Each sub-area is treated as a concentrated conceptual storage. Each storage has a storage delay time:

where:

K(q)=subarea storage delay time (hours) as a function of q
q =discharge (m³/s)
B=storage delay time coefficient
n=storage non-linearity exponent.

Substituting Equation (6) into Equation (1) gives:

The default value for the non-linearity exponent XPRafts is -0.285. XPRafts provides the mechanisms to alter this value, usually in respect to rare events involving significant sub-catchment overbank flood routing, by:

  • directly entering an amended value of "n". A value of zero would indicate linear catchment response and equate with unit hydrograph theory, or
  • entering an "n" = f (Q) rating curve for each sub-catchment.

In this manner XPRafts can simulate either a linear or non-linear response.