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.

Appropriate landscape

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.

Model input

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.

Model output

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, asc38@cornell.edu or Tammo S. Steenhuis, tss1@cornell.edu

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.

1. 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 be entered.
2. Field 1: The water table and failure are calculated for Field 1.
3. Field 2: The water table and failure are calculated for Field 2.
4. Field 3: The water table and failure are calculated for Field 3.

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 user.

Enter the following parameters (replacing existing data) for each of the three fields:

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 Table Output”.

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 Field 1.

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”.