Conclusion and Discussion
The rivers nearby were strongly considered in the building of cities by Chinese ancestor due to its convenient water supply. Accordingly, most Chinese early cities were built along the major rivers basins, especially the Yellow River basin and Yangtze River basin. Nevertheless, there are high flood risks when a city is built near a river for the purpose of convenient water supply. Therefore, it had a major impact on the cities at any period because flood was the major disaster to ancient cities. Consequently, the logic of design and construction of the urban water system was to build a water cycling system in the city, and ensure water cycling which flows in and out of the city. Hence, a perfect situation was the seasonal balance of water storage and drainage in the city. Therefore, when a city was built with water supply through river or canal, the drainage river and sluice system inside the city will be built simultaneously. After water was introduced into the city and cycled inside the city for use, it will be channeled out of the city into that same river source or other rivers. Through this water cycling system, the city met the demands of water supply and reduces the risk of flood. Notwithstanding, not all risk was saved, but was effectively achieved at this point which had been evidenced through centuries. Accordingly, the design and construction of urban water cycling system was one of the most important wisdom of the ancient urban water management of China.
Systematic design and construction for multipurpose usage was another remarkable lesson from the ancient urban water system. Mentioned above, the ancient urban water system is not just used for water supply, but was also used for water storage, personal use, sluice, drainage, transportation, defense and waterscape, etc. Therefore, an excellent city is a city with a good design and construction of urban water system whose water resource is multipurpose e. g. for water supply, storage, drainage, transportation and urban waterscape, etc.（Guo, 2013）. Accordingly, a model of urban water system was created in the Han Dynasty 2000 years ago, and was followed by other popular cities in subsequent centuries.
Finally, it is also worthy to note that many ancient urban water system were constructed strategically in meeting the sustainable demand. This was the reason many urban hydraulic engineering is still functioning today.
The lessons learnt from the ancient urban water system design and construction is that a perfect and sustainable urban water system not only depends on the engineering solution, but on the logic of the water management. It is remarkable that the ancient urban water system was constructed logically, systematically，and for a sustainable use that adapts with the local environment. Hence, the situation of water resource made the urban water system to become a multi-purpose water cycling system in the city. It also gave the city a nice waterscape. The situation of water was remarkably improved in many cities through the water system construction. Thus, some of them even improved from being cities with poor to cities with rich water. For example, the Kunming City had deficiency in water supply before the 13th Century, but they became a city with good water conditions at 19th Century. Hence, it was called“Water City”, by the construction of urban water system which also followed the model of Han Chang' an City（Zheng, 2013）.
Additionally, an ancient Chinese urban water system under construction sys tematically and logically which was adapted with the local water situation, performs the function of water supply, storage, use, transportation, drainage, urban waterscape, etc. Also, it formed an infrastructure in the city, of which most of the old Chinese city was developed based on such infrastructure, even until today.
Unfortunately, along with current urbanization and urban reconstruction，the old urban water system was suspended in many cities. The new urban water system was reconstructed in a way such that, there was no water cycling, single proposal, which even destroyed the old water system. For example, the moats and urban rivers were destroyed in many cities before 1980s. A paradigm was the case in Shaoxi City of Zhejaing Province. Hence, there were 17 urban rivers destroyed for the construction of the urban road in 1952—1977. Furthermore, the major river called Hu He river which crosses the town also serves as an underground river that have caused the urban drainage difficultly in this city（Yan，2012）. Anyway, more and more evidences shown that current urban water problems, especially urban storm disaster, are obviously correlated with the changes of the old urban water system. This was adapted particularly to the local environment in constructing a water cycling system naturally. Consequently, a sustainable urbanization requires sustainable water system design, and its construction depends on the local environment. Therefore, the lessons learnt from history are very valuable in establishing a perfect solution to water supply systems.
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（原载英国《全球环境问题》2015年第1期）