Drought 101: The Urban Water System

If you’re like us, then the rain got your gears turning about our urban water system. We wanted to know where rain goes in a city and what implications that has for the rest of the ecosystem. What better place to start than the basics?

Behold, the Water Cycle

The Hydrologic Cycle   Credit 

The Hydrologic Cycle   Credit 

Fourth grade Earth Science anyone? Evaporation, condensation, and precipitation keep rivers and lakes full, little towns are dwarfed by rolling green hills and mountains, and everyone gets a sailboat! What a pretty picture!

But, no skyscrapers, tract housing developments, or asphalt in sight.
Not quite an up-to-date picture of the California watersheds most of us reside in.

So how do water cycles work in dense urban areas?
Let’s use Los Angeles as an example.

The Urban Water Cycle

In the idyllic verdant water cycle above, rain falls, some of it evaporates, some of it becomes surface runoff that flows into streams, lakes and oceans, and some of it seeps into underground aquifers as groundwater recharge. 

Here’s the thing about fresh water seeping into the ground: IT’S REALLY IMPORTANT. If anyone asks you, you can confidently tell them. Groundwater is like a water reserve to be used when there’s no more rain falling on the surface. Plus, if the groundwater is sucked dry, then we’ve got a real big problem, because it takes decades to replenish. Bottom line is we need to get more water in the ground now to save for later! Now in LA, when rain falls, almost all of it runs directly into rivers and streams, which flows straight to the nearby ocean bay. And thus, we get the characteristic Southland rainstorm: half an inch of water at 9am means a flood in the LA River by 10am and a small island of garbage settling in the Santa Monica Bay by noon. Meanwhile barely any fresh water has had time to seep into the ground. Not good.

In LA, Even Our River is Paved!

Los Angeles River

Los Angeles River

Why does so much of our rain flow right into the Pacific? Because we paved our watershed. In the rainy winters of 1930’s LA, we were looking for any way we could to get the sprawling river water out of the city as fast as possible. Flooding was a big problem. So to reduce flooding, the answer was, pave, pave, and pave. We even paved our river! Ironically, the only flood prevention more effective than accelerated, centralized drainage (i.e. the LA River) is the exact opposite: slow, widely spread reservation of precipitation. After all, non-paved soil not only acts as a filter to groundwater aquifers, it contains organic matter and root systems that absorb and hold water before releasing into surface streams, reducing the intensity of floods. 

The urban planners of the 1930s were set on expanding population and cementing (quite literally) a new grid to control a destructive force of nature. What was not considered was the wrench this would throw in our natural water cycle, and the immense barrier it would create for the natural recharge of underground water.

Long Story Short…

The natural balance of our water system requires an open exchange between rain, surface water, and groundwater. The urban landscape has disrupted that exchange. Pavement shirks water quickly into flooding channels and does not retain or recharge. Rain in a drier climate needs to soak in and seep, not flush away. So, we’re looking to better use our local water, soak our soil, and recharge our aquifers.

What do you think, can we make LA into the next sponge city? We think so.

But, before we can dive into that dream, we must understand how water is allocated by the state’s water districts. Did you know 85% of Southern California's urban water is imported? And that a system of over 412 cities and water districts each manage their own little piece? Next week, let the exploration of the geopolitics of water in California begin! 



A watershed is an area of land that drains to a common point. If the LA River is a drain, then its watershed is the sink, which is mostly the San Fernando Valley and everything along the LA River.

Surface Runoff
All water that flows above ground into a larger body of water (i.e. the Pacific Ocean) and does not recharge into the groundwater.

Groundwater Recharge
All the water that permeates the Earth’s surface and eventually makes its way into our subterranean water supply (i.e.. groundwater).


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