Hydrologic processes controlling herbicide transport in a Missouri claypan watershed

Hydrologic processes controlling herbicide transport are still poorly understood for claypan watersheds in the US Midwest. The presence of a near-surface claypan, a restrictive soil layer of smectitic mineralogy, may play a critical role in controlling herbicide transport to stream water. Data from Goodwater Creek Experimental Watershed (GCEW) (area = 72.5 km²) in central Missouri indicate that atrazine concentrations in stream water peaked during spring storm events, but high concentrations persisted in the baseflow following these events for days to weeks. It is hypothesized that hydrologic pathways exert a major control on atrazine concentrations in stream water. The hypothesis is tested using a combination of a statistical hydrograph model developed by Washington University in Saint Louis using Darcy's law and the diffusion equation and orthogonal data such as electric conductivity (EC). The basin time constant, the single fitting parameter for the model, was approximately 600 minutes or 0.4 days for GCEW. This value is similar to those for other small, non-claypan watersheds in Missouri. Stream flows were simulated very well by the model during the rising limbs of hydrographs for GCEW. Unlike other Missouri watersheds without claypan soils, stream flows in this claypan watershed were always significantly over-predicted for the prolonged falling tails, indicating a possible strong evapotranspiration effect during baseflow. EC values in shallow subsurface water indeed became much higher during baseflow than during storm events, consistent with the evapotranspiration effect on shallow subsurface water. These results suggests that both hydrologic pathways and evapotranspiration exert a major control on stream water quality in Goodwater Creek Experimental Watershed.