Human activities alter Yellow River flow — Captured from satellite data by University of Tokyo group

A research group led by Graduate Student Yuki Ishikawa (at the time of research) of the Department of Civil Engineering at the Graduate School of Engineering and Associate Professor Dai Yamazaki of the Institute of Industrial Science at the University of Tokyo, and others applied a method for estimating river discharge from satellite observation data (satellite-based discharge estimation) in a spatially continuous manner to the main channel of the Yellow River in China, revealing that changes in discharge due to human activities can be captured from space. The findings were published in Geophysical Research Letters.

In basins where in situ discharge observations are limited, it is not easy to determine where along the basin, from the upper to lower reaches, where impacts from human activity are occurring. Satellite remote sensing offers a means to overcome these spatial constraints, but conventional satellites can observe surface features yet have difficulty measuring river discharge, which requires information below the water surface.

In recent years, in anticipation of using two-dimensional water surface elevation data from the SWOT satellite, methods have been developed to estimate river discharge solely from satellite observation data (satellite-based discharge estimation). However, previous research on satellite-based discharge estimation has focused mainly on time-series evaluation compared with in situ observation data, and it remained uncertain whether variations in river discharge due to human activities could be detected.

The research team focused on the advantage of enabling wide-area discharge estimation without using ground observation data and attempted to estimate discharge at 668 continuous locations from upper to lower reaches along the Yellow River main channel based on river width extracted from Landsat satellite images.

As a result, reasonable spatial distributions of river discharge were estimated, and in the upper reaches, the trend of changes in satellite-based discharge within certain sections matched observed discharge, with a clear decreasing trend in discharge particularly evident in areas with active irrigation. Additionally, increases in satellite-based discharge were observed at confluence points with major tributaries. These results suggest that satellite-based discharge estimation can capture discharge variations due to both anthropogenic and natural factors.

Furthermore, by artificially incorporating the effects of water withdrawal due to human activities in the initial estimates of satellite-based discharge, it was revealed that overestimation of discharge could be suppressed in lower reaches where human activity impacts are concentrated, thereby improving the estimation accuracy of satellite-based discharge. On the other hand, it was also confirmed that in sections where river width changes are constrained by levees, as in the lower Yellow River basin, it is difficult to capture spatial discharge changes through satellite-based discharge estimation based on river width. Future work is considering the use of Surface Water and Ocean Topography (SWOT) satellite data that observes water surface elevation in addition to river width. This discovery revealed that the impacts of human activities can be captured from satellite data for rivers worldwide. This can serve as a foundation for understanding actual water use around the world and evaluating the sustainability of water resources and agriculture. Furthermore, it is expected to contribute to the development of plans and policies for river flood and water resource management and ecosystem impact monitoring in basins with limited in situ observations, such as in developing countries and mountainous regions.

Ishikawa commented: "When I learned about the advanced method of estimating discharge solely from satellite observation data, my motivation was to see if I could apply this method to capture the impacts of human activities on rivers, which I had been interested in all along. We live in an age overflowing with data, but basins with adequate hydrological observation data are in the minority, so I believe there is great value in being able to understand anthropogenic impacts on rivers in ungauged basins. In the future, I would like to use the newly available SWOT satellite data to elucidate the interactions between human society and terrestrial water cycles globally from space."

Journal Information
Publication: Geophysical Research Letters
Title: Evaluation of a Width-Based Satellite Discharge Algorithm for Detecting Longitudinal Flow Changes in a Human-Regulated Continental River Basin
DOI: 10.1029/2024GL114191