Vegetative Treatment Areas: Design and Risk Assessment Tool

An existing water budget model was modified to be applied to vegetative treatment area (VTA) design and assessment of discharge risk in sloping hardpan soils, such as occur in the northeastern US. The model predicts runoff wastewater volume from an impervious contributing area (e.g., silage bunker or barnyard) and simulates water movement, moisture content, and water table height in four fields consisting of an upslope field and the VTA (divided into three fields). Ultimately, it predicts surface VTA discharge for a set of hydrologic data and wastewater-contributing area, soil, and landscape parameters that are readily available.

Download the VTA modeling tool here! The model is an Excel spreadsheet. Instructions on how to use the tool are given at the bottom of this page. (download) (4.7 MB)

Full Model Description and Analysis

For a more detailed description of the model and a comparison of predicted and measured data please see: Faulkner, J.W., Z.M. Easton, W. Zhang, L.D. Geohring, and T.S. Steenhuis. 2009. Design and Risk Assessment Tool for Vegetative Treatment Areas. In progress.

For a full description of the original model that was modified, including underlying equations, 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 soil surface within the downslope portion of the VTA, resulting in surface water discharge.

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 VTA may saturate completely, potentially routing wastewater directly to surface water 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. The size of the wastewater-contributing area, as well as an estimate of the curve number (CN) of that area, are also needed

Model output

The model output consists of the time-dependent water table elevations within the upslope field and in three fields consisting of the entire VTA. Failure, or surface discharge from the VTA, occurs when the height of the water table reaches the soil surface in the most downslope field within the VTA (i.e., Field 4). The number of days of VTA discharge are tallied and presented. The model also calculates the total volume of discharge from the VTA.

Questions and comments about the model in Excel: Joshua W. Faulkner, jwf24@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 seven worksheets, and all operations are done in the metric system. Three graphics are also included which illustrate the watertable fluctuations, precipitation, and saturation excess runoff volume for the available meteorological and model-produced data.

1. VTA Parameters: The fields' dimensions and soil moisture conditions can be modified in this worksheet.
2. Climate and Bunker Info: Information regarding the wastewater-contributing area and evaporation and precipitation data are entered here.
3. Field 1: The water table and failure are calculated for Field 1.
4. Field 2: The water table and failure are calculated for Field 2. Wastewater additions to the VTA are made within this field.
5. Field 3: The water table and failure are calculated for Field 3.
6. Field 4: The water table and failure are calculated for Field 4. Failure in this field indicates surface discharge from VTA.
7. Water Table Output: Volume of saturation excess runoff for each field is calculated here.

Data and Parameter Modification: 'VTA Parameters' and 'Climate and Bunker Info' worksheets

In the VTA Parameters worksheet, the parameters for each of the four 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 four fields:

In the Climate and Bunker Info worksheet, the parameters for the wastewater-contributing area can be input (i.e, area and CN). The gray shaded cells on the worksheet indicate which variables can be altered by the user. Furthermore, 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. 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: Only change the parameters in the gray shaded cells. As changing others may cause errors in the operation of the model.

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', and 'Field 4' WORKSHEETS

Each field's worksheet is set up approximately the same. However, in Fields 2, 3, and 4 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 contributing area to the surface of the soil (added to precipitation).

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 Figure".

The water table heights and the volume of saturation excess runoff for each of the fields are viewed on the worksheet entitled "Water Table Output". Cumulative saturation excess runoff is plotted in the 'Cum. Saturation Excess' sheet.