Water Balance in MIKESWMM

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Water Balance in MIKE-SWMM

• Melissa Figurski

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Blocks in SWMM

• Runoff
• Transport
• Extended Transport (Extran)

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Runoff Block

• Simulates quantity and quality of runoff
• Rainfall
• Snowmelt
• Converts to runoff (Inlet hydrographs) and
  pollutant loads (pollutographs)

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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

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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

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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

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Time Step

• WET
• Globally wet
• WETDRY
• No precip, water still in surface storage
• DRY
• No precip or surface storage

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Runoff Loses

• Evaporation
• Evapotranspiration
• Infiltration
• Groundwater flow
• Percolation
• Surface Water Outflow

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Evaporation

• Average Monthly Rate
• Continuous simulation value for all months
• Evaporation Time Series
• Daily, weekly, or monthly
• Loss off the Top

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Evapotranspiration

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

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Infiltration (1)

• Hortons Equation
• fp fc (fo fc)e-kt
• 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

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Infiltration (2)

• Green-Ampt Equation
• For F lt Fs
• f i and F SuIMD/(i/Ks 1) for i gtKs
• For F gt 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

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Groundwater Flow

• Can be routed to a channel/pipe
• Can be lost from system
• Both methods are accounted in continuity check

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Percolation

• Uses Darcys Law for Unsaturated Flow

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Water Balance

• Continuity Balance

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Reference

• Storm Water Management Model
• Version 4, Users Manual.
• Section 4 Runoff Block