Drought onset and propagation into soil moisture  and grassland vegetation responses during the  2012–2019 major drought in Southern California

Warter, Maria Magdalena; Singer, Michael Bliss; Cuthbert, Mark O.; Roberts, Dar; Caylor, Kelly K.; Sabathier, Romy; Stella, John

doi:https://doi.org/10.5194/hess-25-3713-2021

Articles | Volume 25, issue 6

https://doi.org/10.5194/hess-25-3713-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-25-3713-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 6

Research article

|

01 Jul 2021

Research article |

| 01 Jul 2021

Drought onset and propagation into soil moisture and grassland vegetation responses during the 2012–2019 major drought in Southern California

Maria Magdalena Warter, Michael Bliss Singer, Mark O. Cuthbert, Dar Roberts, Kelly K. Caylor, Romy Sabathier, and John Stella

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Final revised paper (published on 01 Jul 2021)
Supplement to the final revised paper
Preprint (discussion started on 29 Sep 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Referee Comment', Anonymous Referee #1, 29 Oct 2020
- AC1: 'Reply on RC1', Maria Warter, 18 Feb 2021
RC2: 'Comment on "Onset and propagation of drought into soil moisture and vegetation responses during the 2012-2019 drought in Southern California" by Warter et al.', Anonymous Referee #2, 09 Feb 2021
- AC2: 'Reply on RC2', Maria Warter, 18 Feb 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (28 Feb 2021) by Nunzio Romano

AR by Maria Magdalena Warter on behalf of the Authors (17 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (01 Apr 2021) by Nunzio Romano

RR by Anonymous Referee #1 (10 May 2021)

Suggestions for revision or reasons for rejection

I think overall it was reasonably well revised and authors have satisfactorily addressed all major concerns. I have few minor points that needs further clarification.

1) In the manuscript, it seems like PET and ET0 are used interchangeably. While this is very common, PET and ET0 can mean two different things. PET is used to describe potential evapotranspiration over a large area, either under uniform vegetation (Penman-Monteith) or no vegetation (e.g. Penman model), under unlimited supply of water. ET0 on the other hand is the amount of water transpired by a short green crop of uniform height (often grass), completely shading the ground, and never short of water. While they both express evaporation power of the atmosphere, the underlying conditions are quite different. Reading at the text on page 8L165 onward, it seems like Penman-Monteith model was used to estimate ET0. I suggest avoiding the use PET due to inherent ambiguities. Also please clarify the reference crop/vegetation used in applying the Penman-Monteith model.
2) Somewhat related to above point, the description AET calculation within the SMBM model requires refinement as I do not think that the equation (3) is interpreted correctly. When ET0 is multiplied with the crop coefficient Kc, it results in actual crop evapotranspiration (see BOX 2 http://www.fao.org/3/x0490e/x0490e04.htm#reference%20crop%20evapotranspiration%20(eto) and not PET as stated. I think the confusion may be originating from Cuthbert et al. (2013) with similar interpretation.

I was unable to find the original paper summarizing the SMBM model (Allen et. Al., 1998 link in the citation is not active). However, here is what I understood from Cuthbert et al. (2013):

1) SMBM takes ET0 as an input and not Kc x ET0 as stated on page 12 L247.
2) It calculates ETc under standard condition as Kc x ET0
3) And finally, AET is calculated by applying a stress factor to ETc (as Ks x ETc) as described in the above FAO link (Box 3). Ks is calculated within the model using the equation 4 in Cuthbert et al. (2013).

I suggest rewriting the equation (3) in the manuscript as:

ETc = kcv1 x ET0

and correctly referring the variables ET0, ETc, and AET throughput the manuscript. Or, taking the equation out completely since nowhere ETc is being used. If you would like to show how AET is calculated the something like this will be helpful:

AET = Ks x Kc X ET0

Where Ks is a stress factor calculated within the model using equation 4 in Cuthbert et al. (2013).
Few minor editorials:
P11L 214: closing bracket is missing.
P16L313 refers PET but Figure 4c is showing ET0
Figure 6 c: you might want to show the R2 on the figure itself.
Equation (4). Use of “aP” as accumulated precipitation is confusing with the coefficient a. Either use “AP” or change the regression coefficient to alpha and beta.
End note reference link broken in the supplementary material.

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RR by Anonymous Referee #2 (11 May 2021)

Suggestions for revision or reasons for rejection

The manuscript has been significantly improved in revision. A few remaining suggested edits are listed below. Line numbers reference lines in the track changes version of the document.

L228: Long sentence with some redundancies – maybe: “In this study we build upon the FAO soil moisture modeling approach by including dynamic interactions between vegetation and climate through the incorporation of remotely sensed data. We use the model to investigate the evolution of soil moisture during the recent California drought and under several potential future drought scenarios.”

Fig. 1: Not sure if this is possible, but if there are photos of the two sites that visually demonstrate the differences in vegetation and condition, these might add interest to Fig. 1 and help to draw the reader in.

L380: “…where data have been continuously recorded…” then “Meteorological data…include…”

L404: “these gaps in the data *are* indicated in our results”.

L409: Suggest some editing here, as “To analyze the seasonality and relationship between soil moisture and vegetation for our study period, we used NDVI computed from red and near infrared surface reflectance data distributed by the USGS for Landsat-5 (Thematic Mapper), Landsat-7 (Enhanced Thematic Mapper) and Landsat-8 (Operational Land Imager) – each with a 16-day acquisition interval and 30-m resolution. Because we are using multiple Landsat instruments, the data from Landsat-5, 7 and 8 were harmonized using the approach of Goulden and Bales (2019)..” L413: Do you mean all cloudy pixels were removed (rather than all cloudy images)?

L415: Suggest “The polygons are based on field surveys made during site installation and on NDVI image analysis, delineating regions of relatively homogeneous NDVI including only grassland vegetation (no trees).”

L460: Can remove sentence “Runoff is assumed..” since this was stated back in L455.

Sec 3.1: Suggest changing to “Climatology of the Drought”

L685: Suggest adding these edits: “The seasonal temperature differences during the March – October dry season between drought periods was +0.7°C between non drought and moderate drought, +1.9°C between non drought and extreme drought and +1.3°C between moderate and extreme drought at the coastal site, and +1.1, +1.9 and +0.8°C for the inland site, respectively.”

Fig 6: in 6b, it is not visually clear that the NDVI varies much between drought classes at the inland site although L771 cites significant differences. Suggest adding legend to Fig 6d and e. It’s not immediately obvious what the 4 lines represent given there are 3 drought classes, and the labels are somewhat ambiguous in their relationship to specific lines.

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ED: Publish subject to minor revisions (review by editor) (23 May 2021) by Nunzio Romano

AR by Maria Magdalena Warter on behalf of the Authors (25 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 May 2021) by Nunzio Romano

Short summary

Intensified drying of soil and grassland vegetation is raising the impact of fire severity and extent in Southern California. While browned grassland is a common sight during the dry season, this study has shown that there is a pronounced shift in the timing of senescence, due to changing climate conditions favoring milder winter temperatures and increased precipitation variability. Vegetation may be limited in its ability to adapt to these shifts, as drought periods become more frequent.