That storage reservoir was used to provide potable water as well as other purposes. It had a cover to keep stuff out that would make the water unsuitable for drinking, but it was damaged in some way. So, the storage reservoir was drained so as to repair the cover. if the caption for a photo published by the LA Times is correct, it was empty in September 2022, nearly 2 1/2 years ago.
I get that something that large is not repaired or replaced in a few hours or days, but 2 or 3 years (I have not seen a start date for the repair project)? And in those 2 or 3 years no one in LA Fire Department thought, "Oh bleep! That 3 million gallons is way too little for more than a simple 1 or 3 house fire"?!!! And no FD official thought to ask how quickly the water in one of those 1M gallon tanks could be replenished by exiting pumps?