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TECHNICAL REPORTS

Landscape and Watershed Processes

Phosphorus Transport through Subsurface Drainage and Surface Runoff from a Flat Watershed in East Central Illinois, USA

^a King Abdulaziz City for Science and Technology, GDRGP, P. O. Box 6086, Riyadh 11442, Saudi Arabia
^b Dep. of Agricultural and Biological Engineering, Univ. of Illinois at Urbana-Champaign, 1304 W. Pennsylvania Avenue, Urbana, IL 61801 USA
^c Univ. of Illinois Extension, Springfield Center, P.O. Box 8199, Springfield, IL 62791 USA

^* Corresponding author (pkalita{at}uiuc.edu)

Received for publication April 23, 2006. A long-term water quality monitoring program was established to evaluate the effects of agricultural management practices on water quality in the Little Vermilion River (LVR) watershed, IL. This watershed has intensive random and irregular subsurface drainage systems. The objective of this study was to assess the fate and transport of soluble phosphorus (soluble P) through subsurface drainage and surface runoff. Four sites (sites A, B, C, and E) that had subsurface and surface monitoring programs were selected for this study. Three of the four study sites had corn (Zea mays L.) and soybeans (Glycine max L.) planted in rotations and the other site had seed corn and soybeans. Subsurface drainage and surface runoff across all sites removed an average of 16.1 and 2.6% of rainfall, respectively. Annual flow-weighted soluble P concentrations fluctuated with the precipitation, while concentrations tended to increase with high precipitation coupled with high application rates. The long-term average flow-weighted soluble P concentrations in subsurface flow were 102, 99, 194, and 86 µg L^–1 for sites A, B, C, and E, respectively. In contrast, the long-term average flow-weighted soluble P concentrations in surface runoff were 270, 253, 534, and 572 µg L^–1 for sites As, Bs, Cs, and Es, respectively. These values were substantially greater than the critical values that promote eutrophication. Statistical analysis indicated that the effects of crop, discharge, and the interactions between site and discharge and crop and discharge on soluble P concentrations in subsurface flow were significant ( = 0.05). Soluble P mass loads in surface runoff responded to discharge more consistently than in the subsurface flow. Subsurface flow had substantially greater annual average soluble P mass loads than surface runoff due to greater flow volume.

Abbreviations: TP, total phosphorus • RT, reduced tillage • NT, no-till • LVR, Little Vermilion River watershed • TDP, total dissolved phosphate