Study shows that hydropower dams reduce the temperature of rivers

A new study, published in the Environmental Research Letters journal, focuses on the impact of hydropower dams on the temperature of rivers in Asia's Mekong River basin.

Scientists from the University of Washington studied 3 major rivers (Sesan, Srepok and Sekong) in the basin that meet together to form the Mekong, the 12^th longest river in the world measuring 4,375 km. Mekong is of high importance for Southeast Asia residents as it provides water for field irrigation and fishing opportunities.

The three rivers examined have seen the construction of at least one dam since 2001 while more similar facilities are expected to be realized. In general, hydropower dams provide renewable energy but have a severe impact on the rivers' flow and ecosystem.

To evaluate the effect of dams in the aforementioned rivers, researchers utilized a large quantity of satellite data. In particular, they used data from the Landsat program, a NASA endeavor that provides the longest acquisition record of Earth’s satellite imagery. The data included heat or infrared radiation measurements of rivers. “With these data, we’re looking at the temperature emissions from the rivers. It’s like night vision: Warmer things give off more emissions, colder things give off less,” Matthew Bonnema, lead author of the study and a postdoctoral researcher at NASA’s Jet Propulsion Laboratory, said.

The research team mentioned that satellite imagery utilization to detect temperature changes is a process that has some drawbacks. Clouds make data accumulation impossible so scientists monitored the conditions only during the dry season.

According to the findings, the downstream temperature was reduced by a maximum value of 2°C in the Sesan River, a year after the construction of 2 dams. Furthermore, Mekong River presented a maximum shift of 0.8°C.

Until now, studies have shown that cooling pattern in rivers exists due to dam constructions, however, they were thought to be localized near the facilities. "...But what we see in the Mekong is like, ‘Wow!'” Faisal Hossain, a co-author of the study and a professor at the department of Civil and Environmental Engineering at the University of Washington, stated.

Moreover, the temperature of the rivers was monitored via sensors between 2004-2011. The temperature patterns in all rivers were similar until 2009. In particular, the rivers experienced a temperature rise at the beginning of the dry season and a temperature drop when the wet season kicked in. However, after the construction of dams in the Sesan and the Srepok rivers in 2009, the rivers stayed cool all year.

The temperature drop could escalate if more dams are constructed and may affect the ecosystem of the region.

Scientists also eliminated the case of the cooling being attributed to other factors. Rainfall incidents were almost the same during the examined period while air temperature increase and deforestation would result in temperature rise rather than drop.