To say it has been an unusual year is 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 was marked by heavy and frequent rain storms, floods, planting delays, bone-chilling winter temperatures, and even “bomb cyclones,” among other weather and water anomalies.
Western areas of the Platte Basin in Nebraska largely were spared from weather calamities seen in other parts of the state, including the deluge that caused flooding along Turkey Creek in Kearney and in several other central Nebraska towns in the Wood River Basin.
However, it has 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 as 19th highest in the reservoir’s history. (An acre-foot is the volume of water that would cover one acre with 12 inches.)
Inflows during summer months made the water year unusual.
Inflows normally are highest in October and May to early June. From October 2018 through May 2019, lake inflows were in line with the normal monthly averages.
Then came summer.
Lake McConaughy inflows in June were twice the normal amount and 2½ times normal in July. They were at 348 percent of normal in August. Those August inflows of 162,843 a-f created the highest monthly total for the year.
Historically, as one would expect in a snowmelt-fed basin, August inflows are at the low point for a water year, trailing only July. Median inflows are 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 — upper and lower North Platte River and South Platte River — that affect river flows into Nebraska.
The subsequent runoff, particularly in the North Platte Basin where Lake McConaughy is located, entered U.S. Bureau of Reclamation reservoirs in Wyoming that already were holding plentiful supplies of carryover storage from last year.
Second, frequent precipitation across much of the Platte Valley suppressed irrigation demand.
Rainfall during the growing season, April through September, totaled 25.44 inches at Central Nebraska Public Power and Irrigation District’s Holdrege headquarters. Averages are 18.63 inches for the past 20 years and 19 inches since 1957.
The frequency of precipitation probably played a more significant part in reducing irrigation demand than the amount of rainfall. Few weeks went by this summer without some rain, which often was enough to dissuade irrigators from starting pivots or opening gates on irrigation pipe.
So once water was released from Lake McConaughy, there was not much demand for it to be diverted into the many irrigation canals along the Central Platte.
Finally, the mid-July tunnel collapse on an irrigation canal that delivers water to the Goshen Irrigation District in Wyoming and Gering-Ft. Laramie Canal in Nebraska’s Panhandle prevented delivery of water to approximately 107,000 acres in the two states.
With abundant water already in storage and an approaching need to make room for next year’s inflows, releases from the North Platte Basin reservoirs in Wyoming that normally would have been diverted into the two irrigation canals continued downstream to Lake McConaughy.
The lake’s lowest elevation of 3,252.5 feet above sea level during the 2018-19 water year was on Oct. 1, 2018, the first day of the water year. The peak was on July 15, 2019, at 3,260.1 feet.
Lake McConaughy had declined to 3,257.9 in mid-August and currently stands near elevation 3,259.0, about 6 feet below full elevation.
Here’s another interesting observation. Lake McConaughy’s elevation of 3,258.7 feet on Aug. 31 was the same as on Aug. 1. According to historical data, that has never happened before in the reservoir’s 79 years.
Although August’s inflows were well short of a record amount, they 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.
Now, with long-range forecasts calling for a cold and wet winter, we will wait and see what Mother Nature has in store for Nebraska in this new water year.
Jeff Buettner is government and public relations manager for Holdrege-based Central Nebraska Public Power and Irrigation District.