Published Tuesday, March 26,  2002 in the San Jose Mercury News


Local rainfall isn't only source of region's water

Special to the Mercury News
So far, the rainfall totals for the Bay Area are a mixed bag, but don't rely on the weather outside to get a sense of how we're doing in terms of water.

A check of rainfall accumulations for Bay Area cities so far this season shows a somewhat mixed picture of above, below and near-normal rainfall. A look at the statewide picture reveals huge regional differences, ranging from more than 120 percent of normal in parts of the Bay Area to just 27 percent of normal near San Diego.

However, local rainfall totals don't tell the whole story. Less than half of the water that is used in the Bay Area actually falls here.

In a typical year, the San Francisco Bay Area uses about 7,115 acre-feet of water for urban, agricultural and environmental purposes. An acre-foot of water is the amount of water that would cover an acre of ground at a depth of 12 inches -- about 2.3 billion gallons. Of the Bay Area's supply of water, about 18 percent goes for urban needs such as residential and industrial uses, 1 percent toward Bay Area agriculture, and the remaining 81 percent is accounted for by environmental uses like freshwater-flow through the delta.

The water supply for the Bay Area is also diverse. Only about 7 percent comes from local reservoirs and groundwater. Another 14 percent is imported from other parts of the state via a series of aqueducts and pipelines which include the Hetch Hetchy, Mokelumne and South Bay aqueducts. Most of this water originates in the snowpack and runoff from the Sierra Nevada -- which is near normal for this time of year. The bulk of this water goes toward meeting the region's urban needs. The remaining 80 percent of the area's water budget is from reused groundwater and stream flow and goes toward meeting the area's environmental needs as it flows back into the Pacific Ocean.

When considering the water picture, the bottom line is that it's important to look at not only the current Bay Area rainfall numbers but also the rainfall and snowpack in the Sierra Nevada. A great Web site from the California Department of Water Resources that summarizes both the California rainfall and snowfall totals can be found at

Q  Climate question: Besides locales adjacent to the Mediterranean Sea, are there places in the world that experience very mild Mediterranean climates similar to ours in central and southern coastal California? Steve Nolan - San Jose

A  Mediterranean climates, also called dry-summer subtropical climates, are fairly rare around the globe. They are found only on the west coasts of continents between about 30 degrees and 40 degrees latitude. Consequently, in addition to the Mediterranean and California, the other areas which fall into this category are central Chile, including Santiago, southwestern Australia near Perth and the western portion of South Africa in the vicinity of Capetown. The climate in all of these locales is influenced by westerly winds that produce mid-latitude cyclonic storms in winter, generally dry summers and overall mild temperatures.

Q  I thought that once before I had found a site that not only gave the wind predictions but also gave the current wave, wind and weather observations for the Bay Area. Are you familiar with such a site? And another question: What is the relationship between the National Weather Service and NOAA? It is rumored among sailors that weather forecasts issued by NOAA in Monterey need to be approved in Washington, D.C., before the forecasts can be put on the NOAA radio broadcast. Is this true, and if so, why?  L. Adam Weiner - San Mateo

A  The site that you have in mind is the Marine Weather page from the National Weather Service at . From this page, you can get the forecast and the current conditions at many coastal sites.

The National Weather Service is a sub-agency of the National Oceanic and Atmospheric Administration, or NOAA, which is a part of the Department of Commerce. And, fortunately, it is not true that local forecasters must have their forecasts approved by Washington, D.C. This rumor may have originated when some of the weather maps that were produced locally started being done from the Marine Prediction Center in Washington, D.C.

Q  I would like to learn more about the science of weather, and I have a couple of books. Do you know of any instructional videos on weather-related topics? John Gordon - Los Altos

A  There are some excellent videos available from the Weather Channel's Weather Classroom series. There are 15 30-minute segments being shown Monday through Thursday at 4 a.m. They are copyright-free for educational use, so set your VCR.

Jan Null, founder of Golden Gate Weather Services is a retired Lead Forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif., 95190. You also can send questions via telephone (510-657-2246), fax (510-315-3015) or e-mail