
The text in this file is excerpted from the ANSWERS User's Manual,
second edition, 1991, which is written by 

David B. Beasley, Ph.D., P.E.
Professor and Head
Biological and Agricultural Engineering
North Carolina State University
Raleigh, NC  27695-7625
Phone: (919) 515-2694
FAX:   (919) 515-6772
internet:   beasley@bae.ncsu.edu

and

Larry F. Huggins, Ph.D., P.E.
Professor and Head
Agricultural Engineering Department
Purdue University
West Lafayette, IN 47907-1146
Phone: (317) 494-1162
FAX:   (317) 496-1115


The excerpts of the manual included here were scanned from a copy of
the document and converted to text with an optical character recognition
program. Thus, any errors could be from this process or from my
oversight during editing.

Although including the entire manual could be helpful to the user
of the ANSWERS-GRASS integration, I (Chris Rewerts) had to compromise
between all-inclusiveness and time, so I selected the sections which
I thought were of the greatest need. They describe how to
derive the soil and landuse catagory parameter inputs needed to
run ANSWERS, and this information was not included in any other
place in the information within or included with the r.answers program.

The sections from the ANSWERS User's Manual included here are:

Appendix A - Soil Parameters (pages 33-38)
Appendix B - Land Use and Surface Parameters (pages 39-40)
Addendum   - Quick guide for determining roughness parameters for
             use in the ANSWERS model (page 55). 

-----------------------------------------------------------------------

APPENDIX A - SOIL PARAMETERS

                      Section A.1

The parameters described in this section are concerned with the
physical description of the soil condition. The parameters do have some
seasonal variation: since the bulk density of a soil does vary somewhat
throughout the year. The total porosity (TP) is a measure of the bulk
density. The field capacity (FP) describes the upper limit of available
water in the soil. It also quantifies that portion of the pore volume
which can contain only gravitational water (above the moisture holding
capacity of the soil). The control zone depth (DF) identifies the
volume of soil (depth) that actually influences the rate of
infiltration at the surface. The antecedent soil moisture (ASM)
quantifies the starting point for the soil moisture-based infiltration
equation. Methods for determining each of these important parameters
will be presented in this section.

Total Porosity (TP).The volume of pore space within the soil is directly
related to the bulk density (weight per unit volume) of the soil. The
total porosity (percent pore space) of a soil is defined as:

         TP = 100 - (BD/PD) * 100

         where:

         TP = total porosity, percent,
         BD = bulk density,
         PD = particle density (assumed to be 2.65).

In some cases, comprehensive soil surveys will contain information on
the bulk density of the mapped soil types. However, most soil surveys,
even the newer ones, do not contain this kind of data. One bit of
information that is available in all soil surveys is the textural class
of the individual soil types. Without specific information on the bulk
density of a particular soil, the modeler can still make a reasonably
good estimate of the bulk density, and thus the total porosity, by
utilizing the textural class definition.  Table A-1 lists bulk
densities and other soil physical properties for several different soil
textural classes. For those classes not listed, an interpolation can be
performed. Very sandy soils or organic soils have much larger ranges of
values.

Field capacity (FP). As the moisture content of the soil is increased,
a point is reached when water begins to drain due to gravitational
forces. Another way of describing this phenomenon is to say that the
moisture holding tension within the soil becomes less than 1/3
atmosphere. The point at which this gravitational drainage begins is
called field capacity (FP) and is expressed as a percentage of
saturation. Saturation occurs when the total pore space within the soil
is filled with water. Thus, a soil with a total porosity of 50 percent
and a field capacity of 70 percent contains 35 percent water (by
volume) at field capacity.

Although some soil surveys contain information about the actual field
capacity of the individual soil types, most soil surveys only state
what the available water capacity of the soil is. Using the information
in Table A-1 and the available water capacity of the individual soils
(A horizon), the modeler can easily estimate the field capacity
(percent saturation) when that information is not available. By
definition, the available water capacity of a soil is that water held
within the pores between field capacity (tension of 1/3 atmosphere) and
the wilting point (tension of 15 atmospheres).  In addition, the
assumption is made that approximately one half of the water in the soil
is unavailable. Thus, if the available water is listed as .15
inches/inch (15 percent by volume), the field capacity of the soil is
twice that amount or .30 inches/inch (30 percent by volume). Further,
if the total porosity has been listed as 50 percent, that means that
the field capacity of the soil is 30 percent divided by 50 percent or
60 percent of saturation.


Table A-1. Some Representative Physical Properties of Soils.
----------------------------------------------------------------------
    Soil         Bulk        Total          Field          Wilting
  Texture       Density*    Porosity*      Capacity*        Point*
                (g/cc)     (% volume)    (% saturation)  (%saturation)
 
Sandy            1.65         38              39             17
             (1.55-1.80)   (32-42)          (31-47)       (10-24)

Sandy loam       1.50         43              49             21
             (1.40-1.60)   (40-47)          (38-57)       (15-26)

Loam             1.40         47              66             30
              (1.35-1.50)  (43-49)          (59-74)       (26-34)

Clay loam        1.35         49              74             36
              (1.30-1.40)  (47-51)          (66-82)       (32-40)

Silty clay       1.30         51              79             38
              (1.25-1.35)  (49-53)          (72-86)       (34-42)

Clay             1.25         53              83             40
              (1.20-1.30)  (51-55)          (76-89)       (37-43)
----------------------------------------------------------------------
    *Numbers in parentheses indicate normal range.
     Adapted from: (Israelsen and Hansen, 1962).


Infiltration Central Zone Depth (DF)

Of all of the parameters used in ANSWERS, this one is the least well
defined and most arbitrary. Essentially, it describes the volume of
soil (depth) that influences the infiltration rate at the surface of
the soil. Experimental data and simulation experience have lead to the
conclusion that the control zone depth (DF) varies with time. However,
not enough information exists to describe a seasonal influence exactly.
In general, the control zone depth is equal to or less than the depth
of the A horizon. For most of the soils that have been modeled in the
Midwest USA, the control depth (DF) has been assumed to be equal to .25
to .75 of the depth of the A horizon.  In general, a starting value of
one half of the A horizon has been used.

Antecedent Soil Moisture (ASM)

The infiltration equation in the ANSWERS model is based on the moisture
content of the soil. Since the infiltration rate will be much greater
when the soil is "dry" than when it is "wet", it is very crucial that
the correct antecedent moisture content be used when simulating actual
situations. For hypothetical or "wet weather" simulations, moisture
contents at or above field capacity will generally be used.

This section details a simple moisture balance approach for determining
the antecedent moisture content in each soil. The form of the moisture
balance equation is:

        ASM = ASML + RAIN - ET - RO - PERC 

        where:

        ASM  = antecedent soil moisture,
        ASML = last known (initial) soil moisture,
        RAIN = daily rainfall,
        ET   = evapotranspiration,
        RO   = runoff,
        PERC = percolation.

In this equation, percolation refers to drainage of gravitational water
(water in excess of field capacity). In order to simplify the moisture
balance calculations, several assumptions are made:

  1. The depth of the soil layer that influences the moisture 
     content is equal to the control zone depth (DF),
  2. The evapotranspiration rate is one half of normal on days  
     that have rainfall in excess of 0.2 inches,
  3. The soil drains down to field capacity within 1 day at the 
     steady state infiltration rate (FC),
  4. When the soil moisture content reaches the wilting point, 
     no additional moisture is lost due to ET,
  5. On days when rainfall is less than 0.3 inches, RO (runoff) is zero,
  6. On days when rainfall is between 0.3 and 0.8 inches, RO  0.10*RAIN,
  7. On days when rainfall is between 0.8 and 1.5 inches, RO  0.15*RAIN,
  8. On days when rainfall is greater than 1.5 inches, RO  0.20*RAIN.

The rate of evapotranspiration can be calculated using any of several
different equations. Each method entails certain assumptions and the
user must determine which equation best serves his purposes and
utilizes his data. The average monthly rates shown in the following
example are representative of cropland rates in northern Indiana.

The antecedent moisture calculations should be started approximately
one month prior to the time to be simulated. Field capacity or any
other reasonable moisture content can be assumed as a starting point.
During the period of calculation, the soil moisture is not allowed to
go below the wilting point.  Once enough rainfall has occurred within
the calculation period to equal or exceed field capacity, the previous
history is wiped out. Table A-2 shows example moisture calculations for
a soil with a total porosity of 50 percent, control depth (DF) of 6
inches, field capacity of 70 percent saturation (wilting point of 35
percent saturation) and a steady state infiltration rate (see Section
A.2) of 0.3 inches per hour. Using the above information and assuming
that an ANSWERS simulation is to be started on June 14, the ASM value
for this soil type would be 67 percent.

                      Section A.2

The parameters which specifically describe a soil's infiltration
response as described by the modified form of Holtan's equation used in
ANSWERS are defined in this section. The steady state infiltration rate
(FC) indicates the rate at which the soil will absorb water when the
soil is saturated.  The difference between the maximum and steady state
infiltration rates (A) combined with the infiltration exponent (P)
helps to describe the typical exponential "drawdown" of the
infiltration rate.  

Infiltration Bate Descriptors (FC and A)

A simple procedure for selecting values for the steady state
infiltration rate (FC) and the difference between maximum and steady
state rates (A)is described here. The user is, of course, free to use
any information he has concerning these values.  Soil survey
information is used in this procedure due to its general nature and
ready availability.


Table A-2. Antecedent Soil Moisture (ASM) Calculation Example.
-------------------------------------------------------------------
Day        Soil Moisture     Rain    ET    Runoff   Percolation
           (% saturation)    (in.)  (in.)  (in.)     (in.)
1 (5/15)        70            .01    .05    0         0
2               69            0      .05    0         0
3               67            0      .05    0         0
4               65            0      .05    0         0
5               64            0      .05    0         0
6               62            3.16   .03    .63       0
7               100           .90    .03    .14       2.26
8               94            .80    .03    .08       .73
9               93            .02    .05    0         .69
10              69            0      ..05   0         0
11              67            .29    .02    0         0
12              76            0      .05    0         .18
13              68            0      .05    0         0
14              67            0      .05    0         0
15              65            .05    .05    0         0
16              65            .27    .03    0         0
17              73            .27    .03    0         .09
18 (6/1)        78            .21    .03    0         .24
19              76            .25    .03    0         .18
20              77            .35    .03    .04       .22
21              79            .39    .03    .04       .28
22              81            .53    .03    .05       .32
23              85            .03    .06    0         .45
24              69            0      .06    0         0
25              67            0      .06    0         0
26              65            0      .06    0         0
27              63            .02    .06    0         0
28              62            1.11   .03    .17       0
29              92            .04    .06    0         .66
30              69            0      .06    0         0
31 (6/14)       67
-------------------------------------------------------------------
                                                                                                  The range of permeabilities for a given soil type (as listed in the 
USDA Soil Survey format) are used in the following manner:

  1. The midpoint of the lower 1/3 of the range is used for FC,  
  2. The midpoint of the upper 2/3 of the range is assumed to be 
     the maximum rate,
  3. The value of A is equal to the maximum rate minus FC.

Assuming a permeability range of 0.2 to 1.5 inches per hour, the
following example illustrates this technique:

     The total range = 1.3 inches per hour                                              
     1/3 of range = 1.3/3 = .43 inches per hour
                          
     Thus, FC equals the midpoint of the lower 1/3 of range 
                 
         FC = .2 + (.43/2) = .42 inches per hour

     The maximum rate is the midpoint of the upper 2/3 of the range

         maximum rate = ((.2 + .43) + 1.5)/2 = 1.07 inches per hour
     
     The A value equals the maximum rate minus FC

         A = 1.07 - .42 = .65 inches per hour

Using the entire range, FC would be 0.2 iph and A would be 1.3 iph. The
method detailed above appears to give more realistic numbers.

Infiltration Exponent (P)

As stated in the section on component relationships, the infiltration
exponent (P) relates the rate of decrease of infiltration capacity to
increasing moisture content. This property varies among soil types and
is most closely related to the textural class of the soil. The heavier
the texture (more clay), the larger the value of P. Conversely, sandy
soils show little change in infiltration rate with increasing soil
moisture content and, thus, have a much smaller value of P.  Table A-3
lists some starting point values for several textural classes.

Table A-3. "P" Values for Various Soil Textures.
---------------------------------------------------------------
    Soil Texture      Suggested Values for "P"

      Clay                 .75 - .80
      Silly clay           .65 - .75
      Clay loam            .60 - .70
      Loam                 .55 - .65
      Sandy loam           .50 - .60
      Sand                 .35 - .50
---------------------------------------------------------------

                       Section A.3

Two different types of information are included in this section. The
USLE "K" factor or soil erodibility of each soil type is described. The
general subsurface drainage characteristics of the watershed are
described by the combination of the tile drainage coefficient and the
groundwater release fraction.  Both of these drainage terms are defined
and methods of parameter value assignment are discussed.

Soil Erodibility - USLE "K" (K)

Most of the newest USDA Soil Surveys contain information on the "K"
factor for each soil type mapped. Other sources of this information
include statewide soil loss handbooks or brochures which are published
by most state SCS offices. Wischmeier et al. (1971) have produced a
nomograph technique for determining the USLE "K" factor based on
textural class and other soil characteristics.

Section A.4 details a method for simplifying the soils description file
presented in this Appendix.  Essentially, the technique involves the
grouping of soils by similarities in drainage response. When soils are
grouped in this manner, the (K) parameter for the various individual
soils may not be equal.  If they are not, one of two methods can be
used to arrive at an "effective" (K).

   1. Use an area weighted average of "K" values within a 
      drainage class, or
   2. Use the value of "K" for the predominant soil(s).


Subsurface Drainage Characteristics

The two parameters which describe the rate of subsurface water movement
are the tile drainage coefficient and the groundwater release
fraction.  The tile drainage coefficient is simply the design value for
tile drainage. Interflow or groundwater release is described by putting
a fraction of the water in the subsurface reservoir into the channel
system at each time step. Experience has shown that the value of the
fraction varies from as little as zero to approximately 0.01. Small
values of the fraction may actually cause an increase in the flow rate
on the recession limb of the hydrograph due to the fact that the
drainage rate from the control zone is greater than the groundwater
movement rate. Thus, the subsurface reservoir of water increases, and
the interflow rate rises accordingly. Higher values of the fraction
will cause the hydrograph to "level off" for a period of time and then
decrease as the rate. of subsurface drainage becomes less than the
interflow rate.

                      Section A.4

In order to reduce the number of soils that must be described in the
"predata" file, a technique has been developed for identifying soils
with similar drainage characteristics. The similar soils are then placed
in a general group with drainage and erosion characteristics which
describe the "average" response of the soils making up the group.

The procedure requires information about the drainage classification
and hydrologic soil group of each soil type. The technique involves the
following:

  1. Soils are listed by their drainage classification (i.e., 
     well drained, poorly drained, very poorly drained, etc.) 
     and by hydrologic group (i.e., A, B, C or D).
  2. The soils are first grouped by drainage classification. 
     Then, those soils are examined for hydrologic group. Soils 
     that have the same drainage classification and hydrologic
     group are considered to have similar responses. It is possible 
     that within one drainage classification there may be soils with 
     two or more hydrologic groups. The soils in each hydrologic  
     group should be considered as a separate soil group.
  3. The soil(s) that predominate the area in each soil group are 
     chosen as representative. A more complex method would be to 
     select the descriptive parameters based on area weighting.


APPENDIX B - LAND USE AND SURFACE PARAMETERS

Section B.1

The information presented in this section involves the extent of crop
cover, the flow retardance of the surface and the relative erosiveness
of the various crops or land uses. The specific land use or crop (CROP)
is simply an identifier that is printed during output. The potential
interception (PIT) and percent cover (PER) are used to describe the
interception of rainfall. Manning's n (N) describes the surface
roughness or retardance to flow. The relative erosiveness parameter
(C) is actually a combination of the USLE "C" and "P" values with
seasonal adjustment.

Interception Parameters (PIT and PER)

A certain amount of the precipitation during any event never reaches
the soil surface. Contact with and storage on vegetation accounts for
this removal and is called interception. The potential interception
volume (PIT) describes the volume of moisture that could be removed if
the area were completely covered by that crop or land use. The actual
percentage of cover (PER) assumes the non-covered area has no
interception. Table B-1 lists some example values for PIT.

Table B-1. Potential Interception Values.
------------------------------------------------------------
       Crop                              PIT (mm)


      Oats				.5 - 1.0
      Corn				.3 - 1.3
      Grass				.5 - 1.0
      Pasture and Meadow		.3 -  .5
      Wheat, Rye and Barley		.3 - 1.0
      Beans, Potatoes and Cabbage	.5 - 1.5
      Woods			       1.0 - 2.5
------------------------------------------------------------

 
Manning's n (N)

The measure of surface roughness or flow retardance used in the flow
equation in ANSWERS is Manning's n. This information, when combined
with element slopes, rainfall, interception, infiltration and routing
considerations, helps yield the solution to the continuity equation,
which is the basis of ANSWERS. There are numerous sources for obtaining
reasonable values of n for channel and overland flow situations.

Relative Erosiveness (C)

This parameter is used in determining how much soil could potentially
erode due to a particular crop or land use, when compared to fallow
ground under identical conditions. It is a direct combination of the
USLE "C" and "P" parameters with a seasonal adjustment. Thus,
conventionally tilled corn at crop stage 1 will have a higher (more
erosive) C value than the same corn at crop stage 2, when there is more
foliage and root structure. Agriculture Handbook No. 537 (Wischmeier
and Smith, 1978) contains information for determining the USLE "C" and
"P" values throughout the year for numerous crops and management
systems.


                      Section B.2

Figure B-1 shows a profile of a section of the soil surface. The
combination of the peaks and valleys yields a certain depressional
storage volume. In addition, the amount of the surface that area that
is inundated at any time is a direct function of the depth of water on
the surface. The infiltration rate within an element is greatly
affected by the amount of pondage within the area.

                                     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
                                .``.              ^
           \                   /    \       /     |
  soil       \                /      \     /      | HU
  surface---> \       ~\    _/        `--\/       |
                \    /  \_-/                      |
                 \__/ _ _ _ _ _ _ _ _ _ _ _ _ _ _ v_ _ _ _ _ _ DATUM

 
                  Figure B-1. Soil Surface Profile.


Surface Storage Descriptors (HU and RC)

The ANSWERS model uses the maximum roughness height (HU) and a
roughness coefficient (RC) to describe the surface storage
characteristics and the ponded surface area. The roughness coefficient
(RC) is, essentially, a shape factor which describes the frequency and
severity of the roughness. The maximum roughness height (HU) is used to
establish the upper limits of the surface roughness and is physically
measurable. Table B-2 shows some typical values for both HU and RC.

        Table B-2. Typical Surface Storage Coefficients.
-------------------------------------------------------------------
      Surface Condition         HU (mm)      RC

        Plowed Ground:

           Spring- smooth        100        .53
           Spring - normal       130        .48
           Spring - rough        130        .59

           Fall - smooth         60         .37
           Fall - normal         70         .33
           Fall - rough          130        .45

        Disked and Harrowed:

           Very smooth           30         .42
           Rather rough          60         .43

        Corn Stubble            110         .59
-------------------------------------------------------------------



                Quick Guide for Determining Roughness Parameters
                         for Use in the ANSWERS Model
-------------------------------------------------------------------------

                          RC*            Manning's n             HU  
                    ---------------    --------------     --------------     
 Land Use or Cover         Starting          Starting           Starting
                     Range   Value     Range    Value     Range    Value
                                                        (Inches) (Inches)
Row Crop

Turn Plowed
  Smooth           .40 - .50  .45    .070 - .100  .085    1.0 - 3.0   1.5
  Cultivated       .45 - .60  .52    .090 - .120  .110    1.5 - 4.0   2.5

Chisel Plowed
  Smooth           .45 - .60  .52    .080 - .120  .100    1.0 - 4.0   2.0
  Cultivated       .55 - .65  .60    .100 - .140  .120    2.0 - 5.0   3.0
No-Till
  Normal Residue   .55 - .65  .60    .100 - .150  .120    1.0 - 4.0   2.0
  Heavy Residue    .60 - .70  .65    .130 - .170  .150    2.0 - 5.0   3.0

Grass or Pasture**
  Poor Cover       .35 - .45  .40    .065 - .100  .080    0.5 - 2.0   1.0
  Average Cover    .40 - .50  .45    .090 - .120  .100    1.0 - 3.0   1.5
  Good Cover       .45 - .55  .50    .100 - .140  .120    1.0 - 3.0   1.5

Small Grains
  Residue Removed  .40  .50   .45    .090 - .120   .100    1.0 - 3.0  1.5
  Incorp'd. Residue.50  .60   .55    .110 - .140   .120    1.5 - 4.0  2.5

Forests or Wooded Areas**
 Light Woods       .45 - .60  .55    .120 - .180   .150    1.5 - 5.0  2.5
 Heavy Woods       .55 - .65  .60    .150 - .250   .200    2.0 - 6.0  3.0

Plowed Ground
Turn Plowed
  Smooth           .25 - .35  .30    .01 - .05     .035    1.0 - 3.0  1.5
  Rough            .65 - .80  .75    .25 - .50     .350    2.0 -12.0  6.0
Chisel Plowed
  Smooth           .35 - .45  .40    .03 - .08     .050    1.5 - 4.0  2.5
  Rough            .60 - .70  .65    .15 - .50     .250    2.0 - 8.0  4.0
Disked
  Smooth           .30 - .40  .35    .03 - .07     .040    1.0 - 3.0  1.5
  Rough            .50 - .60  .55    .10 - .40     .200    2.0 - 5.0  3.5

-------------------------------------------------------------------------

* The RC parameter is an exponent which describes the frequency of
  the surface roughness. The number varies from around .28 to .8. 
  The larger the number, the more sinuous the surface profile (greater
  frequency).

**The additional cover afforded in the average or good categories also
  has an impact on the infiltration characteristics of the soil. This 
  is due to prevention of crusting and to enhancement of soil surface 
  organic matter contents.


-------------------------------------------------------------------------
References cited:

Israelsen, O.W. and V.E. Hansen. 1962. Irrigation principles and practices. John Wiley
and Sons, Inc. NY, NY.

Wischmeier, W.H. and D.D. Smith. 1978. Predicting rainfall erosion losses - a guide
to conservation planning. Agriculture Handbook 537. science and Education Adminstration,
U.S. Department of Agriculture. 58 p.

Wischmeier, W.H., C.B. Johnson and B.V. Cross. 1971. A soil erodibility nomograph for
farmland and construction sites. Journal of Soil and Water Conservation. 26(5): 189-193.
