1. Water Balance in MIKE-SWMM
Melissa Figurski

2. Blocks in SWMM
Runoff
Transport
Extended Transport (Extran)

3. Runoff Block Simulates quantity and quality of runoff
Rainfall
Snowmelt
Converts to runoff (Inlet hydrographs) and pollutant loads (pollutographs)

4. Simulated Events Can be run for minutes to years Less than 2 weeks:
Single-event simulation
Greater than 2 weeks:
Continuous simulation (w/ no time limit)
Calculations are exactly same for both
Except for snow

5. Subcatchment Representation
Up to 200 idealized subcatchments
Impervious with depressions
Impervious without depressions
Pervious with depressions
Each subcatchment drains to a channel/pipe or inlet
Single slope

6. Data Input to Runoff Block
Precipitation
Up to 10 hyetographs for 10 gages
Surface Quantity
Surface Quality
Evaporation Rate
Land Use
For snowmelt – air temp and wind speed

7. Time Step WET WETDRY DRY Globally wet
No precip, water still in surface storage
DRY
No precip or surface storage
Time Step is one second to one year. Different time steps for each

8. Runoff Loses Evaporation Evapotranspiration Infiltration
Groundwater flow
Percolation
Surface Water Outflow

9. Evaporation Average Monthly Rate Evaporation Time Series
Continuous simulation – value for all months
Evaporation Time Series
Daily, weekly, or monthly
‘Loss off the Top’
Where do we get this info? Loss of the top – subtracted from rainfall depths and/or ponded water prior to calc. infiltration

10. Evapotranspiration Upper Zone Evapotranspiration
Fraction of total maximum evaporation rate
Lower Zone Evapotranspiration
Constant over time step

11. Infiltration (1) fp = fc + (fo – fc)e-kt Horton’s Equation
fp = infiltration capacity, ft/sec
fc = min or ultimate value of fp, ft/sec
fo = maximum or initial value of fp, ft/sec
t = time from beginning or storm, sec
k = decay coefficient, sec-1
Where do you get data?

12. Infiltration (2) Green-Ampt Equation For F > Fs: For F < Fs:
f = i and F = SuIMD/(i/Ks – 1) for i >Ks
For F > Fs:
f = fp and fp = Ks(1 + (SuIMD/F)
F = infiltration rate, ft/sec
i = rainfall intensity, ft/sec
F = cumulative infiltration volume, ft
Fs = cumulative infiltration volume for ponding, ft
Su = average capillary suction at wetting front, ft water
IMD = initial moisture deficit for this event, ft/ft
Ks = saturated hydraulic conductivity of soil,. ft/sec
Advantage? What is this whole Fs thing? Newton-Raphson routine

13. Groundwater Flow Can be routed to a channel/pipe
Can be ‘lost’ from system
Both methods are accounted in continuity check

14. Percolation
Uses Darcy’s Law for Unsaturated Flow

15. Water Balance
Continuity Balance

16. Reference Storm Water Management Model: Version 4, User’s Manual.
Section 4: Runoff Block