Drought characteristics in the Shiyang River Basin during the recent 50 years based on a composite index

Abstract

The Shiyang River Basin in Gansu Province is located in a semiarid area of northwestern China. In the context of global warming, the intensity and frequency of the drought in the area have experienced significant changes, which brings serious influences to agriculture in the area. The area of the drought in Shiyang River Basin was expanding in the past 50 years. So the drought has become one of the key and hot topics in climate change research, especially in a semiarid area of northwestern China in recent year, and it is necessary to choose a more effective method to analyze temporal and spatial distribution of drought. Based on the observed data of 5 meteorological stations from 1962 to 2010, the method of a series of mathematical statistic theory, GIS spatial analysis techniques and a composite index (CI) of the meteorological drought was applied to analyze the temporal and spatial characteristics of the drought in the area. According to the national standard "Classification of Meteorological Drought" (GB/ T20481-2006), CI was calculated by using the standard precipitation index of the last 30 days (equivalent to monthly scale), the last 90 days (equivalent to seasonal scale) and the relative moisture index of the last 30 days. According to the day-to-day CI index, the drought process of the stations and the drought events of different periods during the recent 50 years were calculated, and the occurrence frequency, coverage area intensity, and annual mean days of the drought in each station were analyzed to reveal the temporal and spatial pattern of meteorological droughts in the Shiyang River Basin. The results are as follows: (1)The seasonal occurrence frequencies of the meteorological drought in the area generally decreased from north to south but it was also different. The highest occurrence frequency was in summer, and the lowest was in winter; The minimum frequency in four seasons all occurred in the Wushaoling Station. However, the maximum in spring, summer and winter was observed in the Minqin Station with exception of the Wuwei Station in autumn. (2) The large-area drought in the Shiyang River Basin occurred mostly in summer and autumn but the least one in winter during 1962-2010. Meanwhile, there are 22 years and 11years of drought in summer and autumn, respectively,whereas there is 3 years in winter. (3) The drought days of different grades which was negatively correlative with precipitation distribution reduced from north to south in general, and were higher in summer and lower in winter in the Shiyang River Basin. (4) In the Shiyang River Basin, drought is mainly controlled by the Eastern Asian Monsoon and Southwest Monsoon, while the runoff factors should also be considered. This study can reflecet the fact and provide a reference for developing appropriate measures to reduce droughts in the Shiyang River Basin.