Currents Status, Challenges, and Future Directions in Identifying Critical Source Areas for Non-Point Source Pollution in Canadian Conditions

Round 1

Reviewer 1 Report

The article provides detailed information regarding the method of identifying CSAs and points out that HWQ model cannot be fully simulated in all mechanisms, especially under the special climatic condition in Canada. Insights regarding the limitation of the model are also provided and the future directions are well-presented. However, the perspective on developing a “Toolbox” in future research is lacking. As sufficient amount of water quality data is required under the premise of establishing a reliable model, I suggest the authors to include more water quality data in this article. Additionally, how to establish an effective and complete water quality monitoring system also needs to be addressed.

The suggested scheme contains sequences of activities that include sensitivity analysis, calibration, validation, and uncertainty analysis (Line 361) while only 25% of the studies reported the result of sensitivity analysis and 10% of the studies reported result of uncertainty analysis (line 385). Is it the case that sensitivity analysis and uncertainty analysis are not vital in the suggested scheme?

Mentioned in the article that SWAT, AnnAGNPS, BASINS, GIBSI and AGBNPS are the top five HWQ models to simulate CSAs (line 221), I then wonder why GWLF, which is not one of the top five models, is listed in table 1? Also, why BASINS and GIBSI application case are not shown in table 1? BASINS/HSPF is often adopted to identify CSAs of watersheds in the USA and many countries; hence, I recommend the authors to include relevant application cases of BASINS/HSPF.

Author Response

Reply to Reviewer’s Comments

We would like to thank the editor for giving us the opportunity to further revise the manuscript and improve its quality. We would also like to thank the two anonymous reviewers for their time and efforts in providing critical and constructive comments. We believe we have carefully addressed all the issues raised. Detailed responses to comments are provided below, and we have uploaded a clean version of manuscript (where changes are marked as blue-colored text) and a version where track changes are shown.

Reviewer’s comments

Reviewer 1:

The article provides detailed information regarding the method of identifying CSAs and points out that HWQ model cannot be fully simulated in all mechanisms, especially under the special climatic condition in Canada. Insights regarding the limitation of the model are also provided and the future directions are well-presented.

(1) However, the perspective on developing a “Toolbox” in future research is lacking.

Reply: We agree that discussion regarding a ‘Toolbox’ was rather limited. We have now expanded it as: “Development of a ‘Toolbox’ to identify CSAs also needs serious attention and could be a step forward. In fact, Sharpley et al. (2011) envisioned the need and value of such a ‘Toolbox’. Especially given the fact that it is very difficult to answer all pertaining questions related to identifying CSAs of NPS pollution using a single model/method. The envisioned ‘Toolbox’ can host a range of different models/methods, ranging from a simple method such as topographic index to a complex HWQ model. This will provide the end users with a variety of options to try and use different methods/models to identify CSAs. Users may opt to use the simple methods for screening larger watersheds while detailed HWQ models may be used to provide absolute values of P loss and long-term effects of future management scenarios on P loss. The ‘Toolbox’ can be hosted in a dedicated platform or can be made available as a web-based modelling framework such as the Hydrology and Water Quality System (HAWQS) using SWAT framework (Yen et al., 2016). The ‘Toolbox’ essentially offers features such as an automated workflow of input data preparing and processing for an area of user’s choice and output data repository to store model and scenario runs. We believe that provision of such a ‘Toolbox’ renders the redundancies associated with pre-processing of large volume of input data (e.g. spatial and meteorological) which is often time-intensive and error-prone. Recent advances in computing facilities and web technologies also support the idea of making the envisioned ‘Toolbox’ available to the wider public”. Please refer Lines 630 to 647.

(2) As sufficient amount of water quality data is required under the premise of establishing a reliable model, I suggest the authors to include more water quality data in this article. Additionally, how to establish an effective and complete water quality monitoring system also needs to be addressed.

Reply: We agree that little insights were presented regarding the water quality data needs and what can be done to enhance the reliability of model results. We have now added a paragraph which details the important issue raised by the reviewer as: “It has been shown that majority of NPS pollution would get exported in late winter and early spring months (Dickinson et al., 1986; Wall et al., 1988; Zhang et al., 2020) and therefore constitute hot-moments of NPS pollution export. Sampling campaigns should be effective enough to capture the variability in such important time/seasons. Identification of the proper locations for water quality monitoring is equally important too. Thus, a cost-effective and efficient water quality monitoring framework, as suggested by Alilou et al. (2018) is desired. Moreover, detailed HWQ model ideally needs continuous water quality data for proper calibration (and/or validation). However, such data are rarely available because of logistic issues. Often, sporadic water quality measurements are available which hinders proper model calibration (and/or validation) and model results may deem to be unreliable. This is a general problem in NPS pollution modelling. One possible solution is to carryout dedicated sampling campaigns to cover wide range of streamflow/field conditions (e.g. dry, wet, 24-hour, etc.) which helps to verify a model’s ability in respective conditions (Shrestha et al., 2018). However, this requires a team of dedicated crew waiting for such an event to occur which may not always be available (van Griensven et al., 2000). Another possibility is to install automatic samplers to measure some explanatory variables (e.g. specific conductance, turbidity) and using an established relationship (e.g. regression model), other water quality variables (e.g. total suspended solids, total phosphorus) are predicted (Anderson and Rounds, 2010). However, uncertainties associate with such estimations render their use in calibrating (and/or validating) detailed HWQ models. The use of remote sensing technologies in validating HWQ model based CSAs can also be an alternative as shown by Shrestha et al. (in press). In the study, they used oblique aerial images to qualitatively validate CSA of phosphorus in a watershed. Such a technique may be useful for a large watershed which needs water quality monitoring at multiple locations”. Please refer Lines 655 to 678.

(3) The suggested scheme contains sequences of activities that include sensitivity analysis, calibration, validation, and uncertainty analysis (Line 361) while only 25% of the studies reported the result of sensitivity analysis and 10% of the studies reported result of uncertainty analysis (line 385). Is it the case that sensitivity analysis and uncertainty analysis are not vital in the suggested scheme?

Reply: We totally agree that sensitivity and uncertainty analysis are vital in modelling CSAs. A good modelling practice will inherently involves sensitivity and uncertainty analyses. We have indeed explicitly mentioned the same in Lines 376-384 as: “As model calibration does not guarantee the reliability of model predictions for other periods (Daggupati et al., 2015), therefore, the calibrated model needs to be validated. During model validation stage, the model is run using the optimized parameters obtained during the calibration process. The other important component of good modeling practice scheme is uncertainty analysis. Modeling uncertainty arises from multiple sources, which include input data used, parameter estimates, model structure, and even observed data used for calibration and validation (Leta et al., 2015). Traditional modeling practices often involve fitting a single best simulation. However, there is no single best model as there is always an inevitable ‘equifinality’ problem (Beven, 1993). It is, therefore, important to include uncertainty analysis as part of good modeling practices”.

(4) Mentioned in the article that SWAT, AnnAGNPS, BASINS, GIBSI and AGNPS are the top five HWQ models to simulate CSAs (line 221), I then wonder why GWLF, which is not one of the top five models, is listed in table 1? Also, why BASINS and GIBSI application case are not shown in table 1? BASINS/HSPF is often adopted to identify CSAs of watersheds in the USA and many countries; hence, I recommend the authors to include relevant application cases of BASINS/HSPF.

Reply: As we explicitly mentioned in Lines 213-227, we have based out selected from Booty and Benoy (2009)’s thorough analysis. In the paper, the authors assessed several HWQ models and came up with top five models and we followed their recommendations. We thank you for pointing out a case study of GWLF, although not being on the top 5 list, which was included in the Table 1. We have now corrected the mistake. Appreciating the reviewer’s concerns, we have now included applications of BASINS/HSPF and GIBSI in Table 1.

Reviewer 2 Report

It is interesting that I was also recently invited to review another “state-of-the-art” review on water quality modeling for another journal.  Whereas that paper was a shallow, incomplete summary of the topic which offered little insight, the subject paper is the complete opposite of that earlier paper.  This subject paper is a very thorough, concise, and insightful summary of the methods that have been used to determine Critical Source Areas (CSAs) of Non-Point Source (NPS) pollution.  The paper is very well-written and enjoyable to read.  This summary of CSA identification methods is an excellent starting point for any researcher or practitioner looking to study CSAs.  When one also considers this paper’s summary of the special considerations of NPS loading and CSA identification under Canadian hydrological conditions, the paper’s value is doubled.  

Even as well written and interesting as this paper is, I still recommend that the authors deal with the following issues.

• Whereas this reviewer grew up about 30 km from Canada and has worked on some climate change issues in various regions of Canada not all readers will be familiar with the various hydrologic regions of Canada discussed in Section 3.5. Thus, I think the readers would benefit if a map of Canada were included identifying the various hydrological regions discussed in Section 3.5.
• One other issue related to uncertainties and/or difficulties when dealing with tile drains (Section 3.5.4) is that often tile drain systems were built so long ago that the actual details of these systems are unknown making it difficult to model these systems in detail. This issue should be briefly mentioned in Section 3.5.4.
• Line 96 refers to Mekonnen et al. (2016), but there are three papers in the Reference list by Mekonnen et al. (2016), i.e. a, b, and c? So, which one is cited here?  Or is this a citation for Mekonnen (2016) which is included in the Reference list, but not cited in the text of the paper.
• Line 154 refers to SLC (2005) whereas the Reference list includes SLC (2007). Please correct the incorrect date.
• Lines 185, 437, and 448 refer to Easton et al. (2016), but there are two papers in the Reference list by Easton et al. (2016), i.e. a and b? So, which one is cited at these locations?
• Add Karst-Riddoch et al. (2014) cited on Line 205, Hunt and Zhang (1999) cited on Line 246, Schaller and Bailey (1983) cited on Line 289, Sharpley et al. (2013) cited on Line 304, Robson et al. (2014) cited on Line 325, Merwin et al. (1994) cited on Line 414, Mishra and Singh (2004) cited on Line 446, and Gupta et al. (2018) to the Reference list.
• The following recent reference should be added to Table 1. This is an application of SWAT to CSA identification in the watershed of Beijing’s principle water supply reservoir.

Guo, Y., Wang, X., Zhou, L., Melching, C.S., Li, Z., 2020. “Identification of critical source areas of nitrogen load in the Miyun Reservoir Watershed under different hydrological conditions,” Sustainability, 12, 964.

• Line 185 refers to Dunne and Black (1970), but there are two papers in the Reference list by Dunne and Black (2016), i.e. a and b? So, which one is cited here?
• Baginska et al. (2003), Beaulieu et al. (2006), Berzina and Sudars (2010), Bolster et al. (2012), Brunet (2011), Eghball and Gilley (2001), Euliss et al. (1999), Goulet et al. (2006), Hayashi et al. (2003), Heckrath et al. (1995), Johansson and Randall (2003), Jonston (1991), Meals et al. (2012), Mekonnen (2016), MOECC (2016), Peters and Meybeck (2000), Shoemaker et al. (1997), Srinivasan and McDowell (2007), Tiner (2003), van der Kamp and Hayashi (2009), van der Valk and Jolly (1992), Woo and Rowsell (1993), and Young et al. (1994) are included in the Reference list, but they are not cited in the text of the paper. These papers must either be cited in the text of the paper, or be deleted from the Reference list.
• Numerous editorial suggestions have been made throughout the marked manuscript which the authors should consider when preparing the final version of this paper.

Comments for author File: Comments.pdf

Author Response

Reply to Reviewer’s Comments

We would like to thank the editor for giving us the opportunity to further revise the manuscript and improve its quality. We would also like to thank the two anonymous reviewers for their time and efforts in providing critical and constructive comments. We believe we have carefully addressed all the issues raised. Detailed responses to comments are provided below, and we have uploaded a clean version of manuscript (where changes are marked as blue-colored text) and a version where track changes are shown.

Reviewer’s comments

Reviewer 2:

It is interesting that I was also recently invited to review another “state-of-the-art” review on water quality modeling for another journal. Whereas that paper was a shallow, incomplete summary of the topic which offered little insight, the subject paper is the complete opposite of that earlier paper. This subject paper is a very thorough, concise, and insightful summary of the methods that have been used to determine Critical Source Areas (CSAs) of Non-Point Source (NPS) pollution. The paper is very well-written and enjoyable to read. This summary of CSA identification methods is an excellent starting point for any researcher or practitioner looking to study CSAs. When one also considers this paper’s summary of the special considerations of NPS loading and CSA identification under Canadian hydrological conditions, the paper’s value is doubled.

Reply: Thank you so much for your encouragement.

Even as well written and interesting as this paper is, I still recommend that the authors deal with the following issues.

(1) Whereas this reviewer grew up about 30 km from Canada and has worked on some climate change issues in various regions of Canada not all readers will be familiar with the various hydrologic regions of Canada discussed in Section 3.5. Thus, I think the readers would benefit if a map of Canada were included identifying the various hydrological regions discussed in Section 3.5.

Reply: We agree with the reviewer and a figure showing ecoregions of Canada is included (please refer Figure 1).

(2) One other issue related to uncertainties and/or difficulties when dealing with tile drains (Section 3.5.4) is that often tile drain systems were built so long ago that the actual details of these systems are unknown making it difficult to model these systems in detail. This issue should be briefly mentioned in Section 3.5.4.

Reply: We really thank the reviewer for this important insight that we have missed in the submitted version. We have now added these details in the revised version. Please refer Lines 539-540.

(3) Line 96 refers to Mekonnen et al. (2016), but there are three papers in the Reference list by Mekonnen et al. (2016), i.e. a, b, and c? So, which one is cited here?  Or is this a citation for Mekonnen (2016) which is included in the Reference list, but not cited in the text of the paper.

Reply: Thank you so much. In line 96, it should be Mekonnen (2016) and it has been corrected. Other references (Mekonnen et al. (2016 a,b,c) are correctly cited elsewhere in the manuscript.

(4) Line 154 refers to SLC (2005) whereas the Reference list includes SLC (2007). Please correct the incorrect date.

Reply: Thank you; it is SLC (2007) and has been duly corrected.

(5) Lines 185, 437, and 448 refer to Easton et al. (2016), but there are two papers in the Reference list by Easton et al. (2016), i.e. a and b? So, which one is cited at these locations?

Reply: Thank you; the only reference that we need is Easton et al. (2016), as follows.

Easton, Z. M., D. R. Fuka, M. T. Walter, D. M. Cowan, E. M. Schneiderman and T. S. Steenhuis. 2008. Re-conceptualizing the soil and water assessment tool (SWAT) model to predict runoff from variable source areas. Journal of Hydrology, 348(3): 279-291.

(6) Add Karst-Riddoch et al. (2014) cited on Line 205, Hunt and Zhang (1999) cited on Line 246, Schaller and Bailey (1983) cited on Line 289, Sharpley et al. (2013) cited on Line 304, Robson et al. (2014) cited on Line 325, Merwin et al. (1994) cited on Line 414, Mishra and Singh (2004) cited on Line 446, and Gupta et al. (2018) to the Reference list.

Reply: We are sorry that we missed just a basic thing. Full references for Karst-Riddoch et al. (2014), Hunt and Zheng (1999) [not Zhang but Zheng]. Schaller and Bailey (1983), Sharpley et al. (2013), Robson (2014) [not Robson et al. but single author Robson], Merwin et al. (1994), Mishra and Singh (2004) and Gupta et al. (2018) are now given in the reference list.

Karst-Riddoch. T. 2014. Managing new urban development in phosphorus sensitive watersheds. Hutchinson Environmental Sciences Ltd., Bracebridge, ON, Canada.

Hunt, R.J., and C.M., Zheng. 1999. Debating complexity in modeling. EOS Trans. Am. Geophys. Union, 80 (3), p.29

Schaller, F.W., and G. W. Bailey. 1983. Agricultural management and water quality. Iowa State Press. Iowa, USA.

Sharpley, A., H.P. Jarvie, A. Buda, L. May, B. Spears, P. Kleinman. 2013. Phosphorus legacy: Overcoming the effects of past management practices to mitigate future water quality impairment. Journal of Environmental Quality, 42(5): 1308-1326. DOI:10.2134/jeq2013.03.0098

Robson, B.J., 2014. State of the art in modelling of phosphorus in aquatic systems: Review, criticisms and commentary. Environmental Modelling & Software, 61: 339-359.

Merwin, I.A., W.C. Stiles, and H. M. Vanes. 1994. Orchard groundwater management impacts on soil physical properties. Journal of the American Society of Horticultural Sciences. 119(2): 216-222.

Mishra, S.K., and V.P Singh. 2004. Long term hydrological simulation based on the soil conservation service curve number. Journal of Hydrological Process. 18 (7): 1291–1313.

Gupta, A.K., R.P. Rudra, B. Gharabaghi, P. Daggupati, P.K Goel, and R. Shukla. 2018. CoBAGNPS: A toolbox to estimate sediment removal efficiency of WASCoBs–pipe risers and blind inlets. Environment and Natural Resources Research, 8(3): 84-101.

(7) The following recent reference should be added to Table 1. This is an application of SWAT to CSA identification in the watershed of Beijing’s principle water supply reservoir. Guo, Y., Wang, X., Zhou, L., Melching, C.S., Li, Z., 2020. “Identification of critical source areas of nitrogen load in the Miyun Reservoir Watershed under different hydrological conditions,” Sustainability, 12, 964.

Reply: Agreed and the suggested reference, Guo et al. (2020), has been added in the Table 1.

Guo, Y., X. Wang, L. Zhou, C.S. Melching, and Z. Li. 2020. Identification of critical source areas of nitrogen load in the Miyun Reservoir Watershed under different hydrological conditions. Sustainability, 12, 964.

(8) Line 185 refers to Dunne and Black (1970), but there are two papers in the Reference list by Dunne and Black (1970), i.e. a and b? So, which one is cited here?

Reply: Thank you. We have corrected them and cited both papers of Dunne and Black (1970 a,b) in the manuscript at appropriate places.

(9) Baginska et al. (2003), Beaulieu et al. (2006), Berzina and Sudars (2010), Bolster et al. (2012), Brunet (2011), Eghball and Gilley (2001), Euliss et al. (1999), Goulet et al. (2006), Hayashi et al. (2003), Heckrath et al. (1995), Johansson and Randall (2003), Jonston (1991), Meals et al. (2012), Mekonnen (2016), MOECC (2016), Peters and Meybeck (2000), Shoemaker et al. (1997), Srinivasan and McDowell (2007), Tiner (2003), van der Kamp and Hayashi (2009), van der Valk and Jolly (1992), Woo and Rowsell (1993), and Young et al. (1994) are included in the Reference list, but they are not cited in the text of the paper. These papers must either be cited in the text of the paper, or be deleted from the Reference list.

Reply: Thank you. We are again sorry for not being able to address in the previous version. Except Mekonnen (2016) [Line 95], Peters and Meybeck (2000) [Line 34], all have been deleted from the reference list. As for Shoemaker et al. (1997), we actually needed Shoemaker et al. (2005) [see Line 215], This has now been corrected.

Shoemaker, L., T. Dai, J. Koenig, and M. Hantush. 2005. TMDL model evaluation and research needs. National Risk Management Research Laboratory, US Environmental Protection Agency.

(10) Numerous editorial suggestions have been made throughout the marked manuscript which the authors should consider when preparing the final version of this paper.

Reply: We really appreciate edits and suggestions on the annotated PDF. We are also very sorry that you had to spend so much time correcting our mistakes. On our part, these mistakes could have easily been avoided. In the revised version, we have accepted the editorial suggestions that you have made. Thank you.

Round 2

Reviewer 1 Report

no comment