What is “renewable” water?
Renewable water can fall on our roofs and yards. It can fall as snow hundreds of miles away. But it is not water that was stored in the ground 10s of thousands of years ago to be pumped out only once and never again.
If water is compared to money, then renewable water is income, and groundwater is an inherited savings account.
What are our renewable water supplies?
Tucson’s renewable water is rain that falls on Tucson, or that falls someplace else and flows into our aquifers along the front of the Catalina and other mountains, or under the river bottoms of our seemingly dry riverbeds.
How much water flows off a surface, depends on the surface. About 80% of what falls on a hard surface in a year, like a roof or street, runs off. No more than about 15% runs off flat ground.
The biggest factor in harvesting water is that the farther rain is harvested from where it falls, the less there is to harvest. If you harvest rain at the edge of a roof or road, you can get about 5.5 gallons per square foot from our average year (that’s before Global Climate Change). If you wait until it gets to our rivers – the only places there is enough water to soak all the way down to our aquifers – you get about 2 oz.
We have about 40 square miles of rooftops in metro-Tucson, and over 80 SM of paving. If we harvest (and use) water very near where it falls, we could potentially have over 50,000 acre-feet of “new” water that isn’t currently being used for any productive purpose.
When combined with directly used rain and net natural recharge from mountain fronts and river beds, our maximum potential Renewable, Harvestable, Local water supply is close to 260,000 acre-feet per year — compared to 192,000 AF used by all municipalities.
Tucson has a lot of local, renewable water. We just waste it all.
Is CAP renewable?
Yes… but can we count on it?
For the first time in our history, Tucson relies almost entirely on water lifted half a mile in the air and pumped 300 miles across the desert from the Colorado River in an aqueduct called the Central Arizona Project (CAP). The remainder comes from pre-historic water that is stored in the ground beneath our feet (called “aquifers” that are saturated rock and sand that hold “groundwater”).
The 70+ years of law and court rulings known as the Law of the River establishes two kinds of water:
- “Water” – meaning a legal right to water – whether or not it exists, and
- “Wet water” – which means exactly what you think it means.
As the saying goes — “whiskey’s for drink’n and water’s for fight’n”. Water, you might say, ‘is complicated’:
- There are more legal claims to Colorado River “water” than there is actual water (the Colorado River is “over allocated”.). This is because “water” rights were allocated on the basis of what we now know was the wettest 20 year period in at least 1200 years.
- Studies of tree-rings, lakebeds, stalagmites, and others geologic records show that “megadroughts” are natural for this region. There have been several droughts in the last 1200 years that lasted many decades (over 50 years in one case). We are currently in the 14th year of a severe drought that covers most of the Colorado River watershed, and climate scientists expect Global Climate Change will make dry places dryer and hotter.
- Because of the drought and over allocation, storage reservoirs that keep the system working are nearing the point when they can’t serve all “water rights”. The Bureau of Reclamation announced last year that there is a 50:50 chance that they won’t be able to supply all “water rights” by 2016.
- CAP is the largest electrical power user in Arizona. And all of it comes from coal, which makes it one of our largest of greenhouse gas producers.
- An acre-foot of CAP water contains almost 1 ton of mineral salts, so we are importing nearly 100,000 tons of salt per year into our community and adding it to our soil and groundwater along with the CAP water.
Will we use CAP, if it’s available? Yes.
Should we count on it? The Feeding Tucson project will examine whether or not we have to, in order to have a Secure Food Supply.