COLLEGE STATION – According to the experts, when it comes to determining if a drought has ended, one of the most important questions to ask is: Which one?
"Trying to determine when a drought has ended is done the same way a fishhook is removed... very carefully," said Dr. Klaus Wolter, a researcher at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Colorado.
Many factors, he said, including rain, runoff, inflows and social impacts must be considered before determining a drought has indeed ended.
“There isn’t just one specific definition or category of drought,” said Dr. Travis Miller, Texas AgriLife Extension Service statewide agronomist in College Station.
Miller said there are four generally acknowledged types of drought – meteorological, hydrological, socioeconomic and agricultural – each as varied as the Texas weather.
Meteorological drought refers to a deficit in precipitation in a given region over a specific period of time when compared to that same time interval over a historical average for that period. Hydrological drought refers to the effect of reduced precipitation on surface and subsurface water supplies. Socioeconomic drought occurs when reduced precipitation causes an adverse affect on the economy of a region.
And agricultural drought, currently widespread through Central and South Texas, is evidenced by dry stock ponds, bare fields and little or no green-up in pastures which should normally be lush this time of year, Miller said.
“The agricultural economy in drought-stricken areas of the state is impacted because it’s either too dry to plant crops or the crops are withering, and ranchers are feeding livestock hay or other supplementation when they would normally have green grass and full water tanks,” he said.
However, while current agricultural drought has had a significant economic impact on the state with almost $1 billion in losses so far, Miller said meteorological drought is the type most people know about through news and weather reports.
Meteorological drought is usually categorized using the Palmer Drought Severity Index, which reflects both precipitation and other hydrological conditions, or the newer Standardized Precipitation Index, which reflects differences in precipitation only over various time scales.
The standardized index is used by the National Drought Mitigation Center because it makes it possible to identify emerging droughts months sooner than with the Palmer index, said Mark Lenz, a meteorologist with the Austin/San Antonio National Weather Service office in New Braunfels.
“Meteorological drought compares long-term average precipitation – an average of which is determined over 30 years – to precipitation over a specific, shorter period of time,” Lenz said . “These levels can vary widely across Texas and other parts of the country, depending on past precipitation and other weather-related factors.”
Lenz said drought as identified by the U.S. Drought Monitor, which uses data from various national and regional weather reporting and predicting agencies, is expressed as either agricultural or hydrological and is assigned one of five levels or stages -- abnormally dry, moderate, severe, extreme or exceptional.
“Even after the rains in late March, each of the 33 Texas counties which this office serves are still either in extreme or exceptional drought,” he said. “People are often under the impression that after a couple of days of good rains a drought has broken, but that only happens if precipitation over a period of months puts us near the long-term average.”
Lenz said about 10-15 inches of rain over the next several months would put most of South Central Texas back on track and make it reasonable to proclaim the drought has ended.
“That’s only true if those rains are longer, soaking rains as opposed to hard rains where there are flash floods and a lot of runoff,” he said. “While a hard rain might help replace some surface water, it won’t penetrate the subsoil enough to be helpful to farmers and ranchers.”
The lingering drought in extreme South Texas was recently changed from “moderate” to “severe,” according to Nezette Rydell, a meteorologist at the National Weather Service in Brownsville, who also said she does not expect improvement anytime soon.
“The weather patterns are not giving us great hope,” she said. “In general, it’s not looking favorable for the chances of rain.”
Since Jan. 1, the four-county Rio Grande Valley area has received only 1.1 inches of rain, a deficit of about three inches. Ironically though, the area’s reservoirs are filled to the brim, according to Erasmo Yarrito Jr., the Texas Commission on Environmental Quality’s deputy Rio Grande watermaster.
“We received abundant inflows from Mexico, thanks to a rainy cyclone that hit the state of Chihuahua last summer,” he said. “Water levels at the reservoirs behind Falcon and Amistad dams are so high that farmers were able to irrigate between October and March at no charge, meaning that the water they used was not charged against their yearly allocations.”
But despite abundant supplies of water for municipalities, industry and agriculture, growers still require rainfall to leach plant-choking salts from soils that accumulate salinity from irrigation water, said Dr. Juan Enciso, an AgriLife Extension irrigation engineer in Weslaco.
“In addition to leaching salts from soils, rainfall is also needed for dryland growers whose properties don’t have the infrastructure to irrigate,” Enciso said.
“Parts of the central and far western Texas Panhandle as well as parts of extreme West Texas have received sufficient rainfall to say they are now in the process of recovering from drought,” said Dr. John Nielsen-Gammon, Texas' state climatologist, College Station.
Forecast information from Nielsen-Gammon’s office indicates La Nina will weaken significantly in May and June, and that in the fall El Nino will “take over the Pacific Ocean,” creating conditions which should “produce a wetter, cooler Texas climate and give Texas the rain it has been looking for.”
While it’s difficult to determine exactly when a drought is over, there are still points of general agreement, Miller noted.
“I think most experts would concur that a drought is over when rainfall is close to the long-term average, ground moisture is adequate for normal dryland crop growth and to provide sufficient livestock forage, and lakes, reservoirs, ponds and stock tanks are full or nearly full,” Miller said. "However, while a drought may be over, the impact of drought to local economies and to agricultural production can be longer lasting."
The state’s Drought Joint Information Center has a new multi-agency, comprehensive drought information Web site, http://agrilife.tamu.edu/drought. The center is comprised of state and federal agency public information officers, activated by the Governor’s Division of Emergency Management and led by Texas AgriLife Communications.