This is evident in Table 4. The rate of guaranteed water supply could therefore be increased by combining the utilization of river water over a larger area. Our data are presented in Table 5, which shows that abundant, average or deficient water years have occurred simultaneously in all four rivers only five times in forty-seven years. Of these, four were average years and one was abundant. In no years was there a deficiency in all four basins.
Annual runoff is thus basically nonsynchronous in the region's major rivers.
We have also drawn up some simulated annual runoff differential mass curves which are presented in Figure 5. These also make it readily apparent that there is a lack of synchronization which would be beneficial to interbasin water transfer. Differential Mass Curves of the Four Basins Annual variations in evaporation are seldom studied. Since the percentage of precipitation which evaporates is over 80 per cent in the Huang, Huai and Hai basins, we may safely assume that CV E is actually very close to CV P . There is quite a large seasonal variation in water balance elements on the North China Plain.