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 VolumeSelected modified SCS method for the follow reasons:

Reliable and has been used for many years in the USComputationally efficientRequired generally available inputsRelates 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 methodThe 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 dayPrimarily 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 FlowStreamflow 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 FlowVolume 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)