PREDICTING BROMIDE LEACHING UNDER FIELD CONDITIONS USING SLIM AND MACRO

The updated versions of the SLIM and MACRO computer simulation models were evaluated using 3 years of bromide leaching data from a field experiment conducted on Hagerstown silt loam soil (fine, mixed, mesic Typic Hapludalf) with a well developed structure in central Pennsylvania. An application of potassium bromide (KBr) tracer was broadcast to 18 plots at 100 kg ha-1 at planting on May 13, 1988. Eighteen zero-tension pan lysimeters (0.456 m2) were placed at a depth of 1.2-m below the soil surface to collect gravitational water samples. Average pan collection efficiency was 52%, with a CV of 44%. Model simulations of Br- leaching for 1988, 1989, and 1990 were compared with the mean of pan efficiency-corrected measured data. There were no statistical differences between the SLIM and MACRO model predictions and the experimental data for 1988 except for 1 or 2 months for each model. The SLIM model accurately predicted Br- leaching in 1989 but significantly underestimated Br- leaching from October 1990 through February 1991. The MACRO model significantly underestimated Br- leaching from May to November 1989 and performed well in 8 of 12 months in 1990. The overall simulations for 3 years of each model were compared with the overall field measurements of Br- leaching. The statistical evaluation criteria indicated that the SLIM model performed somewhat better than the MACRO model through the 3-year period. The results given in this study indicate that both models have good potential for conservative tracer simulation.