Daily rainfall data requirement to apply the SPI. Minimum number of years for which daily rainfall data is required.
Aktar is right, but in some developing countries that amount of years is hard to achieve, because of data gaps and discontinuities in the observation systems. Several works used 25 or 20 years of data. If that is your case you will have to be careful upon the gamma distribution fit, especially during dry season.
If the data possesses small gaps you can interpolate (choice of best method for interpolation is necessary) the data.
If you intend to use precipitation index for crop planning, then you can even use 10 yrs of rainfall data, as there is very erratic climatic changes taking place all over the world. More variations have been observed in the last decade, there is a shift in normal dates of monsoon (India), increased rainfall intensity, reduced rainy days, more evaporation due to higher minimum and maximum temperatures. These changes are more prominent in last 5 to 10 yrs. Therefore, focus of our research should be on short duration and early maturing, less water requiring varieties. Each passing year has a different pattern of rainfall which are yet to be predicted with precision even while using all our intelligent and highly mathematical models.
But, if you wish to find the index for design of temporary water conservation structures then in my opinion, data of 30 to 50 yrs should be used, if not actual data then by following standard methods to predict high intensity rains, especially if such structures are to be planned in hilly topography having crops or human settlements in the downstream.If the structures are big involving high cost and human life then data of 100 to 200 yrs should be generated for use.
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