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Hydrological droughts are closely related to the meteorological ones in the arid and semi-arid areas. It is of great significance to reveal the response relationship between the meteorological and hydrological droughts in drought warning and water resource management. In this study, the changing patterns of drought were determined in the Xiangjiang River Basin of western China using Copula functions. Daily precipitation data was collected from 40 meteorological stations, while the monthly runoff data was from three hydrological stations (Laobutou, Hengyang, and Xiangtan stations). The standardized precipitation index (SPI) and standardized runoff index (SRI) were adopted to describe meteorological and hydrological droughts. The Pearson correlation coefficient was used to determine the response time to drought. The run theory was used to identify, merge, and remove the drought events. The Mann-Kendall trend test and wavelet analysis were employed to reveal the trend and periodicity of drought in different seasons in the upper, middle, and lower reaches. Furthermore, seven widely used distribution functions were selected to fit the duration and severity sequences of meteorological and hydrological drought that were obtained from SPI and SRI. The optimal parameter estimation was obtained in each marginal distribution using the maximum likelihood. There were strong links between meteorological and hydrological droughts using three Archimedean Copula functions (Frank, Gumbel, and Clayton Copula) by coupling the response probability curve in a Bayesian network model. Finally, the model performance was evaluated using the Akaike information criterion (AIC) and ordinary least squares (OLS). The main results demonstrate: 1) The run theory [0.5, 0, −0.5] was achieved to better identify the drought events, with a matching rate of over 90% between meteorological and hydrological droughts. There was a two-month propagation time from the meteorological to hydrological drought in the whole watershed. The SPI and SRI effectively detected the drought events and better matched them with the historical drought statistics from the different periods (in 1963, 1986, and 2003). 2) The frequency of meteorological drought was higher than that of hydrological drought. The duration and severity of hydrological drought were greater than those of meteorological one, where the downstream area experienced the most severe hydrological drought, but the midstream is the most severely affected by meteorological drought. The difference was attributed to the higher evapotranspiration and high level of socio-economic development in the downstream. 3) There was a tendency toward wetness of the historical data in the basin, which was consistent with the remote sensing monitoring of terrestrial water storage anomaly. However, there was a distinct seasonal variation in the inter-annual wetness of the basin. The upper and middle reaches showed a drier trend in the autumn, while the middle reaches were in a trend toward dryness in the summer. Additionally, there were several outstanding oscillation periods of dry and wet, namely 3-5, 6-10, and 18-21 a. 4) The optimal joint distribution of meteorological-hydrological drought severity in the upstream and downstream area was the Gumbel Copula function, while the Frank Copula function was the optimal joint distribution for the rest. 5) The response probability of hydrological drought duration and severity increased with the increase of characteristic variables in the meteorological drought. There was a stable response probability of hydrological drought duration when the duration of meteorological drought exceeded a specific threshold. This study is of great significance in understanding the mechanism of drought in the Xiangjiang River Basin as well as guiding the drought prevention and fighting work in the region. [ABSTRACT FROM AUTHOR]

River water and sediment characteristics are the main index reflecting the surface hydrological cycle process and soil erosion, and is of great significance to water resources and water ecological environment in the basin. In recent years, influenced by human activities and natural conditions, soil erosion in many watersheds is serious. With the population and economic development, demands for resources and influence on environment by human activities are increasing, and the impact of water and sediment disasters on social and economic development is increasingly prominent and their consequences are closely related to people’s quality of lives. Human activities in Xiang River Basin have a significant impact on the change of runoff and sediment discharge in Xiang River. Through applying the Mann-Kendall test, Pettitt test, and double cumulative curve, we analyzed the relationships between human activities (e.g. water-soil conservations, reservoir constructions, etc.) and annual runoff, flood season runoff as well as non-flood season runoff and sediment transport of the mainstream and tributaries of Xiang River from 1953 to 2014. Among them, the main control stations of the main stream include the Laobutou station in the upper stream, Hengyang station in the middle stream and Xiangtan station in the lower stream. The main representative control stations of the tributaries include Ouyanghai station in Chongling River, Shenshantou station in Zheng River, Ganxi station in Mi River and Daxitan station in Lu River. Results indicated that annual runoff and flood season runoff at main-stream and tributaries stations showed a strong correlation (R2=0.83 in Xiangtan station) with sediment loads, while the correlations were weak in non-flood seasons after 1990. Before 1990, the runoff and sediment loads at main stream stations were basically fluctuating. After 1990, the runoff at main stream stations showed an increasing trend, while the sediment discharge showed a decreasing trend at those stations, and particularly mutated slightly at Laobutou station. Except for several stations, the mutation years and the variations of the runoff and sediment discharge at tributary stations were consistent overall. Water-soil conservations and reservoir construction were verified to be main factors leading to the decrease in sediment discharge in Xiang River basin. Compared to tributary basin, the water-soil conservations had a more significant impact on the reduction of sediment discharge in the main stream of Xiang River. Moreover a strong correlation between reservoir constructions and sediment discharge reduction was found at the main streams as well as tributaries. According to the abrupt change point of sediment transport in Xiang River Basin in 1987 and 1997, the long series of data were divided into three periods: Period A ( 1960-1987 ), Period B (1988-1996) and Period C (1997-2013). Without considering the influence of temperature change, the contribution rate of rainfall and human activities to the reduction of sediment discharge can be calculated by the cumulative slope change rates. Compared with period A, the contribution rate of human activities such as soil and water conservation and reservoir sediment interception to the reduction of sediment transport in periods B and C was 88.58% and 94.01%. That was, human activities were the main factors for the reduction of sediment transport. [ABSTRACT FROM AUTHOR]

In order to better understand the effect of the lake shrinkage on the flood process in a large lake,a quantitative analysis of the relationship between the lake shrinkage and the flooding in Dongting Lake was conducted,for which Jingjiang-Dongting Lake hydrodynamic model was applied and different scenarios of water area in the Dongting Lake were assumed. The results show that the water level and peak discharge will increase,and the peak time will occur earlier with the shrinkage of the lake. The water level in the West and South Dongting Lake would increase by around 2.0 m and the water level in the East Dongting Lake would increase by around 0.4 m if the area of Dongting Lake was shrunk from 2 670 km2 to 1 380 km2 when the 1996-type flood occurred in Dongting Lake. Moreover,the arrival time of the peak water level at Chenglingji station would be about 11 hours earlier,and the peak discharge would increase by about 4 300 m3/s. Hence,the flood disaster would be more severe in the Dongting Lake area if the lake continues to shrink from now on. In addition,the water surface slope changes a little in the West and South Dongting Lake. But the water surface slope increases a lot near the Lujiao station due to the water level in the East Dongting Lake is affected by both of the lake shrinkage and the water level in Yangtze River. [ABSTRACT FROM AUTHOR]