by Jay Lund
People are interested in California water problems, and they ask reasonable questions. Here is a first installment of short science-based answers to some reasonable questions often heard at public and private discussions of water in California. (Longer answers are possible, of course.)
- Why doesn’t California just build desalination plants to end water shortages and leave more water in streams for the environment?
Desalting ocean water is expensive, about $2,000-$3,000/acre-ft. This cost is too high to be economical for almost any crop in California. This cost is also over $1,000/acre-ft more than other sources available to California’s cities (including wastewater reuse, conservation, and buying water from farmers). Providing only 20% of California’s urban water use by desalinating sea water (1.4 maf/year) would cost households at least $3.5 billion/year (about $300/household per year). The environment would benefit more from other expenditures of such money.
- How much water do we lose from evaporation? Wouldn’t reducing evaporation from reservoirs be cheaper than building new reservoirs?
Evaporation is the second largest flow of water in California, following precipitation. Average California precipitation is roughly 200 million acre-ft/year, with roughly 70 maf/year of river runoff, meaning that most precipitation (~130 maf/yr) evaporates quickly back to the atmosphere. Additional evaporation of runoff occurs from agricultural fields, reservoirs, and urban landscapes (evapotranspiration is roughly 26 maf/yr from crops, 2 maf/yr from cities, and 2 maf/yr from reservoirs and canals). Evaporation in all its forms is most of the water that falls on California.
Retaining and reducing evaporation is usually difficult, because it is so widely distributed and driven by the sun, which we all enjoy in California. Farmers often manage irrigation to reduce unproductive evaporation from bare soil. Water system operators sometimes shift water among reservoirs to reduce evaporation. Since the 1950s, researchers have experimented with adding covers and thin layers of floating chemicals to reduce evaporation from reservoirs, but these are rarely economical or environmentally friendly.
- If we are short of water, why don’t we just build new reservoirs?
Just as a refrigerator stores food, but does not make it, reservoirs don’t make water, but only shift it in time. For reservoirs to supply water, they must first fill with water from an earlier wetter time. Even the largest reservoir cannot reliably supply more than its river’s average annual inflow.
Reservoirs are important and attractive because of California’s seasonally variable streamflows and wet and dry years. They can reliably store water from California’s wet winters for the following dry summer, because modest amounts of storage can refill every year. Larger reservoirs become less efficient for storing water from wetter years for dry years, when a reservoir might need several years (or longer) to refill. Large reservoirs for over-year drought storage often refill infrequently, but re-paying for their construction occurs every year.
Increasingly large reservoirs become more expensive and refill less frequently, providing less water per unit of storage expansion and cost. The additional water supplied from larger reservoirs can become very expensive. In addition to these limitations of physics and economics, environmental objections and concerns often arise for new and expanded reservoirs.
- On California’s coast, why don’t we gather fog water?
California’s coast is often foggy and some of its coastal ecosystems receive a sizable share of their water from summer fog. But for humans, the costs of gathering fog water will almost always greatly exceed the costs of alternative water sources or the value of the water use they would supply.
- Why doesn’t California import water from the Pacific Northwest, the Great Lakes, or the Mississippi River? They seem to have extra water.
The Pacific Northwest, Great Lakes, and Mississippi River all have relatively abundant water supplies. These water sources also are all far from California, with mountain ranges in between. Constructing and operating aqueducts, tankers, or railcars to move water great distances is expensive, and moving water (which is heavy) over mountains is very energy-intensive. The cost of moving water these great distances typically exceeds the value of the additional water uses in California (Perrier and Fuji water might be exceptions). Environmental, political, and legal opposition also would likely be barriers to California importing large amounts of water.
Some Larger lessons
Some broader lessons arise from this first set of common questions on California water. First, there are many ways to get water in California, which vary tremendously in cost, availability, environmental impact, and practicality. Second, because so many potential water sources are available in California, it is sometimes said, “There is rarely a shortage of water, but more often a shortage of cheap water.” California is often a dry place, and the relative costs and benefits of different water supplies and demands typically drive the use, rejection, and research for water management options.
Jay R. Lund is Director, Center for Watershed Sciences and a Professor of Civil and Environmental Engineering, University of California – Davis