* Nebraska, Colorado among states with badly depleted soil moisture
* Driest summer on record for Nebraska, preliminary data says
* Heavy rain, snow needed to recharge soils, pastureland
* Chances of below normal winter precipitation in central US-NOAA
Nov 2 (Reuters) - Drought-struck areas of the U.S. Plains winter wheat belt need a deluge of rain and snow this winter to fully recharge parched farmland, an unlikely scenario that means wheat, corn and soybean crops could face a rough new season.
While the worst drought in roughly half a century is slowly loosening its grip on the United States, meteorologists and agronomists warn that the threat has not passed.
Wheat farmers now sowing their last winter seeds, and corn and soybean growers making planting plans for spring, are haunted by the worrisome fact that parts of Nebraska, Kansas and other key agricultural states have the lowest levels of moisture in years. Indeed, Nebraska recorded the driest June, July and August on record and the third-driest September ever, according to preliminary data from the National Climatic Data Center.
Some areas would need 5 to 6 feet of snow on top of more than 15 inches of rainfall over the next few months just to get back to normal, say climate experts. It typically takes about 10 inches or more of snow to make one inch of water.
That would not mark record amounts of precipitation, but would provide much more than normal - and more than can easily be dealt with.
``It's an insane amount of snowfall that would be required... among the top five snowfalls in history,'' said Al Dutcher, a climatologist with the National Drought Mitigation Center in Nebraska. ``That would be a nightmare.''
The heavy rains that hit the eastern United States this week when Hurricane Sandy roared ashore had little to no relief to offer for drought-hit states like Nebraska.
The National Oceanic and Atmospheric Administration (NOAA) in October predicted an ``enhanced chance'' of below normal precipitation for the middle section of the country this winter.
DRY, DRY NEBRASKA
Soil moisture not only is necessary for growth. It also helps insulate plant roots from killing freezes and helps protect a crop from fierce winds that can topple weak stands.
Without adequate soil moisture plants either die outright, or yield poorly, if at all. But just how much rain and snow is required to recharge soil moisture levels to adequate and surplus levels varies widely across the country.
There are so many variables, includng the condition of the soil, and the rate at which water penetrates the soil, that an equal number of inches of rain on different fields the same year, or upon the same field in different years, may still result in material differences in the quantity of water taken into and retained by the soil, according to agricultural researchers.
Yet some generalizations can be made. In northern Wyoming, and Minnesota more than 9 inches of additional precipation is needed, and parts of Oklahoma and Arkansas need between 9 and 15 inches extra, according to QT Information Services, which tracks climate data.
Conditions in the eastern Corn Belt have improved in recent days thanks to heavy storm systems through Illinois and parts of Iowa and Missouri, though much more will be needed. They have the advantage of time, since they aren't planted until spring.
The Plains and areas west will need much more than normal.
``Conditions are about as bad as they've ever been,'' said Brian Fuchs, a climatologist with the National Drought Mitigation Center. ``The impact of the drought and the dryness .. is far from being replenished.''
Though many farm states are suffering, Nebraska is in particularly poor shape. An estimated 78 percent of the state is now plagued by ``exceptional'' drought, the worst level measured by state and federal climatologists. In some areas of Nebraska, roughly 91 percent of the topsoil is considered very short to short of moisture. That compares to a five-year average of 27 percent.
All-important subsoil moisture levels are 97 percent short to very short, compared to an average of 29 percent, according to the state agriculture department.
``We are in a real dire situation,'' said Nebraska farmer Larry Flohr, who along with his son planted about 4,000 acres of winter wheat in early September.
Under normal conditions, the new winter wheat crop should already be well established with strong roots to prepare for winter, but for farmers like Flohr, the wheat has only just now emerged following a few inches of recent snowfall across the bone-dry land.
In Nebraska about 74 percent of the new winter wheat crop has emerged overall, well behind the 91 percent five-year average pace for emergence. Nebraska's wheat is rated 49 percent poor to very poor by the USDA.
South Dakota is another state suffering from drought. The wheat crop there is 23 percent emerged, compared to the average pace of 88 percent. South Dakota's crop is rated 61 percent poor to very poor due to drought by the USDA.
Overall, the U.S. winter wheat crop is rated 15 percent poor to very poor, 45 percent fair and 40 percent good to excellent.
It would take at least 9 inches of additional precipitation in Nebraska over and above normal levels that range from 20 inches to 50 inches, depending on the area, to bring soil moisture levels back to near normal, according to climate experts in the state.
``We probably won't be able to make up those deficits by spring,'' said Brad Rippey, a USDA meteorologist. ``The winter is the dry season, after all, for the High Plains. A couple of big winter snow storms might dent the drought, but it is highly unlikely that we will see drought eradication by spring.''
(Reporting By Carey Gillam; editing by Jonathan Leff and Andrew Hay)