James Glover II / Star staff Alex Doran and Hank Weishaar of the Ventura County Watershed Protection District use a small cable car to cross Matilija Creek and check water flow levels of the creek in Ojai on Feb. 4.

Getting a perfectly accurate rainfall reading is like trying to stop time or attain immortality.

"It can't be done," said Tom Ackerman, director of the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean in Seattle.

Everything from the wind to human error to the type of rain gauge used stands in the way of an accurate reading. Accurate readings provide a way to gauge changes in rainfall amounts from season to season.

People have been measuring rainfall for thousands of years. Some of the earliest uses of rain gauges were around 100 A.D. to determine rainfall amounts in Palestine for agricultural purposes

Since then, rainfall measurements have been used for everything from gauging the size of storm drains to what size a reservoir needs to be to ensure an adequate water supply for a region.

Such measurements are especially important here in dry Southern California during heavy rains, such as the ones that fell across the region during the week of Jan. 21.

So exactly how much rain did we get during this period? It depends on whom you ask.

The National Weather Service pegs the amount of rain that fell on El Rio, an unincorporated community just north of Oxnard, at 3.01 inches. But the Ventura County Watershed Protection District puts it at 4.68 inches. Simi Valley got 6.31 inches, according to the Watershed District, but only 4.59 inches if you accept the Weather Service's data.

Variations in time, location

Part of the difference in rainfall amounts is because the Watershed District measures rainfall over two days from 8 a.m. to 8 a.m., whereas the Weather Service uses the midnight to midnight period.

The two agencies often measure rain from different parts of the county, which leads to different readings.

"If I took 10 different measurements from places that are no closer than half a mile from each other, I would likely get significantly different rainfall amounts," Ackerman said.

The county's terrain of hills, valleys and mountains also produces different readings. Higher elevations tend to get more rainfall.

To compensate for the differences, meteorologists try to get average rainfall amounts over a period time. Readings taken over a longer period of time tend to be more similar than those from a specific day or time.

Gauges closer to the ground are more likely to get better readings than those positioned higher up. A gauge situated on top of a 30-foot roof will, on average, be only 80 percent as accurate as one placed on the ground. Put it on top of a 150-foot tower and it will only be half as accurate. Much of the difference is due to stronger winds higher off the ground.

The ideal place for a gauge is on a flat area close to the ground, away from buildings and other large objects that can block the rain.

But care must be used to make sure leaves and other debris don't block the gauge's opening or clog it in other ways.

Both the National Weather Service and the county Watershed Protection District send out crews on a regular basis to make sure the gauges are working well and are clear of debris.

Depend on actual data

The agencies also carefully monitor the rainfall readings from the gauges. A gauge that provides significantly different readings from other nearby gauges could either be broken or clogged, said Mark Bandurraga, a senior hydrologist with the Watershed Protection District.

But meteorologists and others are reluctant or unwilling to change a rainfall reading to compensate for inaccurate readings.

"It's very difficult to know how to compensate," Bandurraga said. Do you add or take away from a reading? And if so, how much?

Despite all the chances for errors, it's still far better to depend on the actual data than change it, he said.

In the end, imprecision is something weather forecasters and others must accept, Ackerman said.

"Uncertainty is something that climatologists must live with on a daily basis."