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 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.
Peak Runoff Rate Rational Method: A stochastic element is included to allow for a realistic runoff rate. t c 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 t c. Q is a daily runoff. A is the drainage Area.
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 q p = peak rate q p * = peak rate per unit of rainfall There are a set of curves are available (USDA-SCS, 1986) for estimating q p * 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 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
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
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)