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City is an important foundation for human development. Natural disasters, whose impact on cities was influenced by disaster risk, urban vulnerability and resistance, have always posed serious threats to human development. It is important to identify the key factors in urban disaster preventing and urban securing to reduce the losses of disasters and ensure the urban safety. However, it is difficult to assess the urban safety comprehensively, as the characteristics of disasters and cities varied a lot, resulting in the limited efficiency of urban disaster prevention and management. As a result, we built an index system to improve the urban safety assessment method. By taking six types of urban disasters (urban flood and waterlogging, drought, low temperature, sandstorm, earthquake, and rockfall, landslide and debris flow disasters) into consideration, based on the open access data and the convenient methods to calculate weights and indices, this evaluation system is of applicability and simplicity, and it can overcome the low efficiency shortcoming of the current assessment methods. The index system was then applied to 596 cities in the mainland of China to validate its effectiveness and accuracy and understand the situation of urban safety of China. The interaction mechanisms among the three characteristics of urban disasters (disaster risk, urban vulnerability and resistance) were analysed qualitatively and quantitatively, and the ‘compensate effect’ between urban vulnerability and resistance was found. According to the result of the urban safety assessment, the safety situations of most cities in China were not satisfactory, but there was great potential to improve them. Since the major natural disasters and safety degrees varied in different cities, these Chinese mainland cities were classified into six different regions based on our results, and the key points of disaster management in each region were pointed out, in order to help the government to promote Chinese cities’ safety.

As the second largest lake in Africa, the extent of Lake Chad has over 50% seasonal variations with large parts of water under aquatic vegetation. Although a great area shrinkage since the 1960s divided the lake into two parts, namely, the northern and southern basins, it still feeds millions of people from four countries around it. Given sparse in situ measurements, remote sensing with seamless spatial coverage has served for acquiring the long-term area series of Lake Chad. This study retrieves the open (unvegetated) surface water area series of Lake Chad and the total inundation area, including open water and water under macrophytes, combining multiple remote sensing data from around 1980 to 2020 with satellite-based water level and in situ observations at the Bol gauge as validation and auxiliary. Results show that the total inundation area continuously recovered at a rate of 145 km2/year from 1982 to 2020 with large annual fluctuations. Approximately two- to three-month time lags between the open surface water and the total inundation area reveal the hydrological asynchrony of Lake Chad, which can be attributed by the large area of rooted macrophytes. The overspill of the southern surface water flowing over the Great Barrier into the northern lake also has a one-month lag with the open surface water of the southern lake. The Google Earth Engine-based online application can be easily shared for use and data downloaded by the general public, and the open code can be adjusted for any other lakes worldwide.

The Liulin karst system is typical of hydrogeological systems in northern China, with a group of springs as the dominant way of regional groundwater discharge. Surface water leakage into groundwater has been observed in six sections of the rivers in the study area. To extract hydrogeological information from hydrochemical data, 29 water samples were collected from the system. On a trilinear diagram, most of the groundwater samples are clustered around the surface waters, indicating the effect of leakage on their chemistry. R -mode factor analysis was made on seven variables (Na, Ca, Mg, SO 4 , Cl, HCO 3 , and NO 3 ) of the samples and three principal factors were obtained: the F 1 factor is composed of Ca, Mg and SO 4 , the F 2 of HCO 3 and NO 3 , and the F 3 of Na and Cl. These factors are then used as regionalized variables in ordinary Kriging for unbiased estimates of the spatial variations of their scores. Considering regional hydrogeological conditions, the hydrogeological implications of the spatial distribution of the factor scores as related to the effects of the surface leakage are discussed. To evaluate the geochemical processes, the geochemical modeling code NETPATH was employed. The modeling results show that mixing commonly occurs in the system and dolomite dissolution is more important than calcite dissolution. Dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution) is locally important, in the western flank of the system where the surface water leakage has the least effect.