Implementation of Procedure
The procedure will be conceptualized and
described with logic and
mathematical expressions
to form a model,
which will be
coded in a computer program.
A record and description of these
activities is to be documented.
Model Considerations
Structure
The model is to be based on the fundamental erosion processes of:
(a) interrill erosion
(principally detachment by raindrop impact and lateral transport by
thin flow),
(b) rill and concentrated flow erosion
(detachment by flow),
(c) sediment transport by flow,
(d) deposition by flow,
(e) deposition in impoundments,
and concentrated flow hydraulics.
The model is expected to include major modules for:
(a) climate generation;
(b) snow accumulation,
(c) snowmelt,
(d) infiltration,
(e) runoff,
(f) soil temperature,
(g) erosion,
(h) soil moisture,
(i) crop growth,
(j) plant residue,
and
(k) tillage.
Implicit in all of these modules except for the climate module
is the central role of soil and soil properties.
Although the model will include these modules,
it is NOT intended to be used specifically as
a model for crop yield,
water quality,
soil moisture,
runoff,
stochastic (random) climate variables,
wind erosion,
or
erosion and sediment yield from classical gullies,
stream channels,
or large complex watersheds.
Hydrologic Elements
Hydrologically,
the model is to apply to conditions where overland flow
is significant, and runoff and erosion
is not dominated by "partial area"
hydrology.
The model will
consider
lateral subsurface flow
and baseflow using a simple travel time
approach that takes position on the landscape
into account.
It will consider vertical water movement in the root zone
and tile drainage
only
to the extent needed to compute surface runoff
sufficiently accurate for erosion computations.
The hydrologic elements will be:
(a) overland flow
(broad sheet flow and concentrated flow in "furrows"),
(b) concentrated flow in major natural and constructed waterways
(ephemeral gullies plowed over
within the crop rotation,
terrace and diversion channels,
grassed waterways,
and rangeland gullies comparable to within-field
concentrated flow channels),
(c)
small impoundments
(underground tile outlet terrace impoundments, level terraces without outlets, water and sediment control basins,
within-field natural impoundments,
farm ponds,
and other similar within-field structures and features),
(d)
simple return, lateral, and base flow,
and (e) simple tile drainage.
Family of Models
A family of models is permissible
to meet the entirety of the user requirements.
All of the models in the family are intended for use by NRCS, FS, and BLM;
research models are not permissible in the family.
Commonality within the family and with other models
is to be maintained where reasonable.
In so far as possible,
relationships used in simplified versions
in the family are to be derived from those in the most detailed version.
This statement of commonality implies
that when parameter values representing the assumptions used to derive the
simplified relationships are input
into the detailed relationships,
they will give the same result as the simple ones for
a given situation.
Commonality in so far as possible is to be maintained with
components of other models where
particular dominant algorithms are emerging.
The model and its coding are to be modular
to make changing of model modules or submodules simple and easy.
Inputs are to be common across this family of models and
with other models in wide use in NRCS in so far as possible.
Similarities with Other Models
Since this model will have several features that are similar to those in existing
models produced by ARS
and others
that are being used by NRCS,
NRCS has asked for a brief
description of several of these models.
Similarities of this model
with other models
having similar components are described in Appendix 8.
A model is like a tool in a tool chest.
While a pair of pliers can sometimes be used as a wrench,
a wrench of the proper design and size (i.e., the proper tool for the job) is usually chosen to
tighten bolts.
Similarly,
this prediction model is to be developed as the best
tool for predicting erosion by the field user.
Other models are better for other purposes,
e.g., EPIC for estimating the impact of erosion on productivity.
Modification of Model Modules
The various modules or model subcomponents are to
be constructed so that they can be easily replaced.
Also,
the modules are to be constructed in
a way to facilitate their use in combination with other models.
Coding
The main requirements
presently identified for the coding
is that the model and its code will
follow a structured design.
The model will be programmed using structured programming procedures.
The program must be developed in
the programming languages of FORTRAN 77 or C,
and machine dependent routines are to be avoided
to enhance transportability.
The language and other requirements related to programming
are to be developed during further discussions with NRCS.
Also,
mnemonic variable names will be used.
Additional details are to be developed
on computer requirements
such as data base management, menu vs. command driven, and input and output screens.
Documentation
The main delivered product from the project will be a computer program
and documentation
transferred on a tape,
floppy disk,
or electronically between computers.
However,
written documentation is to accompany the computer file.
The documentation will describe the governing logic and mathematical relationships
on which the model is built,
validation of the model,
information needed to install the computer program,
description of inputs and how to obtain them,
and instructions on how to run the program and use the model.
Also,
information needed to maintain the model and its code will also be provided.
"User" type documentation will be jointly developed
by ARS and the user agencies.
Any Comments Would Be Appreciated
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