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SWAT Land Phase of the

Hydrologic CycleKristina Schneider

Kristi Shaw

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Surface Water Runoff

Calculates surface runoff volumes and peak runoffrates using daily rainfall amounts

Runoff volume estimated using a modification of theSCS 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 VolumeSelected 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 iscalculated.

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Peak Runoff Rate

Rational Method:

A stochastic element is included to allow for a realistic runoffrate.

tc can be estimated by adding the surface and channel flowtimes.

Alpha is a dimensionless parameter that expresses theproportion of total rainfall that occurs during t

c.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 distributionand amount, the runoff curve number, and the time ofconcentration

qp = peak rate

qp*= peak rate per unit of rainfall

There are a set of curves are available (USDA-SCS, 1986) forestimating qp* given rainfall distribution (SCS Type I, IA, II,or III), the runoff curve number, and the watershed time ofconcentration.

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Pond / Reservoir Outflow

Three alternative estimation schemes:

The input is measured outflow.

Specify water release rateVolume > emergency spillway is released within one day

Primarily used for small uncontrolled reservoirs.

Reservoir volume > storage extra water is released at aspecified rate

User specified monthly target volumes for the reservoir

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Lateral Subsurface Flow

Streamflow contribution, which originates below thesurface but is above the zone where rocks aresaturated with water.

Calculated simultaneously with redistribution in thesoil profile (0-2m).

Kinematic storage model is used to predict lateral

flow in each soil layer:Accounts for variation in conductivity, slope and soil watercontent

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

systemsshallow, unconfined aquifer which contributes return flow tostreams within the watershed

storage water may replenish moisture in the soil profile in verydry conditions or be directly removed by plant uptake (trees)

water may also seep into the deep aquifer or be removed bypumping

deep, confined aquifer which contributes return flow tostreams outside the watershed (water may be removed bypumping)

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