When it rains in LA, where does all the water go?

The first big rain storm of the season just hit Los Angeles. That got us wondering: where does that water go? A new report from the US Geological Survey has some answers.

First, a little hydrology 101. Water enters the L.A. Basin in three ways:

• Local rain storms
• Snowmelt from the mountains that fills up rivers and streams
• Outdoor irrigation

There are three places water can go:

• The atmosphere, by evaporating from the ground or transpiring from plants
• The ocean, through stormwater runoff or stream flow
• Groundwater, by soaking into the soil

According to the US Geological Survey, just 8 percent of the water that comes into the L.A. Basin ends up as groundwater. That matters because groundwater is the only place water goes where it is accessible to us. US Geological Survey hydrologist and study author Joseph Hevesi calls groundwater a savings account – we can bank rain and runoff there during a wet period and pump it out during a dry spell in the future.

The study found that the amount of water going into groundwater aquifers varies wildly from year to year, with less than 20,000 acre feet in dry years to more than ten times that amount in wet years (an acre foot is approximately enough water to supply 1-2 households for a year).

Surprisingly, more than half of the water that soaks into groundwater aquifers in the L.A. Basin is from lawn watering and other outdoor irrigation – not rainfall. It’s difficult for rain to absorb into the ground because so much of greater Los Angeles is paved or otherwise developed – over one-third of the area on average, according to the study, but up to 99 percent in parts of downtown L.A.

Last year, LADWP imported 87% of its water.

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Los Angeles Department of Water and Power

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Right now, the city of Los Angeles doesn’t rely very heavily on groundwater, stormwater or other local sources of water for its drinking water supply. In 2014-15, L.A. got just 13 percent of its water locally; the rest came from northern California, the Colorado River or the Eastern Sierra.

Mayor Eric Garcetti has an ambitious plan to wean the city off of imported water, which is becoming less reliable as the drought drags on. He wants the Department of Water and Power to get half of its water from local sources by 2035. To do that, the utility is going to need to capture a lot more rain, irrigation and runoff.