CONSERVATION TILLAGE: IMPACT ON AGRICULTURAL HYDROLOGY AND WATER QUALITY IN THE SAGINAW BAY DRAINAGE BASIN (MICHIGAN)

Water borne edge-of-field losses of sediment and nutrients from conservation (chisel plow) and conventional (moldboard plow) tillage sites were investigated. A field scale monitoring program was undertaken for 20 months, from March 1, 1981 to October 1, 1982 on adjacent plots. Flow from overland runoff and subsurface tile discharge was recorded and nutrient and sediment analysis performed. Precipitation characteristics, residue cover, crop stage, and antecedent soil moisture were analyzed to determine the conditions that generated substantial water borne losses from one or both of the study fields. Longterm weather recods were then evaluated to find the likelihood of occurrence of those conditions that appeared to generate runoff and erosion.;The conventionally tilled field lost substantially more sediment, phosphorus and total kjeldahl nitrogen than the conservation field. A large portion of the nitrogen and phosphorus lost from both tillage systems was in a soluble form. Sediment loss from both fields was low; snowmelt runoff generated the largest quantity of sediment on both fields. A single intense storm that occurred on emerging field beans accounted for much of the difference in sediment and phosphorus losses from the two tillage systems.;Based on longterm weather records, conservation tillage practices will cause a larger reduction of phosphorus and sediment losses on sites planted to field beans than corn. Erosive events are not expected from November through May and water borne losses can be reduced by any management practice that diminishes overland flow.;Eleven hydrologic events (tile and/or overland flow) occurred during the sampling period; two resulted from snowmelt and nine from precipitation. The combined volume of overland and tile flow from the two fields was almost identical for the study period, but the conservation tillage field had significantly more subsurface tile flow than the conventionally tilled field. Subsurface tile flow on both fields had significantly lower concentrations of phosphorus, sediment, and total kjeldahl nitrogen than overland flow. Nitrate nitrogen concentrations were higher in tile flow than overland flow.