View Full Table | Close Full ViewTable 1.

Study site characteristics.

 
Value Drainage area Land use, in % of drainage area
Urban Crop Pasture Forest Range
km2 %
Minimum 17 0.0 1.0 0.0 0.0 0.0
25th percentile 204 0.8 30.2 7.8 3.1 0.0
Mean 6,123 1.9 52.8 20.9 15.1 6.0
Median 763 1.6 59.3 14.5 9.4 0.4
75th percentile 2,775 2.8 74.7 31.8 26.5 4.5
Maximum 121,182 5.0 98.8 74.8 48.0 74.9
Standard deviation 17,545 1.3 26.9 17.5 14.2 13.0
Total number of sites 133 133 133 133 133 133



View Full Table | Close Full ViewTable 2.

Final multiple linear regression models of mean annual export of total nitrogen (N) and total phosphorus (P) from agricultural watersheds in the United States.

 
Parameter (units) Model coefficients
Value SE t value p value
Total N
Intercept −4.22 0.508 −8.31 <0.001
Total anthropogenic inputs of nitrogen (kg, log transformed) 0.970 0.0281 34.5 <0.001
Runoff (cm, log transformed) 0.755 0.0539 14.0 <0.001
Slope (%) 0.0650 0.0359 1.81 0.073
Number of reservoirs (number km−2) −243 113 −2.16 0.033
Irrigated area (% of agricultural area) −0.00580 0.00200 −2.93 0.004
Area with subsurface tile drains (% of agricultural area) 0.0105 0.00280 3.70 0.000
Area in the Conservation Reserve Program (CRP) (% of agricultural area) 0.0151 0.00880 1.73 0.086
Soil erodibility factor (unitless, centered) −0.493 0.800 −0.616 0.539
Area in conservation tillage (% of agricultural area, centered) 0.00550 0.00290 1.91 0.059
Soil erodibility factor : Area in conservation tillage† −0.0552 0.0337 −1.64 0.103
Residual SE 0.519
Degrees of freedom 122
Multiple R2 0.93
Total P
Intercept −6.48 0.714 −9.07 <0.001
Total anthropogenic inputs of P (kg, log transformed) 1.07 0.0428 25.0 <0.001
Runoff (cm, log transformed) 0.555 0.0949 5.85 <0.001
Precipitation (in) 0.0188 0.00740 2.53 0.013
Slope (%) 0.0779 0.0540 1.44 0.152
Number of reservoirs (number km−2) −310 173 −1.79 0.076
Area in conservation tillage (% of agricultural area) 0.00600 0.00390 1.55 0.123
Soil erodibility factor (unitless, centered) 3.07 1.04 2.95 0.004
Area in the CRP (% of agricultural area, centered) 0.0532 0.0156 3.41 0.001
Soil erodibility factor : Area in the CRP† −0.413 0.130 −3.19 0.002
Residual SE 0.798
Degrees of freedom 119
Multiple R2 0.86
Interaction term indicated with a colon (:).