Export Coefficient Modelling of Nutrient Neutrality to Protect Aquatic Habitats in the River Wensum Catchment, UK
Round 1
Reviewer 1 Report
This submitted study explores the impact of nutrient pollution from wastewater treatment plants, agricultural runoff, and future residential development on aquatic ecosystems. It focuses on a specific area with intensive agricultural practices in the UK, the Blackwater sub-catchment of the River Wensum. The study develops models to assess the potential of converting crops into mixed woodland or grazing grass to achieve "nutrient neutrality" in the region. The manuscript, in general, is well-written and structured. The reviewer has a few questions that need to be addressed prior to further processing of the manuscript.
Please check the comments below:
1. L15: Mention chemical symbols of nitrogen and phosphorus.
2. Section 2.1: Consider adding a table explaining the characteristics of the study area for a better understanding of the readers.
3. L200-201: Add units.
4. The authors used more than 10 years old data for model development. Using such an old dataset might impact the prediction capability of the developed models. The reviewer is, therefore, concerned about the accuracy of the suggested models. Also, just wondering whether the land use patterns and meteorological conditions in the study are still the same.
Author Response
Please check the comments below:
- L15: Mention chemical symbols of nitrogen and phosphorus.
Symbols N and P now included.
- Section 2.1: Consider adding a table explaining the characteristics of the study area for a better understanding of the readers.
A new table (Table 1) of study area meteorological, hydrological and hydrogeological characteristics is now included to aid the reader. Rainfall and river discharge information duplicated in Tables 3 and 4 has now been omitted.
- L200-201: Add units.
Units now included following Equation 1.
- The authors used more than 10 years old data for model development. Using such an old dataset might impact the prediction capability of the developed models. The reviewer is, therefore, concerned about the accuracy of the suggested models. Also, just wondering whether the land use patterns and meteorological conditions in the study are still the same.
The chosen datasets generated as part of the Wensum Demonstration Test Catchment from a decade ago provided an opportunity to investigate the topic of nutrient neutrality. Given the effort in their collection, these are the only datasets that we hold that contain sufficient detailed information to construct and demonstrate an export coefficient model. However, to assist the reader in placing the modelling results in the present context, the following sentences have been added to Section 2.1 (Study area) to show that there has been some change in hydrological characteristics in the Wensum catchment but little change in land use and cropping patterns that would affect the export coefficient model results:
“Changes in winter (+13%) and summer (-7%) river discharge over the past two decades have increased the risk of diffuse pollution mobilisation and reduced the dilution of point source pollutants, respectively [33]. By 2022, EU Water Framework Directive compliance fell to just 46% for P and 1.8% for N [33].”
“Land use in the upper Blackwater sub-catchment remains relatively unchanged in the last decade and is predominantly arable (75%) with some improved grassland and fresh water (13%), mixed woodland (10%) and rural settlements (2%) (Figure 1) [39-40]. Typical arable crops grown in rotation include winter wheat, winter and spring barley, winter oilseed rape, sugar beet, spring beans and potatoes. Key trends in arable farming in the last decade include less winter oilseed rape because of flea beetle damage [41], particularly in southern England, more barley and less sugar beet. Sugar beet has tended to become restricted to farms that grow this crop successfully and are near one of the four processing factories, as in the case for the upper Blackwater.”
Reviewer 2 Report
This manuscript is well written and organized to explore the export coefficient modeling of nutrient neutrality in catchment, UK. Some minor comments are given as below.
1. The authors can highlight the novelty of this study at the end of “Introduction” section.
2. How did the authors validate the export coefficient modelling with past data?
3. In the “Discussion”, the discussion on model applicability would be needed.
4. In Table 2 and Table 3, usually the riverine discharge is cubic meter per second. Why did the authors use “mm” for riverine discharge?
5. Future work can be described.
Minor editing of English language required.
Author Response
- The authors can highlight the novelty of this study at the end of “Introduction” section.
The novelty of the study has now been included at the end of the Introduction section.
- How did the authors validate the export coefficient modelling with past data?
The export coefficient model is not validated with past data. Instead, model results are interpreted by comparison with the riverine nutrient export, which is expected to be less than the calculated amount of leached N given the catchment characteristics that promote denitrification (mentioned at lines 290-296 in relation to other published studies) and approximately equal for P. The calculation of P leaching from rural septic systems is a source of uncertainty (as mentioned at lines 324-327). The model is only as good as the export coefficients selected and here there is room for improvement, now mentioned in the discussion at the end of Section 3.2 and also in the Conclusions section as an area for further work.
- In the “Discussion”, the discussion on model applicability would be needed.
We agree, the model has been developed for an arable area in eastern England that experiences a temperate maritime climate. A qualification to this effect has been added to the discussion at the end of Section 3.2.
- In Table 2 and Table 3, usually the riverine discharge is cubic meter per second. Why did the authors use “mm” for riverine discharge?
Depth of water-equivalent riverine discharge is a convention used in hydrology for ease of calculating runoff volume (area ´ depth of water). Additional lines are now included in the new Table 1 to also give riverine discharge in m3 s-1 and as a volume in m3 to assist the reader.
- Future work can be described.
Mention of further work is now included in the renamed “Conclusions and further work recommendations” section.
Reviewer 3 Report
Dear Authors,
I wanted to take a moment to express my appreciation for your work. Your research, which delves into the critical issue of nutrient pollution from wastewater treatment and agricultural runoff, highlights the pressing need to safeguard aquatic habitats. The approach you've taken, using an export coefficient model alongside detailed farm business data, to explore nature-based solutions for nutrient neutrality is appreciated. Your study's focus on converting crops to mixed woodland or grazing grass in the Blackwater sub-catchment of the River Wensum, UK, demonstrates a practical way to mitigate nutrient loading resulting from projected residential development. Your research provides a vital contribution to addressing the ecological challenges posed by nutrient pollution. Thank you for your efforts in this field.
I have included a few minor comments that could potentially enhance the clarity of certain specific aspects.
Warm regards
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Author Response
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