Rain Pattern and Soil Moisture Content Effects on Atrazine and Metolachlor Losses in Runoff

Abstract

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 moisture content on herbicide loss in runoff, and used our data to validate the uniform mixing concept for modeling herbicide transfer to runoff. 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 loam 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 were twice those from the other patterns for both herbicides for Cecil soil. The significance of rain pattern effects was reduced for Miami soil, 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 conditions a greater potential for dissolved and sediment‐bound herbicide losses was shown for both herbicides on Miami soil than on Cecil soil. Measured herbicide concentrations in runoff decreased exponentially with cumulative rainfall depth, and were adequately described by the uniform mixing model. Results show that information on intensity distribution and transient infiltration rate is needed to better predict both instantaneous and total herbicide losses during a storm.