Quantifying Evapotranspiration and Drainage Losses in a Semi-Arid Nectarine (Prunus persica var. nucipersica) Field with a Dynamic Crop Coefficient (K_c) Derived from Leaf Area Index Measurements

Round 1

Reviewer 1 Report

Comments to Editor: 

The authors quantified the “evapotranspiration and drainage losses in a semi-arid nectarine field with a dynamic crop coefficient (Kc) derived from leaf area index measurements.” However, the authors have not explained the research problem clearly; therefore, it is strongly suggested first to describe the research problem and then explain how the authors will address their research question. Then, the message of this manuscript will be more effective and more explicit for readers. Based on my comments below, I would recommend a major revision of the manuscript.

Specific Comments

Abstract

The abstract needs to be revised. The authors are advised to add little more details of their study’s importance within the abstract, and then corellate this with their findings.

Introduction

The introduction is not providing enough information on the research background and research gap.

Materials and Methods

This section is fine.

Results

I strongly recommend the authors to write results with exact significant values and then calculate percentage differences between best and worst treatments.

Discussion

I think authors should rethink what they write in the first paragraph and only summarize the main findings in view of the research questions. After this, authors can explore different aspects of the work in subsequent paragraphs and explain how their findings expand the envelope of knowledge, but first of all, authors simply need to state the main results without discussing their why and how or the relationships to the literature. First of all, the reader needs a clear statement on what the study found. Moreover, it is suggested to discuss the main results in a logical way.

Author Response

Reviewer 1

R1C1) The authors quantified the “evapotranspiration and drainage losses in a semi-arid nectarine field with a dynamic crop coefficient (Kc) derived from leaf area index measurements.” However, the authors have not explained the research problem clearly; therefore, it is strongly suggested first to describe the research problem and then explain how the authors will address their research question. Then, the message of this manuscript will be more effective and more explicit for readers. Based on my comments below, I would recommend a major revision of the manuscript.

We would like to thank you for your comments and suggestions. We have revised the manuscript to explain the research problem better by reorganizing and expanding the introduction section (lines 69 – 85).  Then, we present the knowledge gap in the following lines (85 – 86):

“There is a knowledge gap on the derivation of K_c values from frequent plant observations of Prunus persica in field conditions.” 

Specific Comments

Abstract

R1C2) The abstract needs to be revised. The authors are advised to add little more details of their study’s importance within the abstract, and then corellate this with their findings.

Thank you. We added two new sentences (lines 12 - 14) which link the introduction with the aim of the study and highlight its importance:

“The FAO-56 is the most popular method for computing crop evapotranspiration. There is, however, a need for locally derived crop coefficients (K_c) with a high temporal resolution to reduce errors in the water balance.”

Introduction

R1C3) The introduction is not providing enough information on the research background and research gap.

We added more information on the research background and knowledge gap in lines (69-98):

“Allen and Pereira [18] expressed the increase in K_c as a function of the increase in the amount of vegetation, with the use of a density coefficient, which is computed as an exponential function of LAI. They obtained K_c estimates for various crops that were similar to those in the literature. However, they reported that orchard crops can vary widely in plant spacing, tree pruning, and age, thus the relationship between K_c and LAI for these crop types is not singular. According to Netzer et al. [19], the relationship between LAI and K_c can serve as a tool for estimation of K_c in situations in which LAI can be measured directly and can be used for precision irrigation.

Different values and seasonal trends of K_c for Prunus persica trees have been reported in the literature [29,30,31,33]. These studies were conducted with trees planted in weighing lysimeters. Zambrano-Vaca et al. [33] did regular measurements (3-week interval) of the projected canopy area for the determination of K_c and reported that these measurements allow the seasonal adjustment of irrigation scheduling, thus water stress or over-irrigation of young peach trees is avoided. However, Girona et al. [32] showed that the soil water uptake by Prunus persica trees in lysimeters differs from the soil water uptake by trees outside them, during irrigation treatments, due to differences in the root architecture. There is a knowledge gap on the derivation of K_c values from frequent plant observations of Prunus persica in field conditions. 

The seasonal course of K_c has a great effect on the evapotranspiration estimates and subsequently on the quantification of the water balance components. Thus, there is a need to increase the temporal resolution of LAI measurements, in order to capture the dynamic changes of the canopy cover of crops and thus the dynamic changes of the K_c.”

Materials and Methods

This section is fine.

Results

R1C4) I strongly recommend the authors to write results with exact significant values and then calculate percentage differences between best and worst treatments.

We apologize for any possible misunderstanding of your recommendation but we didn’t apply any treatments in this study. Thus, no comparisons can be made between treatments.

Discussion

R1C5) I think authors should rethink what they write in the first paragraph and only summarize the main findings in view of the research questions. After this, authors can explore different aspects of the work in subsequent paragraphs and explain how their findings expand the envelope of knowledge, but first of all, authors simply need to state the main results without discussing their why and how or the relationships to the literature. First of all, the reader needs a clear statement on what the study found. Moreover, it is suggested to discuss the main results in a logical way.

Thank you. We reorganized the results and discussion section as you suggested. We now present in the first paragraph the main results of the water balance computations. We show the results of the soil (SM_fc and SM_wp) and canopy parameters (LAI and c) in the second and third paragraphs, respectively. Then, in the fourth paragraph, we show the K_c estimates (derived from c in the previous paragraph) and we compare our findings with other studies. Finally, in the remaining paragraphs, we present the water balance components in detail and then a discussion on rainfall interception is given, as suggested by reviewer 3.

Reviewer 2 Report

The manuscript contains interestingly described research. The methodology and aim of the research are specified and included in the paper. The conclusion section is very well written. In my opinion, the discussion section should be expanded in this publication. Try to relate the results obtained by you to previously published works. Did you also use the LAI-based method for other plants and compare the results obtained with the FAO method? What were the differences? Is there any similarity to the results you obtained?

At the moment, this work is only missing a good discussion chapter.

Editorial comments

According to the journal's guidelines, every word in the title should be capitalized - please change this.

Please work on the appearance of figure 1. Try to use a better quality satellite photo or another map.

Also, figure 2 could be improved in terms of appearance and graphic sophistication. Try to make this figure look more professional and not stretched.

Author Response

Reviewer 2

R2C1) The manuscript contains interestingly described research. The methodology and aim of the research are specified and included in the paper. The conclusion section is very well written. In my opinion, the discussion section should be expanded in this publication. Try to relate the results obtained by you to previously published works. Did you also use the LAI-based method for other plants and compare the results obtained with the FAO method? What were the differences? Is there any similarity to the results you obtained?

We would like to thank you for your helpful and constructive comments. We have revised the manuscript in order to improve the discussion section as you suggested (please see our reply to R2C2).  This was our first attempt to use the dynamic K_c approach, thus we do not have any results from other plants. However, we compare our results with the findings of other studies (including the FAO method) for the nectarine trees in lines 261-288.

R2C2) At the moment, this work is only missing a good discussion chapter.

We agree. Following your suggestions and those of reviewer 1 and 3, we restructured the results and discussion section and added more information.  We now provide detailed discussion on LAI and c (lines 250 – 256), on the K_c estimates (lines 261 – 288) and on the differences between ET_{a_Kc} and  ET_{a_wb} (lines 379 – 408).

Editorial comments

R2C3) According to the journal's guidelines, every word in the title should be capitalized - please change this.

Thank you. We have now corrected the title.

R2C4) Please work on the appearance of figure 1. Try to use a better quality satellite photo or another map.

Unfortunately, this was the highest resolution image available for this site.

R2C5) Also, figure 2 could be improved in terms of appearance and graphic sophistication. Try to make this figure look more professional and not stretched.

Thank you. Following your suggestions, we changed the figure 2.

Reviewer 3 Report

This paper highlighted that use of a dynamic Kc approach, based on in-situ LAI measurements, soil moisture and meteorological observations, resulted in Kc estimates with a very small MBE and MAE model's performance metrics. In general, the manuscript is well organized and well written. I only have minor suggestions as below:

-- Title: add a name of nectarine, as important for authors.

-- The introduction seems somewhat disconnected in how the authors present their case, hypothesis, background, and approaches. Also, more detailed connections should be drawn between each of the paragraphs to help with the flow and motivation more than simply introducing some key concepts.

-- Line 13: Add a scientific name of nectarine.

-- Line 47: LAI also important in rainfall interception (see for example Sadeghi et al., 2020: Sadeghi, S. M. M., Gordon, D. A., & Van Stan II, J. T. (2020). A global synthesis of throughfall and stemflow hydrometeorology. In Precipitation partitioning by vegetation (pp. 49-70). Springer, Cham.)

-- Line 49: Which kind of relationship? positive or negative relationship? 

-- Line 53: Describe these literature and indicate their findings.

--  Can you explain why you didnt measure rainfall interception, as past research (like Hakimi et al., 2018) highlighted that rainfall interception of Pomegranate orchard is very large (please see this paper: Hakimi, L., Sadeghi, S. M. M., Van Stan, J. T., Pypker, T. G., & Khosropour, E. (2018). Management of pomegranate (Punica granatum) orchards alters the supply and pathway of rain water reaching soils in an arid agricultural landscape. Agriculture, Ecosystems & Environment, 259, 77-85.).

-- Instead figure 8, I would like to see a scatter plot between measured and predicted value.

Author Response

Reviewer 3

R3C1) This paper highlighted that use of a dynamic Kc approach, based on in-situ LAI measurements, soil moisture and meteorological observations, resulted in Kc estimates with a very small MBE and MAE model's performance metrics. In general, the manuscript is well organized and well written. I only have minor suggestions as below:

We thank the reviewer for his thorough revision and for his suggestions.

R3C2) -- Title: add a name of nectarine, as important for authors.

Thank you. We have now added the name (Prunus Persica var. Nucipersica) in the title.

R3C3) -- The introduction seems somewhat disconnected in how the authors present their case, hypothesis, background, and approaches. Also, more detailed connections should be drawn between each of the paragraphs to help with the flow and motivation more than simply introducing some key concepts.

Thank you. We have improved the introduction as you suggested. Please see our reply to R1C3.

R3C4)-- Line 13: Add a scientific name of nectarine.

Thank you. We have now added the name (Prunus Persica var. Nucipersica) in the abstract.

R3C5) -- Line 47: LAI also important in rainfall interception (see for example Sadeghi et al., 2020: Sadeghi, S. M. M., Gordon, D. A., & Van Stan II, J. T. (2020). A global synthesis of throughfall and stemflow hydrometeorology. In Precipitation partitioning by vegetation (pp. 49-70). Springer, Cham.)

We agree that LAI is important in rainfall interception too. We added the following lines (393 - 408) in the discussion:

“The differences between ET_{a_wb} and ET_{a_Kc} (Table 1) may be also due to rainfall interception, which was not measured in this study. Rainfall interception can be an important water balance component (21% of P on a global average) [34]. This percentage varies between different climates, and plant types. According to Sadeghi et al. [34], the largest non-disturbance related differences in relative throughfall typically occur in deciduous trees between the leafed and leafless periods due to large changes in canopy cover fraction. Throughfall during the leafless period can be between 15 and 40% higher than throughfall during the leafed period. In their study on rainfed pomegranate orchards located in arid environment (P = 214 mm), at Saveh city in Iran, Hakimi et al. [35], examined the partitioning of rainfall across stands with different structural characteristics. Rainfall interception ranged from 10% of P in the dense stand (LAI = 3.19) to 19% of P at the heavily thinned stand (LAI = 2.33). Reductions in LAI, crown length, canopy coverage, and tree height maximize throughfall but the additional water input may not be available for tree water use. Preferential flow paths in the soil profile and the increased radiation on the soil surface may increase soil evaporation and drainage losses and thereby reduce the water available for the tree.”

R3C6) -- Line 49: Which kind of relationship? positive or negative relationship? 

We added “positively .....”  in lines (61 - 62)

R3C7) -- Line 53: Describe these literature and indicate their findings.

We added the research findings by previous studies in the following lines (69 - 86):

“Allen and Pereira [18] expressed the increase in K_c as a function of the increase in the amount of vegetation, with the use of a density coefficient, which is computed as an exponential function of LAI. They obtained K_c estimates for various crops that were similar to those in the literature. However, they reported that orchard crops can vary widely in plant spacing, tree pruning, and age, thus the relationship between K_c and LAI for these crop types is not singular. According to Netzer et al. [19], the relationship between LAI and K_c can serve as a tool for estimation of K_c in situations in which LAI can be measured directly and can be used for precision irrigation.

Different values and seasonal trends of K_c for Prunus persica trees have been reported in the literature [29,30,31,33]. These studies were conducted with trees planted in weighing lysimeters. Zambrano-Vaca et al. [33] did regular measurements (3-week interval) of the projected canopy area for the determination of K_c and reported that these measurements allow the seasonal adjustment of irrigation scheduling, thus water stress or over-irrigation of young peach trees is avoided. However, Girona et al. [32] showed that the soil water uptake by Prunus persica trees in lysimeters differs from the soil water uptake by trees outside them, during irrigation treatments, due to differences in the root architecture. There is a knowledge gap on the derivation of K_c values from frequent plant observations of Prunus persica in field conditions.”

R3C8) --  Can you explain why you didnt measure rainfall interception, as past research (like Hakimi et al., 2018) highlighted that rainfall interception of Pomegranate orchard is very large (please see this paper: Hakimi, L., Sadeghi, S. M. M., Van Stan, J. T., Pypker, T. G., & Khosropour, E. (2018). Management of pomegranate (Punica granatum) orchards alters the supply and pathway of rain water reaching soils in an arid agricultural landscape. Agriculture, Ecosystems & Environment, 259, 77-85.).

Thank you. The objective of our study was to estimate K_c. For the derivation of K_c we excluded rainy days because they result in errors (lines 199 – 207):

“The unknown parameters K_c1, K_c2, WA and ps were derived with a two-step parameter derivation approach. First, we derived the parameter K_c1 for the leafless period, where the WA is equal to 1 and I is equal to 0. This was done by minimizing the sum of the daily errors (∑e_{_dry}) and mean absolute error (MAE_{_dry}), for days without P and without water stress (K_s = 1). Also, days with an increase in soil moisture (ΔSM > 0) while P was zero were not included in the parameter derivation. This increase in soil moisture sometimes occurred on days following P, most likely due to lateral soil moisture flows. Secondly, we derived the parameters K_c2, WA and ps for the leafed period by minimizing ∑e_{_dry} and MAE_{_dry}, for days without I, P and ΔSM > 0. “

Also, the estimation of Kc is more important during the irrigation season because we can save water lost due to over-irrigation.  For the irrigation season, we compute the water balances over the irrigated wetted area and we again assumed zero interception. However, the overestimation of the contribution of rain, by ignoring the interception, is compensated by an underestimation of the tree’s uptake of rain (thoughfall) from outside the wetted area. But we agree that this may cause the difference in the water balance computations (see table 1). We have added a paragraph on rainfall interception in the discussion section (please see our reply to R3C5).

R3C9) -- Instead figure 8, I would like to see a scatter plot between measured and predicted value.

We added a new figure showing both the scatter plot (8b) and the changes over time of ET_{a_Kc} and ET_{a_wb} (8a).