The Institute of Hydrology carried out a number of studies on revising the runoff equations produced in the original Flood Studies Report (1975). IH124 was specifically produced to address the runoff from small catchments. (Institute of Hydrology, 1994).
Although shown to be slightly less accurate than more recent FEH based methods, it is still considered to be an acceptable approach for assessing greenfield runoff rates. The IH124 equation estimates Qbar with the following equation:
Qbarrural = 0.00108 x (0.01 x AREA)0.89 x SAAR1.17 x SPR2.17, m3/s