Capacity behind dams doesn’t create new water, and in fact cases evaporation thus reducing average available water in the long term. It’s a pointless expense unless there’s regular surplus water to refill them.

The issue isn’t that farming is using 92% of all water, the issue is farming is extracting water from aquifers faster than it can replenish. This reduces not only the amount stored but also the amount that ends up in streams and rivers.