Threshold determination and hazard evaluation of the disaster about drought/flood sudden alternation in Huaihe River basin, China

Based on the related impact factors of precipitation anomaly referred in previous research, eight atmospheric circulation indicators in pre-winter and spring picked out by correlation analysis as the independent variables and the hazard levels of drought/flood sudden alternation index (DFSAI) as the dependent variables were used to construct the nonlinear and nonparametric classification and regression tree (CART) for the threshold determination and hazard evaluation on bimonthly and monthly scales in Huaihe River basin. Results show that the spring indicators about Arctic oscillation index (AOI_S), Asia polar vortex area index (APVAI_S), and Asian meridional circulation index (AMCI_S) were extracted as the three main impact factors, which were proved to be suitable for the hazard levels assessment of the drought/flood sudden alternation (DFSA) disaster based on bimonthly scale. On monthly scale, AOI_S, northern hemisphere polar vortex intensity index in pre-winter (NHPVII_PW), and AMCI_S are the three primary variables in hazard level prediction of DFSA in May and June; NHPVII_PW, AMCI_PW, and AMCI_S are for that in June and July; NHPVII_PW and EASMI are for that in July and August. The type of the disaster (flood to drought/drought to flood/no DFSA) and hazard level under different conditions also can be obtained from each model. The hazard level and type were expressed by the integer from - 3 to 3, which change from the high level of disaster that flood to drought (level - 3) to the high level of the reverse type (level 3). The middle number 0 represents no DFSA. The high levels of the two sides decrease progressively to the neutralization (level 0). When AOI_S less than - 0.355, the disaster of the quick turn from drought to flood is more apt to happen (level 1) on bimonthly scale; when AOI_S less than - 1.32, the same type disaster may occur (level 2) in May and June on monthly scale. When NHPVII_PW less than 341.5, the disaster of the quick turn from flood to drought will occur (level - 1) in June and July on monthly scale. By this analogy, different hazard types and levels all can be judged from the optimal models. The corresponding data from 2011 to 2015 were selected to verify the final models through the comparison between the predicted and actual levels, and the models of M-1 (bimonthly scale), M-2,M- and M-3 (monthly scale) were proved to be acceptable by the prediction accuracy rate (compared the predicted with the observed levels, 73%, 11/15). The proposed CART method in this research is a new try for the short-term climate prediction.