GLEAMS, Opus, PRZM2 beta, and PRZM3 simulations compared with measured atrazine runoff

High-intensity storms that occur shortly after chemical application have th e greatest potential to cause chemical runoff We examined how effectively c urrent chemical transport models GLEAMS, Opus, PRZM2 beta, and PRZM3 could predict water runoff and runoff losses of atrazine [6-chloro-N-ethyl-N'-(1- methylethyl)-1,3,5-triazine-2,4-diamine] under such conditions, as compared with observations from a controlled field runoff experiment. The experimen t was conducted for 2 yr using simulated rainfall on two 14.6- by 42.7-m pl ots within a corn (Zea mays L.) held on Tifton loamy sand (fine-loamy, kaol initic, thermic Plinthic Kandiudults) under conventional tillage practices. For each plot-year, atrazine was applied as surface spray immediately afte r planting and followed by a 50-mm, 2-h simulated rainfall 24 h later. A si milar preapplication rainfall and four subsequent rainfalls during the grow ing season were also applied. Observed water runoff averaged 20% of the app lied rainfall. Less runoff occurred from freshly tilled soil or under full canopy cover; more runoff occurred when nearly bare soil had crusted. Obser ved total seasonal atrazine runoff averaged 2.7% of that applied, with the first posttreatment event runoff averaging 89% of the total. GLEAMS, Opus, PRZM2 beta and PRZM3 adequately predicted water runoff amounts, with normal ized root mean square errors of 29, 29, 31, and 31%, respectively. GLEAMS a nd PRZM3 predicted atrazine concentrations in runoff within a factor of two of observed concentrations. PRZM2 beta overpredicted atrazine concentratio ns. Opus adequately predicted atrazine concentrations in runoff when it was run with an equilibrium adsorption submodel, but significantly underestima ted atrazine concentrations when it was run with a kinetic sorption submode l.