Hi Kudzai. As you can see ...

Hi Kudzai.
As you can see from the comments here, there are a few different ways of estimating water use with various levels of accuracy. You can get some good results using surveys/questionnaires if they are well constructed and well targeted. You may also be able to use existing data sources. Obviously detailed metered information is desirable, but this is very expensive and the other means may be fit for purpose.
I have quite a few years of experience in water accounting and water survey design and can give you some pointers on what has worked for me in the past. You can email me at stuart.peevor@eibconsulting.com.au
I hope this helps.
Stuart