Maybe, but the formula works. For instance, suppose you want to keep New York in steady water supply for a century: How big should you make the reservoirs? Hurst’s formula gave the answer. From 1826 to 1945, Hurst observed, it had rained an average of forty-two inches a year in New York; and from year to year, the standard deviation was 6.3 inches. According to Hurst’s formula, to store enough water for one hundred-year extremes of drought or flood, you would need reservoirs deep enough to store up to 16.7 times the standard deviation: 105 inches, or two and a half years’ supply. Other
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