Much of the water in the Palisades is provided by three 1 million gallon tanks that sit up in the hills, using gravity to maintain water pressure in the hydrants and homes they supply below.
Pumps forcibly move water from main lines and surrounding reservoirs to those tanks. The tanks were full when the fires started, but the pumps couldn't replenish water in the tanks as quickly as firefighters were using it below. As the tanks depleted, so did the water pressure, until some 20% of hydrants ran dry.
"The hydrants would have run dry anywhere in the world with a fire event like this in the topography where this occurred," said Greg Pierce, director of the UCLA Human Right to Water Lab.