Estimating simazine-treated area in watersheds based on annual stream loads

Area of Science:

• Environmental Chemistry
• Agricultural Science
• Hydrology

Background:

• Current pesticide usage data in the US is inadequate for watershed-scale estimations.
• Accurate pesticide use data is crucial for understanding non-point source pollution in aquatic ecosystems.
• Estimating pesticide-treated crop areas at the watershed level requires improved methodologies.

Purpose of the Study:

• To evaluate an innovative approach for estimating simazine usage on corn (Zea mays L.).
• To assess simazine transport to streams within the Salt River Basin (SRB) of Missouri, USA.
• To compare a novel output-based method with existing input-based national data for pesticide usage estimation.

Main Methods:

• Quantified annual loads of simazine and atrazine (parent + metabolites) in seven SRB watersheds (2005-2017).
• Calculated simazine-treated corn area using the ratio of total simazine load to total atrazine load.
• Utilized atrazine load data as a surrogate for simazine-treated area in the absence of direct data.

Main Results:

• Estimated 3.8-31% of SRB corn area treated with simazine from 2005-2010; four watersheds had <10% treated.
• Long Branch Creek and its sub-watersheds showed consistently higher simazine treatment (≥20% of corn area).
• Results indicated simazine usage one to two orders of magnitude greater and more variable than national estimates.

Conclusions:

• The developed output-based method significantly improves pesticide usage estimation at the watershed scale.
• This approach offers a more accurate and spatially/temporally resolved alternative to national input-based data.
• Findings highlight the importance of watershed-specific monitoring for understanding pesticide application patterns.