The 85th percentile in the context of water balance refers to which concept?

• A. A maximum post-project discharge
• B. A target pre-project storm volume
• C. An historical rainfall average
• D. A drought period benchmark

Answer: (B)

In water balance analysis, a percentile defines the size of a rainfall event you use as a representative case. The 85th percentile is a relatively large, but not extreme, storm that occurs more often than most events—only about 15% of events exceed it. This value is used to set a target pre-project storm volume, meaning you use the rainfall depth for that 85th percentile event to define the baseline pre-development volume of water. The goal is to compare post-development runoff to this pre-development baseline for the same storm, ensuring development doesn’t increase runoff for that representative large rain event. It’s not the maximum discharge, not a simple historical average, and not a drought benchmark.