How to study changes in intermittent flows to a tropical delta using 31 year daily flow data ?

[Aswin kokkat]

Need to understand the long-term changes in the release from upstream storages to the intensively cultivated delta.

Normally flows are from July to Feb months only, over the years flow has reduced to Aug to Jan.

flow data is available for a network of 17 gauging stations in the delta. how can it be studied?

any suggested literature?

Can I do the analysis in Excel? if not please suggest some software?