Abstract: Based on the related impact factors of the precipitation anomaly in summer in the Middle and Lower Reaches of the Yangtze River, 15 atmospheric circulation indicators as the independent variables and the hazard dangerousness level of drought-flood abrupt alternation index as the dependent variable were selected to construct the nonlinear and non-parametric classification and regression tree (CART) model for the hazard evaluation. The time series of data is from 1954 to 2010. Results showed that Arctic Oscillation Indices in summer (AOI_SU) and spring (AOI_SP), Asia Meridional Circulation Index in spring (AMCI_SP), and Asia Polar Vortex Area Index in spring (APVAI_SP) were the four main impact factors, which were proved to be suitable for the hazard evaluation of disaster about drought-flood abrupt alternation through the CART model. The type of the disaster (flood to drought or drought to flood) and the predicted dangerousness level under different diagnosis conditions can also be obtained from the model. The diagnosis thresholds and dangerousness level as follows: 1) When AOI_SU more than 1.11, the disaster of the quick turn from flood to drought is more apt to happen (dangerousness level is 1). 2) When the AOI_SU is less than 1.11 and the AOI_SP is more than -1.11, the disaster of the abrupt alternation from drought to flood will occur (dangerousness level is 6). 3) When the AOI_SU is less than 1.11, the AOI_SP less than -1.11, and the AMCI_SP more than 61, the same situation will happen as the one before this, and the predicted level is 5.5. 4) When the AOI_SU is less than 1.11, the AOI_SP less than -1.11, and the AMCI_SP less than 57, the abrupt alternation from flood to drought will happen, and the predicted level is 1.29. 5) When the AOI_SU less than 1.11, the AOI_SP less than -1.11, the AMCI_SP between 57 and 61, and the APVAI_SP less than 172, the abrupt alternation from flood to drought will happen, and the predicted level is 2.1. The corresponding indicators from 2011 to 2013 were selected to verify the final model through the comparison between the predicted values and actual levels, and the model was proved to be reliable for the close values. The CART proposed in this study provides a new method that can predict the hazard dangerousness level from the disaster of drought-flood abrupt alternation in the middle and lower reaches of the Yangtze River.

Key words: drought-flood abrupt alternation, hazard dangerousness evaluation, threshold diagnosis, CART, the Middle and Lower Reaches of the Yangtze River