To understand how to calculate the return period of drought using the Standardized Precipitation Evapotranspiration Index (SPEI), I recommend referring to the paper titled Article Trivariate frequency analysis of droughts using copulas unde...
In the paper, you should find information on the following key steps:
Calculation of SPEI: The paper should describe how to compute the SPEI values, which are typically based on a combination of precipitation and potential evapotranspiration data.
Return Period Calculation: The paper should detail the statistical methods used to estimate the return period of drought based on SPEI values. This may involve fitting a probability distribution to the data and determining the exceedance probability of the threshold.
Interpretation and Results: The paper should provide guidance on how to interpret the calculated return periods in the context of drought risk assessment and management.