| The Pearl River located in southern China,which is the second largest river in China.The Pearl River Basin has the characteristics of wide basin,complicated topography,changeable climate and unbalanced regional economic development.The southeast costal area,especially the Pearl River Delta area,has a much higher economic development speed than other areas.However,along with the development of economy,the terrestrial ecosystem has been damaged and degraded.Net Primary Productivity(NPP)is an important factor in evaluating carbon balance,which plays a key role in evaluating ecosystem structure and function.Therefore,it is of great significance to study the spatiotemporal variation and its influencing factors of NPP in The Pearl River Basin.Based on CASA(Carnegie Ames Stanford Approach)model theory,this dissertation implemented the NPP estimation for the Pearl River Basin using the data products of MODIS,and the datasets of temperature,rainfall and sunshine time.The main research contents and results of this dissertation are as follows:(1)Combined with the geographical factors,environmental factors and vegetation types of the Pearl River Basin,the calculation of the two parameters of CASA model,photosynthetic effective radiation and light energy utilization ratio was improved.Eventually,the estimation of the NPP in Pearl River Basin from 2004 to 2018 was realized.Through the accuracy estimation,the results of model estimation can be used as the basic data of the NPP analysis in the study area.(2)Based on the built NPP spatio-temporal dataset of Pearl River Basin,and the difference analysis and unitary linear analysis methods,the spatial and temporal variation of NPP in the basin and its sub basins were analyzed.The value of NPP in the Pearl River Basin were high in the east and low in the west,high in the north and low in the south.From 2004 to2018,the average NPP of the whole region was in 959.43g C/(m~2·a),and the average annual NPP ranged from 795 to 1078 g C/(m~2·a).The average annual NPP of each sub-basin was ranked from large to small as Dongjiang River Basin,Hanjiang River basin,Beijiang River Basin,Guangdong-Guangxi-Hainan coastal river basin,Xijiang River basin and the Pearl River Delta basin.The NPP of the Pearl River basin and its sub-basins between 15 year showed a fluctuating rising trend.The variation trend of NPP in the year was down-up-down,with the maximum value of NPP appeared in August and the minimum value in February.In terms of spatial variation,the NPP of vegetation in the Pearl River Basin increased by 94.23%from 2004 to 2018,in which the significantly increased area accounted for 57.23%of the entire basin.The spatial variation trend of each sub-basin was consistent with that of the whole basin.(3)Based on the Maximum Entropy(Max Ent)model,the distribution point data of NPP was taken as the training sample data,and the selected Normalized Difference Vegetation Index(NDVI),meteorological data and soil data were taken as environmental variables to screen the leading impact factors of NPP.The results showed that the four leading factors affecting NPP distribution were NDVI,annual solar radiation,annual rainfall and average annual temperature.Among them,the contribution rate of NDVI to NPP distribution was the highest and showed an increasing trend.(4)Based on Pearson correlation coefficient method,the correlation between NPP and the four leading factors were analyzed one by one.The results showed that NPP and NDVI in the Pearl River Basin were moderately positively correlated,with a correlation coefficient of0.65.The NPP had no correlation with the changes of the other three main factors,but showed different correlations within the small range.Among them,the NPP was positively correlated with solar radiation in the coastal basins of Guangdong,Guangxi and Hainan,the western Xijiang River Basin and some parts of the Pearl River Delta.The NPP in the north of the Xijiang River Basin and most of Hanjiang River Basin were positively correlated with the average annual temperature. |
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