double getSubareaRunoff(int j, int i, double area, double precip, double evap,
double tStep)
//
// Purpose: computes runoff & losses from a subarea over the current time step.
// Input: j = subcatchment index
// i = subarea index
// area = sub-area area (ft2)
// precip = rainfall + snowmelt over subarea (ft/sec)
// evap = evaporation (ft/sec)
// tStep = time step (sec)
// Output: returns runoff rate from the sub-area (cfs);
// updates shared variables Vinflow, Vevap, Vpevap, Vinfil & Voutflow.
//
{
double tRunoff; // time over which runoff occurs (sec)
double surfMoisture; // surface water available (ft/sec)
double surfEvap; // evap. used for surface water (ft/sec)
double infil = 0.0; // infiltration rate (ft/sec)
double runoff = 0.0; // runoff rate (ft/sec)
TSubarea* subarea; // pointer to subarea being analyzed
// --- no runoff if no area
if ( area == 0.0 ) return 0.0;
// --- assign pointer to current subarea
subarea = &Subcatch[j].subArea[i];
// --- assume runoff occurs over entire time step
tRunoff = tStep;
// --- determine evaporation loss rate
surfMoisture = subarea->depth / tStep;
surfEvap = MIN(surfMoisture, evap);
// --- compute infiltration loss rate
if ( i == PERV ) infil = getSubareaInfil(j, subarea, precip, tStep);
// --- add precip to other subarea inflows
subarea->inflow += precip;
surfMoisture += subarea->inflow;
// --- update total inflow, evaporation & infiltration volumes
Vinflow += precip * area * tStep;
Vevap += surfEvap * area * tStep;
if ( i == PERV ) Vpevap += Vevap;
Vinfil += infil * area * tStep;
// --- if losses exceed available moisture then no ponded water remains
if ( surfEvap + infil >= surfMoisture )
{
subarea->depth = 0.0;
}
// --- otherwise reduce inflow by losses and update depth
// of ponded water and time over which runoff occurs
else
{
subarea->inflow -= surfEvap + infil;
updatePondedDepth(subarea, &tRunoff);
}
// --- compute runoff based on updated ponded depth
runoff = findSubareaRunoff(subarea, tRunoff);
// --- compute runoff volume leaving subcatchment for mass balance purposes
// (fOutlet is the fraction of this subarea's runoff that goes to the
// subcatchment outlet as opposed to another subarea of the subcatchment)
Voutflow += subarea->fOutlet * runoff * area * tStep;
return runoff;
}
void getSubareaRunoff(int j, int i, double precip, double evap, double tStep)
//
// Purpose: computes runoff & losses from a subarea over the current time step.
// Input: j = subcatchment index
// i = subarea index
// precip = rainfall + snowmelt over subarea (ft/sec)
// evap = evaporation (ft/sec)
// tStep = time step (sec)
// Output: none
//
{
double tRunoff; // time over which runoff occurs (sec)
double oldRunoff; // runoff from previous time period
double surfMoisture; // surface water available (ft/sec)
double surfEvap; // evap. used for surface water (ft/sec)
double infil; // infiltration rate (ft/sec)
TSubarea* subarea; // pointer to subarea being analyzed
// --- assign pointer to current subarea
subarea = &Subcatch[j].subArea[i];
// --- assume runoff occurs over entire time step
tRunoff = tStep;
// --- initialize runoff & losses
oldRunoff = subarea->runoff;
subarea->runoff = 0.0;
infil = 0.0;
Vevap = 0.0;
Vinfil = 0.0;
Voutflow = 0.0;
Losses = 0.0;
Outflow = 0.0;
// --- no runoff if no area
if ( subarea->fArea == 0.0 ) return;
// --- determine evaporation loss rate
surfMoisture = subarea->depth / tStep;
surfEvap = MIN(surfMoisture, evap);
// --- compute infiltration loss rate
if ( i == PERV ) infil = getSubareaInfil(j, subarea, precip, tStep);
// --- add precip to other subarea inflows
subarea->inflow += precip;
surfMoisture += subarea->inflow;
// --- save volumes lost to evaporation & infiltration
Vevap = surfEvap * tStep;
Vinfil = infil * tStep;
// --- if losses exceed available moisture then no ponded water remains
Losses = surfEvap + infil;
if ( Losses >= surfMoisture )
{
Losses = surfMoisture;
subarea->depth = 0.0;
}
// --- otherwise update depth of ponded water
// and time over which runoff occurs
else updatePondedDepth(subarea, &tRunoff);
// --- compute runoff based on updated ponded depth
findSubareaRunoff(subarea, tRunoff);
// --- compute runoff volume leaving subcatchment for mass balance purposes
// (fOutlet is the fraction of this subarea's runoff that goes to the
// subcatchment outlet as opposed to another subarea of the subcatchment)
if ( subarea->fOutlet > 0.0 )
{
Voutflow = 0.5 * (oldRunoff + subarea->runoff) * tRunoff
* subarea->fOutlet;
Outflow = subarea->fOutlet * subarea->runoff;
}
}
