In addition to the means I suggested to reduce demand, there are lots of others that would better use the appropriated money. One might be to buy farmland that is currently irrigated turn it into a park or wildlife preserve. Another could be to subsidize conservation measures; e.g., give a tax credit for installing low flow or dual flush toilets, or one to car washes that recycle water, in certain counties. Or perhaps fund an ad campaign asking people not to waste water by letting it run while brushing teeth, and similar painless measures.
Some measures don’t require state funding. Pass a law that when washing a car by hand, the hose must have a self-shut off so that it just doesn't run while it's not used. Pass a law that reduces county funding if the county doesn’t have water conservation measures in place, such as requiring every-other day lawn sprinkling.
This could be a real opportunity for Dems to catch the attention of south Idaho farmers. They’re mad at the administration and ready to look at alternatives. Work in a serious manner to address the water curtailment problem. Help farmers now, and help Idaho in the future avoid problems like Georgia is having with its drought.
Update: I was thinking about the wildlife preserve idea I suggested, and it occurred to me that it in particular really holds an opportunity to erode the Republican base in Idaho. First, land and water conservation will appeal to the greenies and environmentalists that already tend to vote Dem, and could energize them. More importantly, conserving water and helping farmers could attract rural voters who currently vote pretty Red. Also, wildlife preserves that are open to hunting and fishing will appeal to that crowd, another typically Red group.