October 25, 2013

And With Normal Snows, Likely to Reach Field Capacity by Spring

Soil moisture recharge across most of the state is better than at the start of the 2013 growing season, according to information presented to the Nebraska Climate Assessment and Response Committee October 23. They met to discuss the current drought status across Nebraska and prospects for more moisture before the start of the 2014 crop season. The committee examined impacts of the Colorado flood, the South Dakota blizzard, and above normal precipitation in October that has led to widespread drought reductions across most of Nebraska.

Figure 1. U.S. Drought Monitor for Nebraska for October 22, 2013.

Although massive flooding along the front range of the Rockies resulted in large infrastructure damage to numerous Colorado communities, flooding on the South Platte watershed scoured the channel of water-consuming vegetation and helped recharge the alluvial aquifer.

It has been over a month since the flood surge reached Nebraska. River flows at Roscoe stand at 650 cubic feet per second, five times the average daily flow for this time of year. Flows from now through the spring snow melt are likely to be significantly greater than the past few years because only a small fraction of the water will be lost to infiltration into the alluvial aquifer.

With the alluvial aquifers in the South Platte and along with the Platte east of Lake McConaughy recharged, normal snowmelt likely will have a greater hydrological impact than we’ve seen since 2011. There is enough space available to hold 2.6 million acre feet of runoff. We could mimic the winter snows of 2010-2011 and still have 400,000 acre feet of storage available by the end of snowmelt.

By all accounts, the winter of 2010-11 produced a snowpack of 140%-150% in the northern branch of the Platte. Two significant snow events have hit the headwater region this October and month to date precipitation averages are running over 150% of normal in north central Colorado to 170% of normal across south central Wyoming. With soils moist going into winter, it is expected that much of the snow melt early next spring could become runoff.

Streamflow rates also are likely to benefit from October moisture events, including the Panhandle blizzard at the start of the month and subsequent rainfall from two systems moving through the state. Soil moisture reports across the entire Panhandle are now above average which may enhance flow rates into Lake McConaughy next spring. Of course, these benefits will be contingent on periodic moisture events.

Soil Moisture Recharge Mostly Good

Soil moisture levels in other areas of the state are also showing significant benefits from the intense storm activity of the past 30 days. A few pockets of central and northeast Nebraska have as much soil moisture in the top 4 feet of the profile as was measured in late April and early May of this year. Only one site in the High Plains Regional Climate Center’s soil moisture monitoring network is currently showing less soil water content than it had at the beginning of March.

In short, current soil moisture water content is four to five months ahead of last year and is within 1 inch of normal for all areas except southwest and south central Nebraska. With normal moisture from now through April 2014, most of eastern Nebraska would have 7-8 inches of stored soil moisture and be at or close to field capacity in the top 4 feet of the profile.

The situation is a little more precarious in south central and southwest Nebraska where they haven’t seen as much moisture as other areas of the state. While soil moisture is higher than in March, these sites all had less than an inch of stored moisture in the top 4 feet of the profile. Our best estimate is that it will take approximately 120% of normal precipitation through next April for this area of the state to return to normal soil moisture levels.

Drought Outlook Improves for Nebraska

The positive impacts of recent storm activity can be seen on the latest U.S. Drought Monitor for Nebraska. The October 24 Drought Monitor indicates that 13.35% of the state is not classified in drought, 38.8% is abnormally dry, 20.46% is experiencing moderate drought, while 27.39% of the state is experiencing severe drought or worse.

If storm activity continues through the winter at the same intensity we saw in early October, snowpack levels will be sufficient to provide the necessary runoff to bring reservoirs on the northern branch of the Platte back to near capacity. However, if the winter is mostly dry, abnormally dry or moderate drought conditions are likely to appear in these areas.

Next Week’s Forecast

Another round of precipitation appears to be bearing down on the central Plains early next week. A substantial push of Artic air is projected to arrive as early as Monday across western Nebraska, moving into eastern Nebraska Tuesday. There is the potential for moderate snow accumulations (3-6 inches) across the western third of the state, with minor accumulations possible further east. The models show the next significant chance of moisture arriving about November 8.

Weather models are indicating that the recent trend of one significant storm system every six to seven days will continue through at least mid-November. The moisture would improve prospects for next growing season, but likely will push the end of corn harvest into late November or early December.

Al Dutcher
UNL State Climatologist