Satellite images reveal link between urban growth and changing rainfall patterns

Satellite images reveal link between urban growth and changing rainfall patterns

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Stanford, USA, July 2, 2007: Satellite images have been used to demonstrate a link between rapid city growth and rainfall patterns, as well as to assess compliance with an international treaty to protect wetlands. The results have been published in two studies co-authored by Karen Seto, assistant professor of geological and environmental sciences and a fellow at the Woods Institute for the Environment at Stanford University.

The study published in the July online issue of the journal Global Environmental Change, showed that inclusion in an international environmental agreement did not significantly improve the health of a coastal mangrove habitat in a wetland preserve in Vietnam. Another study published on May 15 in the Journal of Climate, the researchers found that rapid urban growth has caused drier winters in the Pearl River Delta of China. Both findings are based on an analysis of satellite images of Vietnam and China, which NASA has been collecting through its Land Remote-Sensing Satellite (Landsat) Program for more than 30 years.

Seto and her colleagues analyzed satellite imagery and found that urban areas in the Pearl River Delta, China, increased more than 300 percent from 1988 to 1996. In the Journal of Climate study, the researchers compared this rapid urban growth with monthly temperature and precipitation data from 16 meteorological stations. Their analysis revealed a direct correlation between the rapid growth of cities and a decrease in rainfall during the winter dry seasons from 1988 to 1996.

In the Global Environmental Change study, the researchers focused on Vietnam, a signatory of the Convention on Wetlands of International Importance Especially as Waterfowl Habitat, drafted in Ramsar, Iran, in 1971. The goal of the treaty was to protect wetlands by promoting sustainable use of resources found there. To date, more than 150 countries, including the United States, have signed the convention. The Florida Everglades and portions of the Ganges River in India and the Red River Delta in northern Vietnam are included on the convention’s list of wetlands of international importance.

The growth of aquaculture in recent years has threatened coastal mangrove forest habitats in the Red River Delta. In response, the Vietnamese government has established protected areas, such as the Xuan Thuy Natural Wetland Reserve, which was designated a Ramsar site in 1988.

Seto and her colleagues concentrated on Xuan Thuy and a nearby reserve that is not included in the Ramsar treaty. The researchers analyzed a series of Landsat images taken between 1975 and 2002 which revealed that both reserves experienced increased fragmentation of mangrove forest habitat with increased aquaculture. Contrary to expectations, the scientists found that aquaculture developed at a faster rate at the Xuan Thuy treaty site than at its non-Ramsar neighbour.

These results showed that satellite technology is a cost-effective means of assessing wetland health, Seto said, noting that the cost of acquiring the satellite images and conducting field studies totaled less than $5,000.