Precipitation during the winter recharge period from October 1 to April 30 is critical to Nebraska crop production and ultimately determines the degree of drought risk for the next growing season. This year there has been a large variance in the amount of precipitation across the state, with some areas receiving above normal moisture while others have been exceptionally dry.
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Areas with above normal precipitation since October 1 include the southern Panhandle, southwestern and northeastern Nebraska, and a small area centered around Lancaster County. Areas with significantly below normal precipitation include south central, southeast, and portions of east central Nebraska, mainly south of Interstate 80. Two additional pockets of concern include portions of west central Nebraska near North Platte and the northern third of the Panhandle. Remaining areas of the state are only slightly drier than normal.
Statistical analysis of precipitation for selected locations across the state from October 1 to February 25 indicates the wide variation in precipitation trends. Lincoln received 6.29 inches of moisture, representing the 60th wettest period in 80 years of valid data. The following list includes the station, precipitation in inches during this period, and ranking out of years of available information (low value dry, high value wet): Omaha (3.34, 8/58); Norfolk (5.77, 44/58); Columbus (3.72, 31/102); Grand Island (4.20, 53/100); Red Cloud (2.82, 28/100); McCook (4.13, 58/81); North Platte (1.80, 14/57); Valentine (1.61, 9/57); Chadron (1.95, 18/83); Scottsbluff (3.62, 82/101); and Sidney (4.36, 78/87).
Soil moisture reserves drop with warmer temperatures
Temperatures during the period have been exceptionally warm, with only the mid-November
through mid-December and mid-February period experiencing below normal temperatures. From
late December through early February over 50 consecutive days of above normal temperatures
were recorded for all weather reporting sites in Nebraska. January temperatures averaged a full
8-12 degrees above normal for Nebraska. These temperatures were so high that a frost layer
didn’t develop in soils, which, coupled with little precipitation, increased surface drying during
January.
Records from the High Plains Regional Climate Center soil moisture monitoring sites indicate
that many sites across eastern Nebraska are at historical minimums for soil moisture, while
western sites are at historical maximums. Only stations with at least five years of data were
examined. All monitoring sites are over grass and may not be representative of conditions under
fields dedicated to row crop production. Cultivation practices, field slopes, and residual crop
cover can have a dramatic influence on soil moisture when compared to levels under grass.
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A further investigation into the dry soil moisture readings across eastern Nebraska indicate that abnormally warm conditions into mid-November increased water demand of cool season grass species. Minimum temperatures were insufficient to promote senescence. Soil moisture monitoring sites across eastern Nebraska showed a marked decrease in reserves, even with several moisture events. Western Nebraska reserves increased as native warm season species did enter senescence and did not require moisture for continued growth.
Soil moisture studies conducted at Mead in the early 1990s found that approximately 70% of the moisture received from October through April was captured in the soil profile. If there was 10 inches of moisture, the soil profile would capture 7 inches.
Using this precipitation/soil moisture relationship, most of the eastern third of Nebraska has between 2.50 and 3.50 inches of available moisture stored in soil profiles. Areas immediately surrounding Lancaster County, as well as northeastern Nebraska, have 4-5 inches of available soil moisture and the central third has 1.5-2.5 inches. Over the western third of the state, the southern Panhandle and southwestern corner have 2.5-3.5 inches of available moisture, while the northern Panhandle has 0.75-2.50 inches.
Precipitation probabilities
If normal moisture is received during the next two months, soil moisture values would increase
by 4 inches in the eastern third of the state, while the western two-thirds of the state would
receive an additional 3-4 inches. The probability that precipitation deficits incurred from
October-February could be alleviated during the next two months range from 15% in south
central Nebraska to 35% in the southeast. The Sandhills region has a 25-40% chance of erasing
deficits, while the northern Panhandle has a 15-25% chance.
Areas of the state with above normal moisture for October-February show above normal chances
for normal precipitation during the October-April period. Over the southern Panhandle and
southwestern corner of the state, there is a 60-80% likelihood of receiving normal moisture for
the period, while the northeastern corner of the state has a 55-70% chance of receiving normal
precipitation. The above normal area around Lancaster county has a 60-70% chance of receiving
normal precipitation for the October-April period. These probabilities are based on a calculation
that looks at the accumulated precipitation from October to February and the additional
precipitation needed in March and April to bring the October-April period back to normal.
Estimated effects of snow-pack
Snow-pack levels across the central and northern Rockies has shown significant improvements
during the 2005-06 winter as compared to the 2004-05 winter. Much of this region has surplus
snow-pack and stream flow projections are calling for above normal flows during the spring and
early summer. The Platte River Basin in northern Colorado, as well as south central and
southeast Wyoming, have remained above normal for the entire winter.
Current projections of snow-pack levels, which reach their statistical peak in mid-April, call for
levels in the Platte basin to average 120-150% of normal. Stream flow projections are for rates
of 110-140% of normal for April through June. Deep snow levels in north-central Colorado
could lead to localized flooding, depending on the rate of snow melt. All projected forecasts
assume normal precipitation distributions for the rest of the winter.
Within the northern branch of the Platte, projections indicate 1.2 million acre-feet of runoff. The
Platte watershed reservoirs in Wyoming hold about 2 million acre-feet. If full irrigation
allocations are delivered to Wyoming and Nebraska Panhandle irrigators, then 200,000 acre-feet
of water will be added to Wyoming reservoirs by the end of the 2006 productions season. It is
unlikely that the reservoirs will completely fill this year, unless significant moisture events
blanket the region for the next two months. With proper conservation and water limits, the
Wyoming reservoir system could reach full pool with a couple more winters like this year.
Reservoir levels and projected releases
Lake McConaughy continues to show improvement over its fall 2004 low of 18% of capacity.
As of February 25, the elevation was at 3217.3 feet above sea level, compared to 3212.2 feet
above sea level at this time in 2005. Currently Lake McConaughy holds 638,000 acre-feet of
water, compared to 552,000 acre-feet last year at the same time. The lake is currently at 36% of
capacity and should increase throughout the spring. The amount of rise in the lake will depend
on conservation methods employed to minimize water releases, the amount of spring
precipitation events, and snowfall east of Casper, Wyoming.
Water release projections for Lake McConaughy users are projected to be 8.4 inches per acre of
deliverable water. This forecast was updated in October and increased from the September
forecast of 6.7 inches.
It’s too early for irrigation district managers to determine the extent of irrigation allocations for
western Panhandle users. Although projections are that runoff could provide enough water to
deliver full allocations, conservation tools may be used to increase reservoir storage levels in
Wyoming to offset declines the past five years. Preliminary estimates of releases to
Republication River Basin irrigators are for 1.5 inches to Harlan reservoir users, 0.50 inch to
Enders reservoir users, 6 inches to Trenton/Hugh Butler users, and 8 inches to Cambridge users.
Unlike the Platte River Basin, the Republican River Basin doesn’t receive mountain snow-pack
and is entirely dependent on snow storm activity in east central Colorado and rainfall within the
basin. It will take at least several years of above normal precipitation and strict groundwater
management to repair the cumulative damage from this six-year drought. The only way
individual reservoirs within the basin could recover to normal levels would be if a major
precipitation event resulted in flash flooding of the magnitude seen around the Grand Island area
last spring.
Al L. Dutcher
State Climatologist
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