Several projects have been conducted in the Little Vermillion River in east-central Illinois. The Little Vermilion River (LVR) drains approximately 190 mi2 of agricultural land before forming the Georgetown Reservoir which serves as the primary water supply for a population of 5000. Since 1991, 15 sites have been established to track important water quality parameters in the river, subsurface drainage, field runoff, and effluent from filter strip and wetland test areas.
An initial study compared water quality from the various subsurface-drained sites. Of particular interest is the difference in nitrate-N concentrations in drainage from corn-soybean fields versus a field in continuous meadow for over 10 years: drainage from the seven fertilized fields had concentrations averaging 10 times greater than those measured in meadow drainage. The meadow will soon be placed back into corn-soybean production and monitoring will continue to observe changes in nitrate-N output.
Additional study focused on the effectiveness of N application and tillage methods in reducing the movement of nitrate-N to surface and subsurface water outlets. Nitrate-N concentration from the seven cropped fields varied considerably depending upon the management systems used and, in particular, the level of N fertilizer applied. Preplant, anhydrous-N application systems with average annual application of 96 lbs/acre had a mean concentration of 16.8 mg/L ; while side-dress and manure application systems, with average annual N application of 82 lbs/acre had a mean concentration of 10.2 mg/L. The mean concentration of nitrate-N from the continuous meadow was 1.1 mg/L.
In a third study, flow and nitrate-N outputs from the field tile systems were compared to outputs measured in the drainage ditch in the upper reach of the LVR. Field subsurface drainage was found to be a useful predictor of surface ditch flow and nitrate-N loading. Ditch response was considered as a combination of contributions from field drainage systems and from flow entering directly from the sides and bottom of the ditch itself. Runoff appears to play a minor role in the LVR watershed. Attempts have been made to model the two flow components using Hooghoudt's equation. The relative contributions of field drain flow and direct ditch flow may be a function of the distribution of different soil types in the watershed and may also vary over time with rainfall. Contributions of N from direct ditch flow are unknown.
Although many questions remain concerning how to predict nitrate-N response at the watershed outlet and eventually reduce concentrations at the Georgetown Reservoir, educational efforts are under way to help encourage reduced applications of nitrogen fertilizer. The Champaign County Soil and Water Conservation District recently interviewed producers in the Champaign County portion of the LVR watershed to determine the extent to which University of Illinois fertilizer recommendations were being followed. Interview results were examined with LVR monitoring data for the period from May 1993 to December 1995. It was determined that the N applied in excess of the recommended amount (the quantity in excess of the economic optimum application rate) was close to N load measured in the drainage ditch at the outlet of the Champaign County LVR subwatershed.
Flow in the Little Vermillion River (which, for the most part is a
man-made ditch) is considered as a combination of inputs from field
subsurface drainage systems and from the sides and bottom of the channel
itself. Overland flow contributions are assumed to be minimal.
Comparison of mean nitrate-N concentration observed in subsurface
drainage below fields fertilized with differing rates of N and
application methods, and continuous meadow.