View Full Table | Close Full ViewTable 1.

Comparison of hydrologic discharge, and loads of NO3–N and total P from Tipton Creek, Iowa (TC325), and from two tile-drained subbasins (TC240 and TC242).

 
Gauge station Drainage area Mean discharge NO3–N load Total P load
ha mm yr−1 kg ha−1 yr−1 g ha−1 yr−1
TC325 19,850 280 31.1 581
TC240 1,700 187 33.1 170
TC242 156 252 38.2 195



View Full Table | Close Full ViewTable 2.

Cumulative discharge at four gauging stations during the September 2006 rainfall-runoff event, Tipton Creek, Iowa, expressed on a unit-area basis, with estimated source contributions based on hydrograph separations.

 
Gauge site Description Discharge Surface runoff Tile discharge Groundwater discharge
mm
TC325 Watershed outlet 5.93 1.26 4.07 0.60
TC240 Tile outfall 4.30 0.53 3.77 0
TC242 Tile outfall 1.92 0.46 1.46 0
TC240 + TC242 Area-weighted average 4.10 0.52 3.58 0
TC101 Surface flume 5.28 5.28 0 0



View Full Table | Close Full ViewTable 3.

Cumulative loads of four contaminants observed at four gauging stations during the September 2006 rainfall-runoff event, Tipton Creek, Iowa, expressed on a unit-area basis. Nutrients and sediment loads are apportioned to major source-pathways for TC325, TC240, and TC242, based on results of source-pathway separation methods discussed in the text.

 
Gauge site Description NO3–N Total P E. coli Sediment
g ha−1 g ha−1 109 ha−1 Mg ha−1
TC325 Watershed outlet 940 27.3 12.50 0.016
Overland sources 4 X† X 0.004‡
Subsurface (tile + groundwater) 936 X X X
TC240 Tile outfall 890 13.5 3.98 X
Surface intakes 3 10.2 X X
Subsurface tiles 887 3.3 X X
TC242 Tile outfall 276 7.6 0.45 X
Surface intakes 3 6.7 X X
Subsurface tiles 273 0.9 X X
TC240 + 242 Area-weighted average 839 13.0 3.70 X
TC101 Surface flume 11§ 66.4 8.33 X
X, not determined.
The remaining sediment load is attributed to channel sources.
§ Includes nondetectable concentrations in multiple samples, assumed to be 0.15 mg NO3–N L−1