Texas--> pay Louisiana $15 billion and endow a bunch of scholarships to every college in Louisiana, in exchange for...
An allowance of X acre feet per year to be pumped from the Mississippi River downstream of where it's East bank exits the State of Mississippi (that way we only have to deal with one state, not Miss and Ark as well).
Build California style massive water pipelines and pumps to fill up lakes in East Texas, then build pipelines and pumps to fill up Central and North Texas lakes. Plenty of maintenance, and de-silting, and other civil engineering work will obviously be required. State can charge localities and farmers for the water. Once built, it will more or less pay for itself.
Texas' long term water problem solved.