The Nonlinear Boussinesq baseflow method is similar to the Recession baseflow method but assumes that the channel overlies an unconfined aquifer which is itself underlain by a horizontal impermeable layer (Szilagyi & Parlange, 1998). Through the use of the one-dimensional Boussinesq equation, an assumption that capillarity above the water table can be neglected, and the Dupuit-Forcheimer approximation, it is possible to parameterize the method using measurable field data. 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).
Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the initial baseflow type and value, threshold type and value, the characteristic subsurface flow length [ft or m], saturated hydraulic conductivity of the aquifer [in/hr or mm/hr], and drainable porosity of the aquifer [ft/ft or m/m].
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 characteristic subsurface flow length corresponds to the mean distance from the subbasin boundary to the stream, which can be estimated using GIS. The saturated hydraulic conductivity of the aquifer can be estimated from field tests or from the predominant soil texture. An upper limit of the drainable porosity of the aquifer corresponds to the total porosity minus the residual porosity. The actual drainable porosity depends on site-specific conditions.
