SWMM - Computational Methods - Infiltration

infiltration is the process of penetration of rainfall into the soil surface in the unsaturated soil zone of permeable sub-Catchment areas. SWMM offers four options for infiltration modeling:

• Classic Horton Method - This method is based on empirical observations that show that infiltration decreases exponentially from an initial maximum rate to some minimum rate over the course of a long rainfall event. Input parameters required by this method include maximum and minimum infiltration rates, a decay coefficient that describes how quickly the rate decreases over time, and the time it takes for a fully saturated soil to dry out completely (used to calculate recovery of infiltration rate during dry periods).
• Modified Horton Method - This is a modified version of the classic Horton Method that uses the cumulative infiltration in excess of the minimum rate as its state variable (instead of time along the Horton curve ), providing a more accurate estimate of infiltration when low rainfall intensities occur. It uses the same input parameters as the traditional Horton Method.
• Green-Ampt method - This method for modeling infiltration assumes that there is a sharp wetting front in the soil column, separating soil with some initial moisture content below from the saturated soil above. The necessary input parameters are the initial soil moisture deficit, the hydraulic Conductivity of the soil and the suction head at the wetting front. The rate of moisture deficit recovery during periods of drought is empirically related to hydraulic Conductivity.
• Modified Green-Ampt method - This method modifies the original Green-Ampt procedure by not depleting the moisture deficit in the upper surface soil layer during the initial periods of low rainfall as was done in the method original. This change can produce more realistic infiltration behavior for storms with long initial periods, where rainfall intensity is below the soil's saturated hydraulic Conductivity.
• Curve Number Method - This approach is adopted from the NRCS (SCS) Curve Number method for estimating runoff. It assumes that the total infiltration capacity of a soil can be found in the soil's tabulated curve number. During a rain event, this capacity is depleted based on cumulative rainfall and remaining capacity. The input parameters for this method are the curve number and the time it takes for a fully saturated soil to dry completely (used to calculate infiltration capacity recovery during periods of drought).

SWMM also allows the seepage recovery rate to be adjusted by a fixed amount on a monthly basis to account for seasonal variation in factors such as evaporation rates and groundwater levels. This optional monthly soil reclamation pattern is specified as part of a project's evaporation data.