Flood Forecasting

Water, Water, Everywhere

Three severe storms in a week drenched Northern California last winter. Forecasts of rainfall and river levels were good enough to keep loss of life and property damage to a minimum.

For residents of California's Central Valley, the New Year's Day 1997 rainstorms and floods were devastating: eight deaths, nearly $2 billion in damages. It could have been worse. For the hydrologists, meteorologists and engineers at the Flood Operations Center in Sacramento, the storms proved a keen test of their forecasting skill. Author Edwin Kiester, Jr., reports how their forecasts allowed flood-fighters to work efficiently, saving countless dollars in flood damage.

More than half of Northern California's precipitation ends up in the Central Valley, the huge basin extending south from Shasta Dam. During the 19th century, flooding of the Sacramento and San Joaquin rivers and their tributaries regularly turned the area into an inland sea. Today levees, dams, weirs and bypasses channel and store the river water and snowmelt, allowing some control during floods. Using rain gauges and Doppler radar, as well as satellite pictures and special computer models that take into account the effect of nearby mountains on precipitation, Central Valley forecasters advise local dam operators on predicted rainfall and water flow. The dam operators then juggle reservoir inflows and releases. During the New Year's flooding, forecasters' and operators' round-the-clock work ensured that dams did not overflow, and resulted in precipitation predictions that were exceptionally close to what actually fell.

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