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SWAT – Land Phase of the Hydrologic Cycle

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

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

  2. 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

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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

  8. 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)

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