The time series of stability and periodic tests are primarily considered in this study. It can be observed that the feature
of non-stationary is clearly detected from the precipitation series {Yt } in Fig. 3. In order to eliminate the impact of periodic
variation, a time interval for 12 months of season difference can be used. The defined lag/ difference operator ∇12 = 1−B12,
the model can get a new sequence dsy basic for stationary series (Fig. 4). Combining the correlation coefficient (AC) with
the partial autocorrelation coefficient (PAC) in Fig. 2, the new series meets the stability requirement. We can carry out the
model for the orders.