| 11 Jun 2026
Multi-Sensor Remote Sensing Evidence of Connection between Deep Aquifer Recharge in California's Central Valley and Sierra Nevada Snowmelt
Abstract. California’s arid Central Valley (CV) relies heavily on groundwater extracted from deep aquifers (>50 m) and imported surface water to sustain approximately one-quarter of the United States’ food production. While the recharge to deep aquifers in CV is hypothesized to be influenced by hydrologic processes in the adjacent Sierra Nevada, direct observational evidence at regional scales remains limited.
Here, we present an integrated analysis of multi-decadal geodetic remote sensing, hydrologic, and climate datasets, including groundwater levels, GRACE-derived water storage, GNSS and InSAR deformation, and snowmelt observations, to investigate the spatiotemporal relationships between mountain hydrology and deep aquifer dynamics in the CV. A consistent sequence of seasonal signals emerges across independent datasets: peak groundwater levels in deep CV aquifers occur approximately one month after peak water availability from Sierra Nevada snowmelt, while peak groundwater storage inferred from GRACE lags groundwater levels by approximately three months.
These phase delays indicate that groundwater systems respond to mountain hydrologic forcing through temporally distinct processes, with pressure signals propagating more rapidly than changes in bulk water storage. A simplified first-order diffusion analysis shows that the observed lag between snowmelt and the groundwater-level response is feasible, given plausible pressure propagation timescales in fractured mountain bedrock.
Together, these results provide observational evidence consistent with a hydraulic connection between high-mountain aquifers and deep basin aquifers via mountain block recharge (MBR). Our findings highlight the importance of incorporating mountain-driven recharge processes and pressure dynamics into regional hydroclimate models and groundwater management strategies, particularly in snowmelt-dominated and water-stressed regions.
GENERAL COMMENT
The paper addresses relevant scientific questions that fall well within the scope of the journal. The manuscript lays the groundwork for further studies and investigations aimed at characterizing important recharge mechanisms in valley aquifers and their interactions with adjacent mountain systems. The results presented appear sufficient to support the interpretations and conclusions drawn by the authors, who also provide appropriate credit to previous work while clearly highlighting the novel contributions of the present study. In addition, the title accurately reflects the content of the paper, and the abstract provides a concise and comprehensive summary of both the analyses performed and the main findings.
A few references appear to be missing throughout the manuscript: these are indicated in the “Technical corrections” section below. I would also like to comment on the substantial amount of the supplementary material provided. In my opinion, including such a large amount of additional material may make it more difficult for readers to identify the key results and conclusions of the study. I would therefore strongly encourage the authors to consider reducing the supplementary material by removing sections that are not essential for understanding the analyses presented in the manuscript.
SPECIFIC COMMENTS
1. It seems to me that the statement in lines 38-40 (“A direct consequence [...] (Galloway et al., 2000; Herrera-García et al., 2021).”) would benefit from additional context. It appears that a word may be missing (e.g., “deep confined aquifer exploitation” or “deep confined aquifer depletion”), which could help clarify why this statement is included at this point in the manuscript. If the discussion concerns both aquifer recharge and depletion, the term “replenish” in line 40 might be better replaced by wording referring to both filling and depletion processes. Otherwise, if the discussion concerns recharge only, the sentence in line 38-40 might be more appropriately relocated elsewhere in the manuscript.
2. Lines 94-95. The connection between the low groundwater flow velocity and the expectation that deep lowland aquifers are recharged by lateral flows originating at higher elevations is not entirely clear to me. Could the authors please provide some clarification on this point?
3. Line 158. I am not entirely sure that the term “data-driven” (also al lines 164 and 640) is appropriate in this context, although it may simply reflect my limited familiarity with the method. Could the authors please provide some clarification on this point?
4. Lines 158-159. For greater clarity and completeness, the authors may whish to briefly justify here the choice of the methods adopted in the analyses presented in the manuscript.
5. Line 181. The authors may consider including a brief explanation of what is meant by mascon cells. Readers who are not familiar with the topic may otherwise find this part difficult to follow.
6. Lines 193-198. If I understood correctly, this section refers to SnS rather than SoS, as currently written in lines 193 and 197. SoS is instead discussed in lines 186-191. If this is the case, I suggest correcting the terminology accordingly.
7. Lines 204-207. The purpose and relevance of the analysis described in these lines within the context of the present study are not entirely clear to me. The authors may wish to provide additional context or motivation.
8. Lines 219-220. For greater clarity and transparency, I would suggest explicitly stating the additional selection criteria based on the distance from pumping wells and on the historical stability of water levels.
9. Lines 222-223. On what basis was this method selected for outlier detection and removal? Is this a standard procedure in similar studies?
10. Lines 226-227. Is the requirement for time series spanning at least 3 years and containing less than 3 months of missing data an additional criterion for selecting the wells used in the subsequent analyses? This is not entirely clear from the current wording.
11. Line 233. The meaning of the term “unique” in this context is not entirely clear to me.
12. Lines 241-242. I am not entirely clear on why an increase in water mass would lead to subsidence, whereas a decrease in mass would result in surface uplift. I would intuitively expect the opposite, although this may simply reflect my limited familiarity with the topic. In any case, I would suggest including a brief explanation in the manuscript to facilitate understanding for readers who may not be fully familiar with the underlying processes.
13. Similarly, I would suggest expanding slightly on the explanation of the poroelastic deformation process described here.
14. Line 232. If the quantities hp2 and hp1 mentioned here correspond to h2 and h1 in Figure 1, the authors may consider adopting a consistent notation throughout the manuscript to facilitate the comprehension of the process described.
15. Lines 333-334. To what extent are the assumptions mentioned here realistic and applicable to the considered study area?
16. Line 480. The rationale for selecting k=0.3 m2/s is provided later in the manuscript (line 575). However, I would suggest either moving this explanation to this earlier point or briefly adding it here when the value is first introduced, so that the choice is immediately justified.
TECHNICAL CORRECTIONS
17. Lines 58-59. In my opinion, “water for later use” reads more naturally than “water for use later”. Please consider changing the text.
18. Line 94. There is a repetition of “However”, which is also used in line 92. I would also suggest starting a new paragraph at this point in order to give greater prominence to the subsequent discussion of the two recharge mechanisms (MFR and MBR).
19. Line 107. If the “recent study” referred to here is the one cited in the following line (Armengol et al., 2024), it would be preferable to state this explicitly here. Otherwise, please include the appropriate reference.
20. Lines 136-138. For completeness, the authors may consider to include references for GRACE, GNSS and InSAR.
21. Line 156. It may be preferable to use a different term instead of “feasible”, for example “plausible” or “realistic”.
22. Line 156. After point (3), please correct the singular/plural agreement by using either “have a […] signature” or “have […] signatures”.
23. Line 179. If possible, please include a reference for the RL06 Level-3 product from NASA’s Jet Propulsion Laboratory.
24. Line 203. The abbreviation “SN” for Sierra Nevada is not introduced previously in the text and only appears later in the caption of Figure 6. The authors may consider either to define it when first used or to remove it.
25. Line 252. The expression “the most severe effects” may be clearer than “the effects more severe”; the authors may wish to consider this modification.
26. Line 257. If possible, please include a reference for the GipsyX software.
27. Linnes 276-277. The acronym LOS (line of sight) is already defined in line 266 and does not need to be introduced again here.
28. Lines 285. In most references containing two authors, the names are separated by “&”, whereas here they are separated by “and”. Please verify consistency throughout the manuscript.
29. Please verify that the figures included in the supplementary material appear in the same order in which they are first referenced in the main manuscript.
30. Figure 5. I would suggest swapping the positions of the maps currently shown in Figures 5b and 5c, so that the panels are ordered row-wise rather than column-wise, consistently with the other figures in the manuscript.
31. Please aim to maintain consistency throughout the manuscript in the way multiple figures are cited within the text. For example, in line 468 Figures 6e and 6g are separated by a conjunction (“Fig. 6e and 6g”), whereas in line 471 Figures S13b and S13c are separated by a comma without repeating the figure number (“Fig. S13b,c”). Similarly, line 491 uses “Figs.” Instead of “Fig.” before a list of two figures.
32. Line 482. Please add a space between “100” and “m”, so that the text reads “100 m” rather than “100m”.
33. Line 624. The suffix “st” in “21st” should be superscripted.
34. Line 651. There currently appear to be unnecessary parentheses around the two references cited.