Table 7. Summary statistics for source shares of total nitrogen and total phosphorus load for National Hydrography Dataset stream reaches in the United States Pacific Northwest (2002 conditions).


[Total nitrogen and phosphorus load, source shares: The contribution from each source as a percentage of the total load due to all upstream landscape nutrient loading. Abbreviations: NHD, National Hydrography Dataset; –, not applicable because this was not a source used in the model]


Source category	Total nitrogen load (source shares)							Total phosphorus load (source shares)
	Mean	Percentiles						Mean	Percentiles
		10th	25th	50th	75th	90th			10th	25th	50th	75th	90th
Atmospheric deposition	17.3	3.38	6.51	11.5	19.6	38.0		–	–	–	–	–	–
Urban sources1	2.78	0.00	0.00	0.0	0.5	6.3		1.73	0.00	0.00	0.00	0.02	1.26
Farm fertilizer	12.4	0.00	0.00	0.0	3.11	65.6		7.26	0.00	0.00	0.00	0.62	19.1
Livestock manure2	17.1	0.45	2.31	7.9	21.4	53.0		37.6	1.66	10.4	30.2	62.4	87.4
Geologic sources3	–	–	–	–	–	–		53.1	3.88	17.8	58.0	85.6	96.8
Forestland4	49.9	0.00	7.89	62.6	80.5	88.4		See footnote5
Springs and power returns	0.18	0.00	0.00	0.00	0.00	0.00		0.12	0.00	0.00	0.00	0.00	0.00

¹ Contribution from developed land, point sources, and nitrogen leaching from septic tanks.


² Contribution from all livestock manure.


³ Contribution from geologic phosphorus exported from forest, scrub, and grass land.


⁴ Contribution from asymbiotic nitrogen fixation in forests and symbiotic nitrogen fixation by red alder trees.


⁵ Phosphorus export from forest land is included in the geologic phosphorus source category.


First posted July 17, 2013