Hi! I'm comparing two 50-year SPEI (Standardized Precipitation Evapotranspiration Index) time series created from model and observed data (1950-1999) and averaged over a large area. Both time series are annual (i.e., 50 values each). My goal is to assess very simply the reliability of the model-based SPEI before analyzing future projections for mid- and late-21st century.

What is a good aggregation period on which to perform this correlation?

I tried the annual data (i.e., 50 values each), as well as 5- and 10-year averages (i.e., 10 and 5 values) but got higher correlation coefficients the longer the aggregation period, ranging from -0.02 (annual) to 0.2 (5-year) to 0.95 (10-year), which tells me I can get any correlation coefficient depending on the aggregating period.

I would appreciate any advice from more experienced users on how to choose a "good" aggregation period. As a note, my future projections represent 25-year averages (i.e., 2041-2065 and 2075-2099).

Thank you very much!

Toni