Calibration of MODIS-Derived Cropland Growing Season Using the Climotransfer Function and Ground Observations

Round 1

Reviewer 1 Report

This manuscript used meteorological factors to correct SOS and EOS in MODIS LSP products based on ground meteorological station data. The correction results showed that the proposed method was effective and had important application value in Northeast China. Because the spatial resolution of MODIS LSP product is 500m, it is more suitable for the application  in Northeast China. However, the growth periods of different crops are obviously different, which should have a greater impact on the results of this manuscript, especially EOS. It is hoped that the author can further seek methods to obtain more accurate correction models. There are also some minor problems that need to be corrected, as follows:

Line 17 add abbreviation “NEC” for Northeast China

Line 105 replace ‘Collection’ with ‘Version’

Line 188 averaged Smod or Sgrd was calculated through Eq.3?

Line 290 Fig.7 does not clearly illustrate the description of the caption. Scatter plots of GDD10 vs ΔEOS, CL vs ΔEOS y should be plotted separately.

Author Response

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Reviewer 2 Report

This study proposes a new approach to calibrate the satellite derived surface phenology (LSP) observations products by developing a climotransfer function (CTF) based on a polynomial regression of the satellite-ground observation difference in key crop phenophases (growing season start and end) against climatic factors in Northeast China.

This is a very interesting study producing results that should attract interest of readers. It used long-term crop trial data to examine how a statistical model can be developed to calibrate the satellite data for more accurate spatiotemporal monitoring of cropland phenology that will allow more meaningful applications in climate change and food security. Therefore, I recommend publication of this study once the issues noted on the attached manuscript (for example the discussion section is too long and could be greatly reduced) would be taken care of. The use of English is fine.

Comments for author File: Comments.pdf

Author Response

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Reviewer 3 Report

This study endeavor to establish a new method to calibrate the satellite land surface phenology products by developing a climotransfer function. The MODIS-derived end of season was calibrated using a model fitted to the satellite-ground difference in end of season versus two climatic factors, namely, the growing degree-days on the base temperature of 10 degree centigrade and cloud cover and cloud cover.

The manuscript is overall well-written and it is a pleasure to read it. However, I do have several concerns that need to be addressed before this paper can be published.

 

Major:

1.     Why using a "climotransfer" function instead of others? This wasn't adequately justified in the Introduction section. Also, how have previous researchers have addressed this issue should be elaborated in Introduction. Actually several paragraphs in Discussion may be more suitable to present in Introduction, e.g. second paragraph in 4.1 and first paragraph in 4.2.

2.     The authors have come up with one climotransfer function as the best-performing model. How large do you think this function can be applied with considerable accuracy based on MODIS observations? The northeast China is large already. For example, how many such functions may be needed to calibrate the LSP products across China?

3.     The authors have mixed the different crop types present in northeast China and proposed one climotransfer function for all of them. Would it be more accurate if each crop type has its own calibration function, considering that their climatic requirements are different?

4.     An overview description of the method should first be given in 2.2.1 instead of listing the several steps directly, which can be quite confusing.

5.     Figs. 10 and 11 should be in Results section.

 

Minor:

1.     Line 93-95: (4) is unnecessary to be listed here as it's not the focus of this study.

2.     It may be more reasonable to move the last paragraph of Introduction to Discussion.

3.     Line 129-130, is this dataset public? If so, please give the download website here.

4.     Line 138, how can the threshold of 50% be connected to the EVI2 crossing 15% seasonal amplitude?

5.     Line 162, where is the spatial-explicit crop production map? This is quite important information and should be presented if available.

6.     Line 176, why second-order polynomial function?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The revised  manuscript can be accepted for publication