Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 152

8.3.4.1 Dissolved Mass Transfer from the Upper Soil

The complicated nature of the flux at the soil or sediment surface is usually
characterized through the use of a mass transfer coefficient, an empirical coeffi-
cient that relates the concentration gradient to mass transport (Choy and Reible
1999 ). The transfer rate of dissolved species from the soil to the water column is
affected by the concentration gradient across the water-soil interface as well as
flow conditions in the water column. This rate is computed with the NSM and then
incorporated into the GSSHA-NSM integration as an external source/sink flux.
Mass transfer theory states that the mass flux of a given species under a given set
of flow conditions can be expressed as:

S d ¼ k e C d2 = / C ð Þ ð 8 : 10 Þ
where S d is mass transfer flux of a dissolved species [ML -2 T -1 ], C d is dissolved
concentration of a species in the water column, C d2 is dissolved concentration of a
species in the soil layer in terms of mass of the substance per bulk volume of the
soil layer [M/L 3 ], k e is mass transfer coefficient between water column and soil
layer [L/T], and / is porosity of the soil layer.

8.3.4.2 Leaching

Nitrate is highly mobile as discussed previously and subject to leaching losses
when both soil NO 3 - content and water movement are high. NO 3 - leaching from
soils must be carefully controlled because of the serious impact that it can have on
the groundwater. Movement of NO 3 - through soil is governed by bulk flow which
results in the movement of nitrate with the flow of water, molecular diffusion
which results in the movement of nitrate due to the concentration gradient, and
hydrodynamic dispersion in the soil due to the heterogeneity and internal structure
of the soil. Leaching of NH 4 + is usually insignificant.
Phosphorus is mainly bound to the fine soil particles. Only a small fraction of
the phosphorus in the soil is present in the dissolved phase. However, some dis-
solved phosphorus is transported with runoff, and small amounts of phosphorus
can reach the ground water through leaching. The amount of percolating phos-
phorus is controlled by the phosphorus adsorptive capacity of the soils above the
aquifer (Nelson and Logan 1983 ). Transport of dissolved phosphorus involves the
same processes as those described for N: convection, diffusion and hydrodynamic
dispersion. These flux terms are computed through the GSSHA-NSM integration.

8.3.4.3 Erosion and Sedimentation

The erosion and sedimentation of sediments and associated pollutants are two
important processes in water quality modeling. Sediment detachment by surface