The Middle Reaches Of The Yellow River Drought And Its Impact On Runoff

Based a comprehensive introduction to the nature of the middle Yellow River, the drought and the drought-related research, the frequency of occurrence and characteristics of changes of droughts analyzed. According to the relationship between the drought index and runoffs, a formula which computes natural runoffs is established, and it is used to analyze the impact of the exacerbated droughts on the runoffs of the middle Yellow River. At the same time, by use of the drought-and-flood levels of the basin, some information of tree growth rings and their relationships with drought index, a formula computing the drought index of summer season of the middle Yellow River is proposed, and the corresponding series of drought index are extended to 1575 years. With the aid of some statistical methods, the historical evolution law of droughts in summer seasons of the middle Yellow River, and the trends of changes for the next 30 years are also examined. In addition, a discussion concerning the reinstate computation of the natural runoffs in literatures used for this research is also carried out. The main conclusions that can be drawn from this paper are the followings: (1) The probabilities that droughts may occur in middle Yellow River are relatively high; (2) The ratio between occurrences and non-occurrences of droughts is estimated to be 7 to 3 for early summer and fall, while this ratio is 6 to 4 for midsummer; (3) From 1955, only in year of 1997, there were three consecutive seasons that sever droughts occurred, while there were no special cases that floods occurred in three consecutive seasons (only in two consecutive seasons); (4) The probabilities that droughts may occur are increasing with respect to the incremental of years; (5) The probabilities that most sever and sever droughts occur in summer after 1986 are three times more than those before 1986, and the drought index of fall always keeps high values, which means there are no fall floods in the middle Yellow River; (6) In recent years, the tendency of droughts is becoming more and more serious, and droughts are now having significant influences on runoffs. The magnitude of influences from droughts on natural runoff volumes, which is proportional to that from human's activities in general, now reaches 5.22 billion m~3 from 1986, constituting 54.9% of the total influences. (7) For the next 30 years, except for recent drought years that may occur sporadically, and those that may dominate the early and middle ages of 2020's, the rest majority years are perhaps non-drought or flood years. Especially the years of middle and late ages of the century, and of the late ages of 2020's are considered in greater probabilities to be flood years. (8) There is a possibility that the natural runoffs of the middle Yellow River used in this paper are Small in figures.