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Laboratory studies provide the best opportunity for studying water and pesticide partitioning for a range of soil conditions while maintaining reproducible conditions. We developed and evaluated a laboratory technique for determining pesticide partitioning among infiltration, runoff, and sediment. Simulated rainfall (44 mm h -1 ) was applied to a sandy clay loam 14 days after pesticide (atrazine, chlorpyrifos, and 2,4-D) application. The laboratory technique provided controlled test conditions and reproducible results on the partitioning of rainfall, runoff, and pesticides at the soil surface and on sediment and pesticide transport. Average atrazine and chlorpyrifos half-life (t 1/2 ) values determined from corresponding concentrations in the 0- to 1-cm soil layer were approximately 30 and 10 d. Atrazine, chlorpyrifos, and 2,4-D concentrations remaining in the 0- to 1-cm surface layer decreased logarithmically with rainfall duration. Atrazine and 2,4-D concentrations in the 0- to 1-cm layer were correlated positively with those associated with splash (S), runoff (R), and sediment yield (E) (R 2 = 0.69 to 0.99). Relationships for all pesticide concentrations in the 0- to 1-cm layer and infiltration (INF) were correlated negatively. Relatively poor correlations (R 2 = 0.10-0.63) were found between chlorpyrifos losses and corresponding concentrations in the surface layer even though positive correlations were found between R and E (R 2 = 0.91) and E and S, (R 2 = 0.77). Measured effective partitioning coefficients (K deff ) increased gradually for each event, and were at least 31 times greater than reported K d values for atrazine, chlorpyrifos, and 2,4-D. The methodologies and experimental design described provide a means of evaluating how changes in pesticide concentrations in the surface layer are influenced by soil, rainfall, and pesticide characteristics, which will enhance the development of process-based models to predict pesticide fate and transport for event-based storms.

Many pesticide fate and transport models, including the GLEAMS model, overestimate pesticide degradation during its later stages of residence in soil. Except for runoff events shortly after pesticide application, models using equilibrium sorption kinetics often underestimate sediment-transported pesticides. To address concerns about transport of low levels of pesticides by runoff to sensitive ecosystems over annual or longer time cycles, GLEAMS was modified using a two-compartment pesticide-state model and algorithms for adjustment of degradation rates for temperature and soil water contents. Two pesticide pools, labile and nonlabile, were linked using first-order kinetics, with the forward and reverse rate constants between these pools as the only two additional inputs required for the model, GLEAMS-TC. GLEAMS-TC was calibrated successfully using 3 years of field data. Long-term pesticide persistence in soil was represented. Comparing model predictions with observed data, GLEAMS-TC simulated observed pesticide sediment transport, whereas GLEAMS underestimated observed data by a factor of 4. Sensitivity to rates of pesticide exchange between the two pools and impact on runofflosses were demonstrated with 50-year simulations. GLEAMS-TC is a research tool for investigating changes in pesticide state in soils as a function of exposure time, environmental variables, and interrelationships between pesticide degradation, mobility, and fate and transport in the environment.

Rainfall intensity (I), antecedent water content, and slope steepness affect soil loss from interrill areas. Objectives of this field study were to (i) test the validity of the assumption that interrill soil loss is a function of I 2 , (ii) determine effect of antecedent water content on interrill erodibilities (K i ) for two slope steepnesses, (iii) evaluate the slope factor used in the water erosion prediction project (WEPP), and (iv) evaluate the product of I, flow discharge, and slope steepness as a possible interrill erosion model. Four soils, ranging in texture from sandy loam to clay, were exposed to sequences of simulated rainfall at three intensities (50, 70, and 100 mm h −1 ). Soil loss and runoff were measured from airdried and prewetted flat plots and from air-dried and prewetted ridged plots [...]