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Soil drought expected to end; hydrologic drought to be long-term issue

Reduced stream flow occurs with hydrologic drought
<p> Reduced stream flow occurs with hydrologic drought.</p>
The drought that has covered most of the Southwest for two years is coming to an end. A hydrologic drought could persist for years. Average rainfall for several years will not refill reservoirs. &nbsp;

The drought that has covered most of the Southwest for two years is coming to an end.

With the onset of an El Nino weather pattern and decline of La Nina, weather forecasters are saying that the coming winter will be cooler and wetter than normal. That situation should persist into March, says Steve Lyons, National Weather Service, San Angelo, Texas.

Lyons, speaking Thursday at the Big Country Wheat Conference in Abilene, said little will change over the next 90 days. “We predict warm and dry conditions until about Christmas, and then it turns cooler and wetter.”

The last 90 days, to no one’s surprise, has featured below average rainfall and above average temperatures.

“We’ve been running much above normal temperatures,” he said, “much like last year, and that does not help with evaporation.”

Those evaporation losses expose a situation that has longer-term and possibly more severe ramifications than the soil drought that has covered the region for two years. A return to more normal—or slightly cooler and wetter conditions this winter—could break the drought. But a hydrologic drought could persist for years.

A hydrologic drought results when surface water—including reservoirs and streams— are depleted faster than they can be replenished because of prolonged drought. Lyons says a hydrologic drought exists when catchments are depleted to zero to 30 percent of full. When reservoirs get that low, “It takes a long time to fill them back up. One rain event will not be enough. We need widespread rainfall.”

Hydrologic drought is evident in much of West Texas. Near San Angelo, the O.C. Fisher and E.V. Spence reservoirs are at zero percent; Twin Buttes is at 5 percent. Near Abilene, Abilene Lake is at 11 percent; Hords Creek Lake is at 9 percent and Lake Stamford is at 35 percent.

“It may take several years to replenish these lakes,” Lyons said. “And that is if the drought doesn’t get worse.”

That also assumes wetter than average years. With a “wetter than normal,” prediction for the coming winter, a situation the region “has not seen for awhile,” reservoirs will not make appreciable progress to refill, Lyons said.

Trouble for cities

“This could mean trouble for some cities. We have to get a lot of rain for hydrologic drought relief. High temperatures, low rainfall and high winds equal high evaporation,” he said. Those conditions have persisted for the last two years.

Now, with catchment levels as low as they are, a “typical year with average rainfall will not be enough to maintain the reservoir levels.”

In fact, Lyons said, several years of just average rainfall would result in reservoir levels continuing to drop. An event such as a hurricane that hovers over the area for several days, dropping up to 20 inches of water and creating massive flooding, might be necessary for a quick refill. Otherwise, the region will need several years of above average rainfall to bring reservoir levels up to normal capacity.

“Rainfall has to soak the soil first, before water can begin to run off and into streams that carry it to the reservoirs.” He said to “pre-condition” soil for runoff would require something on the order of a two-inch rain event.

Lyons said the current drought, now afflicting more than half of the United States, is “bad, but not as bad as the Dust Bowl Days of the 1930s.”

But it’s bad enough with 87 percent of the U.S. corn acreage, 85 percent of soybean acreage, 63 percent of hay production and 72 percent of cattle operations experiencing drought. “The current drought monitor, which has created a big hoopla because of the effect on the Midwest, shows drought extending into Texas. Conditions in West Texas are not good,” Lyons said.

He said forecast for the next 90 days shows possibility for “some improvement” for south and east Texas. For the rest of the state, the drought likely will persist until at least November with “an equal chance of above and below average rainfall and temperatures.”

Beginning in December, Lyons said, El Nino should become the dominant force in Southwest weather. “That forecast is not perfect. El Nino could fizzle, but it seems pretty certain that we will have an El Nino event. The magnitude, however, is not as certain.”

In an El Nino cycle, the Southwest typically has more snowfalls and some bigger ones.

Regardless of how much snowfall the region receives this winter and regardless of whether the drought breaks by Christmas, without a significant amount of precipitation, either in a massive and possibly destructive event or above average precipitation over the course of several years, the area’s hydrologic drought will continue.

“Eventually, all kinds of water restrictions will be employed if things don’t change,” Lyons said.

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