the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2024
Can large-scale tree cover change negate climate change impacts on future water availability?
Abstract. The availability of fresh water over land may become increasingly scarce under climate change, and natural and human-induced tree cover changes can further enhance or negate the water scarcity. Previous studies showed that global tree cover change can have large impacts on water availability under current climate conditions, but did not touch upon the implications of global tree cover change under climate change. Here, we study the hydrological impacts of large-scale tree cover change (climate-induced changes in combination with large-scale afforestation) in a future climate (SSP3-7.0) following an interdisciplinary approach. By combining data from five CMIP6 climate models with a future potential tree cover dataset, six Budyko models, and the UTrack moisture recycling dataset, we can disentangle the impacts of climate change and future tree cover change on evaporation, precipitation, and runoff. We quantify per grid cell and for five selected river basins (Yukon, Mississippi, Amazon, Danube, and Murray-Darling) if tree cover changes enhance or counteract the climate-driven changes in runoff due to their impact on evapotranspiration and moisture recycling. Globally averaged, the impacts of climate change and large-scale tree cover change on runoff are of similar magnitude with opposite signs. While climate change increases the global runoff, the changing tree cover reverses this effect which overall results in a limited net impact on runoff relative to the present climate and current tree cover. Nevertheless, locally the change in runoff due to tree cover change and climate change can be substantial with increases and decreases of more than 100 mm yr-1. We show that for approximately 16 % of the land surface, tree cover change can increase the water availability significantly. However, we also find that, for 14 % of the land surface, both tree cover change and climate change might decrease water availability with more than 5 mm yr-1. For each of the selected catchments, the direction and magnitude of the impacts of climate change and tree cover change vary, with dominating climate change impacts in all basins except the Mississippi River basin. Our results show that ecosystem restoration projects targeting an altered tree cover should consider the corresponding hydrological impacts to limit unwanted (non-)local reductions in water availability.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-2015', Michael Roderick, 01 Oct 2024
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RC2: 'Comment on egusphere-2024-2015', Anonymous Referee #2, 20 Oct 2024
This study investigates the hydrological impacts of large-scale tree cover changes and climate changes using the Budyko framework with data from CMIP6 models, tree cover datasets, and the UTrack moisture recycling dataset. The authors claim that this study provides a first estimate of the impacts of tree cover change under climate change on global hydrological fluxes over land. However, the novelty of the methods and conclusions is significantly overstated. Numerous studies have already addressed this topic, and the methodological flaws prevent me from recommending this paper for publication at its current stage. My major concerns are as follows:
- The study estimates ET and Q using the Budyko equations, assigning fixed parameters for scenarios with and without tree cover. This approach predetermines the impact of tree cover change on the water balance, reducing the subsequent calculations to a numerical exercise with limited scientific value. Moreover, the implicit assumption behind this method—that changes in Budyko parameters are solely related to tree cover—is problematic. Climate factors, such as snow proportion and rainfall intensity, also influence Budyko parameters and affect the estimates of ET and Q. At a minimum, the authors should validate their approach by demonstrating its ability to estimate ET and Q using observational data.
- The logic behind combining historical UTrack data with projected future precipitation to represent the scenario of ‘Climate change with tree cover change with altered moisture recycling’ is highly confusing. These datasets cover different time periods, and the authors themselves admit that "The UTrack dataset does not account for the feedbacks of altered tree cover." Additionally, forest-climate feedbacks are complex and can affect net radiation through changes in albedo, surface temperature, and other factors, thereby influencing PET. Why do the authors focus solely on precipitation changes without considering the potential impacts on the energy balance?
- There is a lack of coherence between the simulation results and tree cover datasets. The authors use the global tree cover dataset provided by Hansen et al. (2013) as the tree cover data for the ‘present’ scenario, but the climate models they use for the ‘present’ scenario do not incorporate this tree cover dataset. The same issue arises for future simulations with potential tree cover changes. In conjunction with my first point, it seems that the study's methodology is more of a numerical combination of different datasets rather than a robust scientific analysis. This raises doubts about the reliability of the results.
- The analysis is overly simplistic. While the paper devotes significant space to introducing tree cover change, it notes that tree cover change "is not used for calculations and is solely used to visualize the differences in tree cover between the present climate and CC, and the CC+TCC scenarios." The final result only reiterates that while climate change increases global runoff, changing tree cover reverses this effect, leading to a limited net impact on runoff compared to the present climate and current tree cover. This conclusion has been well documented in the literature, and the paper does not offer any additional insights.
- On a positive note, the authors have acknowledged many of these issues in their discussion. However, merely acknowledging these limitations is not enough to mitigate their impact on the reliability and scientific integrity of the paper. A more rigorous scientific approach is needed to explore this important and interesting topic.
Citation: https://doi.org/10.5194/egusphere-2024-2015-RC2 -
RC3: 'Comment on egusphere-2024-2015', Anonymous Referee #3, 25 Oct 2024
The study by Engel et al. presents a very interesting approach to examining the hydrological impacts of large-scale tree cover change under future climate scenarios. The interdisciplinary method you employ, combining data from multiple CMIP6 climate models, Budyko models, and the UTrack dataset, provides a good initial estimate of how climate change and tree cover shifts may influence water availability.
The author’s acknowledgment of the limitations, particularly the inability of the UTrack dataset to capture energy balance changes in both current climate (CC) and future tree cover change (TCC) scenarios, is well-placed. I appreciate that you have addressed these important limitations in detail within the methodology and discussion sections, providing clarity on the scope of your findings.
Despite these constraints, the manuscript still offers valuable insights into the potential hydrological consequences of tree cover change at a global and regional scale. The authors highlight the complex interplay between climate-driven and vegetation-driven effects on runoff. Future studies that could take a more complex approach and employ fully coupled models and could build on your findings to provide an even more comprehensive understanding.- Visualization of Table 1: Consider redrawing Table 1 as a flow chart to clarify the workflow. This could improve the reader's understanding of your methodology at a glance.
- Clarity in Methodology: The beginning of the methods section could be more accessible. I suggest explaining the necessity of including multiple Budyko models for the uncertainty estimate earlier in the section to guide readers through your approach. Streamlining Table 1, possibly by replacing it with a simplified flow chart, and moving the detailed Table 1 to the appendix could help improve clarity.
- Use of the UTrack Dataset: The decision to use the UTrack dataset at a 1° resolution, when it is available at 0.5°, warrants an explanation. Additionally, a recent preprint (https://www.researchsquare.com/article/rs-4177311/v2) has highlighted a potential issue with the water balance in the dataset, which should be acknowledged, particularly since it is being utilized for water balance estimations. Ensuring that the global water balance checks out would strengthen the validity of your analysis. Furthermore, here are two papers that may be relevant, as they utilize the UTrack dataset for basin-level estimations and also account for the impact of land use changes. These references could provide additional context and support for your analysis (https://www.nature.com/articles/s44221-024-00291-w, https://doi.org/10.1029/2023EF003837).
Citation: https://doi.org/10.5194/egusphere-2024-2015-RC3
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