Based on GEE to research the spatiotemporal impact of cascade dam construction on river water temperature in the upper reaches of Yangtze River

Abstract

[Objective] Water temperature is a key factor affecting the evolution of river ecosystems, playing an important role in the conversion of biogenic elements, biological habitat, and other physical, chemical, and biological processes. Hydropower development has obstructed the continuity of rivers and altered the transmission of river water temperature along the river,especially under the cascade hydropower development, which has seriously affected the rhythm of river water temperature. It is necessary to reveal the impact patterns of cascade dam construction on water temperature in the Yangtze River mainstream. [Methods] Based on the Google Earth Engine(GEE) platform, Landsat-5 TM, Landsat-7 ETM+, and Landsat-8 TIRS remote sensing datasets were utilized to retrieve surface water temperature of the Yangtze River from 2000 to 2020 through atmospheric radiative transfer models. The retrieved result were validated by comparison with measured data from Zhutuo hydrological station,and the spatiotemporal characteristics of surface water temperature before dam construction( 2000—2005) and after dam construction(2015—2020) were analyzed in the Liyuan-Guanyin section, Wuduode-Xiangjiaba section, and Three Gorges Reservoir area. [Results] The result show that the average annual surface water temperatures before the construction of dams in the Liyuan-Guanyin section, Wuduode-Xiangjiaba section, and Three Gorges Reservoir area were 21. 58 ℃, 21. 31 ℃, and 17. 79 ℃, respectively. After the construction of dams, the water temperature in the middle reaches of the Jinsha River reservoir area decreased significantly to 15. 47 ℃, with an average decrease of 5. 43 ℃ in the dry season and an average decrease of 2. 46 ℃ in the rainy season. The average annual water temperature downstream of the dams was close to the natural river water temperature, and the correlation with the surrounding measured air temperature was significant. Water temperature within the reservoir areas exhibited fragmented distribution, with maximum temperature differences observed at the heads of cascade dams.The Three Gorges Reservoir area exhibited phenomena of “cold retention” and “temperature retention”. The Nash efficiency coefficient between remote sensing inversion and measured data reached 0. 907, with a root mean square error of 1. 24 ℃,indicating high reliability of the result. [Conclusion] The continuity of river water temperature has been disrupted by cascade dam construction, significantly weakening the temperature transmission effect. The water temperature in the head of each reservoir area and the backwater area showed a significant correlation, but the correlation between the water temperature in the reservoir area and the water temperature discharged from dam was weak. Remote sensing method can effectively retrieve river surface water temperature, providing method ological support and theoretical references for river water temperature inversion and watershed water temperature management.

Keywords

river water temperature / cascade hydropower development / Landsat satellite / Google Earth Engine / hydropower station / Three Gorges Reservoir / water ecosystem