Water resource crises are an increasing threat to human survival and development. To reveal the nature of water resource issues under changing situations, the water resources system needs to be studied from a macro and systematic perspective. This report develops a water resources system into a water resources compound system that is constantly evolving under the combined action of the development, resistant, and coordination mechanisms. Additionally, the water quotient is defined as a quantitative representation of the sustainable development state of the water resources compound system. Four cities in China, Beijing, Fuzhou, Urumqi, and Lhasa, were selected as the study areas. The differences in the three types of mechanisms and the water quotient of the water resources compound system of each city in 2013 were compared. The results indicate that the different subsystems that comprise the compound system of a given area have different development mechanisms and resistant mechanisms. There are clear differences in the mechanisms and the water quotients for the water resources compound systems of different regions. Pertinent measures should be taken into account during integrated water resource management to improve the sustainable development status of regional water resources compound systems.

The process of urbanization takes up a lot of wetlands, profoundly changing the natural connection of surrounding river–lake systems, all the while causing serious damage to the environment of connected catchments. Urban systems and river–lake systems are not isolated and static, there is a relation between them which is constantly changing. Based on the idea of system research, the urban system is simplified into four subsystems: environment, infrastructure, social, and economic. These four components interact together, influencing the river–lake system to form a compound system. This paper aims to reflect the features and evolution laws of the compound system, by building a Collaborative Development Model to study the changing of the compound system in Wuhan, China over a 10-year period. The results show that by implementing the Donghu Lake Ecological River Network Engineering Project, the damaged river–lake system in Wuhan showed some improvement. However, in order to improve the sustainability of the compound system in Wuhan, the status of the river–lake system, social system and environment system, which are still comparatively substandard, should be constantly improved. The Collaborative Development Model could also be used in other cities and regions, to provide the basis for sustainable development.

We chose Jiangxi Province in China as the case study, which frequently suffered from mountain torrents disaster. We selected 14 indexes of flood risk assessment and random forest algorithm to construct a mountain torrents disaster risk assessment model. The optimal model which has overall accuracy of about 90.0% was used to map the risk zoning of mountain flood disaster in Jiangxi Province. Our mapping results indicated that the area of high risk and very high risk zones accounts for 33.0% of the total area of Jiangxi Province, and the regions with high risk are mainly distributed in that vicinity of mountains with high rainfall, heavy rainstorm and high potential of farmland production, etc.