SWAT – Land Phase of the Hydrologic Cycle

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SWAT – Land Phase of the Hydrologic Cycle. Kristina Schneider Kristi Shaw. 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.

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### SWAT – Land Phase of the Hydrologic Cycle

Kristina Schneider

Kristi Shaw

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
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.
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.

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.
Pond / Reservoir Outflow
• Three alternative estimation schemes:
• The input is measured outflow.
• Specify water release rate
• Volume > emergency spillway is released within one day
• Primarily used for small uncontrolled reservoirs.
• Reservoir volume > storage extra water is released at a specified rate
• User specified monthly target volumes for the reservoir
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
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)