The regular pattern and underlying mechanisms of seawater intrusion in the Modaomen channel in the Pearl River Estuary of China.

In recent decades, the frequency and severity of inland river seawater intrusion has increased in the Pearl River Estuary (PRE) of China. Based on time series of water salinity data, river discharges, tides, and wind records for the period 2003-2015, this study aims to examine the regular pattern and underlying mechanisms of the seawater intrusion in the Modaomen channel of the PRE. Toward this end, the autocorrelation and spectral analysis are used to quantify the periodicity of runoff, tide, and water salinity and to infer the regular patterns of seawater intrusion. The Pearson correlation and the LMG metric are used to examine the effects of influencing factors on seawater intrusion. Results indicate that seawater intrusion in the Modaomen channel has distinct annual (369 days) and half-monthly (14.8 days) cycles, corresponding well with annual and half-monthly tidal cycles. Spatially, the frequency, severity, and periodicity of seawater intrusion decrease with increased landward distance away from the channel mouth. The prevention of seawater intrusion by incoming river flow becomes more remarkable as freshwater flows downstream. Shifts in wind speed and direction can strengthen or weaken seawater intrusion, depending on if wind is in the opposite or same direction of river flow. Further analysis using the LMG metric suggests that river discharge plays a dominant role in governing seawater intrusion. Overall, river discharges, tides, and wind are three major forces triggering the inland river seawater intrusion and account for, respectively, 46%, 15%, and 15% of the variation of water salinity in the Modaomen channel. The study is featured in not only identifying the major factors in driving seawater intrusion but also quantifying their relative contribution. Thus, the findings are important for water resources manager to predict and manage inland river seawater intrusion in the PRE of China.

The variation of vegetation greenness and underlying mechanisms in Guangdong province of China during 2001-2013 based on MODIS data.

This study explored the spatiotemporal variation of vegetation greenness and the mechanisms underlying this variation in the subtropical region of Guangdong in China during 2001-2013 to obtain a better understanding of vegetation response to climate and land use/cover changes in warm-humid regions. Satellite-based vegetation indices, land use/cover data and observed weather records during 2001-2013 were used. Univariate and multivariate linear regressions were conducted to quantify the trends and variations in vegetation greenness and the relationships with climate and land use/cover changes. The results indicated that the annual mean greenness trended upward significantly in eastern and western Guangdong and downward significantly in northern Guangdong. The patterns of significant positive or negative relationships between vegetation greenness and climatic factors were observed at the sub-regional scale. In addition, our results showed that (i) vegetation greenness in Guangdong was more sensitive to changes in temperature than in precipitation, (ii) spring temperature had an important time-lag effect on seasonal mean greenness in the following summer and autumn, and (iii) winter greenness depended largely on vegetation growth in the previous autumn. Furthermore, the conversion of grasslands and croplands to evergreen forests resulting from afforestation increased the vegetation greenness in eastern and western Guangdong. Overall, our results suggest that afforestation plays a dominant role in increasing vegetation cover/greenness in Guangdong whereas the effects of land use/cover change on vegetation growth are subject to climatic conditions. Thus, a better understanding of terrestrial vegetation dynamics requires considering both climate and land use/cover changes.