Herbicide transport to surface runoff from a claypan soil: Scaling from plots to fields

Streams and drinking water reservoirs throughout the claypan soil region of Missouri and Illinois are particularly vulnerable to herbicide contamination from surface runoff during spring. This study follows a plot-scale study conducted on claypan soils to quantify and compare edge-of-field herbicide losses from a corn–soybean rotation under mulch tillage and no-tillage systems. The objectives of the present study were to confirm at field scale (34.4 ha [85 ac] and 7.8 ha [19.3 ac]) the plot-scale findings (0.37 ha [0.92 ac]) on the effects of tillage and herbicide incorporation on herbicide transport and to evaluate the applicability of plot-scale exponential models in calculating atrazine and metolachlor concentrations as a function of application rate, runoff volume, and days after application at the field scale. Herbicide transport to surface runoff was studied (1997 to 2001) from two fields with cropping systems similar to those on the plots. Field 1 (F1) was a mulch tillage corn–soybean rotation system with surface-applied herbicides, which are then incorporated. Field 2 (F2) was a no-tillage corn–soybean rotation system with surface-applied herbicides that were not incorporated. During each event, runoff volumes were measured, and water samples were collected and analyzed for atrazine and metolachlor concentrations. The percentages of applied atrazine and metolachlor transported to surface runoff from no-tillage (F2) were 3.2 and 2.0 times those from mulch tillage (F1), respectively. Throughout the study period, 1.0% and 3.2% of total atrazine and 1.0% and 2.0% of total metolachlor applied to F1 and F2 were lost to surface runoff, respectively. Similar to the results from the plot study, the model performed well in calculating field atrazine concentrations from both mulch and no-tillage systems with coefficient of determination ≥ 0.70 and Nash and Sutcliffe efficiency ≥ 0.64. However, model performance in calculating metolachlor concentrations was poor for both tillage systems (Nash and Sutcliffe efficiency < 0.35). When the model was modified to include cumulative temperature instead of days after application, performance in calculating atrazine and metolachlor concentrations was improved, particularly metolachlor concentrations at the field scale. The coefficient of determination and Nash and Sutcliffe efficiency values for metolachlor relative to cumulative temperature and days after application were 0.62 and 0.61 versus 0.41 and −0.13 for F1, and 0.73 and 0.55 versus 0.53 and 0.34 for F2, respectively. Overall, the study confirmed plot-scale results that atrazine concentrations and losses were greater for a no-tillage system than for a mulch-tillage system, in which the herbicide was incorporated. The study also showed that the model developed using plot-scale data was applicable in calculating concentrations at the field scale, particularly for atrazine.