter is characterized by a slower mean velocity, a larger spreading, and smaller concentration values. For s olutes subject to Linear equilibrium sorption, the sorption mechanism compe nsates for the effects of the lower boundary condition. The larger the sorp tion coefficient, the less the difference between lysimeter and field soil. However, large differences are found in the case of strongly convex nonlin ear sorption isotherms.