SWMM 5.2.2 Code for LID Function greenRoofFluxRates

2 / 2

This code is a function called "greenRoofFluxRates" that calculates flux rates from the layers of a green roof. It takes in two input arrays, "x" and "f", which represent the vector of storage levels and the vector of flux rates, respectively. The function uses several intermediate variables, including "availVolume" and "maxRate". It also makes use of several properties of the green roof, such as "soilThickness", "storageThickness", "soilPorosity", "storageVoidFrac", "soilFieldCap", and "soilWiltPoint".

The code first retrieves the moisture levels from the input vector "x" and converts them to volumes. It then calculates the ET rates, soil layer perc rate, and storage (drain mat) outflow rate. The code also checks for the case where the unit is full and limits the rates accordingly. The Surface infil is adjusted so that the soil porosity is not exceeded. The code also finds the surface outflow rate and computes overall layer flux rates.

A table of the variables used in this function and their descriptions is as follows:

Variable Name	Description
x[SURF], x[SOIL], x[STOR]	Input vector of storage levels
f[SURF], f[SOIL], f[STOR]	Output vector of flux rates
surfaceDepth	Moisture level variable for the surface layer
soilTheta	Moisture level variable for the soil layer
storageDepth	Moisture level variable for the storage layer
availVolume	Intermediate variable for available volume
maxRate	Intermediate variable for maximum rate
soilThickness	Property of the soil layer representing its thickness
storageThickness	Property of the storage layer representing its thickness
soilPorosity	Property of the soil layer representing its porosity

Variable Name	Description
storageVoidFrac	Property of the storage layer representing its void fraction
soilFieldCap	Property of the soil layer representing its field capacity
soilWiltPoint	Property of the soil layer representing its wilting point
SurfaceVolume	Volume of the surface layer
SoilVolume	Volume of the soil layer
StorageVolume	Volume of the storage layer
SurfaceInflow	Inflow rate to the surface layer
SurfaceEvap	Evaporation rate from the surface layer
SoilPerc	Percolation rate out of the soil layer
SoilEvap	Evaporation rate from the soil layer
StorageExfil	Exfiltration rate out of the storage layer
StorageDrain	Drain flow rate out of the storage layer
SurfaceInfil	Infiltration rate into the soil layer
SurfaceOutflow	Outflow rate from the surface layer

The code uses several helper functions:

• getEvapRates(SurfaceVolume, 0.0, availVolume, StorageVolume, 1.0) : to calculate evaporation rate
• getSoilPercRate(soilTheta) : to calculate soil percolation rate
• getDrainMatOutflow(storageDepth) : to calculate storage (drain mat) outflow rate
• getSurfaceOutflowRate(surfaceDepth) : to calculate surface outflow rate

The final output of the function is the overall flux rate for each of the three layers: surface, soil, and storage.

void greenRoofFluxRates(double x[], double f[])

//

//  Purpose: computes flux rates from the layers of a green roof.

//  Input:   x = vector of storage levels

//  Output:  f = vector of flux rates

//

{

    // Moisture level variables

    double surfaceDepth;

    double soilTheta;

    double storageDepth;

    // Intermediate variables

    double availVolume;

    double maxRate;

    // Green roof properties

    double soilThickness    = theLidProc->soil.thickness;

    double storageThickness = theLidProc->storage.thickness;

    double soilPorosity     = theLidProc->soil.porosity;

    double storageVoidFrac  = theLidProc->storage.voidFrac;

    double soilFieldCap     = theLidProc->soil.fieldCap;

    double soilWiltPoint    = theLidProc->soil.wiltPoint;

    //... retrieve moisture levels from input vector

    surfaceDepth = x[SURF];

    soilTheta    = x[SOIL];

    storageDepth = x[STOR];

    //... convert moisture levels to volumes

    SurfaceVolume = surfaceDepth * theLidProc->surface.voidFrac;

    SoilVolume = soilTheta * soilThickness;

    StorageVolume = storageDepth * storageVoidFrac;

    //... get ET rates

    availVolume = SoilVolume - soilWiltPoint * soilThickness;

    getEvapRates(SurfaceVolume, 0.0, availVolume, StorageVolume, 1.0);

    if ( soilTheta >= soilPorosity ) StorageEvap = 0.0;

    //... soil layer perc rate

    SoilPerc = getSoilPercRate(soilTheta);

    //... limit perc rate by available water

    availVolume = (soilTheta - soilFieldCap) * soilThickness;

    maxRate = MAX(availVolume, 0.0) / Tstep - SoilEvap;

    SoilPerc = MIN(SoilPerc, maxRate);

    SoilPerc = MAX(SoilPerc, 0.0);

    //... storage (drain mat) outflow rate

    StorageExfil = 0.0;

    StorageDrain = getDrainMatOutflow(storageDepth);

    //... unit is full

    if ( soilTheta >= soilPorosity && storageDepth >= storageThickness )

    {

        //... outflow from both layers equals limiting rate

        maxRate = MIN(SoilPerc, StorageDrain);

        SoilPerc = maxRate;

        StorageDrain = maxRate;

        //... adjust inflow rate to soil layer

        SurfaceInfil = MIN(SurfaceInfil, maxRate);

    }

    //... unit not full

    else

    {

        //... limit drainmat outflow by available storage volume

        maxRate = storageDepth * storageVoidFrac / Tstep - StorageEvap;

        if ( storageDepth >= storageThickness ) maxRate += SoilPerc;

        maxRate = MAX(maxRate, 0.0);

        StorageDrain = MIN(StorageDrain, maxRate);

        //... limit soil perc inflow by unused storage volume

        maxRate = (storageThickness - storageDepth) * storageVoidFrac / Tstep +

                  StorageDrain + StorageEvap;

        SoilPerc = MIN(SoilPerc, maxRate);

                

        //... adjust surface infil. so soil porosity not exceeded

        maxRate = (soilPorosity - soilTheta) * soilThickness / Tstep +

                  SoilPerc + SoilEvap;

        SurfaceInfil = MIN(SurfaceInfil, maxRate);

    }

    // ... find surface outflow rate

    SurfaceOutflow = getSurfaceOutflowRate(surfaceDepth);

    // ... compute overall layer flux rates

    f[SURF] = (SurfaceInflow - SurfaceEvap - SurfaceInfil - SurfaceOutflow) /

              theLidProc->surface.voidFrac;

    f[SOIL] = (SurfaceInfil - SoilEvap - SoilPerc) /

              theLidProc->soil.thickness;

    f[STOR] = (SoilPerc - StorageEvap - StorageDrain) /

              theLidProc->storage.voidFrac;

}