NUMERICAL AND FIELD-EVALUATION OF SOIL-WATER SAMPLED BY SUCTION LYSIMETERS

Porous cup suction-lysimeters are widely used for extracting soil wate r in solute transport monitoring, but It is not clear how the sampled concentration can change with the ambient solute concentration, or wit h applied suction inside the lysimeter. This research was designed to numerically evaluate soil water sampled by suction lysimeters and to t est simulation results with field observations. An axisymmetric three- dimensional finite element model (SWMS_2D) was used to simulate soil w ater sampled by suction lysimeters in a Zimmerman fine sand (mixed, fr igid Argic Udipsamments). To test the simulated results, KBr was unifo rmly spread on the soil surface. After 183 mm rainfall, suction was ap plied to the lysimeters (with ceramic cups at a depth of 0.8 m) to wit hdraw soil water; soil column samples were also removed from the surro unding soil and sectioned to determine solute concentration in situ us ing a bromide electrode. Simulation showed that when there is a concen trated solute band near a porous cup, as can occur with banded fertili zers, the concentration of sampled soil water in the lysimeter is cons iderably higher than the resident concentration in the soil at the sam e depth. The influence of a concentrated solute band on the concentrat ion of sampled soil water decreased as the distance to the cup increas ed. Variations in applied suction (25, 35, and 45 kPa) appear to have little influeuce on the amount and concentration of field lysimeter ex tracted soil water. The measured amount of water in the lysimeter (78. 4 mL) was comparatively more than the simulated amount (53.9 mL) when the mean water content of nine soil columns was used as an initial con dition for the simulation, but was within the range of simulated amoun ts (32.5-91.5 mL) when the lower and upper 95% limits of the measured soil-water content profiles were used as initial conditions for the si mulations. Measured Br- concentration of the sampled water (3.13 x 10( -4) M) was lower than the simulated concentration (4.66 x 10(-4) M) wh en the geometric means of each layer from the nine soil column Br- con centrations were used as the initial conditions. The difference betwee n measured and simulated concentration may have been due to variabilit y in soil hydraulic properties.