Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 146

where z = elevation of the soil surface [L], q b = bulk density of soil or bed sed-
iments [M/L 3 ], C sb = concentration of sediment at the bottom boundary [M/L 3 ].
8.3 Nutrient Cycling Simulation
There are two components to simulate water quality. The first component is for
transport of reactive or nonreactive materials throughout the watershed, both
insoluble and dissolved. The second component is a flexible biogeochemistry that
addresses the water quality state variables and transformation processes. Water
quality state variables included in GSSHA can either be transported by advection-
dispersion processes or storage routing depending on the water engines. Con-
ceptually, three hydrologic domains and associated nutrient pathways in the
watershed were modeled: (1) subsurface soils, (2) overland flow, and (3) channel
flow. Currently GSSHA includes: (1) subsurface soil nitrogen module, (2) sub-
surface soil phosphorus module, (3) soil plant dynamic module, (4) overland flow
nitrogen module, (5) overland flow phosphorus module, and (6) in-stream water
quality module.
8.3.1 Nitrogen Cycle
The nitrogen cycle represents one of the most important nutrient cycles found in
terrestrial ecosystems which includes stores of nitrogen found in the atmosphere,
where it exists as a gas (mainly N 2 ) and other major stores of nitrogen including
organic matter in soil and the oceans. Nitrogen in soil and water can be present in
organic or inorganic forms and in either dissolved or particulate forms. The
inorganic forms of nitrogen include nitrate (NO 3 - ), nitrite (NO 2 - ), exchangeable
ammonium ( 4 + ), and fixed ammonium. The activities of humans have severely
altered the nitrogen cycle. Some of the major processes involved in this alteration
include: the application of nitrogen fertilizers to crops and increased deposition of
nitrogen from atmospheric sources. A schematic representation of the watershed
nitrogen transport and transformation processes involved in the nitrogen cycle is
given in Fig. 8.2 a.
8.3.1.1 Nitrogen Transformations in Soils
Once in the soil, the nitrogen will transform through the processes of minerali-
zation, immobilization, volatilization, nitrification, denitrification, plant uptake,
nitrogen fixation, and sediment sorption. Most plants can only take up nitrogen in
two solid forms: ammonium ion (NH 4 + ) and the nitrate ion (NO 3 - ). Ammonium is