Basic Concepts and Equations
The Recession baseflow method is designed to approximate the typical behavior observed in watersheds when baseflow recedes exponentially after an event (Chow, Maidment, & Mays, 1988). This method is intended primarily for event simulation. However, it does have the ability to automatically reset after each storm event and consequently may be used for continuous simulation.
Unless parameters are carefully chosen, this method is not guaranteed to conserve mass (e.g., precipitation losses < baseflow volume).
This method defines the baseflow at time t, Qt [ft3/sec or m3/sec], as:
where Q0 = initial baseflow at time zero [ft3/sec or m3/sec] and k = recession constant. Within HEC-HMS, k is defined as the ratio of the baseflow at time t to the baseflow one day earlier and must be positive and less than one.
Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the initial baseflow type and value, recession constant, and threshold type and value. The initial discharge type can be specified as either a discharge rate [ft3/sec or m3/sec] or a discharge rate per area [ft3/sec/mi2 or m3/sec/km2]. The discharge rate method is most appropriate when there is observed streamflow data at the outlet of the subbasin for determining the initial flow in the channel. The discharge rate per area method is better suited when regional information is available. The threshold type can be specified as either a ratio to peak or a threshold discharge [ft3/sec or m3/sec]. If the threshold type is set to ratio to peak, the baseflow will be reset when the current flow divided by the peak flow falls to the specified value. If the threshold type is set to a threshold discharge, the baseflow will be reset when the receding limb of the hydrograph falls to the specified value, regardless of the peak flow during the previous storm event.
The recession constant, k, depends upon the source of baseflow. If k = 1.0, the baseflow contribution will be constant, with Qt = Q0. Otherwise, to model the exponential decay typical of natural undeveloped watersheds, k must be less than 1.0. The following table shows typical values proposed by Pilgrim and Cordery (1992) for basins ranging in size from 120 to 6500 square miles (300 to 16,000 square kilometers) in the U.S., eastern Australia, and several other regions.
Flow Component | Recession Constant, k |
|---|
Groundwater | 0.95 |
Interflow | 0.8-0.9 |
Surface runoff | 0.3-0.8 |
The recession constant can be estimated if gaged flow data are available. Flows prior to the start of direct runoff can be plotted and an average of ratios of ordinates spaced one day apart can be computed. This is simplified if a logarithmic axis is used for the flows as the recession model will plot as a straight line.
The threshold ratio to peak or discharge value can be estimated from examination of a hydrograph of observed flows. The flow at which the recession limb is approximated well by a straight line defines the threshold ratio to peak or discharge value.
