Coupled Hydrologic Model Takes Cue From Atmosphere

February 17, 1998

Philadelphia, Pa. -- A linked atmospheric and hydrologic model system that simulates storm events and river basin response may allow researchers to study flood and drought regimes and the effects of climate change, according to a Penn State researcher.

"The hydrologic models are driven by the meteorological models," says Dr. Zhongbo Yu, research associate in Penn State's Earth System Science Center.

Yu developed his simulation for the West Branch of the Susquehanna River basin. He analyzed data from the storm of April 14 to 22 in 1986, using hourly precipitation data for a 96-hour period obtained from the meteorological record and from the Penn State/National Center for Atmospheric Research Mesoscale Meteorological Model Version 5 (MM5).

"We want to see how the entire system responds hydrologically," Yu told attendees today (Feb. 17) at the annual meeting of the American Association for the Advancement of Science in Philadelphia. "We can use either historic meteorological data about a storm, or have the atmospheric model generate simultated precipitation data from atmospheric conditions and feed that information into the hydrological models."

The model is geographically nested, beginning with a domain of the Eastern United States, going down to a more localized segment of the East and then focusing on the West Branch of the Susquehanna River basin. Data for the various scales of the model are recorded for 12- and 4-kilometer square sections, while the smaller hydrologic model scales down to 1-kilometer square sections. The hydrologic models cover four regimes -- soil hydrology, terrestrial hydrology, groundwater hydrology and channel ground-water interaction model.

The soil hydrology model looks at evaporation, vegetation cover and type, runoff and infiltration. The terrestrial hydrology model charts overland flow in all directions and channel flow of permanent or temporary rivers and streams. The ground water hydrological model tracks ground water flow in the aquifer. The fourth hydrologic model -- the channel ground-water interaction model -- analyzes interactions between the stream system and the ground water.

Yu found that the initial pre-storm conditions of the ground were not known. The model was therefore run using both moderate soil moisture and wet soil moisture. The moderate soil moisture initial conditions most closely followed the actual hydrologic regime.

When the actual storm data was fed to the hydrologic models, the results are good. When the atmospheric model's predictions were fed to the hydrological model, the results are not as good.

"The finer the scale of data, the better are the results we get with the hydrological models," says Yu. "However, it already takes two days to run the model on a Cray computer. Using the finest resolution, it could take a long time to run."

The researchers compromise, using the finest resolution data that will provide simulation results in a reasonable time. Yu notes that at this time the meteorologic and hydrologic models run independently and will eventually need to be run simultaneously so that feedback information can be passed between parts of the coupled model system.

"The problems of scaling from 12 to 4 to 1 kilometer and from 1 to 4 to 12 kilometers can be handled with a variety of mathematical approaches," says Yu. "We would like to look into these specifically in the future."

**aem**

EDITORS: Dr. Yu may be reached at (814) 865-1781 or yu@essc.psu.edu by email.
-end-


Penn State

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