Microclimate Characteristics and Evapotranspiration Estimates of Cucumber Plants in a Newly Developed Sunken Solar Greenhouse

Round 1

Reviewer 1 Report

The manuscript is in general well written and presented. The topic of research is interesting for the research community but it has also practical application for the greenhouse industry. Below there are some points that needs to be clarified or improved:

• The legends in Fig. 1 should be corrected in order to be readable
• The used relationship for the bulk crop resistance is based only to solar radiation, thus it is not take into account variation in VPD. This could be valid in ventilated greenhouses with high solar load but when we are in winter conditions the vairation of VPD is crucial for the determination of stomatal resistance. Has this checked by the authors before adopting the current formula?
• In L197, which was the resolution of cup anemometer. Due to the usual low air velocities inside the greenhouse cup-anemometers are rarely used for air velocity measurements.
• For the boundary layer resistance, since measurements of air velocity were available inside the greenhouse why this resistance was not evaluated using Eq 8 and it was taken as constant?
• From Fig. 2 it is difficult to obtain information for the average climate conditions during the experiment. Maybe it is better that information to provided in a table with min, max, average and standard deviation values.
• In Fig 5 instead of the ration of inside to outside I would prefer the variation of inside VPD over the experimental period.
• Fig 6 did not add much information
• Fig 10 present the decoupling coefficient and boundary layer resistance with relation to air velocity; however the air velocity measurements are not used for the estimation of the above two parameters but a constant value was selected
• A practical discussion on how the present results can be used in order to improve the design and management of such types of greenhouses is missing.

Author Response

Please see the attachment. Thanks.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript presents an interesting subject that is clearly within the scope of the journal. The manuscript is well written and all sections are clearly presented and discussed.

Two major points have to be considered:

• Since the reference/control greenhouse that is the SG is missing, I would suggest that the authors present a clear comparison of their results with those observed in the literature for SG in the same region. The case of outside climate or crop can not be considered as reference.
• If someone compares Eqns (1) and (2) for ETo, it can be seen that Eqn (2) considers an air velocity of 0.7 m/s for the greenhouse environment. However, the authors found that the internal air velocity was on average 0.1 m/s. Based on the above, the results of Eqn (2) seem questionable. Thus, considered that several estimations and comparisons done in the study are related to the results of Eqn (2), I suggest that the authors reconsider the use of Eqn (2) as presented.

Author Response

Please see the attachment. thanks.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear authors,

congratulations for your work, no further revision is suggested.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The paper presents research related to the micro-climate inside a sunken solar greenhouse in the North of China, a solar greenhouse where the inside soil surface is lowered 1 ~ 2 m below outside and in which the cucumber crop was planted for 7 seasons. The paper presents also an innovative idea because due to the construction of the solar greenhouse, in the cold periods of the year it can be used for planting crops, which also brings a substantial gain if you plant crops that are hard to find in that period. But it also has the disadvantage that in hot periods of the year it is almost unusable due to the high temperatures inside. I think that authors or other international researchers must find solutions with low economic impact to make the solar greenhouse usable during this period as well. I also offered a possible solution in the comments made on the paper (see the attached document)

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1#

Thank you very much for giving us these helpful comments and suggestions. The followings are the point-to-point replies to all the comments.  

1. Lines 6-13, You forgot to write the e-mail address for each author.

Reply: We added the email information for all authors.

2. Line 15, The abstract should have a maximum of 200 words. Please reduce it

Reply: We rewrote the abstract and now the total number of words is 196, meeting the requirement of the journal.

3. If the properties from table 1 and table 2 was investigated by you, explain in few words how you determined. If not please put a reference there

Reply: Thanks. We added the description of methods for each measured item in Tables 1 and 2. They are presented as notes just following the tables, in Lines 110-112 and lines 115-120.

4. Specify the thickness of the plastic film

Reply: The thickness of the plastic file covering the ground surface is 0.1 mm. We added this information on line 130.

5. Line 197, Rephrase with ”....by Fernández et al. in papers [24,25]...”

Reply: The sentence was rephrased to:” by Fernández et al. in papers [20,21]...” in Line 221. Thanks.

6. Lines 219 and 221, Maybe you should put a reference here!

Reply: We added the references [23]. Thanks.

6. Lines 231 and 234, Write the relationship in a more aesthetic form, with fraction and radical.

Reply: Thanks, we re-prepared these two equations as suggested by the reviewer.

7. Lines 307-312, If you say ”The highest increases in daily average, maximum, and minimum temperatures inside the SSG were found in January. They were 14.7, 17.9, and 15.6 oC....” the readers will understand and that the average value was 14.7, the maximum value is 17.9 and the minimum is 15.6. This is wrong, obviously you do not get 14.7 as an average from temperatures between 17.9 and 15.6. I recommend you to write ” The highest increases in daily minimum, maximum and average temperatures....”

Reply: Thanks for your suggestion. The daily average temperature offsets the large variation of daily maximum and minimum temperatures, and finally showed a smaller increase range. To reduce the confusion of description related to the increases in daily average, maximum and minimum temperatures, in the revised version we only showed the data of maximum and minimum temperatures. These two data sets, of maximum and minimum temperatures, are enough for showing the temperature improvement inside the SGG. See the revision on lines 361-366.

8. Lines 362-363, You should use a shade net during this period to have crops in the solarium. By lowering the temperature I think you can put crops like green onions, green garlic, radishes.

Reply: Thanks for your suggestion. This SSG is designed and mainly used in winter to grow offseason vegetables. During most of the summer, the SSG is fallowed and prepared for the autumn season cultivation, which generally starts on September. As you suggest, shading the greenhouse is a perfect way to reduce internal heat load during summer. In this situation, other heat - tolerate vegetables can be planted. This option will be the subject of a future study where we plan to examine ways to efficiently use the SSG at year scale.   

9. Line 392, It is more appropriate to use correlation coefficient instead determination coefficient.

Reply: Thanks. We calculated the correlation coefficient and obtained 0.78. This coefficient was used to replace the determination coefficient. The description sentence can be found on Lines 448-451.

10. Lines 432, 436, If the data and charts are not shown why did you talk about them?! Maybe you should attach these as an appendix to the paper

Reply: We added two figures as supplementary material to further show the relationships between soil temperature at different soil depths.  The Suppl. Fig. 1 illustrates the relationships between daily maximum (a) and mean (b) soil temperatures at 0.1 and 0.2 m depth to those at the soil surface (0 m depth) inside the sunken solar greenhouse. Suppl. Fig. 2 shows the relationships between daily maximum (a), minimum (b) and mean (c) soil temperatures at 0.2 m depth to the corresponding soil temperatures at 0.1 m depth.

11, Line 526, Marked on “data not shown”

Reply: We added Suppl. Fig. 3 to show the relationship between outside ET_o and the outside solar radiation.

12, Line 544, Marked on “both data not shown”

Reply: We added Suppl. Fig. 4 to show the relationships between inside ET_o and outside temperature and vapor pressure deficit (VPD).

13, Lines 577-578, You should put this relation in editor equation

Reply: We did it in the text. Thanks.

14. References

Reply: Thank you very much. We corrected it.

Supplementary materials

Suppl. Fig. 1  The relationships between daily maximum and mean soil temperatures at 0.1 and 0.2 m depth to those at the soil surface (0 m depth) inside the sunken solar greenhouse. Ts,mean-10, Ts,mean-20, Ts,max-10 and Ts,max-20 on figures are daily mean soil temperatures at 0.1 and 0.2 m depth,  and daily maximum soil temperatures at 0.1 and 0.2 m depth, respectively.

Suppl. Fig. 2  The relationships between daily maximum (a) minimum (b) and mean (c) soil temperatures at 20 cm depth to their corresponding soil temperatures at 10 cm depth.

Suppl. Fig. 3 The relationships between daily ET_o and solar radiation outside the SSG.

Suppl. Fig. 4  The relationships between inside daily ET_o and outside daily mean air temperature (a) and vapor pressure deficit (b).

Author Response File: Author Response.pdf

Reviewer 2 Report

1. Explain, how SSG can enhance crop growth and yield, through improvement of microclimate. It has to be well discussed and shown as this would be the scientific benefit of the paper. It is obvious that microclimate in SSG would be improved.

2. How ET/ETc (inside-outside) is estimated when wind speed was not available, please clarify as observed data were shown available in 2018-19.

3. How net radiation is computed, inside/outside. Outward solar radiation, incoming and outgoing longwave radiation, emissivity, cloud fraction etc, shall be discussed as net radiation governs ET.

Author Response

Response to Reviewer 2#

Thank you very much for giving us these helpful comments and suggestions. The followings are the point-to-point replies to all the comments.

1. Explain, how SSG can enhance crop growth and yield, through improvement of microclimate. It has to be well discussed and shown as this would be the scientific benefit of the paper. It is obvious that microclimate in SSG would be improved.

Reply: Thanks for this comment. In this study, we did not set another traditional solar greenhouse as the control. Consequently, we could not directly show how the cucumber plant growth and yield are improved in the SSG compared to the SG. However, we measured microclimate and cucumber yield in seven seasons from 2014-2019. Results showed that the temperature inside the SSG was greatly improved in winter compared to traditional SGs. Cucumber plants favor humid and moderate temperature conditions. We found that days with air temperature falling into the optimal temperature range for cucumber accounted for 80% ~ 90% of days in winter. Therefore, this improved microclimate could greatly enhance cucumber growth.

In the revised MS, we added references to two more papers (Sun et al, 2019[reference 36]; Amer et al., 2009[reference 37]) to show that cucumber yield in the SSG is much higher than those in traditional SGs in China and other regions. Also, we found the yield in the SSG is approximately 3 to 4 times higher than the mean cucumber productions in China and world reported by FAO [1] (see another new reference [reference 39]). We also showed that the economic benefit from cucumber production in this SSG is 25 times higher than that in the wheat-maize rotation system in North China.

Therefore, the higher yield and economic benefits in the SSG compared to that in published papers could indirectly confirm that the cucumber plant growth was greatly improved.

The revised sentence is in lines 298-308.

2. How ET/ETc (inside-outside) is estimated when wind speed was not available, please clarify as observed data were shown available in 2018-19.

Reply: Thanks for this comment. In this study, air velocity inside the SSG was measured in the last two experimental seasons using an ultrasonic anemometer (Model: ATMOS 22, METER Group Inc, USA). However, we found that inside air velocity varied slightly and averaged 0.1 m s^-1. Therefore, in the last two seasons, the measured air velocity was used to estimate inside crop evapotranspiration ET. In the other five seasons, the mean value of 0.1 m s^-1 was used. The revised sentences are in Lines 260-261.

3. How net radiation is computed, inside/outside. Outward solar radiation, incoming and outgoing longwave radiation, emissivity, cloud fraction etc, shall be discussed as net radiation governs ET.

Reply: Thanks. You are right. The net radiation is the main factor that controls the ET, especially in the SSG. Therefore, we take more attention to calculate net radiation inside and outside the SSG. Net radiation is calculated as the sum of net shortwave radiation (R_ns) and net longwave radiation (R_nl). In this study, we measured both inside and outside shortwave radiations (R_s). The net inside shortwave radiation was estimated by taking an albedo of 0.23 which is typical for a hypothetical grass reference crop, using the approach by Allen et al. [2] as R_ns=(1-0.23)R_s.

For estimating the outside R_nl at daily scale, the method proposed by Allen et al. [2] was adopted as:

(Seeing the attached file)

Where σ is the Stefan-Boltzmann constant (4.903 10^-9 MJ K^-4 m^-2 day^-1); T_max,K is the daily maximum absolute temperature (K); T_min,K is the daily minimum absolute temperature (K); e_a is the actual vapour pressure (kPa); R_s/R_so is the relative shortwave radiation (≤1); R_s is measured solar radiation (MJ m^-2 day^-1); and R_so is the clear-sky radiation (MJ m^-2 day^-1). The R_so is a constant for a given site and specific day and was estimated by the latitude (34^o27’3.5’’N), longitude (113^o31’58.5’’E) and the elevation above sea level (110.4 m) at this experimental site for each day of a year, using the method by Allen et al. [23]

This method was also used to estimate inside net longwave radiation at a daily scale. With the measured inside solar radiation and estimated net radiation, we found net radiation is linearly related to solar radiation with a slope of 0.676 (see the following figure). Then this rate was used to estimate hourly inside net radiation with the measured inside solar radiation.

In the ET_c calculation process, we have two assumptions. One is that the albedo of the cucumber canopy is 0.23, the same as the reference grass in the ETo calculation. Another is that the outward and downward longwave radiations from the cover of the greenhouse are the same. Therefore, the effect of the cover on net longwave radiation is not considered.  Both assumptions could induce errors. Therefore, we explain that the crop ET_c inside SSG is roughly estimated. The calculated basal crop coefficient of 0.83 falls into the range (0.75-1.15) proposed by Allen et al. [2], which means that the estimated ETc is acceptable.

The revised sentences regarding the calculation of net radiation are given in Lines 233-249 and related discussion is in Lines 653-659.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Most of the comments are addressed. Minor English grammar editing/ check is suggested.