Influence of Reduced Rainfall Seasons on the Runoff and Leaching Losses of Mobile Agricultural Pesticides and on their Runoff/Pesticides and on their Runoff/Leaching Ratios

Authors: Southwick Jr, Lloyd; Grigg, Brandon; Fouss, James; Kornecki, Ted

Submitted to: Mississippi Water Resources Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2002
Publication Date: 5/30/2002
Citation: Southwick Jr, L.M., Grigg, B.C., Fouss, J.L., Kornecki, T.S. 2002. The Influence of Reduced Rainfall Seasons on the Runoff and Leaching Losses of Mobile Agricultural Pesticides and on their Runoff/Pesticides and on their Runoff/Leaching Ratios. Proceedings of the 32nd Mississippi Water Resources Research Conference, Raymond, MS. pp. 118-123.

Interpretive Summary: Since 1985 we have been studying surface runoff and rainfall infiltration leaching losses of agricultural pesticides. During typical rainfall seasons (average annual rainfall 60 inches) rainfall events occur about every week in the spring and early summer. For commonly-applied herbicides such as atrazine, simazine, and metolachlor this rainfall pattern removes up to 5% (occasionally higher) of application in runoff and about 0.2% in leachate into subsurface drains three feet deep. During the unusually dry growing seasons of 1998-2000, rainfall was less than 25% of the amount observed in 1995-1997, typical rainfall years. During the dry years, runoff losses of the herbicides were only about 10% of those during typical rainfall seasons. Leaching losses, however, were about twice as high. Herbicide residues on the soil surface (top inch) lasted 23-34% longer than measured in 1995-97. These observations indicate increased residence time of these herbicides in the soil profile. But the longer persistence during the low-rainfall years did not translate into extended availability for leaching: a 4-inch rain two months after application in 1999 removed only 0.02-0.06% of application of the herbicides in runoff and leachate.

Technical Abstract: In our studies of runoff and leaching of agricultural pesticides in southern Louisiana in normal rainfall years (the 30-year average for which is 1500 mm), atrazine, metolachlor and similarly mobile chemicals have left the field in runoff to the extent of about 2% of application (sometimes higher). Soil leachate contained about 1/10th this amount. Most of the runoff losses occurred within the first 60 days after application. For example, in 1995 165 mm of rainfall in 33 days caused four runoff events, amounting to losses for atrazine of 2.7% of application and for metolachlor of 1.8%. For this season, leaching of atrazine over a period of 155 days amounted to 0.10% of application and to 0.09% for metolachlor. Similar results were observed for 1996 and 1997. The much reduced rainfall we experienced in 1998-2000 led not only to a reduction in total losses from the field in runoff and leachate, but also to a decrease in the ratio of runoff to leachate losses. In 1998, 77 mm of rain in 16 days produced in runoff 0.87% of application losses of atrazine and 0.37% for metolachlor. Through day 151 no other runoff occurred. For 56 days, 106 mm of rainfall led to 0.49% of application losses of atrazine in leachate, and to 0.25% of losses for metolachlor, no other leachate occurring to day 151. The ratio of runoff to leachate for atrazine dropped from 27 in 1995 to 1.8 in 1998. Likewise, for metolachlor the runoff/leaching ratio decreased from 20 to 1.5. Persistence of atrazine in the top 2.5 cm soil layer was 37% greater in 1998, and metolachlor persisted 24% longer that year, compared to 1995. The indicated longer persistence of these chemicals in the soil did not translate into greater availability for leaching during rainfall: a 100-mm rain two months after application in 1999 removed only 0.02-0.06% of application of the herbicides.