Temporal and spatial distribution and hazard assessment of hail disasters during crop growth period in Xinjiang

Abstract: Hail is a serious meteorological disaster that occurs frequently in the growing period of crops in Xinjiang. Based on 2 629 records of 3 major disaster elements: the occurrence times, disaster area and economic loss of hail disaster in 1951－2017, the temporal and spatial distribution of hail disaster in Xinjiang was analyzed. Based on the entropy weight method (EWM), the disaster loss index which can comprehensively express the 3 disaster information was constructed. According to the gamma distribution function of the disaster loss index, the classification criteria of the damage grade of hail disaster were given objectively and quantitatively. Then the harm of hail disaster was classified and evaluated. The reasons for the regional, seasonal and interannual variation of hail disasters in Xinjiang were explained with water vapor pressure and air mass index. Air mass forecast model of hail disasters was established based on the logical regression. The results showed that: 1) The annual number of occurrences, the area and the economic losses of hail disasters in Xinjiang were 39 times, 75 485 hm2, and 109.96 million yuan. The hail disasters concentrated from May to August, i.e. in the crop growth period, and mostly in June; Bozhou, Kuitun-Manas Valley, Zhaosu County, Aksu Prefecture and Kashgar Prefecture on both sides of the Tianshan Mountains and adjacent areas had the most occurrences of hail disasters, large area and heavy economic losses; the largest areas affected by the single hail were Manas River Basin and Aksu Prefecture. The area that suffered the most economic losses by the single hail was the northern part of Bazhou and Kashgar Prefecture. The 3 major disaster factors all showed a linear increase trend. The propensity rates were 1.2 times/year, 2 589 hm2/year, and 3.36 million yuan/year. All of them showed abrupt changes in 1983, 1981, and 1979, respectively. The mean values were significantly different before and after the abrupt changes, and the mean value after the abrupt changes was 3.9 times, 5.7 times and 3.6 times of that before the abrupt changes, respectively. 2) Using the disaster loss index, the hail disaster was classified into 4 hazard levels: general, heavier, serious and extra serious. The extra serious disaster area was located in Tacheng, Bozhou, Kuitun-Manas Valley, Zhaosu County, Aksu Prefeccture, and Jashi. Aksu Prefecture ranked first, followed by Bozhou. In recent years, the hail disasters in the extra serious disaster-hit areas have been on the rise, requiring major defense. Multi defense lines should be added to the moving path of hail cloud, and the density of anti-aircraft guns and rocket launchers should be increased to carry out large-scale anti-hail catalytic operation for the continuous moving and developing hail clouds in order to minimize the harm of hail disaster to the maximum extent. 3) Vapor pressure and air mass index were good indications of the water vapor conditions and atmospheric stability when the hail disaster happened. In the areas, seasons and years where or when water vapor pressure and air mass index were high, more hail disasters occurred, and vice versa. Vapor pressure and air mass index were the main reasons for the impact of disasters. The number of occurrences of disaster and the area planted with crops both had an impact on the affected area. The contribution rates of the former 2 to the linear growth trend of the latter were 93% and 7%, respectively. The water vapor pressure, air mass index and planting area were used as the factors, and an annual forecasting model forecasting high or low frequency of the occurrence times of hail disaster was established. The fitting accuracy of the model was 73% and the fitting effect was good.