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 procedure produces monthly estimates of sediment, phosphorus and nitrogen loads based on a daily
time step for computing runoff and ground water discharge. A computerized version is available from
the first author. Monthly nutrient loads in stream flow are determined by summing the following (mass
balance) components of the dissolved and solid-phase watershed nutrient loads in kilograms, for month
m.
L Dm = D Pm + D Rm + D Gm + D um
(1.1.13)
LSm = SPm + SRm + SUm
(1.1.14)
Where:
LDm = total dissolved nutrient mass
DPm = dissolved point source nutrient mass
DRm = dissolved rural nutrient mass
DUm = dissolved urban nutrient mass
LSm = total solid-phase nutrient mass
SPm = solid-phase point source nutrient mass
SRm = solid-phase rural nutrient mass
SUm = solid-phase urban nutrient mass2
1.1.1.9 Cropland and Forest Dissolved Nutrient Loading
We describe the dissolved nutrient loading component of the GWLF (Haith and Shoemaker,
1987) model for rural source areas. The model is based on the CNE for runoff estimation and earlier
applications of the dissolved loading function are documented by Delwiche and Haith (1983) and Haith
and Tubbs (1981). The event based model applies to agricultural, forest, and construction land use
pollutant runoff.
The watershed is divided into source areas based on soil type, vegetative cover, management
practice and hydrologic response. Each source area is assumed to make an independent load
contribution to the watershed outlet. Nutrient export for each source area is summed over the
precipitation event. Loads area then transported to the receiving watercourse in runoff. The method
assumes a conservative transport mechanism allowing no attenuation of nutrients from "edge-of-field" to
the watershed outlet (Haith and Tubbs, 1981).
2
Dr. Douglas A. Haith, Department of Agricultural Engineering, Riley-Robb Hall, Ithaca, NY
14853, 607-255-2802
1.1-12
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