Overland Flow/Runoff

Overland Flow is the transfer of surface water from an adjacent cell. It is calculated using the following formula in versions of the Natural System Model NSM 4.2 and below, or South Florida Water Management Model SFWMM 3.6 or less:

Overland Flow Volume:

In later version of these models the following formula is used:

Overland Flow Volume:

The parameters used in the calculation are given in Figure 8. This calculation is sensitive to how the matrix of grid cell are processed. To avoid accumulation of error calculate in two phases, process cells from West to East for half the day and from North to South for the next half day. Vegetation and other factors may inhibit flow. This is simulated using the Manning's Roughness Equation.

The values used for Manning's n are based on a 1950's study by the U.S. Army Corps of Engineers.

Runoff is the volume of surface water that is transfered to a canal or to an adjacent cell. Runoff is not calculated in the SFWMM nor the NSM. District models that simulate runoff utilize an extension of the Soil Conservation (SCS) method. The SCS method is a combination of surface runoff and subsurface infiltration rate. Surface runoff occurs when the rainfall rate is greater than the infiltration rate. Subsurface or interflow flow occurs when infiltrated rainfall reappears on the surface due to an impermeable layer. Channel runoff is considered negligible. A runoff curve number, CN, used to predict direct runoff is selected based on antecedent conditions, soild type, and land use practices. An index of Antecedent Moisture Conditions, AMC, estimates the soil wetness for a specific storm based on the five-day antecedent rainfall depth and whether the storm occurs in wet or dry season. The AMC classification is shown in Figure 9. There are four different SCS classifications of hydraulic soil groups, see Figure 10. The landuse description, landuse treatment, the SCS hydraulic soil group are used to do a table lookup (See Table 1. Runoff Curve Numbers and Initial Abstractions for Landuse Types (McCuen 1982)) in "Unsaturated Component for an Integrated Surface-Ground Water Flow Models (ET-RECHARGE PROGRAM)", pp 22-24) and generate the runoff curve number CN. That table assumes AMC group II conditions, therefore if the conditions warrant a different AMC classification, the adjustment to the runoff curve number can be calculated by doing another table lookup (See Table 2. Adjusted CN Depending on Antecedent Moisture Conditions (McCuen 1982) in "Unsaturated Component for an Integrated Surface-Ground Water Flow Models (ET-RECHARGE PROGRAM)", p 25). For cells with multiple land use, a composite curve number is calculated by weighing each curve number according to its area.

The prediction for total direct runoff depth, Q, can be calculated using the Soil Conservation Service equation:

where S is the maximum cell storage and Ia is the initial abstraction, and P is the cummulative depth rainfall and is defined by the SCS method as:

S =(100/CN) - 10.

The curve number CN varies from one to 100. The higher the number the higher the runoff. The initial abstraction estimated by the SCS method is not used in the SFWMM, instead they use the value

Ia = 0.07 inches for Impervious Areas , and

Ia = 0.11 + IF[1000/CN)-10] for Pervious Areas,

where IF , initial filtration, is set to 0.10 for CN > 75, 0.18 for 50 <= CN <= 75, 0.25 for CN < 50, for the pervious area. In free water surfaces, such as Lake Okeechobee, Q = 0 because it is assumed that there is no overflow from the lake.