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

3. 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

4. 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.

5. 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.

6. 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.

7. Pond / Reservoir Outflow
Three alternative estimation schemes:
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

8. 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

9. 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)