Another example of non-uniform behavior observed in the preliminary study was the unpredictable nature of system performance following a very light late summer or autumn rain. In some cases, a light rainfall can cause system performance to fall sharply. For example, a performance drop after a light end of summer rain was noted for all of the Northern California systems in the preliminary study.
The anomalous behaviors described in this section suggested that further investigation of the amount of rain to clean systems was necessary, and that care should betaken when selecting the beginning and end of the defined soiling period to avoid unpredictable end of summer behavior.