New Evidence Sponge Effect Tropical Forests
December 18, 2013

More Evidence For ‘Sponge Effect’ In Tropical Forests Of Panama

Lee Rannals for - Your Universe Online

New research on tropical forests is adding more evidence to a controversial phenomenon known as the 'sponge effect.' Teachers working at the Smithsonian Tropical Research Institute in Panama found tropical forests soak up water during storms and release stored water during droughts, which is known as the sponge effect. This phenomenon is at the center of a debate about how to minimize flood damage and maximize water availability in the tropics.

Scientists measured the amount of runoff from pastureland, abandoned pastureland and forested land as part of a large-scale land-use experiment in the Panama Canal watershed during nearly 450 tropical storms. Data collected indicates land-use history has complex, long-term effects.

"We measured large differences in hydrologic response between watersheds with different land-use histories and land cover," said Fred Ogden, STRI Senior Research Associate and Civil Engineering Professor at the University of Wyoming. "Our ultimate objective is to better understand these effects and include this improved understanding in a high-resolution hydrological model that we are developing to predict land-use effects in tropical watersheds."

Robert Stallard, hydrologist at STRI and the United States Geological Survey who developed the statistics for data analysis, said the result for storm peaks is spectacular. "Storm-water runoff from grazed land is much higher than from forested land. The results are clearest after big storms,” Stallard explained.

The team also found that forests released more water than grasslands and mixed-use landscapes during the late dry season, which highlights the importance of forests in regulating water flow throughout the year in seasonal climates.

Some scientists have questioned the validity of the sponge effect argument because it lacks supporting evidence.

"One of reasons why there isn't more scientific evidence for the sponge effect is that you have to take what nature dishes out," said Stallard, who is a strong proponent of the sponge effect. "It requires a long-term institutional commitment to get good results. The USGS, STRI, University of Wyoming and the Panama Canal Authority have the resources to do that."

In 1997, a severe drought forced Panama Canal authorities to impose draft restrictions on ships passing through. A major storm system in December of 2010 stopped shipping in the canal for 17 hours. These events show how land use in the watershed not only affects world commerce but also water availability of Panama’s major urban areas.

The team said their study is relevant to land use decisions throughout the topics where more than 50 percent of forests are now 'secondary' forests that have grown back after logging or on abandoned pastureland.

"Our project aims to clearly quantify environmental services such as water flow, carbon storage and biodiversity conservation that decision makers will consider as they evaluate projects from forest restoration to watershed management," said Jefferson Hall, Smithsonian staff scientist and project director.