Ponds can lose water by seepage (infiltration/exfiltration into the groundwater) or by evaporation. Any water lost by seepage or evaporation does not show up in downstream links and the flow is considered as lost from the system. In the properties of a pond element you will see the attribute called "Pond Seepage Method" in the "Infiltration/Inflow & Seepage" section. You can choose between a few different methods depending on which numerical solver you are using.
By default, "None" will be selected which means no flow loss/seepage is modeled.
Implicit and GVF-Convex solvers:
- Constant Flow: Models a Constant infiltration Flow rate, as specified in the "Constant Flow" field (regardless of pond surface area)
- Average Seepage Loss: An Average Infiltration rate in depth per unit time which is multiplied by the area of the pond water surface (not the wetted perimeter) at that time step to determine the infiltration rate. The depth is based on the current full water depth; adjustment is made for varying water depth at the banks.
- Evaporation is not supported. See more here: Using the Average Infiltration (Seepage) method for ponds
Note: Pond Infiltration is ignored when the pond is not connected to an outlet structure. See: No seepage results from pond in SewerGEMS or CivilStorm
In applying either method, the models also has a stability filter when the pond water depth is below 0.5 ft so that the seepage loss rate will linearly reduce to zero as the depth decreases to zero.
Explicit (SWMM) solver:
- Green-Ampt: The Green- Ampt infiltration method is used to compute the pond seepage loss, in which suction head, conductivity and initial deficit data are required.
Note: - this method considers the surface area over the sides, not the total wet cross section area of the sides. The SWMM solver takes into account variation of depth over the pond banks, by taking the depth average when computing the infiltration rate using the Green-Ampt method (see more details further below).
- Suction Head: Average value of soil capillary suction along the wetting front (inches or mm)
- Conductivity: Soil saturated hydraulic conductivity (in/hr or mm/hr)
- Initial Deficit: Fraction of soil volume that is initially dry (i.e., difference between soil porosity and initial moisture content) The initial deficit for a completely drained soil is the difference between the soil's porosity and its field capacity. Typical values for all of these parameters can be found in the Soil Characteristics Table.
Detailed steps for the Green-Ampt pond seepage method:
1. First, the infiltration rate is computed over the bottom of the pond using the full depth of the ponded volume.
2. Next, the infiltration flow rate is computed by multiplying the bottom area of the pond by this infiltration rate.
3. Next, it determines the surface area over the banks by subtracting the bottom area from the total surface area at the current pond depth.
4. Then, the infiltration rate through the banks is calculated by using the average depth (Current Depth / 2.0).
5. Next, the bank infiltration flow is determined by multiplying the surface area over the banks by the infiltration rate computed in step 4.
6. Finally, it adds together the bank infiltration and the bottom infiltration to get the total infiltration.
- Evaporation loss is supported, a few evaporation types can be defined in the climatology data (under Components - SWMM Extensions) and a user specified evaporation factor for the pond. See more here: Modeling evaporation in a pond?
Reporting flow or volume loss from seepage
Pond infiltration losses can be viewed in a few different ways. First, you can view or graph the attribute "Flow (Seepage Loss)" for your pond. Alternatively, you can view the volume of infiltration loss in the "Ponds" tab of the Hydraulic Reviewer.