Review of ‘Unusual’ 2018-19 Water Year

Review of ‘Unusual’ 2018-19 Water Year

To say it has been an unusual year is perhaps an understatement.

The 2018-19 water year ended on Sept. 30 (a water year runs from Oct. 1 to Sept. 30 of the next year) and it was a year marked by heavy and frequent rain storms, floods, planting delays, bone-chilling winter temperatures, and even “bomb cyclones,” among other anomalies in terms of weather and water.

While this part of Nebraska was largely spared from the calamities that befell other parts of the state (except for the deluge that caused flooding along Turkey Creek in Kearney and the Wood River flooding that struck several other central Nebraska towns), it has also been an unusual year for the water supply at Lake McConaughy.  While total water year inflows were above average, the 1.19 million acre-feet barely cracked the Top 20, finishing at 19th highest in the reservoir’s history.  (An acre-foot is the volume of water that would cover one acre with 12 inches.)

But it was the inflows during the summer months that made the water year unusual.  Normally inflows are highest in October and then in May and early June.  In fact, from October of last year through May, inflows were pretty much in line with the normal monthly averages.

Then came summer.  Inflows to Lake McConaughy during June were twice the normal amount; more than two and a half times normal in July; and 348 percent of normal in August.  In fact, the 162,843 acre-feet (a-f) that flowed into Lake McConaughy in August was the highest monthly total for the year.  Historically, as one would expect in a snowmelt-fed basin, inflows during August are near the low point for the year, trailing only July (median inflows of 46,815 a-f in August and 45,718 a-f in July).

Several factors converged to yield this outcome.  First, mountain snowpack in Colorado and Wyoming was above average in all three basins – the upper and lower North Platte River and the South Platte River – that affect river flows into Nebraska.  The subsequent runoff, particularly in the North Platte Basin in which Lake McConaughy is located, entered U.S. Bureau of Reclamation (USBR) reservoirs in Wyoming that were already holding plentiful supplies of carryover storage from the last year.

Second, frequent precipitation across much of the Platte Valley suppressed demand for irrigation.  Rainfall during the growing season (April through September) collected in Central’s Holdrege gauge totaled 25.44 inches, compared with the 20-year average of 18.63 inches and 19.0 inches since 1957.

However, the frequency of precipitation perhaps played a more significant part in reducing irrigation demand than the amount of rainfall.  Few weeks went by this summer without some amount of rain, which was often enough to dissuade an irrigator from starting his pivot or opening the gates on his pipe.

And finally, a mid-July tunnel collapse on an irrigation canal that delivers water to the Goshen Irrigation District in Wyoming and the Gering-Ft. Laramie Canal in Nebraska’s Panhandle, prevented delivery of water to about 107,000 acres in the two states.  With abundant water already in storage and the approaching need to make room for next year’s inflows, releases from the USBR reservoirs that normally would been diverted into the two canals continued downstream to Lake McConaughy.

Lake McConaughy’s lowest elevation (3,252.5 feet above sea level) during the 2018-19 water year actually occurred on Oct. 1, 2018, the first day of the water year.  The reservoir’s peak elevation occurred on July 15 at 3,260.1 feet, declining to elevation 3,257.9 in mid-August and currently stands near elevation 3,259.0, about six feet below full elevation.

And here’s an interesting observation:  Lake McConaughy’s elevation of 3,258.7 feet on Aug. 31 was the same as it was on Aug. 1.  A check back through the data reveals that that has never happened in the reservoir’s 79 years.  While August’s inflows were well short of a record amount, the monthly total did rank fifth behind 2010, 1973, 2011 and the record of more than 328,000 a-f in 1983.

So if you’ve noticed quite a bit more water flowing down the Platte River this summer, that’s the explanation.  A lot of water going into Lake McConaughy, and once it was released, not much demand for it to be diverted into the many irrigation canals along the central Platte.

With long-range forecasts calling for a cold and wet winter, one wonders what Mother Nature has in store for Nebraska in the new water year.

###

Jeff Buettner

    Facebook
    Twitter
    LinkedIn
    Instagram