Emergence From the Complex Interactions of Epidemics, Droughts, and Floods: Insights From Ming and Qing Dynasties of China During 1470-1911.

With the many health implications of droughts and floods known, and the many adverse secondary and tertiary effects of the Covid-19 pandemic still lingering, it is important to study the complex interactions of epidemics, droughts, and floods. To gain insights into this issue, we have constructed epidemic, drought, and flood indices for the Ming and Qing dynasties of China in a period of more than 400 years. Using adaptive fractal analysis, we find that the time series of epidemic, drought, and flood indices possess long-range correlations of different degrees in different regions of China. More importantly, the scaling behavior for the cross correlations between the epidemic and the drought indices in Northern China is characterized by a non-stationary emergent behavior rather than by a long-range correlation, while this scaling behavior is close to the boundary of stationarity and non-stationarity in the Central China. This scaling is up to about 16 years, highlighting that on average, outbreak of large-scale epidemics may occur not shorter than once every 32 years. Interestingly, the emergent behavior can be characterized as a Zipf's law for the ranked size of the epidemics, mostly in the Northern China, and sometimes also involving some regions in the Central China.