Evaluation level construction and analysis of risk on rice flood in Southwest China

Increasing of extreme precipitation has been witnessed in Southwest China in the past several decades. The frequent occurrence of extreme weather has aggravated the formation of regional flood hazards in this region. As the main crop in Southwest China, rice is generally considered to be seriously threatened by flood. Nowadays, researches on rice flood however cannot be connected with disastrous weather effectively, which has brought great difficulties for rice flood monitoring accurately and timely. Therefore, it is of great importance to establish the evaluation level of rice flood based on the metrological method so as to provide a technical support for rice flood monitoring, prevention and mitigation, and agricultural disasters' insurance management. Given above background, based on a data set of daily precipitation from 1961 to 2012 in 193 meteorological stations, associated with historical disaster and phenophase data of rice in Southwest China (Sichuan, Chongqing, Guizhou and Yunnan), the evaluation of the flood related to single cropping rice based on the metrological method was conducted in this study, and meanwhile, the rice flood risk was consequently estimated according to the rice flood evaluation indicators, in order to gain detailed information of the characteristics of rice flood disaster in Southwest China. Historical rice flood disasters were represented through the coupling with precipitation data. Afterwards, 27 rainfall sets of rice flood samples were built in the context of the combinations of different growth stages (transplanting-tillering, jointing-booting and tasselling-maturity), flood levels (light, moderate and heavy) and precipitation duration days (1 d, 2 d, ≥3 d). Kolmogorov-Smirnov (K-S) method was used in the distribution fitting of the rainfall sets, and 27 normal distribution functions were established. Each indicator of rice flood evaluation level (light, medium and heavy) was built using the statistical analysis, with the lower limit of 95% confidence interval as the critical threshold. The evaluation level of rice flood disaster has been validated by the reserved independent samples of rice flood. According to the rice flood indicators, the risk index of rice flood disaster was calculated based on the data in 193 metrological stations in Southwest China. The results were as follows: rice flood disaster caused by the rainfall was more intense in tasseling-mature stage, followed by the jointing-booting stage and transplanting-tillering stage. The precipitation threshold of rice flood in transplanting-tillering stage was 120, 130 and 150 mm respectively for the flood disaster lasting for 1 d, 2 d and ≥3 d. It became 150, 160 and 180 mm for jointing-booting stage, and 170, 200 and 230 mm for tasselling-maturity stage respectively. The rice flood level indicator could better reflect the actual level of damage caused by flood disaster in rice planting and be used in monitoring and assessing disaster damage in regional scale. The identification results based on rice flood level indicator were detected to be basically consistent with the historical occurrence levels of rice flood disasters, with 66.7% assessment of flood levels being completely consistent with the historical records, and calculation errors of all validation samples being within one level. High flood risk existed in transplanting-tillering stage and furthermore, higher rice flood risk from transplanting to tillering stage was mainly concentrated in southern area of Yunnan Province and Liuzhi Special Zone, Guizhou Province with the rice flood index of more than 1.5. Flood risk decreased in Guizhou and the northeast area of Yunnan, while it increased in mid regions of Sichuan Province in jointing-booting stage, compared to the early growth stage of rice. Southern Yunnan and mid Sichuan were detected with higher rice flood risk in tasselling-maturity stage. Overall, high-risk areas of rice flood in whole rice growth stage included south and northeast areas of Yunnan, southern Guizhou, and Chengdu, Meishan and Deyang in Sichuan Province. The evaluation level of rice flood firstly established in this paper could be used in the future monitoring and evaluation of rice flood in South China. The results of rice flood risk analysis could provide basic evidence for the prevention and mitigation of rice flood.