Apart from the necessity of ensuring a west to east flow of water, the canal designers also concerned about many other important problems of city design. Through the canal system, the supply of water to the city for domestic and manufacturing use was assured. The waterways also acted as a drainage system capable of dealing with the heavy rainfall in this region of China where the annual average is 1,100 to 1,200 millimeters. (Human waste from the city was collected and used as fertilizer.) The presence of large areas of water which the canals provided also served to temper the environment. In the summer months, this was achieved by an increase in humidity brought about less by the direct presence of water—which, due to the effects of intense evaporation when day temperatures are highest, give of little moisture—than by the growth of vegetation in the proximity of the canals. The environment was thus tempered by trees and plants which provided a rich secondary source of moisture. This factor alone must have done much to stimulate the great garden tradition of the city which was to flourish during the later dynasties. The readily available and evenly distributed water supply was also a major advantage in the control and isolation of major fires. From the evidence of the construction of the canals as they exist today, it can be observed that the many steps or small quays which give access to the canal from each property are actually built into the canal retaining walls. Although it is probable that in the course of 750 years, many of the canals must have been rebuilt and repaired many times, it also strongly suggests that a large proportion of the actual property boundaries were predetermined as a part of the initial setting out of the basic ground plan of the city.