Air, Water, and Wavelets

Wavelet analysis and the impact of rising air temperatures on streams

Sunflower River
Sunflower River in Sunflower MS, located within the Lower Mississippi River Basin. Photo by Ying Ouyang, USFS.

We know air temperature has increased over the last 15-20 years, and that it will continue to do so on an unprecedented scale. But we are still learning exactly how this air trend impacts water. As it turns out, air temperature is linked to changes in our streams — affecting things like flow, soil moisture, and surface runoff.

“The correct estimate of stream flow is essential to water resource management, water supply planning, environmental protection, and ecological restoration,” says Ying Ouyang, a research hydrologist with the U.S. Forest Service. “Because climate change can have significant effects on stream flow, water resource managers must be able to assess potential risks and opportunities, and where appropriate, implement good practices to adapt for future climatic conditions.”

Ouyang led a study that was recently published in the Journal of Environmental Management. SRS research plant pathologist Ted Leininger and other scientists from Mississippi State University, the Agricultural Research Service, and the Chinese Academy of Forestry contributed to the study. They used the wavelet analysis technique to identify how changing air temperature affected forest stream flows in the Lower Mississippi River Alluvial Valley (LMRAV).

In the frontier days, the LMRAV was considered a region that was rich in water and supported high standards of living and biodiversity. But in the past several decades, this region has experienced increasing water stress. Agricultural irrigation, climate change, land use conversion and a rise in population are to blame. Extensive water usage has led to overdrafts and declines in water resources. These are likely to become more severe in the future.

LMAV
The Lower Mississippi River Alluvial Valley (LMRAV) is located in the floodplain of the Mississippi River which begins in Cairo, Illinois and continues south through Missouri, Kentucky, Arkansas, Tennessee, Mississippi, and Louisiana. Image courtesy of USGS.

Now that we have a better understanding of how climate change impacts water quantity and quality, Ouyang says there is a greater need to gauge how the forest stream flow in watersheds is responding to the change in air temperature.

The scientists looked at data collected from four surface water monitoring stations in the LMRAV located near headwater areas. These spots had very few land use disturbances and provided long-term data records, which are good for climate change analysis.

Using this data from the past 60 to 90 years, researchers found that the climate in the LMRAV did get warmer as time elapsed. Significantly, the streams were drier due to the warmer air temperature.

The wavelet analysis technique used in this study provided information that descriptive statistical analysis could not. So, it seems the best way to estimate these temporal patterns is to perform the wavelet transformation around their mean values. This discovery may be useful to other researchers, who can apply the same approach to identify temporal trends for other climate and water-related variables.

Read the full text of the article.

For more information, email Ying Ouyang at youyang@fs.fed.us

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