Catalina Island News

Facts about Catalina's Water Supplies

Catalina Island Conservancy

The prolonged drought is creating shortages throughout California, and Catalina is experiencing similar challenges.  The big difference is our water system is completely independent of any mainland system.

As we explained in last week’s column from the Catalina Island Conservancy, Southern California Edison (SCE) has a monopoly to obtain and deliver fresh water on the Island. In this, our second column, we examine the physical and biological realities of the Island’s water.

As anyone who has lived on Catalina Island knows, it is a place where the average rainfall is rarely average. The large variability in the amounts of annual rainfall has significant impacts on our water supply.

With the exception of when the desalinization plant is producing water, Avalon’s water supply comes from underground aquifers, which are layers of permeable soil and rock that can hold water. The amount of water that “infiltrates” the aquifer (or trickles through the ground and into the water-retaining soil and rock) varies greatly from one year to the next, depending almost entirely on rainfall. This process, which is known as recharging the aquifer, determines how much drinking water is available on the Island.

In 2008, SCE sponsored a study of how water enters, moves through, is stored and leaves the Middle Canyon watershed upstream of Thompson Reservoir dam (the hydrology of the system).  To visualize this process, imagine the watershed as a 3 x 12 x 24 inch rectangular cake pan that is tilted so that water would flow down the length of the pan. The pan has holes in its downhill side and three sponges inside:  a thin 1/16-inch sponge on the top, a 1/4-inch sponge in the middle and a 2-inch sponge on the bottom.

The thinnest sponge represents surface soils and vegetation. The middle sponge represents the shallow alluvial soil part of the aquifer, and the thick sponge represents the deep bedrock part of the aquifer. Water can only enter the “pan” from rain.  Water can leave the “pan” by flowing out the bottom, over the bottom or by evaporation, which is technically called evapotranspiration because plants “pump” water from the ground into the air in order to photosynthesize (transpiration) and transpiration is responsible for much more water “loss” than evaporation alone.

During a normal rain year, the “thinnest sponge” has mostly dried out by August and the “middle sponge” has started to dry out on top.  The “thick sponge” is still full of water and leaking.  After several years of drought, the upper “sponges” only become damp when it rains and dry out quickly by summer. Little or no water gets to the bottom sponge.

During very rainy years (like big El Nino years), the pan can completely fill with water, saturating all the “sponges” and spilling water over the pan’s edge.  The big rain years that overfill the pan happen frequently enough that the amount of water leaving the “pan” from evapotranspiration and “leaking” out of the “thick sponge” is basically constant year to year and available to the Island’s plants and animals. (If it wasn’t, Catalina Island would become a true desert ecosystem, rather than a chaparral ecosystem.)

The bottom line is the system completely recharges only in a big rain year – or approximately every 4-8 years.  Between the big rain years, the system is drawing down water (or partially recharging during, say, an average rain year).  The natural ecosystem (plants, wetlands, surface flow in streams, animals, etc.) uses the same amount of water every year. During drought years, precipitation deficits are made up by accessing water stored in the “sponges” (tapping into groundwater directly through root systems or the discharge of groundwater at the surface in streams, springs and seeps.)

Human beings entered into this natural cycle by, in effect, inserting straws into the “middle sponge” of our “pan” ecosystem and pumping water from Middle Canyon into Avalon Canyon.  The recent proposal to install a bedrock well in Middle Canyon would mean inserting a new straw or straws into the “thick sponge” and pumping water from it.  The unanswered question is whether this can be done without harming the natural hydrologic balance of the Middle Canyon ecosystem, especially during drought years, when the natural ecosystem is most at risk.  This proposal will be addressed more fully in the next article in this series.

Five-part water series from the Catalina Island Conservancy
Part 1: Answers to Common Questions about Catalina's Water Supply
Part 2: Facts about Catalina's Water Supplies
Part 3: The Future of Water on Catalina Island
Part 4: Ways to Increase Catalina’s Water Supply
Part 5: Making a Difference in Catalina's Water Future

By John J. Mack
John J. Mack is the Catalina Island Conservancy’s chief conservation and education officer. For more information about the Conservancy, please visit  The Catalina Island Conservancy has been an active participant in the Catalina Island Consortium, a group of Island stakeholders that has sought to maintain an affordable and sustainable fresh water supply for the Catalina Island community.  The Consortium, working with SCE and CPUC, was recently successful in averting significant water rate increases on the Island.