| 08 Aug 2025
More trees, more rain? The unexpected role of forest and aquifers on the global water cycle
Abstract. Forests’ influence on rainfall has been debated since antiquity, with historical observations suggesting that deforestation reduces precipitation. While early scientists believed that forests attract rain, later research provided conflicting views, and modern climate models remain inconclusive on forests’ role in regional precipitation at continental scale. A difficulty of global climate model is the need to integrate processes associated to multiple scientific field, like meteorology, hydrology and forest ecology. As a result, the numerous constitutive equations, the complexity of numerical codes and the variability of natural situations make difficult to provide a quantitative water cycle model controlled by a small number of key parameters. Assuming water mass conservation between ocean, atmosphere, ecosphere and continental aquifers, we revisit this problem and we build a simplified physical scheme of water transfer across these reservoirs. Assembling a limited number of constitutive equations into a numerical code, we propose a parsimony model able to simulate rainfall distribution patterns. Starting from initially different vegetation conditions (sparse and dense), we identify the parameters modulating steady state continental precipitation for both situations. First, we show that sparse vegetation patterns do not lead to uniform and large continental precipitation. Rather, continental moisture is spatially limited by atmospheric dispersion and wind, thus providing a decay of precipitation as a function of coastal distance as observed in most cases worldwide. Second, our model predicts that large and widespread continental precipitation results from the combination of dense vegetation patterns and deep aquifers low hydraulic conductivity. This ecological/hydrological interaction is due to the slowness of horizontal water flow, allowing underground reservoir level to increase enough for reaching plant roots, inducing in turn vegetation evapotranspiration and atmospheric moisture. Overall, our model shed new light on the balance between atmospheric water vapor transport by wind and dispersion, vegetation evapotranspiration and underground water flow. Noticeably, the efficiency of the water cycle and its related precipitation distribution not only depends from atmospheric parameters but is also largely modulated by vegetation dynamics and underground aquifers behavior. We finally discuss how forest growth or destruction may alter continental precipitation at various time scales.
The manuscript presents a model linking forests, aquifers, and rainfall distribution. The core ideas seem new and potentially useful: presenting complex climate–vegetation feedbacks in this simple framework has possible value, especially as a conceptual tool. The positive framing of forests as sustaining inland rainfall is also constructive and welcome. However, the current paper is hard to follow (I am not sure I grasp all the details) and unsuitable for publication without revision.
Major:
Framing and focus – The paper claims to “test the biotic pump” but does not engage with its defining mechanism. Instead, the model explores a conceptual model of forest–aquifer–rainfall interactions. For example, the sentence “We test the biotic pump” could be replaced with “We propose a simplified model of how forests and aquifers may jointly sustain inland rainfall.”
Structure, coherence and clarity – The argument is dense and difficult to follow. Assumptions, derivations, and conclusions are jumbled together. Methods and results should be separated, with explanatory text alongside equations. For instance, when presenting the conservation equations, explain each term before moving to outcomes.
Literature – The review of prior work is incomplete and risks misleading readers. For example, the manuscript cites criticisms of the biotic pump but omits published rebuttals of this criticism and various other studies that have advanced the debate. At minimum—if the focus on the biotic pump is maintained—the authors should add references to both critiques and responses, note further advances, and explain where their model and implied results sit relative to these debates.
Validation – Does the model capture real patterns and processes? We don’t know. Even a simple comparison figure or discussion would help.
Specific sections:
Introduction – Lacks a clear framing of the research gap. It blends critiques of the biotic pump with broad statements about forests and hydrology, which makes it difficult to identify a clear focus. The introduction should be restructured to identify the unresolved question and state precisely how the new model contributes.
Section 2 and Figure 1 – The purpose is unclear. It is not evident whether these results reproduce earlier work or simply serve as context/background. As written, the section is difficult to follow, and the connection to the rest of the paper is weak. If Figure 1 is meant to replicate established results, this should be stated explicitly and properly cited; if it introduces something new, the text should highlight what differs from prior publications and why this matters. In either case, the caption should clarify the intent. It did stimulate questions (not sure if relevant): If a comparison with past publications is intended then we need to know are the data suitable to show what is intended? How do these patterns compare if other data sources are used? What are the differences and why?
Results – The results are presented with little explanation of the logic linking assumptions to outcomes. It is opaque where it needs to be clear. Statements such as “forests amplify rainfall inland” are asserted without the required step-by-step justification. This section should be rewritten to guide the reader through the results.
Discussion – The discussion is underdeveloped. The authors should expand this section to situate their findings within some broader debate(s) and to clarify whether their model supports, contradicts, or simply complements the biotic pump hypothesis (a clearer goal would help too).
Conclusions – This section seems more a restatement of intent than a synthesis. Asserting that “forests sustain rainfall” seems too general (we know that already). The conclusions should restate the core contribution and new insights with clarity and precision.
Minor:
Terminology is inconsistent; clarify early whether “biotic pump” refers to the original condensation-driven hypothesis or a broader forest–rainfall linkage.
Long, complex sentences should be broken into shorter, clearer ones.
Overall : contains some stimulating ideas about important topics but needs substantial work on framing, clarity, literature, and validation before it might be published.