the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Measuring evaporation across canopy phenophases of a natural forest: Miombo forest, Southern Africa
Henry Zimba
Miriam A. J. Coenders-Gerrits
Kawawa E. Banda
Bart Schilperoort
Imasiku A. Nyambe
Nick C. Giesen
Hubert H. G. Savenije
Abstract. Atmospheric water demand drives forest evaporation controlled by the plant physiological properties within available moisture storage thresholds. The pattern and magnitude of African Miombo Forest transpiration across dry season canopy phenophases are unknown. This is because estimating forest evaporation in African ecosystems continues to be a challenge as flux observation towers are scant, if not completely lacking in most ecosystems like the Miombo Forest, one of Africa’s largest woodland formations. Moreover, in the Miombo Forest, satellite data-based evaporation products (i.e., GLEAM, MOD16, SSEBop and WaPOR) show significant discrepancies in both pattern and amounts of evaporation especially during the dry season canopy phenophases. Despite the main limitations with estimation of forest evaporation the development and application of the distributed temperature sensing (DTS) system is providing deepened insights and improved accuracy in forest energy partitioning for evaporation assessment. In this study the Bowen ratio distributed temperature sensing (BR-DTS) approach is used to partition available energy and estimate evaporation across three Miombo Forest canopy phenophases covering the entire 2021 dry season and early rain season. Furthermore, four satellite evaporation products are compared to the field observations. Results show that evaporation appears to follow the net radiation and air temperature pattern with the lowest values observed during the most net radiation and air temperature depressed periods and highest values during the peak net radiation and air temperature. Evaporation continues to rise even during the driest period in the dormant leaf phenophase when canopy cover is said to be at its minimum. This is possibly facilitated by the retention of about 70 percent canopy cover during the dry season which transpires within the adapted thresholds constrained by physiological properties of each Miombo Forest species with access to ground water and vegetative water storage. This goes to show that during the dry season Miombo species may not be as water stressed as imagined. When compared to field observations all four-satellite evaporation products underestimate evaporation with only the WaPOR showing a similar pattern of evaporation during the dry season. The differences between field observations and satellite-based evaporation products can be attributed to the model structure, processes as well as inputs.
Henry Zimba et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2022-303', Anonymous Referee #1, 08 Nov 2022
The authors present a well written manuscript on their research which describes the measurement of evaporation in a Miombo forest using the BR-DTS approach. The research has a clear aim and justification, with a throrough description of the methodological approach.
The results of the research are presented clearly and discussed adequately.
The attached PDF includes a few minor grammatical corrections, but I would suggest an additional round of language editing.
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AC1: 'Reply on RC1', Henry Musonda Zimba, 10 Dec 2022
The reviewers observations and comments are highly appreciated.
Our brief response is here attached
- AC3: 'Reply on AC1', Henry Musonda Zimba, 05 Feb 2023
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AC1: 'Reply on RC1', Henry Musonda Zimba, 10 Dec 2022
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RC2: 'Comment on hess-2022-303', Anonymous Referee #2, 10 Nov 2022
In this paper the DTS method was used to estimate total evaporation in a Miombo Woodland in Mpika, Zambia. These results were compared with four remote sensing products.
I commend the project team for the installation and maintenance of the equipment at what must have been a challenging site. The measurements in this particular vegetation type is where the paper makes a significant contribution. A second contribution is that remote sensing models do not do a great job of estimating ET accurately in tall, heterogenous natural vegetation types. In fact, I wondered whether this is not two separate papers with the first being the DTS measurements and the second the remote sensing? However, separately, they may be a bit sparse?
It would have helped me to understand the canopy better, if there had been a temporal graph of LAI, preferably measured monthly. It could be a satellite derived product though if no measurements were taken.
The Atmos41 was not the ideal choice of instrument to validate the DTS air T and RH measurements and it is designed to be a low maintenance station and not really for research grade experiments. Radiation shielding is the main problem and this is mentioned where the data is assessed.
This work makes a useful contribution but the grammar and language needs improvement throughout. It really made it difficult for me to review as it detracted from the scientific content. So, my suggestion is major revision of the grammar and manner in which sentences are structured. Generally, I’m happy with the paper structure and even the paragraphs are structured well. The paragraphs explaining why the RS models have problems are quite long and hard going to read through. I suggest possibly trying to put the info into a table to reduce the length of the paragraphs?
I think the title needs to be reviewed or considered. Much of the paper is about RS but this is not reflected in the title? Consider replacing the word ‘measuring’ with ‘estimating’? Did you actually measure evaporation? You measured temperatures really?
Please note the pdf attached with 232 comments and edits. I gave up on grammar issues at the start of section 3.5. I would think that most of the comments can be addressed relatively easily.
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AC2: 'Reply on RC2', Henry Musonda Zimba, 12 Dec 2022
We find the reviewers observations and comments extremely helpful. We have made effort to address the issues raised. If we missed any of the comments it’s not intentional. Additional comments are in the attached pdf document.
Reviewers comment
In this paper the DTS method was used to estimate total evaporation in a Miombo Woodland in Mpika, Zambia. These results were compared with four remote sensing products.
I commend the project team for the installation and maintenance of the equipment at what must have been a challenging site. The measurements in this particular vegetation type is where the paper makes a significant contribution. A second contribution is that remote sensing models do not do a great job of estimating ET accurately in tall, heterogenous natural vegetation types. In fact, I wondered whether this is not two separate papers with the first being the DTS measurements and the second the remote sensing? However, separately, they may be a bit sparse?
Our response
We appreciate the observations concerning splitting the paper into two. There is actually another paper focused on remote sensing.
Reviewers comment
It would have helped me to understand the canopy better, if there had been a temporal graph of LAI, preferably measured monthly. It could be a satellite derived product though if no measurements were taken.
Our response
We did not take measurements of LAI and NDVI. However, we took photographs of the top of the canopy cover across phenophases. The temporal graph of LAI and NDVI has been added in the remote sensing paper. Nevertheless, the graph can be added in this paper if that will help with the understanding of the canopy. This information is also available in Zimba et al. (2020) (Zimba, Henry, Coenders-Gerrits, M., Kawawa, B., Savenije, H., Nyambe, I., & Winsemius, H. (2020). Variations in canopy cover and its relationship with canopy water and temperature in the miombo woodland based on satellite data. Hydrology, 7(3). doi: 10.3390/HYDROLOGY7030058). The graph in the remote sensing paper is more closer representation of the LAI and NDVI because it is based on the flux foot print.
Reviewers comment
The Atmos41 was not the ideal choice of instrument to validate the DTS air T and RH measurements and it is designed to be a low maintenance station and not really for research grade experiments. Radiation shielding is the main problem and this is mentioned where the data is assessed.
Our response
This was a challenge. In the absence of “research grade equipment” we used the ATMOS-41to observe if there were major differences in the DTS temperature measurements.
Reviewers comment
This work makes a useful contribution but the grammar and language needs improvement throughout. It really made it difficult for me to review as it detracted from the scientific content. So, my suggestion is major revision of the grammar and manner in which sentences are structured.
Our response
This can easily be done
Reviewers comment
Generally, I’m happy with the paper structure and even the paragraphs are structured well. The paragraphs explaining why the RS models have problems are quite long and hard going to read through. I suggest possibly trying to put the info into a table to reduce the length of the paragraphs?
Our response
The recommendation is highly appreciated.
Reviewers comment
I think the title needs to be reviewed or considered. Much of the paper is about RS but this is not reflected in the title? Consider replacing the word ‘measuring’ with ‘estimating’? Did you actually measure evaporation? You measured temperatures really?
Our response
A very important observation. This will be reviewed.
Reviewers comment
Please note the pdf attached with 232 comments and edits. I gave up on grammar issues at the start of section 3.5. I would think that most of the comments can be addressed relatively easily.
Our response
We appreciate the comments and edits. The grammar will be corrected.
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AC4: 'Reply on AC2', Henry Musonda Zimba, 05 Feb 2023
We find the reviewers observations and comments extremely helpful. We have made effort to address the issues raised. If we missed any of the comments it’s not intentional. Reviewers comments are denoted by RC while our responses are denoted AC. Additional comments are in the attached pdf document.
RC
In this paper the DTS method was used to estimate total evaporation in a Miombo Woodland in Mpika, Zambia. These results were compared with four remote sensing products. I commend the project team for the installation and maintenance of the equipment at what must have been a challenging site. The measurements in this particular vegetation type is where the paper makes a significant contribution. A second contribution is that remote sensing models do not do a great job of estimating ET accurately in tall, heterogenous natural vegetation types. In fact, I wondered whether this is not two separate papers with the first being the DTS measurements and the second the remote sensing? However, separately, they may be a bit sparse?
AC
We appreciate the observations about splitting the paper into two. We are glad to say that there is already a paper speaking to this observation. The paper is focused on comparing satellite-based evaporation estimates at basin scale and across the various phenophases of Miombo forest. It will be assessed under HESS Journal.
RC
It would have helped me to understand the canopy better, if there had been a temporal graph of LAI, preferably measured monthly. It could be a satellite derived product though if no measurements were taken.
AC
We did not take measurements of LAI and NDVI. However, we took photographs of the top of the forest canopy across phenophases. The temporal graph of LAI and NDVI has been added in the remote sensing paper. Nevertheless, we have also added the same graph in this paper. We have also included additional photographs (Figure A3 in the appendices). We believe this will help with the understanding of the phenophase dynamics in the Miombo forest canopy cover. This information is also available in Zimba et al. (2020) (Zimba, Henry, Coenders-Gerrits, M., Kawawa, B., Savenije, H., Nyambe, I., & Winsemius, H. (2020). Variations in canopy cover and its relationship with canopy water and temperature in the miombo woodland based on satellite data. Hydrology, 7(3). doi: 10.3390/HYDROLOGY7030058).
RC
The Atmos41 was not the ideal choice of instrument to validate the DTS air T and RH measurements and it is designed to be a low maintenance station and not really for research grade experiments. Radiation shielding is the main problem and this is mentioned where the data is assessed.
AC
This was a challenge. In the absence of “research grade equipment” we used the ATMOS-41 to observe if there were major differences in the DTS temperature measurements.
RC
This work makes a useful contribution but the grammar and language needs improvement throughout. It really made it difficult for me to review as it detracted from the scientific content. So, my suggestion is major revision of the grammar and manner in which sentences are structured.
AC
We had the entire manuscript grammar edited. We believe the grammar issues raised have been adequately addressed.
RC
Generally, I’m happy with the paper structure and even the paragraphs are structured well. The paragraphs explaining why the RS models have problems are quite long and hard going to read through. I suggest possibly trying to put the info into a table to reduce the length of the paragraphs?
AC
It is difficult to shrink the information into a table and convey the same information. However, to make the section more concise we have revised the long sentences and some components. The revised paragraphs should now be easily understood.
RC
I think the title needs to be reviewed or considered. Much of the paper is about RS but this is not reflected in the title? Consider replacing the word ‘measuring’ with ‘estimating’? Did you actually measure evaporation? You measured temperatures really?
AC
A very important observation. We reviewed the title and it now includes the remote sensing component. We believe the new title covers the important aspects of the paper.
RC
Please note the pdf attached with 232 comments and edits. I gave up on grammar issues at the start of section 3.5. I would think that most of the comments can be addressed relatively easily.
AC
We have responded to each of the 232 comments in the pdf document. We addressed all the issues raised as suggested by the reviewer. The entire manuscript has been checked for grammatical errors and necessary corrections have been made.
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AC4: 'Reply on AC2', Henry Musonda Zimba, 05 Feb 2023
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AC2: 'Reply on RC2', Henry Musonda Zimba, 12 Dec 2022
Henry Zimba et al.
Data sets
ZAMSECUR Project Miombo Forest, Zambia, Southern Africa Henry Zimba, Hubert H.G. Savenije, Nick van de Giesen, Miriam CoendersMiriam Coenders, Bart Schilperoort https://doi.org/10.4121/20492934.v1
Henry Zimba et al.
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