SWMM 5.2.2 Code for LID Function biocellFluxRates

 This code is a function called "biocellFluxRates" that calculates flux rates from the layers of a bio-retention cell LID. 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 LID layers, such as "soilThickness", "soilPorosity", "soilFieldCap", "soilWiltPoint", "storageThickness", and "storageVoidFrac".

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 exfiltration rate out of the storage layer. The code also checks for underdrain flow and limits the perc rate and exfiltration rate by available water. The surface infil is limited by unused soil volume. The code also checks for special cases where the storage layer is not present, both layers are full, or either layer is not full and limits the rates accordingly.

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
soilPorosity	Property of the soil layer representing its porosity
soilFieldCap	Property of the soil layer representing its field capacity
soilWiltPoint	Property of the soil layer representing its wilting point
storageThickness	Property of the storage layer representing its thickness
storageVoidFrac	Property of the storage layer representing its void fraction
SurfaceVolume	Converted surface moisture level to volume
SoilVolume	Converted soil moisture level to volume
StorageVolume	Converted storage moisture level to volume
SurfaceInfil	Rate of inflow to the surface layer
SurfaceEvap	Rate of evaporation from the surface layer
SoilPerc	Rate of percolation from the soil layer
SoilEvap	Rate of evaporation from the soil layer
StorageExfil	Rate of exfiltration from the storage layer
StorageDrain	Rate of underdrain flow from the storage layer
Tstep	Time step
theLidProc	Pointer to LID process data

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

//

//  Purpose: computes flux rates from the layers of a bio-retention cell LID.

//  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;

    // LID layer properties

    double soilThickness    = theLidProc->soil.thickness;

    double soilPorosity     = theLidProc->soil.porosity;

    double soilFieldCap     = theLidProc->soil.fieldCap;

    double soilWiltPoint    = theLidProc->soil.wiltPoint;

    double storageThickness = theLidProc->storage.thickness;

    double storageVoidFrac  = theLidProc->storage.voidFrac;

    //... 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);

    //... exfiltration rate out of storage layer

    StorageExfil = getStorageExfilRate();

    //... underdrain flow rate

    StorageDrain = 0.0;

    if ( theLidProc->drain.coeff > 0.0 )

    {

        StorageDrain = getStorageDrainRate(storageDepth, soilTheta, 0.0,

                                           surfaceDepth);

    }

    //... special case of no storage layer present

    if ( storageThickness == 0.0 )

    {

        StorageEvap = 0.0;

        maxRate = MIN(SoilPerc, StorageExfil);

        SoilPerc = maxRate;

        StorageExfil = maxRate;

        //... limit surface infil. by unused soil volume

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

                  SoilPerc + SoilEvap;

        SurfaceInfil = MIN(SurfaceInfil, maxRate);

    }

    //... storage & soil layers are full

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

    {

        //... limiting rate is smaller of soil perc and storage outflow

        maxRate = StorageExfil + StorageDrain;

        if ( SoilPerc < maxRate )

        {

            maxRate = SoilPerc;

            if ( maxRate > StorageExfil ) StorageDrain = maxRate - StorageExfil;

            else

            {

                StorageExfil = maxRate;

                StorageDrain = 0.0;

            }

        }

        else SoilPerc = maxRate;

        //... apply limiting rate to surface infil.

        SurfaceInfil = MIN(SurfaceInfil, maxRate);

    }

    //... either layer not full

    else if ( storageThickness > 0.0 )

    {

        //... limit storage exfiltration by available storage volume

        maxRate = SoilPerc - StorageEvap + storageDepth*storageVoidFrac/Tstep;

        StorageExfil = MIN(StorageExfil, maxRate);

        StorageExfil = MAX(StorageExfil, 0.0);

        //... limit underdrain flow by volume above drain offset

        if ( StorageDrain > 0.0 )

        {

            maxRate = -StorageExfil - StorageEvap;

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

            if ( theLidProc->drain.offset <= storageDepth )

            {

                maxRate += (storageDepth - theLidProc->drain.offset) *

                           storageVoidFrac/Tstep;

            }

            maxRate = MAX(maxRate, 0.0);

            StorageDrain = MIN(StorageDrain, maxRate);

        }

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

        maxRate = StorageExfil + StorageDrain + StorageEvap +

                  (storageThickness - storageDepth) *

                  storageVoidFrac/Tstep;

        SoilPerc = MIN(SoilPerc, maxRate);

        //... limit surface infil. by unused soil volume

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

                  SoilPerc + SoilEvap;

        SurfaceInfil = MIN(SurfaceInfil, maxRate);

    }

    //... find surface layer 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;

    if ( storageThickness == 0.0 ) f[STOR] = 0.0;

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

                   theLidProc->storage.voidFrac;

}