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About the Reservoir Data

The data provide a snapshot in time of basin, lake and reservoir capacities in the western states of Arizona, California, Colorado, Idaho, Montana, New Mexico, Nevada, Oregon, Utah, Washington, and Wyoming.  Most reservoir water in these states is used for hydropower and agriculture. Capacity statistics are useful for making release and allocation decisions, and are an indicator of a basin's water budget. Is it fair to graph water levels for reservoirs in Arizona (dry) and Oregon (wet) in the same graph? In which states would the distribution of reservoir levels differ significantly from that of all western states, as displayed in the graph? In what ways would the histogram change in drier or wetter years? What would the histogram look like in March or April? How about in August or September?

The diagram of water levels is a classic example of a negatively skewed or left-skewed histogram. In a negatively skewed histogram, the mean lies to the left of the median as it is more sensitive to the extremely low basin levels. An excellent statistical exercise is to transform the horizontal scale so that the histogram is more symmetrical (closer to normal), and then compute the mean and standard deviation.

Source: National Water and Climate Center (NWCC), part of the Natural Resources Conservation Service - an  agency of the US Department of Agriculture.  The web site is  http://www.wcc.nrcs.usda.gov/ .

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