Table 3. Model Statistics for the total nitrogen National Hydrography Dataset SPARROW model developed for the United States Pacific Northwest.


[The p-values for the source and aquatic loss variables are based on a one-sided t-test; the p-values for the land-to-water delivery variables are based on a two-sided t-test. Abbreviations: NLLS, non-linear least squares; R², coefficient of determination; RMSE, root mean squared error; cm, centimeter; kg/yr, kilogram per year; kg/km²-yr, kilogram per square kilometer per year; kg, kilogram; km²; square kilometer; m², square meter; mm, millimeter; m²-day, square meter per day; <, less than; –, not applicable]


Parameter	Model coefficient units	NLLS estimate of model coefficient	Standard error of the model coefficient	Probability level (p-value)	90 percent confidence interval for the model coefficient		Nonparametric bootstrap estimate of model coefficient
					Lower	Upper
Sources
Forestland (west)1 (km2)	kg/km2-yr	25.5	9.8	0.0104	3.7	41.6	26.8
Forestland (east)2 (km2)	kg/km2-yr	71.6	10.7	< 0.0001	49.5	113.5	76.2
Point sources3 (kg/yr)	dimensionless	1.03	0.38	0.0075	0.40	1.48	1.04
Developed land4 (km2)	kg/km2-yr	337	69	< 0.0001	253	409	334
Nonsewered population5 (number)	kg/person-yr	1.17	0.65	0.0735	0.24	1.75	1.08
Atmospheric deposition6 (kg/yr)	dimensionless	0.026	0.012	0.0322	-0.082	0.042	0.007
Farm fertilizer7 (kg/yr)	dimensionless	0.069	0.012	< 0.0001	0.046	0.097	0.072
Livestock manure8 (kg/yr)	dimensionless	0.072	0.030	0.0172	0.025	0.143	0.083
Red alder trees9 (m2)	kg/m2-yr	0.28	0.10	0.0054	0.08	0.41	0.26
Springs and power returns10 (kg/yr)	dimensionless	0.96	0.35	0.0069	0.43	1.20	0.91
Land-to-water delivery
Precipitation11 (mm)	–	1.29	0.14	< 0.0001	1.11	1.78	1.38
Solar radiation12 (watt-hours/m2-day)	–	7.85	1.83	< 0.0001	-0.68	11.03	6.59
Arid land irrigation13 (percent)	–	0.015	0.005	0.0012	0.006	0.031	0.035
Base flow index14 (percent)	–	-0.022	0.005	0.0001	-0.031	-0.009	-0.021
Aquatic loss
Free-flowing streams	–	–	–	–	–	–	–
Impoundments	–	–	–	–	–	–	–
Model diagnostics
R2 of yield	–	0.908	–	–	–	–	–
RMSE	–	0.398	–	–	–	–	–
Number of observations	–	172	–	–	–	–	–

¹ Area of forest land, west side of Cascade Range, 2001.


² Area of forest land, east side of Cascade Range, 2001.


³ Surface-water discharges from permitted wastewater facilities, 2002.


⁴ Area of developed land, 2001.


⁵ Population without sewer service, 2001.


⁶ Wet and dry atmospheric deposition of oxidized and reduce nitrogen, 2002.


⁷ Commercial fertilizer applied to agricultural land, 2002.

⁸ Manure from cattle in dairies and feedlots and grazing livestock, 2002.


⁹ Basal area of red alder trees, circa 2002.


¹⁰Surface-water discharge from springs and power returns, 2002.


¹¹Natural log of mean annual precipitation (1971–2000).


¹²Natural log of mean annual solar radiation (1991–2005); interaction limited to the forest land source terms.


¹³Percentage of catchment containing irrigated agriculture; interaction limited to the farm fertilizer source term.


¹⁴Mean percentage of streamflow due to base flow.

First posted July 17, 2013