Dear Peter, The already ...

Published May 26, 2017 20:12 by Jorge Jódar, Senior Hydrogeologist en Hydromodel Host, S.L.

Dear Peter,

The already given answerts are pretty good, nevertheless, if you still have doubts perhaps you should go to following reference in which the use of SPI is clearly explained:

Tallaksen, L. M., & Van Lanen, H. A. (2004). Hydrological drought: processes and estimation methods for streamflow and groundwater (Vol. 48). Elsevier.

Best regards