The issue is with the water laws in the west, which never were designed to take into account the variability of precipitation year to year in the west, as well as being created during some of the highest water years on record.

Basically, farmers "own" that 80% of water — under current water laws that cannot be cut, they cannot be forced to use less. Instead, they are actually incentivized to use more because, if they don't, then they can lose some of that percentage.

As such, farms in the west are growing things like alfalfa, crops that need a high amount of water so that the farmer actually uses all his water rights. Much of those crops are then sold to China, rather than being things we need for food in the US.

So, yes, we need Congress to have the will to fight the farmers and recreate water rights for the West — though that comes with its own set of issues that it will get as messed up in the next generation of laws as it was in this one. Of course, it seems Congress doesn't want to be seen as "anti-farmer" so has no desire to try to fix the water laws, even with the issues being caused in the west.

As such, the up to 15% of residential usage is the only place where you can conserve, as the farmers desperately want/need to hang on to their water rights (to keep from losing their farms, particularly in the bad water years) and can't be forced to conserve.