On a number of occasions I have been unfortunate enough to drive Highway 5 between San Francisco and the LA basin. I have observed that water is aquaducted (turning a noun into a verb!) from Northern California to Southern California for drinking, and presumably into the Central Vally for irrigation.
I was astonished to see that the aquaduct is open, and is little more than a canal. Given the heat of the Central Valley, wouldn’t the evaporative losses along 300+ miles be staggeringly high? Surely the extra water draw and pumping costs alone would be high enough to warrant a ‘lid’ or cover over the drain?
Really? In GQ?
Anyone know flow rates/quantity and associated costs of sourcewaters when the aqueducts were built versus the costs today? Or how those costs would compare to installing covers? There’s a network in southern Arizona, too, I believe that’s also open. The question came up then, too. I’ll see if I can find my notes.
The sources below suggest about 400 miles of open aqueduct, 110 feet wide. That’s 22 million square meters of surface area, or about 2000kg/sec of water evaporated according to this evaporation calculator using pre-filled assumptions. The aqueduct’s capacity is 370 cubic meters of water per second, or about 370,000 kg/sec. That’s a loss of about 0.5% to evaporation, if the math is correct.
The second link in particular says: “One of the issues with the aqueduct is that a lot of people talk about the benefit to evaporation,” said Mr. Torres, who noted he had talked to a number of companies over the years about putting solar panels on the canal. “The reality is that there’s an insignificant loss of water from evaporation on the aqueduct.”
The aqueduct is seldom (read: almost never) flowing at capacity, and in an overallocated system the fraction of evaporation is less important than the total volume of evaporation. Contrary to Torres’ claim that the loss is insignificant, there’s an enormous volume of water lost. There’s just not a very efficient way to reduce it. Covering the aqueducts with solar panels (or anything else) is expensive and would make maintenance more difficult, and the cost-benefit ratio on that probably doesn’t work out just yet.
On a smaller scale, covering canals does work - the USGS and USDA have in the past (I don’t know if the most recent farm bill included this, but previous ones did) offered grants for agricultural producers to do just that. Water law works against this, though - a producer who reduces evaporative losses from his water allocation doesn’t get to keep the water he’s saved; it just gets added back to the state’s total water budget. Water law in the western US hasn’t changed that much since the late 19th century, and it doesn’t really keep pace with modern concerns about conservation or water quality very well.
Even as water rights get more expensive and the (theoretical) cost benefit ratio gets more favorable, I’m not sure anything would get covered there. Because the water supply for the aqueduct system is already strained, evaporation savings couldn’t be used to grant additional water rights, and there’s not a lot of incentive for big-money, publicly-funded conservation projects these days.