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Year-to-year dynamics in weather affect both the timing of application and the potential hydrologic transport of pesticides. Further, the most commonly used pesticides dissipate in the environment during the growing season. Interactions among these factors - hydrology, timing of application and dissipation kinetics - hinder the detection of temporal trends in transport. It is increasingly important to be able to discern such trends, to judge effectiveness of management practices or to determine whether observed changes were caused by management or weather. In previous work, a cumulative vulnerability index was developed to account for these three factors. It explained 63% of annual variation in atrazine load in the Goodwater Creek Experimental Watershed (GCEW). The objectives of the current work were (i) to generalize the cumulative vulnerability index to explicitly account for variation in watershed size, area treated with atrazine and average application rate; (ii) to test the overall performance on watersheds showing such variation; and (3) to test whether the generalized index properly accounted for the additional input parameters. The generalized index was tested using data from GCEW (73.7 km2) and seven additional watersheds in the northeast Missouri claypan region that varied in size from 212 to 1180 km2 and from 4% to 23% of watershed area planted to corn or sorghum. Across 32 site-years, the generalized index explained 84% of variation in annual atrazine load. Further, tests of residuals showed no dependence on either watershed area or fraction of area planted to corn and sorghum, indicating that these parameters were properly integrated into the index. The performance of the index supports the conclusion that data obtained from GCEW is representative of the Mark Twain Lake Basin and likely the entire Central Claypan Major Land Resource Area. Published 2012. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]

Lerch, R.N., E.J. Sadler, K.A. Sudduth, C. Baffaut, and N.R. Kitchen, 2010. Herbicide Transport in Goodwater Creek Experimental Watershed: I. Long-Term Research on Atrazine. Journal of the American Water Resources Association (JAWRA) 1-15. DOI: 10.1111/j.1752-1688.2010.00503.x Atrazine continues to be the herbicide of greatest concern relative to contamination of surface waters in the United States (U.S.). The objectives of this study were to analyze trends in atrazine concentration and load in Goodwater Creek Experimental Watershed (GCEW) from 1992 to 2006, and to conduct a retrospective assessment of the potential aquatic ecosystem impacts caused by atrazine contamination. Located within the Central Claypan Region of northeastern Missouri, GCEW encompasses 72.5 km of predominantly agricultural land uses, with an average of 21% of the watershed in corn and sorghum. Flow-weighted runoff and weekly base-flow grab samples were collected at the outlet to GCEW and analyzed for atrazine. Cumulative frequency diagrams and linear regression analyses generally showed no significant time trends for atrazine concentration or load. Relative annual loads varied from 0.56 to 14% of the applied atrazine, with a median of 5.9%. A cumulative vulnerability index, which takes into account the interactions between herbicide application, surface runoff events, and atrazine dissipation kinetics, explained 63% of the variation in annual atrazine loads. Based on criteria established by the U.S. Environmental Protection Agency, atrazine reached concentrations considered harmful to aquatic ecosystems in 10 of 15 years. Because of its vulnerability, atrazine registrants will be required to work with farmers in GCEW to implement practices that reduce atrazine transport. [ABSTRACT FROM AUTHOR]

Variability in soil landscapes and their associated properties can have significant effects on erosion and deposition processes that affect runoff and transport of pesticides. Simulation models are one way in which the effects of landscapes on these processes can be assessed. This simulation study evaluated the effects of variations in landscape position on runoff and dissolved atrazine utilizing a calibrated farm- and field-scale Agricultural Policy/Environmental eXtender (APEX) model. Twelve agricultural plots (18 m x 189 m) in the Goodwater Creek watershed, a 7250 ha agricultural area in north-central Missouri, were simulated. Plots were treated with three tillage and herbicide management systems for two grain crop rotations. Each plot contained three landscape positions (summit, backslope, and footslope) along with two transition zones. Runoff was measured and samples were collected from 1997 to 2002 during the corn year of the crop rotations. Runoff samples were analyzed for dissolved atrazine. The model was calibrated and validated for each plot with event data from 1997 to 1999 and from 2000 to 2002, respectively. APEX reasonably simulated runoff and dissolved atrazine concentrations, with coefficients of determination (r²) values ranging from 0.52 to 0.98 and from 0.52 to 0.97, and Nash-Sutcliffe efficiency (NSE) values ranging from 0.46 to 0.94 and from 0.45 to 0.86 for calibration and validation, respectively. The calibrated model was then used to simulate variable sequencing of landscape positions and associated soil properties as well as variable lengths of landscape positions. Simulated results indicated that the runoff and the atrazine load at the plot outlet increased when the backslope length increased while keeping the steepness constant. The maximum simulated runoff among different sequences of landscape positions occurred when the backslope position was located adjacent to the outlet. Results from this study will be helpful to managers in placement of conservation practices on sensitive landscapes for improvement in water quality. [ABSTRACT FROM AUTHOR]

Conventional cultivation of claypan soils leads to soil and water quality degradation because of high runoff and associated soil erosion. The Goodwater Creek Experimental Watershed, which is part of the USDA Agricultural Research Service Benchmark Conservation Effects Assessment Project, Watershed Assessment Studies, was established to address these issues. Plot studies have highlighted trade-offs between erosion control and herbicide or nutrient runoff. There is a need for long-term field-scale evaluation of combined practices that reduce sediment, nutrient, and herbicide losses by runoff. A 36 ha field located in Missouri was under a conventional corn (Zea mays L.)-soybean (Glycine max L.) system from 1993 to 2003 with fertilizer application and tillage prior to planting in the spring. A precision agriculture system defined by two main management zones was implemented from 2004 to 2014: wheat (Triticum aestivum L.) and soybean in 60% of the field, and corn and soybean in the remaining 40%. The system included no-till, cover crops, atrazine split-applications based on weed pressure, variable rates of nitrogen (N), and variable rates of fall-applied phosphorus (P). The objective of this study was to compare runoff water quality from the two management systems, based on flow and load duration curves, cumulative distribution functions, and conclusions from replicated plot studies. The precision agriculture system did not affect annual runoff, but it did increase the frequency of low flows. Sediment losses were reduced by 87% as a result of no-till and cover crops. Atrazine and P losses were lower than expected, despite the lack of incorporation into the soil. Atrazine losses were possibly lower because of the wheat area acting as a buffer, greater atrazine adsorption and retention in the field, and faster decay. Dissolved P losses as a fraction of applied remained the same, likely because of greater adsorption and lower runoff risk when applying P. Finally, nitrate-N (NO3-N) losses decreased and resulted in an overall decrease of N losses despite a slight increase of ammonium-N (NH4-N) losses. Explanations included a greater soil water content, a different timing of N applications, and N uptake by cover crops. Building on these successes, an aspirational management system is proposed to further improve on the performance and practicality of the precision agriculture system. [ABSTRACT FROM AUTHOR]

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 ad) the plot-scale findings (0.37 ha [0.92 ad) 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 (P2) were 3.2 and 2.0 times those from mulch tifiage (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 iii calculating field atrazme concentrations from both mulch and no-tillage systems with coefficient of determination ⩾ 0.70 and Nash and Sutciffe 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 Sutciffe 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. [ABSTRACT FROM AUTHOR]

Soils that naturally have a significant runoff component because of low permeability, such as claypans or steep slopes, are especially susceptible to herbicide losses in runoff. For these soils, seasonal losses as impacted by management practices are not well quantified. The objectives of this study were to evaluate the effect of three cropping systems on herbicide loss in surface runoff and develop a model that calculates herbicide concentration. Cropping System 1 (CS1) was a mulch tillage corn-soybean rotation system with herbicides surface applied then incorporated. Cropping System 2 (CS2) was a no-till corn-soybean rotation system with herbicides surface applied and not incorporated. Cropping System 5 (CS5) was a no-till corn-soybean-wheat rotation system with split herbicide application in 1997 and 1999 and no incorporation. The study was conducted on 0.37 ha (0.92 ac) plots equipped with flumes and automated samplers. During each runoff event, runoff volumes were measured, and water samples were collected at equal flow increments and analyzed for atrazine [2-chloro-4-ethylamino-6-isopropylamino-s-triazine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethy/acetamide]. Averaged over years, atrazine and metolachlor losses from CS2 were 2.2 and 1.6 times those from CS1, respectively. Atrazine loss to surface runoff from CS1, CS2, and CS5 accounted for 1.6, 2.5, and 5.7% of the total atrazine applied, respectively. Metolachlor loss to surface runoff accounted for 1.8, 2.0, and 2.0% of the total applied for the three cropping systems. Herbicide concentrations were extremely high in the first runoff event measured after application, particularly when it occurred within a few days after application. A generalized model was developed to account for the effects of time after application, runoff volume, and application rate on herbicide concentration in runoff. Overall, the study showed that accounting for incorporation, split application, runoff volume, and timing of runoff events relative to the day of application can increase the confidence in calculations of the amount of herbicide transported to surface runoff. [ABSTRACT FROM AUTHOR]