Hi Sion
In forcing the relationship through the origin you're also assuming that you have a linear response (of a transformed variable?) down to zero flow. In your data, I was wondering what is your minimum flow value and what is its exceedence? Are you interested in predicting down to zero flow, and if so, do you think the data warrant that extrapolation? Even though zero flow and zero concentration may be logical, the linear relationship may break down at very low flows which may be hypothetical anyway. So I guess what I'm trying to say is that the linear relationship with the intercept may be valid over the range of observed x. This is a general issue: in your specific case your slope is not significant.
cheers
Mike Dunbar
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