This manuscript details the results of a modeling exercise to assess the fate of nitrogen and grey water footprint size in agricultural systems under different management practices. The authors use the Agricultural Policy Environmental eXtender (APEX) model to assess crop yields and nutrient leaching/ runoff, and results are compared against a more simplistic, less-data intensive, expert-based approach for estimating N loads from agricultural areas. The assessments are based on climate and agricultural characteristics of a farming region in Spain. Results indicated that for low levels of nitrogen application, the simple approach yields approximately the same results as the more data-intensive modeling endeavor. At higher levels of nitrogen application, the simple method underestimates nitrogen leaching/runoff. The modeling results also explored multiple types and combinations of management practices, finding there are inevitable tradeoffs between crop yield and water pollution, and water pollution and water consumption.
This is an interesting paper with potential implications for applied agricultural management to reduce water pollution problems while minimizing negative impacts to crop yields. The following are a few comments that could help improve this work if incorporated.
- Lines 79-84 provide an overview of studies that have explored the impacts of crop yield and N management via various agricultural management practices. However, what would be more helpful to the reader is an overview of those study findings, and how these findings informed the present work.
- I realize that validating model results with field experiments is difficult, and the authors clearly explain why this is so (Lines 90-94). I also appreciate the author’s comparison of their model results with those of Berenguer et al 2009- particularly given that the APEX results are only really comparing Tier 1 and Tier 3 levels. Given the lack of field data available, this multi-model comparison is very important. However, I feel the discussion and comparison of model results is short and lacks depth. There is brief mention of modeling results based on EU studies, and research by Berenguer et al 2009, but digging in and really discussing the differences and similarities between these results would add greatly to this work.
- At the beginning of Section 2.3, the authors state that part of the reason for evaluating Tier 3 vs Tier 1 is to see if “the simpler estimation approach (Tier 1) as applied to previous studies” is an accurate measure. I’m not sure what these “other studies” are in particular, maybe they’re the ones I mention in the comment above, but either way, there could be much value added to explicitly showing the comparisons between Tier 1 results and Tier 3 results, if possible?
- In line 36, “fertilizer” should be “fertilizers”.
- The authors use the spelling “freshwater” and “fresh water” in this manuscript (see lines 38 and 39 for examples). One spelling version should be selected and used consistently throughout.
- In line 157, I suspect the authors meant “Phenological”, not “Phonological”.
- Equations 4 & 5 (Lines 220-223) reference work done by Franke et al 2013. In these equations, there is a variable s¬_i that is a “score for leaching runoff potential”, while I realize this is information from another paper, it would be helpful to discuss how these values are developed, even if briefly.
- In Figs 8 & 9, I assume the “calculated, tier-1” numbers (in blue) are mean values? Please specify in the body of the manuscript for clarity.