the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Sep 2026
GC Insights: On The Definition of Aridity in Geoscience Research
Abstract. Aridity is a fundamental concept used across geoscience to describe water-limited environments, yet the scientific community currently uses multiple contradictory approaches to communicate aridity science. Here, we present the scope of aridity definition divergence, showing that out of 1000+ assessed studies aridity, A, is commonly defined as both the ratio of precipitation, P, to evaporative potential, EP, as A = P/EP (32 % of papers) and as its mathematical reciprocal A = EP/P (23 % of papers), or is not defined at all (29 % of papers). Based on this split between opposing definitions, we suggest standardization of the meaning of aridity for more effective interdisciplinary communication, accurate modeling, and metanalyses pertaining to water scarce regions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3062', Anonymous Referee #1, 13 Oct 2026
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RC2: 'Comment on egusphere-2026-3062', Anonymous Referee #2, 27 Oct 2026
In this manuscript, authors classify roughly 1100 Web of Science–indexed studies by how they define aridity. They find that P/EP (32%) and its reciprocal EP/P (23%) are both common, and that 29% of studies do not define aridity at all. They summarise the history of aridity indices, show how usage varies by field, over time and by citation rate, and recommend that the community adopt EP/P so that higher aridity means drier conditions.
This is a clear, timely insight that suits the Geoscience Communication Insights format. I like the field breakdown (Fig. 1b). This would be specially useful for readers moving between hydrology, ecology and climate science. I would also support the intuitiveness-based argument (higher aridity = drier) as it makes a stronger case for EP/P.
However, I have minor concerns before I can recommend this for a publication. The main changes I ask for are: (i) a short explicit methods paragraph; (ii) removal of the citation-based argument for Epot/P; (iii) a direct discussion of the established P/Epot infrastructure; and (iv) correction of a few weaker supporting arguments . My detailed concerns are listed below.
General comments
- Authors seem to use citation count as a causal measure of output which needs to be toned down. Citations should not be used to justify the standard. The statement that Epot/P “leads to more impactful geoscience” (around line 100) is a causal claim that citation counts cannot support. Also this can be a misleading inference because two confounders can explain much of the this difference. First, highly cited Budyko-framework hydrology papers dominate the Epot /P group. Second, according to the supplementary table, P/Epot’s share rises from about 13% of studies published before 2010 to about 42% of those published since 2023, and recent papers have had less time to collect citations. So the difference can be biased and trend in citations would say that P/Epot is a better choice. When studies are compared within 5-year publication windows, the difference in citations per year is small and changes sign between windows. The heavy tail in Fig. 1c that authors observe may also depend on a few individual papers. Thus, caption should explain how zero-citation papers were handled in the log-normal fit, and any claim of a difference, if authors want to highlight, should be backed by a test statistic with things that I discussed above taken into account.
- I think authors should also give some weight to already established P/EP infrastructure. For example:
- P/Epot is the aridity index behind the UNESCO/UNEP dryland classes, which are now quite heavily used in policy, are well recognized for example, in UNCCD reporting.
- The Trabucco and Zomer datasets (2008, 2022) map P/EPot, and many ecology and land-use papers rely on them.
- The authors’ own data (Fig. 1a) suggest P/Epot is becoming more common and is being cited more.
A recommendation to adopt Epot/P therefore asks part of the community to invert thresholds it already uses which thus has implications (or one can call them costs of switching) to an existing infrastructure going up to policy, not just science. This manuscript would be more persuasive if it addressed this directly. One option would be to keep (P/Epot) for the UNEP version and call it humidity index (HI), and use “dryness index” or “aridity index” (AI = Epot/P) for the Budyko version, as much of the hydrology literature already does. The authors note this aridity vs aridity index confusion in the manuscript, but do not use it in their recommendation.
3. The correlation argument (L104–108, Fig. 1a inset) applies equally to both definitions but authors try to make the case for one version using it. The loss of Pearson correlation under a reciprocal transform is symmetric. A variable that is perfectly linear in P/Epot loses the same correlation with Epot/P. Thus, the inset is a good reason to state the definition clearly, or to use rank-based statistics, but it is not evidence for one definition over the other. Please reframe it that way.
4. A practical reporting recommendation would be benefitial. I think in the end, whatever convention is adopted, the clearest communication gain would come from asking authors to always report:
- the formula and symbol;
- the Epot method (e.g. Thornthwaite, Penman–Monteith, Priestley–Taylor, pan evaporation, Rn/λP);
- the averaging period;
- whether the index is a ratio of long-term means or a mean of annual ratios.
Differences in how Epot is estimated arguably cause as much (if not more) disagreement between studies as the direction of the ratio (see. Milly and Dunne, 2016; Greve et al., 2019; Scheff et al., 2021, all of which appear in the authors’ samples!). A one-sentence checklist would give readers something they can use even if they do not adopt Epot/P.
- I would also encourage authors to describe the search and classification protocol which is only sketched right now. I am wondering whether requiring “evap*” in search biases the sample towards Budyko-type hydrology papers, which tend to use Epot/P. Several widely cited studies that use P/Epot or 1−P/Epot do not seem to be included (e.g. Feng and Fu, 2013; Huang et al., 2016; Berdugo et al., 2020). So please add one small but explicit paragraph covering things like:
- the exact WoS query string and which fields were searched (topic, title or abstract);
- inclusion and exclusion criteria;
- what counted as “defined” (for example, does citing the UNEP index without a formula count?);
- what falls under “Other”;
- how papers using more than one definition, or ambiguous papers, were classified;
This will make the study more transparent and reproducible.
Bibliography:
Feng, S. and Fu, Q. (2013). Expansion of global drylands under a warming climate. Atmos. Chem. Phys., 13, 10081–10094.
Huang, J., Yu, H., Guan, X., Wang, G. and Guo, R. (2016). Accelerated dryland expansion under climate change. Nature Climate Change, 6, 166–171.
Berdugo, M., Delgado-Baquerizo, M., Soliveres, S., et al. (2020). Global ecosystem thresholds driven by aridity. Science, 367(6479), 787–790.
Citation: https://doi.org/10.5194/egusphere-2026-3062-RC2
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- 1
In the manuscript “On The Definition of Aridity in Geoscience Research”, the authors assess how aridity is defined across more than 1,000 studies indexed in Web of Science. They show that aridity is commonly defined as both Ep/P and its inverse P/Ep, while a substantial fraction of studies does not provide an explicit quantitative definition. They further highlight how the use of these definitions developed through time and among research areas and argue that EpP should become the common definition.
It is my assessment that the topic is relevant and well suited for Geoscience Communication. The manuscript is concise and generally well written and addresses a genuine problem in interdisciplinary geoscience. I agree with the authors that clearer and more consistent communication would be beneficial. However, I believe that the interpretation of the observed differences between disciplines and the resulting recommendation require further discussion. In addition, some aspects of the literature assessment need clarification. However, I think this a worthwhile and timely short communication, and I recommend publication after addressing my comments below.
Major comments
My first main point is that the disciplinary differences shown in Figure 1 deserve more discussion. The manuscript currently frames the coexistence of Ep/P and P/Ep mainly as an inconsistency (partly due to historical reasons) that should be resolved by agreeing on a single definition. However, both formulations may have advantages and disadvantages depending on the scientific perspective. I find it particularly interesting that Ep/P is more common in water-resources studies, whereas atmospheric-science studies tend to use P/Ep. I think this, at least partly, reflects also contrasting perspectives. Water-resources research often focuses on atmospheric evaporative demand relative to atmospheric water supply to the land surface, for which Ep/P is more intuitive. Atmospheric research may more commonly consider precipitation relative to the ability of the atmosphere to draw water from the surface. I therefore suggest discussing whether the disciplinary differences identified in Fig 1 partly also reflect different perspectives rather than only inconsistencies. I agree that Ep/P has the intuitive advantage that aridity increases with increasing values (I am hydrologist, so I may be biased as well). However, this does not necessarily make the inverse formulation less relevant, especially in disciplines where P/Ep may have an established physical interpretation. I suggest the authors still recommend Ep/P as a common definition, but this recommendation should acknowledge the reasons why other communities may use P/Ep. I guess a practical recommendation to always state the underlying equation and indicate the direction of increasing aridity may be at least as important as defining a common ratio.
My second point concerns the methodology of the literature assessment. The manuscript provides the search terms and the total number of publications, but some essential additional information is missing: When was the Web of Science search performed? Which publication types were considered? Have you applied any language or location restrictions? Did you check the classifications independently and manually (or at least a sample of it)? I do not think you need to provide a very detailed protocol for this short communication, but one or two additional sentences may be helpful.
My third point concerns the use of citation rates as an argument for Ep/P. The statement that this definition "leads to more impactful geoscience" is not really supported by the numbers. Citation rates differ between research fields, publication year, journal, etc. and may also be impacted by individual highly cited studies. I suggest removing that argument.
Minor comments
Line 54: What do you mean by "Man in the Biosphere Program"?
Line 80: I don't really get the meaning of that sentence. Consider revising it.
Line 100: I suggest replacing "leads to more impactful geoscience" with a neutral framing (see my comment above).
Line 119: In my opinion, the wording "severe errors" may be too strong. Consider using more cautious wording.
Line 136: "Preformed" should "be performed"