A hurricane can dump a lot of rain . . .

From New Orleans, at the Joint Assembly of the American Geophysical Union

The large masses of warm, moist air that fuel hurricanes also prime those windstorms to drop a lot of precipitation in a short time, a phenomenon that residents of Puerto Rico experienced in spades when Hurricane Georges struck their island in 1998. Now, new hydrological analyses indicate just how much storm runoff and sediment washed into the surrounding waters in the wake of that storm.

In the course of a normal year, the 8,700-square-kilometer island of Puerto Rico gets about 1.6 meters of rain, says Matthew C. Larsen, a hydrologist with the U.S. Geological Survey in Reston, Va. That’s about 14 billion cubic meters of precipitation. About 6 billion m3 of that water recharges the island’s aquifers, but the other 8 billion m3 runs off the island in streams, carrying around 5.9 million metric tons of sediment.

In September 1998, however, Hurricane Georges swept over the island, dumping an islandwide average of 0.3 m of rain—more than 2 months’ worth of precipitation in a mere 2 days. The deluge triggered landslides, flooding, and severe erosion. Data from flow meters in streams indicate that more than 1 billion m3 of runoff reached the ocean in those 2 days, along with 2.4 million metric tons of sediment, says Larsen. That’s about 40 percent of the average annual sediment load and amounts to about seven large dump truck loads of sediment from each square kilometer of the island.

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