In studying long-range precipitation variability and its impact on
streamflow, hydraulic engineer Jurgen Garbrecht and meteorologist Jeanne
Schneider examined National Oceanic and Atmospheric Administration (NOAA)
records dating back to the late 19th century. They found that precipitation
over the entire United States from 1971 to 2000 was about 4 percent
higher than from 1895 to 2001. In some regions, such as the northern
Great Plains, it was even wetter. "This confirms that, for at least
the last three decades of the 20th century, water supply systems were
operating under favorable conditions, leaving users to expect high water
availability," says Garbrecht.
Meanwhile, hydrologist Michael W. Van Liew is applying hydrologic models
to better predict streamflow response to rainfall. "These improved
models will be great tools for effectively managing reservoir storage,"
he says. His exploratory workconducted in the Little Washita Experimental
Watershed in southwestern Oklahomadramatically illustrated an
effect of excess precipitation.
"We studied streamflow during fall months that received more rainfall
than average," says Van Liew. "When precipitation was 20 percent
greater than average, streamflow increased by 39 percent; but when precipitation
was 40 percent greater than average, streamflow increased by 96 percent."
Van Liew says this phenomenon, which he attributes mostly to increased
runoff due to ground saturation, may instill a false sense of comfort
regarding water supplies. "These streamflow volumes, which many
communities may have grown accustomed to or even dependent on, will
not be there when drier rainfall patterns assert themselves again,"
he says. "But this knowledge can be put into models to frame forecasts
that can help reservoir managers meet water demands and anticipate when
or whether streamflow will refill reservoirs."
Schneider says there's great promise in integrating dependable, long-term
precipitation forecasts into water-resources decisionmaking. "We
need to find out how seasonal precipitation forecasts might affect the
underlying land and streams," she says. "We can do this by
modeling soil moisture and streamflow response for a range of historical
and forecasted conditions."
Her team has examined the usefulness, dependability, and effectiveness
of the experimental long-term precipitation forecasts issued monthly
by NOAA's Climate Prediction Center. "We found that in the desert
Southwest, California, the Pacific Northwest, and along the Gulf and
southern Atlantic coasts, the forecasts have offered some dependable
predictions of departures from normal conditions," Schneider says,
"but they had little utility in other areas of the country."
A Glimpse Into the Future
Hydrologist John X. Zhang is relating this research directly to agriculture.
He's using seasonal climate forecasts and climate-change projections
to measure the effects of short- and long-range variations on water
runoff, soil erosion, and winter wheat production.
"If we translate seasonal climate forecasts to a range of possible
agricultural outcomes, farmers and ranchers will be able to make better
management decisions," he says. "To test this theory, we're
showing the potential effects of climate changewell into the futureon
soil erosion and winter wheat productivity in central Oklahoma."
Zhang took actual changes in precipitation and temperatures between
1950 and 1999 and those projected for 2056 to 2085 and incorporated
them into a climate generator. He then constructed five climate-change
"The scenarios offer a glimpse of how soil erosion and crop production
may change if various climate factors change," says Zhang. "For
instance, an increase in atmospheric CO2 concentration of
50 percent increased wheat yield dramatically, but an increase in temperature
along with increased CO2 considerably decreased this yield
and increased soil loss.
"Overall results indicate that conservation tillage and no-till
systems would be effective in controlling soil erosion in the region
under the climate change projected in these scenarios."
Steiner says that while seasonal climate forecasts and information
on precipitation trends are already old news to the National Weather
Service, changes are needed in how the information is analyzed and presented
so that agricultural producers can reap the full benefits of this knowledge.
"We've taken some steps toward closing this gap," she says.
"Tailoring our planning and management tools to include known variations
of precipitation and seasonal forecasts will lead to better agricultural
production, higher profitability, better conservation, and, ultimately,
more reliable estimates of water quantity and quality for all."By
Agricultural Research Service Information Staff.
This research is part of Water Quality and Management (#201) and
Global Change (#204), two ARS National Programs described on the World
Wide Web at www.nps.ars.usda.gov.
Jean L. Steiner is
at the USDA-ARS Grazinglands Research
Laboratory, 7207 W. Cheyenne St., El Reno, OK 73036; phone (405)
262-5291, fax (405) 262-0133.
"Less Rain in the Forecast? Tailored Climate Data May Help
Water Managers Be Prepared" was published in the July
2004 issue of Agricultural Research magazine.