SWAT

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SWAT Land Phase of the Hydrologic Cycle

• Kristina Schneider
• Kristi Shaw

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Surface Water Runoff

• Calculates surface runoff volumes and peak runoff
  rates using daily rainfall amounts
• Runoff volume estimated using a modification of
  the SCS Curve Number Technique.
• Two options for estimating the peak runoff rate
• The modified Rational formula
• The SCS TR-55 method

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Surface Runoff Volume

• Selected modified SCS method for the follow
  reasons
• Reliable and has been used for many years in the
  US
• Computationally efficient
• Required generally available inputs
• Relates runoff to soil type, land use, and
  management practices
• R is rainfall Q is daily runoff
• The modification deals with how Soil Water
  content is calculated.

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Peak Runoff Rate

• Rational Method
• A stochastic element is included to allow for a
  realistic runoff rate.
• tc can be estimated by adding the surface and
  channel flow times.
• Alpha is a dimensionless parameter that expresses
  the proportion of total rainfall that occurs
  during tc.
• Q is a daily runoff.
• A is the drainage Area.

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Peak Runoff Rate

• The SCS TR-55 method
• The peak runoff rate is dependent on the rainfall
  distribution and amount, the runoff curve number,
  and the time of concentration
• qp peak rate
• qp peak rate per unit of rainfall
• There are a set of curves are available
  (USDA-SCS, 1986) for estimating qp given
  rainfall distribution (SCS Type I, IA, II, or
  III), the runoff curve number, and the watershed
  time of concentration.

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Pond / Reservoir Outflow

• Three alternative estimation schemes
• Input is measured outflow.
• Specify water release rate
• Volume gt emergency spillway is released within
  one day
• Primarily used for small uncontrolled reservoirs.
• Reservoir volume gt storage extra water is
  released at a specified rate
• User specified monthly target volumes for the
  reservoir

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Lateral Subsurface Flow

• Streamflow contribution, which originates below
  the surface but is above the zone where rocks are
  saturated with water.
• Calculated simultaneously with redistribution in
  the soil profile (0-2m).
• Kinematic storage model is used to predict
  lateral flow in each soil layer
• Accounts for variation in conductivity, slope and
  soil water content
• Allows for flow upward to an adjacent layer or to
  the surface

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Return Flow

• Volume of streamflow originating from groundwater
• SWAT partitions groundwater into two aquifer
  systems
• shallow, unconfined aquifer which contributes
  return flow to streams within the watershed
• storage water may replenish moisture in the soil
  profile in very dry conditions or be directly
  removed by plant uptake (trees)
• water may also seep into the deep aquifer or be
  removed by pumping
• deep, confined aquifer which contributes return
  flow to streams outside the watershed (water may
  be removed by pumping)

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