Septic Systems Research:
Water Budget Model for Septic Disposal Field Design
A water budget model was developed to evaluate septic
systems performance in sloping hardpan soils such as occurring
in the northeast US. The model simulates water movement, moisture
content and water table height in three fields consisting of the
septic effluent disposal field and the up-slope and down-slope
fields. Ultimately, it assesses the probability of failure for
a set of hydrologic data, septic loading rates, and soil and landscape
parameters that are readily available.
Full Model Description and Analysis
For a full description of the model and analyses that have been
made to validate the model, please see the publication: A.S.
Collick, Z.M. Easton, F.A. Montalto, B Gao, Y.J. Kim, L. Day,
and T.S. Steenhuis. Hydrological Evaluation of Septic Disposal
Field Design in Sloping Terrains, 2006. Journal of Environmental
Engineering 132: 1289-1297.
Purpose of model
The purpose of the model is to track the height of the water
table in each field to assess the probability of failure due to
the water table reaching the level of the septic disposal distribution
pipes in the septic field or above 0.2 m below the ground surface
in the upslope and downslope fields.
The model itself is valid for all sloping terrain with a restrictive
or impermeable layer at some depth. On these soils under prolonged
rainfall conditions, the septic and downstream field may saturate
completely causing the septic system to fail and effluent is likely
to be routed directly to streams via overland runoff.
Daily precipitation and at least monthly evaporation are the
required meteorological data. The dimensions of each of the fields
and the soil moisture conditions of the fields’ soils are
also necessary for the operation of the model.
The model output consists of the time-dependent water table elevations
within the septic drainage field, the upslope field, and downslope
field. Failure occurs when the height of the water table is rises
above 20cm below the soil surface in the upslope and downslope
fields and/or exceeds the depth of the septic disposal pipes.
The number of failure days are tallied and presented.
Questions and comments about the model in Excel: Amy
S. Collick, email@example.com
or Tammo S. Steenhuis, firstname.lastname@example.org
Using the Model (An Excel Workbook)
Note: The file containing the model
can only be initially opened in “Read-Only format”.
Once downloaded, it can be saved as the user’s own file.
The model is contained within four worksheets, and all operations
are done in the metric system. Two graphics are also included
which illustrate the watertable and precipitation fluctuations
for the available meteorological and model-produced data.
- Parameters: All the data necessary for the operation of
the model can be found in this worksheet. The fields’
dimensions and soil moisture conditions can be modified in
this worksheet. Evaporation and precipitation data can also
- Field 1: The water table and failure are calculated for
- Field 2: The water table and failure are calculated for
- Field 3: The water table and failure are calculated for
Data and Parameter Modification: PARAMETER WORKSHEET
In the Parameter worksheet, the parameters for each of the three
fields are isolated allowing the model user to change the dimensions
and conditions of each field separately. The gray shaded cells
on the worksheet indicate which variables can be altered by the
Enter the following parameters (replacing existing data) for
each of the three fields:
Depth of Root Zone (RZ)
Depth of RZ to Imp Layer, D
Depth of Initial Watertable, d
Field Slope, α
Soil Moisture Conditions
Saturated hydraulic conductivity, Ks
Drainable Porosity, μ
Shape Parameter, λ
Wilting Point Moisture Content, θwp
Field Capacity Moisture Content, θfc
Saturated Moisture Content, θs
Note: Only change the parameters
in the gray shaded cells. As changing others may cause errors
in the operation of the model.
The model user may change the meteorological data utilized in
the model. The evaporation data must be in monthly form. It may
be put into the sheet as either monthly pan evaporation (gray
shaded column) with a pan factor or monthly potential evaporation
(orange shaded column). The precipitation data is limited to a
maximum of 5 years of data and can be put in to the worksheet
in two or three columns (gray shaded columns): first for date,
second for precipitation in inches, and third for precipitation
in centimeters. Depending on the units of the user’s precipitation
data, precipitation data can be added as inches or as centimeters
in the appropriate columns. However, the model uses the column
in centimeters to run.
Note: The user’s meteorological data must
replace the existing data in the color shaded columns. Changing
other data may cause errors in the operation of the model.
Model Operation: FIELD 1, FIELD 2, FIELD 3 WORKSHEETS
For a full description of the development of the model and how
it operates, please see the publication: A.S. Collick, Z.M. Easton,
F.A. Montalto, B Gao, Y.J. Kim, L. Day, and T.S. Steenhuis. Hydrological
Evaluation of Septic Disposal Field Design in Sloping Terrains,
2006. Journal of Environmental Engineering 132: 1289-1297.
The parameters of each of the fields and the meteorological data
used in the model can be found in the first worksheet titled “Parameters”.
All changes or modifications can be made from this worksheet and
the resulting failure tallies can be viewed. The water table heights
for each of the fields are viewed on the worksheet entitled “Water
Each field’s worksheet is set up approximately the same.
However, in Fields 2 and 3 the downward flow from the root zone
includes the outflow from the adjacent upslope fields. Furthermore,
Field 2 includes the addition of the wastewater from the septic
tank to the soil moisture from the root zone and the outflow from
The water table height is determined simultaneously in 3 columns
depending on precipitation, evaporation, field characteristics,
and existing soil moisture conditions. The column marked “Final
Water Table Height” is the height that is copied to the
worksheet entitled “Water Table Output” and graphed
to the accompanying graphic of “Water Table”.