An important new development in hydrological data analysis in the last decade is the application of wavelet analysis. Here, wavelet theory is used to study the complexity and multi-scale periodicity of the hydrological time series of the Dalai Lake Basin in Inner Mongolia. Two large rivers, the Kelulun and the Wurxun, are the main inflows to Dalai Lake, which is currently shrinking. The annual and monthly flows of the Kelulun River are shown to vary more than those of the Wurxun River, and the monthly flows of the two rivers vary much more than their annual flows. Db5 wavelets are shown to be more suitable for annual flow calculations, whereas Db4 wavelets are more suitable for monthly flow calculations. Multi-scale wavelet analysis of the annual and monthly flows of the Kelulun and Wurxun rivers shows that the variation of the two rivers is similar and has a 25-year cycle, 12 years of wet and 12 years of drought periods, and our results show that both rivers are expected to transition into a wet period beginning in 2012. Therefore, the Dalai Lake Basin, which has been in a drought period since 2000, is expected to gradually transit into a wet period from 2012 onward.
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