If you include greywater reuse with the rainfall you have doubled off the roof area you have more water and more nutrients. This could be included in the calculation. Martin Anda Research Associate Remote Area Developments Group Tel 61 8 9360 6399 Fax 61 8 9310 4997 Environmental Technology Centre Tel 61 8 9360 6123 Fax 61 8 9310 4997 Murdoch University Murdoch WA 6150