| Based on the spring precipitation data of 12 observatories in 1961-2004 in Qinghai province,the variation characteristics of spring precipitation on Qinghai plateau is analyzed,and the time-frequency characteristics of spring precipitation over Qinghai agricultural area is also analyzed by using linear trend analyze method and wavelet analysis method.The results show that the spring precipitation over the east area of Qinghai plateau had outstanding wavy character;its variation was large in the 1960s and 1990s and little in the 1970s and 1980s.The spring precipitation over the east area of Qinghai plateau generally increased weakly.From the 1980s to now,the spring precipitation decreased weakly,but increased in the rest of time.For the large time scale of equal to or more than 10 years,the characteristic of dry/wet-vicissitude in spring over the agricultural area of Qinghai was very significant in last 44 years,and 6 dry/wet-vicissitude periods occurred during these years,the replace point appeared in 1964,1975,1983,1993,and 2001,the period of 10-12 and 16 years is obvious.The variation of spring precipitation for the time scale below 10 years over the agricultural area of Qinghai in last 44 years was rather complicated,the replace point and the relative wet/dry-vicissitude periods got more often,the period of 6-8 years was very clear.Heavy drought stage usually appeared when the whole oscillation period was heavy negative phase-shift.In recent years,the spring precipitation change over the east area of Qinghai plateau has got more often in both the large time scale and the short time scale. |