基于MODIS的长江中游河段悬浮泥沙浓度反演

MODIS-BASED RETRIEVAL AND CHANGE ANALYSIS OF SUSPENDED SEDIMENT CONCENTRATION IN MIDDLE YANGTZE RIVER

监测和预报悬浮泥沙浓度的沿程分布和时间变化,无论是对于河流水利工程还是河流生态和环境保护都具有重要意义。卫星遥感反演同步性好、速度快、周期短,可以实时和全面地观测大尺度悬浮泥沙分布。旨在建立基于中分辨率成像光谱仪(MODIS)影像的长江中游河段悬浮泥沙浓度反演模型,并利用建立的模型反演2002～2009年长江中游河段丰水期的悬浮泥沙浓度,分析其在时间和空间上的变化特征。研究揭示,MODIS Terra影像红波段与悬浮泥沙浓度具有显著的相关性(R2=0.877,n=125,RMSE=40.57mg/L),可以用于长江中游丰水期悬浮泥沙浓度的反演。长江三峡工程经历三次蓄水,坝下游宜昌至汉口段悬浮泥沙含量显著减少,荆江河段、洞庭湖及城陵矶至武汉江段下降最为显著。洞庭湖来水来沙是长江中游城陵矶以下江段主要的悬浮泥沙来源之一,从预测结果可知,洞庭湖来水在城陵矶汇入长江后与江水混合以至形成数十公里的混合带,至洪湖以下江段逐渐混合均匀。

Monitoring and forecasting of the distribution and temporal change of SCC （suspended sediment concentration）is very important both for river water conservancy project and river ecology and environment protection. Retrieving SCC by remote sensing can compensate for the deficiency of the traditional time- consume sampling method which acquires discrete data at high cost, and it provides information of SCC nearly in real-time at large-scale. So far, the Three Gorges Project has been used nearly ten years, meanwhile the impoundment of the reservoir will fundamentally change SCC of the suspended of the Yangtze River water in time and space, its influence is a matter of concern. This study took the middle reaches of the Yangtze River as the study area. It aimed to develope the Moderate-resolution Imaging Spectroradiometer （MODIS） image-based SSC retrieval model for Middle River, use the model to predict the SSC of Middle Yangtze River during the flood period from 2002 to 2009, and analyze its spatial and temporal change characteristics. The results revealed that., the red band of MODIS Terra was significantly related to the SCC,and it may be applied to retrieve the SSC of Middle Yangtze River during the flood period. This study selected three images. After inversion,three sediment concentration profiles from 2002 to 2009 were obtained, in order to contrast SCC of the Yangtze River in wet season before and after the impoundment of the Three Gorges Project. During 2002--2009 years, the SCC from the Yangtze River downstream of the dam was reduced significant. In addition, the sediment from the Yangtze River into the Dongting Lake was also reduced significantly. SCC mainly concentrated in the 300 -- 400 mg/L from Yichang to Chenglingji in the period of wet season in 2002,and it reduced to 150--200 mg/L or so in 2009. During 2002--2009 period,24.2% of the total area of the concentration reduced by 100--150 mg/L,48.4% of the total area of the concentration reduced by 150--200 mg/L. The Dongting Lake inflow was one of the most important sources of suspended sediment in the middle reaches of the Yangtze River under Chenglingji. It contained more suspended solids than that of the Yangtze River as we can see from the forecast results. It formed dozens of kilometers mixed area in the Yangtze River when the Dongting Lake inflow got into the Yangtze in Chenglingji and mixed with the river. This study proved that MODIS data was feasible for inland water suspended matters monitoring. The linear model was simple and feasible. The correlation coefficient R2 of the model was 0. 877,and the root-mean-square error was 40.57mg/L. Thus, it can be promoted as a suspended matters romote sensing retrieval method. The model is area-specific and it needs further tested to identify whether it can be applied to other areas. These results would serve as a reference for studies of the evolution and regulation of rivers and lakes in the lower Yangtze reaches.