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Harvey An ‘Unprecedented’ Disaster, Could Unload 25 Trillion Gallons Of Water

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As I sit in my house in Houston, TX, I try to wrap my mind around the staggering numbers seen thus far by Harvey. Estimates are that up to 25 trillion gallons of water could fall on the state of Texas as a result of this historic storm.

Harvey has forced scientists outside of their comfort zone and into a realm of theoretical, given the unprecedented rainfall totals. Experts will commonly discuss the need to build architectural structures to withstand one hundred year floods or one thousand year floods. These numbers are based historical records and probability distributions.

When determining the anticipated severity of a 100-year flood, scientists determine the 1% annual exceedance probability. Probability distribution functions are built on stream gauge data from around the United States, measuring stream flow under normal and extreme conditions. This builds a normal distribution, from which scientists can determine the upper 1% probability. Since the 1% probability flood has a 1 in one hundred chance of occurring, scientists deem this the one hundred year flood. A 500-year flood would have a 0.2% chance of occurring in any given year.

When dealing with the extreme ends of the probability distribution function, there are errors in calibrating the magnitude and likelihood of a scenario. This is because there are simply fewer data points supporting the extreme flooding and extreme drought conditions. Measurements and data will center around the mean and thus extreme variations from the mean become less reliable.

That is why scientists find themselves largely in uncharted territory with flooding from Hurricane Harvey. Estimates are that up to 50 inches of rain could fall in parts of Texas over the course of a few days. That is larger than the entire year’s average rainfall in Houston.

To put into perspective, the estimated total rainfall in Texas as a result of Harvey, at 25 trillion gallons of water would equal rainfall over the entire United States of 0.38 inches.

Below is a table of every state in the United States and Washington D.C. along with how much water would cover each state if the equivalent Harvey rainfall fell in the state.

State Ranking  Area (Cubic M)   Harvey Rainfall Across State (In) 
Alaska 1  1,717,854,000,000  2.2
Texas 2  695,621,000,000  5.4
California 3  423,970,000,000  8.8
Montana 4  380,838,000,000  9.8
New Mexico 5  314,915,000,000  11.8
Arizona 6  295,254,000,000  12.6
Nevada 7  286,351,000,000  13.0
Colorado 8  269,601,000,000  13.8
Oregon 9  254,805,000,000  14.6
Wyoming 10  253,336,000,000  14.7
Michigan 11  250,494,000,000  14.9
Minnesota 12  225,171,000,000  16.5
Utah 13  219,887,000,000  16.9
Idaho 14  216,446,000,000  17.2
Kansas 15  213,096,000,000  17.5
Nebraska 16  200,356,000,000  18.6
South Dakota 17  199,742,000,000  18.7
Washington 18  184,665,000,000  20.2
North Dakota 19  183,112,000,000  20.3
Oklahoma 20  181,035,000,000  20.6
Missouri 21  180,533,000,000  20.6
Florida 22  170,304,000,000  21.9
Wisconsin 23  169,639,000,000  22.0
Georgia 24  153,909,000,000  24.2
Illinois 25  149,998,000,000  24.8
Iowa 26  145,743,000,000  25.6
New York 27  141,299,000,000  26.4
North Carolina 28  139,389,000,000  26.7
Arkansas 29  137,732,000,000  27.1
Alabama 30  135,765,000,000  27.4
Louisiana 31  134,264,000,000  27.7
Mississippi 32  125,434,000,000  29.7
Pennsylvania 33  119,283,000,000  31.2
Ohio 34  116,096,000,000  32.1
Virginia 35  110,785,000,000  33.6
Tennessee 36  109,151,000,000  34.1
Kentucky 37  104,659,000,000  35.6
Indiana 38  94,321,000,000  39.5
Maine 39  91,646,000,000  40.7
South Carolina 40  82,932,000,000  44.9
West Virginia 41  62,755,000,000  59.4
Maryland 42  32,133,000,000  115.9
Hawaii 43  28,311,000,000  131.6
Massachusetts 44  27,336,000,000  136.3
Vermont 45  24,901,000,000  149.6
New Hampshire 46  24,216,000,000  153.9
New Jersey 47  22,588,000,000  164.9
Connecticut 48  14,357,000,000  259.5
Delaware 49  6,447,000,000  577.9
Rhode Island 50  4,002,000,000  931.0
District of Columbia 51  176,750,000  21,079.5

 

At the end of this historic storm it will surely be ranked as one of the largest natural disasters to hit the United States.

Trevor Nace is a geologist, Forbes contributor, founder of Science Trends, and adventurer. Follow his journey @trevornace. Contact Trevor at trevornace@gmail.com


Source: https://www.forbes.com/sites/trevornace/2017/08/28/harveys-unprecedented-flooding-could-unload-25-trillion-gallons-water/#65a9858772b8

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