Sinks: I think it is too difficult to come up with a good relationship between gpm and gpd for sinks. For the cooking water example you mentioned (BTW, hot tap water should never be used for cooking or drinking), or filling a sink for washing dishes, as well as complications like waiting for hot water, it is difficult to justify much reduction in gpd for sink aerators. In the studies I've seen, the average reduction in in sink gpd was not statistically significant.
Showers:I think it is reasonable to assume a linear relationship between gpm and gpd. There are reasons that it wouldn't be linear: 1) the volume of hot water wasted waiting for the shower water to warm up would be fixed; 2) there may be some take-back where people take longer showers if their showerhead is low-flow; 3) some people take showers until the hot water runs out. These can probably be ignored for lack of good data. See Home Energy magazine article: http://www.homeenergy.org/show/articl...
Baths: N/A; Low-flow fixtures should not change bath water use
BEopt actually used to have low-flow fixtures as an option, but we took it out because of these complications.
Here are links to the studies I've seen:
- http://www.aquacraft.com/sites/defaul...
- http://www.aquacraft.com/sites/defaul...
- http://www.aquacraft.com/sites/defaul...
