Effect of fall tillage following soybeans and the presence of gravel filters on runoff losses of solids, organic matter, and phosphorus on a field scale

This study has two primary components: (1) Effect of fall tillage on pollutant losses in snowmelt runoff following soybeans and (2) Gravel inlet hydrology and filtration capacity. The first part of this study evaluated the effect of no fall tillage following soybeans compared to fall moldboard plowing on snowmelt runoff and losses of total solids (TS), organic carbon, and phosphorus. A paired watershed design was used to evaluate the effect of these management practices. Results show that snowmelt runoff and associated losses of total solids, chemical oxygen demand (COD), and biochemical oxygen demand (BOD) were less with fall moldboard plowing than with no fall tillage. Phosphorus losses depended on the form. There was less soluble phosphorus (SP) lost from the moldboard plow treated watershed compared to a watershed with no fall tillage. More particulate phosphorus was lost from the fall moldboard plowed watershed. In areas where snowmelt runoff is an important part of total runoff, some tillage in the fall would be preferable to no tillage in order to reduce losses.; The hydrology of gravel inlets under field conditions was characterized during major rainfall runoff. The capacity of the entire drainage system limited flow more than individual components of either surface or gravel inlets by generating back-pressure in the outlet pipe. Pollutant loading was examined by monitoring concentrations "above and below" gravel. An estimation of gravel longevity was provided. A historical climatic context approach suggested that gravel inlet in this study would have a 99% probability to function for at least 10 years. Two methods were used to estimate trapping efficiency: mass concentration and particle count. The particle count method showed that detachment occurred early in the event and mostly with the clay size category. The laser diffractometer technique allowed the particle counting approach to estimate number of particles retained in or detached from the media. The pattern of solids retention within pairs of samples showed that large filtration values are associated with concentrations >500 mg 1-1 themselves concurrent with intense rainfall bursts. Overall trapping efficiency of the gravel was 20% based on total solids concentration.