An engineering firm summarized a study Monday of the groundwater “mound” below land that is irrigated by the Central Nebraska Public Power and Irrigation District.
"Groundwater mound" describes underground water that has accumulated below and next to Central's irrigation service area near Holdrege since the system was built in 1941.
The study, conducted by EA Engineering Science and Technology, Inc., will help Central make management decisions in the operation of hydro-irrigation projects.
Dale Schlautman of EA Engineering's office in Lincoln presented the study to the board of directors of the irrigation and public power district. Schlautman said the study evaluated groundwater data from 1954 -- before widespread irrigation well development -- to 2013.
The study also provided a more detailed evaluation of data since 2000.
The area's groundwater supplies currently "are right on the edge of sustainability, a critical point where the mound is no longer rising and some areas are experiencing a slow decline,” said Central's Natural Resources Manager Mike Drain.
“The eastern part of the irrigated area is perhaps at more risk of future declines," Drain said.
The mound grew most rapidly during the 1970s, before the greatest increase in registered wells and widespread use of center pivots. The general long-term trend until 2000 was increasing volume, with occasional periods of decline. The early 2000s brought six consecutive years of decline, and drought, followed by gradual recovery until 2012 when another significant decline occurred.
In general, the mound in the western part of Central's service area -- the area surrounding the E65 supply canal and the Elwood Reservoir -- has risen slightly and at a more constant pace. That area also has had the lowest amount of groundwater irrigation development.
The mound in the middle reach of the Phelps Canal north of Holdrege and east to the Minden area has experienced more fluctuation over the years and is no longer increasing, according to the study.
Schlautman said the study shows that evapotranspiration -- the use of water by crops and other vegetation – as well as diversions of water into the irrigated area are the primary factors in the mound's size, shape and volume.
"The mound is very sensitive to relatively small changes in either of those factors," he said. " It appears that even small reductions to current surface water diversions would lead to slow declines in the groundwater mound."
Central civil engineer Cory Steinke said the study reinforces the district's understanding that the mound is a very complex system that is influenced by many factors.
"The study's focus was on what has happened to the mound over time, not necessarily on the reasons why these changes have occurred,
"Many factors have an effect on the mound, from our customers' on-farm irrigation efficiency and Central's conservation efforts to allocations of water supplies, precipitation, and groundwater development,” Steinke said. “In recent years we have diverted significantly less water into the area due to changes in irrigation practices and shortages of storage water in Lake McConaughy. It's not a surprise to see how these changes have affected the mound, but it is useful for management decisions to know more about where those effects occur and how much they affect groundwater supplies."
Steinke also reported that Lake McConaughy was at elevation 3235.6 feet (991,500 acre-feet or 56.8 percent of capacity). He said Inflows have been around 800 cubic feet per second, or about 67 percent of normal for this time of year, but ice conditions on the river have made accurate measurements difficult.
Steinke said early snowpack accumulation in the Colorado and Wyoming mountains that feed Lake McConaughy are about average.
Also, the Central board of directors:
• Authorized the next phase for construction of two regulating reservoirs near the Platte River. Steps in what is expected to be a two-year process include planning for engineering, water rights permitting, work with consultants, negotiations for land acquisition, and project management.
• Accepted bids totaling $88,320 from Platte Valley Auto Group of Lexington for the purchase of a sedan, two 1/2-ton pickups, and a 3/4-ton pickup.
• Accepted a $96,444 bid from Van Diest Supply Co., of McCook for weed control chemicals.
• Approved water service agreements with Keith-Lincoln Irrigation District, Paxton-Hershey Water Co., and Platte Valley Irrigation District for supplemental water supplies for 2014.
• Accepted a $762,089 bid from Crane Sales and Service of Omaha for the purchase of a mobile crane.
• Approved a $122,542 work order for the purchase of substation equipment in the switchyards at the Jeffrey, Johnson No. 1 and Johnson No. 2 hydroplants.
• Approved a contract with Black & Veatch Corp., of Burlington, Mass., to conduct a study of the merits of joining the Southwest Power Pool (SPP). SPP is a regional transmission organization, mandated by the Federal Energy Regulatory Commission to ensure reliable supplies of power, adequate transmission infrastructure, and competitive wholesale prices of electricity.