The Qingjiang River Basin, which is 423 km long in the Hubei province, China, is the first large tributary of the Yangtze River below the Three Gorges. The Qingjiang River Basin surface water area monitoring plays an important role in the water resource management strategy and regular monitoring management of the Yangtze River watershed. Hydropower cascade exploitation, which started in 1987, has formed three reservoirs including the Geheyan reservoir, the Gaobazhou reservoir, and the Shuibuya reservoir in the midstream and downstream of the Qingjiang River Basin. They have made a great impact on surface water area changes of the Qingjiang River Basin and need to be taken into account. We monitor the Qingjiang River Basin surface water area changes from 1973 to 2010. Ten scenes from the Multispectral Scanner System (MSS), seven scenes from the Thematic Mapper (TM), and two scenes from the Enhanced Thematic Mapper Plus (ETM+) remote sensing data of Landsat satellites, the normalized different water index (NDWI), the modified NDWI (MNDWI), and Otsu image segmentation method were employed to quantitatively estimate the Qingjiang River Basin surface water area in the 1970s, 1980s, 1990s, and 2000s, respectively. The results indicate that the surface water area of the Qingjiang River Basin shows a growing trend with the hydropower cascade development from the 1980s to the first decade of the 21st century. The study concluded the significance of human activities impact on surface water spatiotemporal distribution. Surface water accretion is significant in most parts of the Qingjiang River Basin and might be related to the constructed cascade hydropower dams.
River discharge is an important parameter in understanding water cycles, and consistent long-term discharge records are necessary for related research. In practice, discharge records based on in situ measurement are often limited because of technological, economic, and institutional obstacles. Satellite remote sensing provides an attractive alternative way to measure river discharge by constructing an empirical rating curve between the parameter provided by remote sensing techniques and simultaneous ground discharge data. River width is a popular parameter for constructing the empirical curve, since change in river discharge can be represented by a change in river width. In some rectangular channels, however, river width does not change significantly with river discharge, so an alternative parameter is necessary. We analyze a novel technique using river island area as an indicator of discharge. A river island often has a flat terrain, and its area decreases with higher discharge. This technique is validated by three river islands in the Yangtze River in China. All 61 remotely sensed images acquired by the HuanJing (HJ) satellites from 2009 to 2010 were correlated with corresponding in situ discharge of the nearby Zhicheng hydrological station. The performance of fitted curves for inferring river discharge is validated using 36 HJ images taken in 2011, and the influence of remotely sensed imagery and river islands is discussed. All three river islands can be used as indicators of river discharge, although their performances are much different. For the river island with the best result, the mean accuracy of the estimates is less than 10% of the observed discharge, and all relative errors are within 20%, validating the effectiveness of the proposed method.
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