3. Results

3.1 Data analysis

The first step in any time-series analysis and forecasting is to plot the observations against time, to gain an insight into possible trends and/or cycles associated with the temporal evolution of datasets. For our PDSI time series, Fig. \ref{div-640074}a shows that the series presents important inter-annual and decadal variability, with smooth changes in its structure and turning points which help in orienting the choice of the most appropriate forecasting method (after \citealp{Chatfield2000}).