.. _simplifiedmodel:
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Simplified Model of Nitrogen and Phosphorus Transformation and Transport
=======================================================================

The simplified model consists of three components (flow model, transport
model, and nutrient (nitrate (NO\ :sub:`3`), ammonium (NH\ :sub:`4`),
and phosphate (PO\ :sub:`4`\ :sup:`3-`)) load estimation) implemented in six
separate modules as follows:

-  The proto-module (0-Preprocessing) aids in preparing soil datasets
   derived from the Soil Survey Geographic Database (SSURGO), which
   includes hydraulic conductivity, porosity, texture, and spatial
   distribution.

-  The first module (1-Groundwater Flow) consists of a simplified flow
   model that uses smoothed topography to approximate the water table
   and calculate the magnitude and direction of groundwater seepage
   velocity.

-  The second module (2-Particle Tracking) calculates flow paths based
   on the first module's results of velocity direction and magnitude.

-  The third module, the Vadose Zone MODel (3-VZMOD), is optional and
   estimates the vertical soil flow and transformation processes for
   ammonium (NH\ :sub:`4`), nitrate (NO\ :sub:`3`), and phosphate (PO\ :sub:`4`\ :sup:`3-`)
   within the Vadose Zone below the OSTDS. This estimation is based on
   the number of OSTDS points, soil conditions, and the depth between
   the drainfield and the water table.

-  The fourth module (4-Transport) uses an analytical solution to the
   advection-dispersion equation to simulate the movement of
   NH\ :sub:`4`, NO\ :sub:`3`, and PO\ :sub:`4`\ :sup:`3-` plumes. The Transport Module warps
   plumes along their paths to reflect realistic environmental conditions.

-  The fifth module (5-Load Estimation) calculates the mass load-input, 
   -output, and -removal for NH\ :sub:`4`, NO\ :sub:`3`, and PO\ :sub:`4`\ :sup:`3-`
   for both surface water bodies and plumes that do not reach water features.

The modules are each described in detail below.

Subsections:
- :ref:`preprocessing` for preprocessing steps.
- :ref:`groundwaterflow` for understanding groundwater flow.
- :ref:`particletracking` on particle tracking.
- :ref:`vzmod` for vertical zone modeling.
- :ref:`transport` for transport modeling.
- :ref:`loadestimation` for load estimation techniques.