2014-15 Water Year Inflows Exceeded “Normal”

In the wild and wacky world of water, a review of data from the 2014-15 water year (which ended Sept. 30), is an interesting – if not particularly enlightening — exercise, as I’ll demonstrate below. It’s difficult to know what, if any, conclusions can be drawn.

The 2014-15 water year ended up as the 11^th highest in terms of inflows to Lake McConaughy (see table below), which means it ranked above “normal.”

“Normal” inflows, depending upon how you choose to look at them, are either understood to be the “average,” (or “mean”), which is a number that is calculated by adding quantities together and then dividing the total by the number of those quantities; or the “median,” which is defined as “the value in the center of the distribution for an array of data.”

One problem with using the average to define “normal” is that the values can be skewed by very high or very low data.  Those impacts, of course, are lessened as the data set grows larger.

So perhaps we should use median annual inflows, which produces a number right in the middle of the data set, as an indicator of “normal.”

But is that really “normal?” What, indeed, is “normal?”

According to Webster’s Dictionary, the definition of normal is “conforming to the standard or the common type; usual; regular; natural.”

Hmm. Not sure that’s helpful, particularly given the unpredictability of Nebraska’s weather and water supplies in the Platte River watershed.

Perhaps the second definition in the dictionary would be more appropriate: “Serving to establish a standard.” That might be more helpful as we seek conditions that conform to expectations.

For the sake of comparison, the historical median annual inflow into Lake McConaughy through the recently ended water year is 913,234 acre-feet. But the average annual inflow over that period is 1,020,504 acre-feet, which is a difference of 107,270 acre-feet, or almost 12 percent. For perspective, that’s like getting another October’s worth of inflows during a year, and October is historically the month when inflows, on average, are the highest.

But let’s take a look at another set of numbers, just for fun, of course. We’ve mentioned that the historic median annual inflow is 913,234 acre-feet. That’s over a period of 74 years. If we look at the median inflow over shorter periods of time, we find the following: The 30-year median – back to the 1985-86 water year – is only 758,071 acre-feet; the 10-year median is even lower at 723,595 acre-feet, but the 5-year median – bolstered by a couple of good water years and offset by a couple of below normal (there’s that word again) years – is 819,673 acre-feet, although still significantly less than the historic median. Does that mean that “normal” is a moving target, that it changes with time and circumstances? How can something so transient be referred to as “normal?”  Can “normal” change?  (Well, obviously.  It’s no longer considered “normal” to wear “disco” outfits, but that’s another story.)

So again we have to ask, “What is normal?” One of my favorite answers to this question, which I find fitting given weather on the Great Plains, is that normal is somewhere in the middle of two extremes. If that’s the case, then the only years when inflows to Lake McConaughy ended up in the “normal” range were 1957-58 when inflows were 916,900 acre-feet, or perhaps 1977-78 when inflows were 909,567 acre-feet.

After all that, it appears that we’ve only had two years of “normal” inflows in the last 74 years!

So when looking at inflows to Lake McConaughy, I guess you could use the saying from the movie “Forrest Gump,” when the title character’s mother advised him: “Life is (substitute “Inflows are…”) like a box of chocolates; you never know what you’re going to get.”

(Note that nearly all of the inflow years that populate the “Bottom 10” occurred recently, during the first decade of the 21^st century.)