SOIL PROCESSES AND CHEMICAL-TRANSPORT - TEST OF THE LEACHP MODEL FOR PREDICTING ATRAZINE MOVEMENT IN 3 MINNESOTA SOILS

Leaching Estimation and Chemistry Model-pesticide (LEACHP) was evaluat ed for predicting atrazine 2-chloro-4-ethylamino-6-isopropylamino-s-tr iazine) movement in sandy loam (sandy, Typic Hapludoll), silt loam (fi ne-silty, Typic Hapludoll), and clay loam (fine-loamy, Typic Haplaquol l) soils under moldboard and chisel plow tillage practices. Atrazine d istribution in the root zone was measured by analyzing soil cores. Mod el predictions were compared with observed atrazine concentrations in the soil profiles on various dates after application (two to three gro wing seasons). Model performance was evaluated by testing whether mode l predictions fall within a specified factor of true values and using Goodness-of-Fit tests (maximum error, root mean squared error, coeffic ient of determination, modelling efficiency, coefficient of residual m ass). LEACHP predicted depth of peak atrazine concentration in the soi l profile accurately in all three soils (under both moldboard [MB] and chisel plow [CP] treatments and at all sampling dates). It predicted depth of atrazine movement and atrazine concentrations in the soil pro file reasonably well in the Estherville soil (sandy, mixed, mesic Typi c Hapludoll) except some disparities between measured and predicted co ncentrations in 1988 because of the difficulty in recovering represent ative samples. In Port Byron (fine-silty, mixed, mesic Typic Hapludoll ) and Webster soils (fine-loamy, mixed, mesic Typic Haplaquoll), depth of atrazine movement and atrazine concentrations in the soil profile were predicted reasonably well during relatively dryer years (1988 and 1989). In a relatively wet year (1987), however, the model did not pr edict significant amounts of atrazine at depths >45 cm, whereas, 2.5 t o 27.5 mu g kg(-1) of atrazine were detected in soil samples during ea rlier sampling dates (43, 73 DAA in Port Byron and 14, 43 DAA in Webst er soils). Overall performance of the model was similar in both tillag e treatments (MP or CP). Considering the broad range in soil propertie s and climatic conditions used in testing, the model performed well.