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Over the past three years of severe drought, California has accumulated a rain “debt” equal to a year’s worth of precipitation, NASAresearchers report in the Journal of Geophysical Research — Atmospheres. The state is roughly 20 inches behind in total precipitation, the scientists calculate, which is the average amount expected to fall in the state in a single year. The deficit has been driven primarily by a lack of extreme precipitation events known as atmospheric rivers — water vapor-rich air currents that move inland from the Pacific Ocean — which, in an average year, provide 20 to 50 percent of California’s precipitation. The researchers found that California also had a 27.5-inch precipitation deficit between 1986 and 1994. However, the state’s population, industries, agriculture, and water demand have grown significantly since that time.