Tuesday, June 29, 2010

State finally starting to dry out?









The HPRCC publishes a map every Monday showing the soil moisture conditions at AWDN stations across the state of Nebraska. The Soil Moisture Index (SMI), as featured in the maps, is an index applied to the volumetric water content at the depths of 10, 25, and 50 cm. It is averaged over those three depths and scaled such that 5.0 represents field capacity and -5.0 represents the wilting point. Measurements are made under grass covered, rain fed conditions and may not necessarily be representative of soil moisture conditions in fields with standard row crops.

Two weeks ago most of the state was coming off a five day period where 3-9 inches of rain fell and many AWDN sites had SMI's that suggested the soil at these locations had reached saturation.

Soil moisture lesson of the Day:

For those unfamiliar with the difference between saturation and field capacity, a saturated soil represents a situation where all pores of the soil are completely filled with water and additional rainfall will ultimately run off. Field capacity represents the soil after water has drained from the larger pores but still exists in the smaller pores. A soil profile at field capacity is considered "full" but a small amount of rainfall at this condition would not automatically run off.

But back to the topic at hand..
As I alluded to earlier, most stations in the state were very moist and flash flooding was an imminent threat. In the following week to 10 days, there was a slight shift in the overall weather pattern and western and north-central Nebraska were able to dry out and as of yesterday, a few sites in these areas have an SMI below 0.0, indicating that the soil profile is closer to the wilting point than to field capacity. In other words, the roots of plants will have to work harder to overcome tension, which in turn can cause a stress on vegetation. Prolonged stress can be particularly detrimental to the common row crops we have in Nebraska, so if you see some farmers with their pivots on in this area this week, know that they aren't necessarily wasting water.

On the other hand, the southern and eastern regions of the state are just now getting to dry out. But don't wish for this sunny, dry weather to persist for too long. Ten days with no rainfall would probably cause the SMI to be near or possibly even slightly lower than 0.0 at many of the sites that have just recently received so much rain.

So the moral of the story for today is this: Yes, we've had copious amounts of rain this spring and summer, but it will be all for nothing if the rest of the summer is dry and especially if it's dry and hot. So don't look at your local farmer like he's lost his mind if you hear him wishing for rain a week from now, because we will probably need it; else pivots (west of Hwy 77) and lawn sprinklers everywhere will start getting a workout.

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