RAIN PATTERN AND SOIL-MOISTURE CONTENT EFFECTS ON ATRAZINE AND METOLACHLOR LASSES IN RUNOFF

Herbicide concentration in runoff varies dramatically within a storm, therefore storm pattern is postulated to have a significant impact on herbicide loss. We evaluated the effects of storm pattern and soil moi sture content on herbicide loss in runoff, and used our data to valida te the uniform mixing concept for modeling herbicide transfer to runof f, Atrazine and metolachlor were surface applied to air-dried soil at rates of 1.12 and 2.24 kg/ha, respectively. Two soils (Cecil sandy loa m and Miami silty loam), four storm patterns (uniform, advanced peak, intermediate peak, and delayed peak), and two moisture levels (wet and dry) were used, Dissolved herbicide losses from the advanced peak wer e twice those from the other patterns for both herbicides for Cecil so il, The significance of rain pattern effects was reduced for Miami soi l, but metolachlor loss from the advanced peak was 1.9 times that from the delayed peak. Rain pattern affected sediment-bound herbicide loss , but no consistent trend was shown for both soils. Under wet conditio ns a greater potential for dissolved and sediment-bound herbicide loss es was shown for both herbicides on Miami soil than on Cecil soil, Mea sured herbicide concentrations in runoff decreased exponentially with cumulative rainfall depth, and were adequately described by the unifor m mixing model, Results show that information on intensity distributio n and transient infiltration rate is needed to better predict both ins tantaneous and total herbicide losses during a storm.