|I was very pleased to see the study from Multsch et al. who have taken on such an important challenge to assess the irrigation potential of Brazilian crops in the Cerrado region. The study looks specifically at agricultural practices in 2012 and then estimates blue water scarcity from irrigation expansion assuming that irrigation will be applied to current rainfed agriculture. While I applaud the goals of the study, I have some reservations on the methods and results which need further clarification and review. |
My concerns mainly are:
-Concerns over the definition of irrigation expansion
-Concerns over modeling choices and water scarcity
-Lack of a sensitivity analysis and deeper insight into study limitations
Concerns over the definition of irrigation expansion
The definition of irrigation expansion is solely based on current practices (and I appreciate the reference to this assumption on L-175-176), however the use of irrigation is only expected until the onset of the rainy season, particularly in central and south-western Cerrado. We would expect the CONAB general planting dates and season to change to adapt to irrigation practices. For instance, irrigation has been applied in the first month of soybean development as a means to anticipate the growing season, but also to include a third crop in the dry season (Lathuillière et al., 2018). I also strongly suggest that the authors consider an interannual scarcity estimate, with a focus on the dry season when we would expect irrigation practices to kick-off. This additional piece of information would be more informative since more in line which the practices that we expect.
Concerns over modeling choices and water scarcity
Despite great detail provided in the methodology, there are still many open questions on some important modeling choices. The region presents important seasonality in rainfall and hydrologic years are typically represented between September and August. Did the study consider calendar year 2012? I think the years should appear in Table A2 as key parameters into the model (this information would also improve the reproducibility of the study). With respect to water scarcity, there seems to be a slight deviation from the typical consumption-to-availability ratio calculations in which availability is often defined including environmental flow requirements (Boulay et al., 2018; Mekonnen and Hoekstra, 2012) which was not mentioned in the study, other than Q95 already include all currently allocation blue water flows within the basins. This has typically been done by comparing Q50 and Q90 flows (previously unallocated) (Smakhtin et al., 2004).
Lack of a sensitivity analysis and deeper insight into the study limitations
There are some modeling choices that I expect would have quite a large impact on the results and would suggest that the authors consider a sensitivity analysis on at least the use of the factor of 2 used to derive water withdrawal from consumption (L-200) and the planting dates provided by CONAB. Planting dates are known to change based on the onset of the rainy season (Arvor et al., 2014) which is strong evidence for the use of a window of planting dates for the 2012/2013 season based on precipitation regimes in each basin. The study would also greatly benefit from self-criticism of the results, perhaps in a supplemental material, where all assumptions and limitations are described to offer the reader with a critical view of the results. This information is somewhat described at the beginning of the Discussion section, but the information provided seem more like a justification of the modeling choices rather than an assessment of how these choices might affect the main conclusions of the study (and thus require more discussion). Some additional key points that merit mention are:
-The lack of other water sources in the scarcity calculation; this is mentioned in the introduction (L-89), but this information can easily be forgotten as an important factor to be considered. In fact, I would expect the irrigation expansion to occur primarily as a function of groundwater availability and water available in small dams previously used for cattle drinking water (Rodrigues et al., 2012) rather than surface water availability.
-The importance of currently existing infrastructure and the most recent study from ANA on irrigation (https://www.ana.gov.br/noticias/ana-e-embrapa-identificam-forte-tendencia-de-crescimento-da-agricultura-irrigada-por-pivos-centrais-no-brasil/ana_levantamento-da-agricultura-irrigada-por-pivos-centrais_2019.pdf)
-Some comparison of results to previous work on crop water use would be helpful, using global and regional studies
P-2, L65: Please provide a reference for the terms “green” and “blue” water.
P-3, L.67: The authors should recognize the timely nature of water resources in Brazil as well given the wet and dry seasons, particularly in the Cerrado.
P3, L82: A definition of water consumption is warranted and its relation to water scarcity in this paragraph, particularly considering the definition from ANA related to water availability and water withdrawal.
P3, L83: Please confirm that you are considering an “irrigation” expansion scenario (it does appear on L87). Is the nature of the assumption needed here (instead of the methods)? In reality, irrigated cropland becomes rain-fed as soon as the rainy season begins, even in the case of crops that are not typically irrigated (Lathuillière et al., 2018).
P3, L91: a separate section called “Data” prior to the methods seems a little odd
P3, L94: What years were selected to derive ET based on the meteorological parameters? I understand that daily data is needed, but are you then using a 30-y average (from the 1980/2013 time series)? How is this data related to the year 2012, and how does it match discharge calculations from ANA described on P7, L188? This was answered by authors in response to R1, but why have a mismatch between consumption and availability?
P4, L103: please provide scientific names for all crops. All “beans” should be “bean”
P4, L113: “demands”, this term should be replaced by “consumption” as the study clearly does not look at demand, but consumption of water.
P4, L121-122: The link sends to a login page, not a download page to the model: is this correct?
P6, L162: the symbols for water content at field capacity and wilting point are reversed. Also, equation (6) does not show water content at wilting point.
P6, L185: If Q95 already includes all regulated flows, can you confirm that it does not include environmental flow requirements? This inclusion is an important component of deriving blue water scarcity following several methods such as (Hoekstra et al., 2012)
P7, L187: It is important to mention that blue water availability does not take into account any environmental flow requirements, which have often been used to derive a blue water scarcity ratio as per (Boulay et al., 2018; Hoekstra et al., 2012).
P7, L200: I am unsure I fully understand where the ratio of 2 comes from. I understand the need to factor in non-crop water uses, but how is this factor applied? And applied to what? The water scarcity level? The blue water consumption estimate? The response to previous comments for Reviewer 1 does not help in understanding this factor.
P7, L214: It is unclear how the allocation of available blue water and water consumptive use was carried out for municipalities that are contained in two different basins. Please clarify your choices.
P11, L338: “parameter” typo
P14, L418: I suggest you call this “moisture recycling”
Figure 4: I am a little confused with the data presented here. Do you mean that the information in d, e and f (which are labeled as rainfed, please consider re-labeling) is the scarcity should irrigation be applied to rain-fed agriculture only? I take that these area are basically those where crop ET < ETc for the existing mix of crops in each municipality.
Table A2: There are many acronyms in this table that need to be defined (IRR, Corn 1st, etc.): Please confirm that these results represent an average of the entire Brazilian Cerrado following the spatial variability in planting dates, precipitation and soil texture described in the methods. Why is ET in mm and Green water in m3/y? In fact, the time component should be expressed per crop cycle (which is shorter than a year). This table would benefit also from additional information on planting dates and total development cycle to better understand ET (or at least reference Table A1 and A2).
Arvor, D., Dubreuil, V., Ronchail, J., Simões, M., Funatsu, B.M., 2014. Spatial patterns of rainfall regimes related to levels of double cropping agriculture systems in Mato Grosso (Brazil). Int. J. Climatol. 34, 2622–2633. doi:10.1002/joc.3863
Boulay, A., Bare, J., Benini, L., Berger, M., Lathuillière, M.J., Manzardo, A., Margni, M., Motoshita, M., Núñez, M., Pastor, A.V., Ridoutt, B., Oki, T., Worbe, S., Pfister, S., 2018. The WULCA consensus characterization model for water scarcity footprints: assessing impacts of water consumption based on available water remaining (AWARE). Int. J. Life Cycle Assess. 23, 368–378. doi:10.1007/s11367-017-1333-8
Hoekstra, A.Y., Mekonnen, M.M., Chapagain, A.K., Mathews, R.E., Richter, B.D., 2012. Global Monthly Water Scarcity: Blue Water Footprints versus Blue Water Availability. PLoS One 7, e32688. doi:10.1371/journal.pone.0032688
Lathuillière, M.J., Dalmagro, H.J., Black, T.A., Arruda, P.H.Z. de, Hawthorne, I., Couto, E.G., Johnson, M.S., 2018. Rain-fed and irrigated cropland-atmosphere water fluxes and their implications for agricultural production in Southern Amazonia. Agric. For. Meteorol. 256–257, 407–419. doi:10.1016/j.agrformet.2018.03.023
Mekonnen, M.M., Hoekstra, A.Y., 2012. A Global Assessment of the Water Footprint of Farm Animal Products. Ecosystems 15, 401–415. doi:10.1007/s10021-011-9517-8
Rodrigues, L.N., Sano, E.E., Steenhuis, T.S., Passo, D.P., 2012. Estimation of small reservoir storage capacities with remote sensing in the Brazilian Savannah region. Water Resour. Manag. 26, 873–882. doi:10.1007/s11269-011-9941-8
Smakhtin, V., Revenga, C., Doll, P., 2004. A Pilot Global Assessment of Environmental Water Requirements and Scarcity. Water Int. 29, 307–317. doi:10.1080/02508060408691785