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Nitrate Leaching to Subsurface Drains as Affected by Drain Spacing and Changes in Crop Production System

^a Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907
^b USDA-ARS National Soil Tilth Laboratory, Ames, IA 50011
^c USDA-ARS Soil Drainage Research Unit, Columbus, OH 43210

View larger version (38K): [in a new window]   Fig. 1. Layout of subsurface drain spacing experiment on the Southeast Purdue Agricultural Center (SEPAC) experimental drainage field. Surface elevation contours are shown in meters.

View larger version (23K): [in a new window]   Fig. 2. Measured flow-weighted mean annual nitrate N concentrations in drainflow, and regression predictions with 95% confidence limits, for the 20-m west plot over the 15-yr study. The regression model is listed in Table 4.

View larger version (25K): [in a new window]   Fig. 3. Measured annual nitrate N load differences between the 5- and 20-m east plots, and regression predictions with 95% confidence limits, over the 15-yr study. Loads from the two plots are significantly different when the confidence limits do not include the 0 difference line. Regression: y = (yr)/(0.0799 + 0.000061yr4); R2 = 0.78.

View larger version (29K): [in a new window]   Fig. 4. Box plot diagrams of monthly fraction of annual (a) rainfall, (b) drainflow (20-m west plot), and (c) nitrate N load (20-m west plot), for 1986 through 1999 (1985 nitrate measurements did not begin until April, see text).

View larger version (27K): [in a new window]   Fig. 5. Plot of drainflow and nitrate N concentration over a 2-mo period in March to May of 1998 for the 20-m west plot.