In annual data observations, the transitional temporal period between low and high frequencies is probably between the 3-year cycle and the longest El Nino cycle of about 7.5 years long. For purposes of this study, the transition point is assumed to be the 3-year cycle. It is found that decadal forecasting can be quite accurate given a good and adequately long tree-ring series. The result from the use of annual river flow series is not so clearcut for long-term drought forecast. This may only be ascribed to the inadequate length of the data series.
River flows from large basins have an inherent advantage over that of tree-ring series.
The large river runoff series has much less high frequencies or noise whereas tree-ring series since it only represents a very small drainage area is indeed composed of numerous high frequencies. That reality makes annual river flows appear to be quite predictable from one to ten years as long as no global or large regional events like huge volcanic eruptions occur. Mathematically, whether observation series are noisy or not, they are easily reconstructed with the use of sinusoidal series as represented in the spectral density spectrum of the observation. Observations containing large number of high frequencies or noise are practically unpredictable. However, under a decadal time period like the occurrence of long term drought, reaching an accurate forecast is quite attainable with the use of the mathematically defined forcing function.