If you dig a ditch specifically to be filled with rain water, then in what sense is taking rainwater from that ditch not harvesting rainwater? Is it no longer

If you dig a ditch specifically to be filled with rain water, then in what sense is taking rainwater from that ditch not harvesting rainwater? Is it no longer rainwater if it touches a ditch? Maybe it's no longer rainwater if it touches a barrel. "Haha no States have laws against harvesting rainwater, only barrel water, you fools!"