Time as the critical factor in the investigation of the relationship between pollutant wash-off and rainfall characteristics

• Use of lumped parameters to analyse pollutant wash-off was found to be inadequate.
• Innovative parameter sets proposed for studying rainfall and wash-off relationship.
• Runoff volume related rainfall parameters influence the initial pollutant wash-off.
• Kinetic energy related rainfall parameters influence the latter part of wash-off.

The approach adopted for investigating the relationship between rainfall characteristics and pollutant wash-off process is commonly based on the use of parameters which represent the entire rainfall event. This does not permit the investigation of the influence of rainfall characteristics on different sectors of the wash-off process such as first flush where there is a high pollutant wash-off load at the initial stage of the runoff event. This research study analysed the influence of rainfall characteristics on the pollutant wash-off process using two sets of innovative parameters by partitioning wash-off and rainfall characteristics. It was found that the initial 10% of the wash-off process is closely linked to runoff volume related rainfall parameters including rainfall depth and rainfall duration while the remaining part of the wash-off process is primarily influenced by kinetic energy related rainfall parameters, namely, rainfall intensity. These outcomes prove that different sectors of the wash-off process are influenced by different segments of a rainfall event.