In their indignant Letter, Houlbrooke et al. declare our paper to be a very poor attempt at predicting the concentration of the runoff of dairy farms. But our paper never once uses the word ‘concentration’ and states explicitly in the third sentence that the aim of the paper is to quantify the amount of effluent coming off dairy farms. We were concerned only with quantity, not quality. In the field of ecological economics, in which we were working, such a number is vital and, before publication of this paper, was not available. We could not reasonably have entered more caveats than we did as to the level of imprecision of this country-wide average. But, at the end of the day, we needed the best reasoned method we could devise for evaluating this number, however inaccurately. This allows economists to do input/output analyses. Furthermore, however imprecise these economic analyses are (and they are very imprecise) it is necessary for policy planning that such high-level analyses should be available.
Recognising that Houlbrooke et al.'s Letter caught the wrong sow by the ear, it is still incumbent on us to deal with the criticisms. We are surprised that, even with this attack before us, as we review our paper, we cannot see any possible improvement except to make our paper simpler to understand.
Loose use of terms, incorrect estimations of volumes of ‘effluent water’
We are entitled to use the term water ‘effluent’, meaning water ‘flowing out’ (according to the Oxford English Dictionary), to describe the water flowing out of dairy farms and we make it very clear (refer to Fig. 1 and Fig. 2) that the farm dairy effluent (FDE) is only one contributing stream to the total outflow. We are not responsible for researchers (Houlbrooke et al. 2010; Shams et al. 2010) who fail to read carefully and understand the paper and misquote its conclusions. We also make it clear that water effluent arising from irrigation (not irrigation of FDE as the Letter mistakenly assumes) is by far the largest contributor to the total water effluent volume. What we did was an overall mass balance. To point out that FDE is only applied over a small portion of the land is to say that the Letter does not understand the concept of a mass balance.
The Letter points out that an average of drainage rate is meaningless because this varies from region to region (as we note in our paper). What the Letter does not perceive is that we require an average value of water volume, effluent from farms.
Repeated reliance on limited or weak assumptions
We acknowledge this point in every section of the paper from the Abstract to the Discussion, and emphasise that the volume estimation is imprecise. The section on Materials and methods devotes a whole paragraph to the question of using an average drainage rate, and states:
This is a poor assumption, since drainage will be affected by a number of factors such as the type of soil, the land topography, etc. and would vary considerably within the different regions of New Zealand.
No account for nutrient attenuation … lack of relevant literature
This was not a paper about the concentration of water effluent, therefore nutrient attenuation and all the articles cited by Houlbrooke et al. related to nutrient attenuation were not relevant to the paper.
In conclusion
In conclusion, we applaud Houlbrooke et al. for their patriotic vigour in cherishing the country's image but deplore any contention that science should have a spin put on it by ‘experts’ to present a green, politically correct face.