land to water
TRANSCRIPT
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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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