Use of Areal Reduction Factor in Modelling and its Implication

Many observational studies of rainfall identify both high intensity rainfall clustering in small areas and rainfall intensity decreasing with distance from the point of highest intensity. It is also found that as the rainfall intensity increases, the variation of intensity over distance also decreases.

The reduction of high intensity rainfalls with increasing areas is a key issue in many hydrological problems and small scale variability can lead to biases. There seems to be differing views on how Areal Reduction Factors are applied to point rainfall which can lead to under estimation of flows at subcatchment level in large catchments (over 1,000 ha).

The areal distribution of rainfall in a large catchment can be accounted for by selecting several points for rainfall input to catchment model which reflect elevation range and orographic characteristics of catchment. In detailed modelling, a large catchment is divided into smaller sub-catchments, then, any areal reduction factor should be related to the subcatchment size. If the hydraulics of the conveyance system is then addressed accurately, some separation of sub-catchment peaks will be achieved that partly addresses the effects of distributed catchments.

A number of project examples from Auckland and Northland are used to demonstrate the application of Areal Reduction Factors.