To reflect the ‘ultra conservative assumptions’ (NERC 1975) required for PMF estimation, adjustments are made to the rainfall and rainfall-runoff model. These adjustments are summarised in Table 2.
Components of the FSR/FEH rainfall-runoff model for standard design and PMF events
| Component . | FSR/FEH standard design . | FSR/FEH PMF . |
|---|---|---|
| Rainfall | FSR or FEH99 | PMP Winter: additional input from snowmelt and rainmelt. |
| Loss Model | Static PR | Static PR, increased due to antecedent conditions. Winter: additional antecedent rainfall from snowmelt and rainmelt. Winter: Frozen ground; SPRHOSTa is set to a minimum 53%. |
| Routing | Triangular unit hydrograph, controlled by Tpb | Triangular unit hydrograph, reduce Tp by a third. |
| Baseflow | Static baseflow | Static baseflow linked to increased CWI. |
| Component . | FSR/FEH standard design . | FSR/FEH PMF . |
|---|---|---|
| Rainfall | FSR or FEH99 | PMP Winter: additional input from snowmelt and rainmelt. |
| Loss Model | Static PR | Static PR, increased due to antecedent conditions. Winter: additional antecedent rainfall from snowmelt and rainmelt. Winter: Frozen ground; SPRHOSTa is set to a minimum 53%. |
| Routing | Triangular unit hydrograph, controlled by Tpb | Triangular unit hydrograph, reduce Tp by a third. |
| Baseflow | Static baseflow | Static baseflow linked to increased CWI. |
aSPRHOST is the standard percentage runoff derive using the HOST soil classification.
bTp is the unit hydrograph time-to-peak.