Not lawn grass, please
"I found myself thinking up ways to purify the regions air (one of the first would be a vigorous landscape plant watering program and green lawn tax credit)"
Lawn grass is spectacularly unsuited for the climate anywhere in the US, let alone anywhere approaching desert. It comes from Scotland, of all places! Keeping it alive in America takes ridiculous amounts of water and fertilizer. Make it "tax credit for prairie wildflowers". Better yet, TAX the lawns. I bet significant part of California's energy use goes into the bloody things.