Process Grade Indicator Construction and Evolution Characteristics of Late Rice Flood in Hunan

Focusing on the late rice in Hunan, daily precipitation data during 1961-2010 from 68 meteorological stations and phenophase data from 17 agrometeorological observation stations in Hunan are analyzed, and 125 late rice flood process precipitation samples are recognized, including disasters of 3 growth stages (transplanting-tillering, jointing-booting, blooming-maturity) and 3 flood grades (light, moderate, severe). Quantile-quantile plot, Shapiro-Wilk test and Student's t-distribution are employed for the suitability test and critical value calculation of flood process precipitation samples from each flood disaster sample sets. And then, late rice flood disaster grade indicators during different growth periods are determined by critical values and verified by independent samples. Temporal-spatial evolution characteristics of late rice flood disaster in Hunan are analyzed based on the constructed flood level indicators, M-K test and ArcGIS. Results show that, there is high consistency between indicator verification result and history record, indicating the constructed flood level indicators can reflect the actual late rice flood disaster situation. Thresholds of the same flood grade in different growth periods are different, ascending from transplanting-tillering stage, jointing-booting stage to blooming-maturity stage. Main occurrence years of Hunan late rice flood are 1961, 1969, 1980, 1987, 1988, 1994 and 1997. Late rice flood disaster is most serious in the 1960s and the 1990s, and the total flood frequency mutated in 1994 and declined afterwards. The total flood frequency of late rice is highest in transplanting-tillering stage, followed by jointing-booting stage, and blooming-maturity stage is the lowest. Light flood has the highest incidence rate during blooming-maturity period, while moderate and severe flood both has the highest incidence rate during jointing-booting period. The total flood frequency during transplanting-tillering and blooming-maturity periods decrease after the year of 2000, but are still similar to the 1990s during jointing-booting stage. The flood-prone areas are located in Chenzhou and Yueyang, severe floods mainly located in mountain area in Loudi and Chenzhou, and areas with relatively less flood are mainly located in central and southern Hunan (the Hengshao Basin). The occurrence of late rice flood disaster gradually decreases from the 1960s to the 1980s, then increases in the 1990s, and decreases in the 2000s. The flood-prone area of each grade and total all moves from the north to the south in Hunan these years.