Study On The Changes In Rainfall Patterns In Xi’an

Since the 1980 s, Xi’an has entered a rapid and stable development stage of urbanization, and the city is expanding. Extreme rainfall and waterlogging occur frequently, which has higher demands for the urban rainwater discharge facilities. Among recent studies and focus on climate change, there are more concerns about the studies on rainfall patterns characteristics. To identify the changes in rainfall patterns will provide theoretical support for the design of urban stormwater drainage network.This paper collected the self-recording rainfall data of 54 years totally in Xi’an from 1961 to 2014, made segmented statistics of the average annual rainfall and number of rainfall days, heavy rain amount, storm amount and its number of rainy days, rainfall intensity and standardized rainfall index SPI and so on, and also carried out analysis on the multiple time scales and trends of the rainfall patterns in Xi’an. The main conclusions are as follows:(1)The average annual rainfall of 54 years in Xi’an is decreasing in the overall trend and the average annual number of rainy days is also decreasing in the overall trend, while the reduction rate of rainy days is greater than that of the rainfall amount. The concentration of rainfall is increasing. The extreme rainfall occurs mainly as heavy rain, which occurred in September at most, while storms often occur in July. The total average annual number of days of heavy rain and the rainfall amount are rising, and the rate of increase of heavy rainfall is greater than the increase of the number of days of rainfall. The average total number of days of storms and the average annual storm rainfall tend to increase; the rate of increase of the storm rainfall is greater than that of the number of days of storms.(2)The droughts in Xi’an are mainly light droughts and moderate droughts. The droughts after 1995 were mainly moderate droughts. The frequency of occurrence of light droughts and moderate droughts from July to September is relatively low compared with other months, which is consistent with the conclusion that months from July to September are the months in which rainfall occurs frequently; the segmented average annual SPI index distribution trend for 54 years is consistent with the segmented average annual rainfall changing trends in Xi’an.(3)The same rainfall intensity has different return periods at different times, and the return period of the same intensity of rainfall in recent years is declining in segment; under the same conditions of return period, the short duration rainfall intensity presents a cycle change of five times totally in which it rose first and then decreased, and the shorter the duration, the more obvious the periodic variation; the rainfall intensity of short duration and long-duration presents a decreasing trend with the increase of the years as a whole.(4)With the increase of sampling years, the precipitation process of the 60 min short duration rainfall in Xi’an has a moderate trend, and the rainfall pattern of the 120 min short duration rainfall was changed from bimodal peaks into single peak, while the concentration of rainfall presents an increasing trend, mainly occurring in the 5 ~ 30 min periods.