Multi-taper method of analysis of periodicities in hydrologic data

Many investigators have reported the existence of periodicities, other than the annual and semi-annual, in hydrologic and climatic data. These results are based on the spectral analyses of data. Spectral analysis has several well known drawbacks, to overcome which the multi-taper method (MTM) of spectral analysis was developed. The MTM has the additional advantage that the spectral components can be tested for statistical significance. In the present study, the MTM is used to investigate the periodicities in the annual streamflow, rainfall, temperature and tree-ring data in the midwestern United States. The results indicate the existence of weak periodicities of periods less than 50 years. Considerable evidence exists for drifts in these periodic components.