Study on the Spatial–Temporal Variation of Groundwater Depth and Its Impact on Vegetation Coverage in Ejina Oasis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors Study Groundwater Depth and Its Impact on Vegetation Coverage in Ejina Oasis. The relationship between them influenced the ecosystem and therefore on the environment. They clarify that the two rivers contributed to the aquifer recharge, which reflected to very shallow water depth than far from them. The following items suggested

1 the west Juyan lake has a higher water surface area than those in the east lake. The NDVI concentration was lower in the west lake, why.

2 the soil geology was very important that affect on water leakage. The soil characteristics should be mentioned here. 

3 the western lake indicates strong correlation between NDVI and water depth, but it has no NDVI concentration.

4 the manuscript contain some comments do them 

Comments for author File: Comments.pdf

Author Response

Comments 1: the west Juyan lake has a higher water surface area than those in the east lake. The NDVI concentration was lower in the west lake, why.

Response 1: Thank you very much for your professional question, which is also a very interesting scientific question. The West Juyan Lake shown in Figure 1 only indicates the management boundary of the lake area, not the actual water area of the lake area. The area of the lake area in this article is based on the lake surface area in the historical period, which was delineated by the lake manager and has been used to this day. According to our investigation, West Juyan Lake has basically dried up since the 60s of the last century. Since 2000, seasonal artificial water transport in the midstream has been obtained, and a small amount of water has been replenished. However, it is still in a state of intermittent drying. Since the negligence of our work has caused your misunderstanding, we have refined and modified Figure 1.

Comments 2: the soil geology was very important that affect on water leakage. The soil characteristics should be mentioned here.

Response 2: Thank you very much for your comments and suggestions, we have added the relevant content of Ejina soil texture.

Lines 344-352:

Due to the influence of the special soil texture of Ejina, Infiltration recharge by surface runoff is the main way to renew shallow groundwater in the bank area of the Ejina River. Different soil textures have different capillary water properties, which determine soil water holding capacity and infiltration [25]. According to field investigations, the riverbed of Ejina River is mainly composed of sandy sediments, with a good infiltration property. The downstream areas such as Gurinai and Juyan lake are mostly composed of lacustrine deposits, and the soil texture is mainly clay and sand. The infiltration of groundwater into the riverbed is significantly linearly related to the river discharge.

Comments 3: he western lake indicates strong correlation between NDVI and water depth, but it has no NDVI concentration.

Response 3: According to our investigation, analysis and understanding, there are two main causes of this problem. First of all, the lake water in West Juyan Lake is now in a state of intermittent dryness, because of its low rainfall, the lake water is mainly due to artificial water transport in the middle reaches, and it is greatly affected by seasonality. Thirdly, due to its strong evaporation and high surface soil salinity, the plant types in the West Juyan  area are mainly drought-tolerant and saline-alkali tolerant plants, and the vegetation coverage is low, and the vegetation is mainly low. Therefore, the groundwater condition in this area has become the main constraint on vegetation growth, especially the impact on vegetation cover, which is also the reason for the relatively strong correlation between NDVI and water depth.

Comments 4: the manuscript contain some comments do them.

a. In spite of The west Juyan lake has high area, it has very little to no vegetation cover. It is a point of interest why it happens, is west lake has high salinity or what reasons for that.

Response (a): This question is basically the same as Comments 1 and 3, and I would like to explain again that it is also Figure 1 of this article that caused your misunderstanding of this point. Figure 1 has now been modified. According to our research, the lake is now intermittently dry, and due to its low rainfall, the lake water is mainly due to artificial water transportation in the middle reaches, and it is greatly affected by seasonality. Thirdly, due to its strong evaporation and high surface soil salinity, the plant types in the West juyan area are mainly drought-tolerant and saline-alkali tolerant plants, and the vegetation coverage is low, and the vegetation is mainly low.

b. recolor high VC with same color. Fig. 2b was not clear.

Response (b): As you suggested, we have refined the color of the legend in Figure 2 and adjusted the clarity of Figure 2b. Once again, we apologize for the negligence of our work.

c. 

The vegetation coverage was illustrated within 2018, 2020, and 2023 periods.You illustrate the groundwater depth within 2018-2020 and (2020-2023) periods. Comments:it is best to illustrate the groundwater depth in 2018, 2020, and 2023 periods, like vegetation cover for comparable as you did in Fig. 5.

Response (c): Thank you very much for your suggestions. According to our survey, the groundwater in the Ejina area is greatly affected by surface water, which is a seasonal river, that is, the groundwater depth is mostly in an unstable and changing state. Therefore, we discuss the differences in groundwater depth changes in two different time periods through Figure 4a, and then illustrate the annual variation characteristics of groundwater in this area. At the same time, we discuss the groundwater depth contour plots for 2018, 2020, and 2023 through Figure 5a. Therefore, we do not think it is necessary to discuss and explain the distribution of groundwater depths in 2018, 2020 and 2023.

d. the west Juyan lake has strong correlation in spite that they have no vegetation cover, correct them.

Response (d): This question is basically the same as the previous question, and we have modified it according to your suggestion. Thank you again for your valuable comments and professional suggestions.

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the current study, the authors investigated the spatial and temporal variations in groundwater depth and their influence on vegetation coverage in Ejina Oasis, northwest China. Using NDVI data from 2018 to 2023 alongside groundwater depth monitoring, the researchers identified patterns and correlations between groundwater levels and vegetation health. They found that vegetation in Ejina is generally low, mainly concentrated along riverbanks, and that both groundwater depth and vegetation coverage fluctuate annually. An optimal groundwater depth (around 4-6 meters) was identified, where vegetation coverage is highest. When groundwater is either too shallow or too deep, vegetation coverage diminishes. The findings highlight the fragility of Ejina’s ecosystem and provide important insights for water resource management.

The present investigation provides valuable insights into the dynamics between groundwater depth and vegetation coverage. The findings add to our understanding of how groundwater availability affects vegetation health, which is crucial in arid and semi-arid forest regions. Although there are some issues that need to be addressed and a minor revision is advised.

   1)     The NDVI acronym should be explained sooner in the manuscript and not in Line 54.

   2)     While the introduction provides background on groundwater's importance to arid ecosystems, discussion of previous studies in similar arid zones would significantly add to the manuscript.

  3)     Lines 32-343: “When the depth of groundwater… vegetation growth.” The authors here and elsewhere in the manuscript state that shallow water depth is correlated with decreased NDVI values. Unfortunately this argument isn’t supported enough in the manuscript. In section 3.3.2 the discussion that correlates the water depths and NDVI index doesn’t present significant statistical values that would suggest that for depths lower that 4.8m the NDVI index decreases.   Furthermore Figure 5b shows a strong variation of NDVI index for lower depths, and doesn’t indicate low NDVI values. The authors are advised to either remove the argument or provide stronger support.

   4)     Figure 5b. There is no label for the horizontal axis  (NDVI)

  5)     Including comparisons of current NVDI application with other cases in literature could increase the applicability of this research.

Author Response

Comments 1: 1.The NDVI acronym should be explained sooner in the manuscript and not in Line 54.

Response 1: Thanks for the expert advice, it has been modified.

Lines 28-30: Further analysis indicates that vegetation growth and coverage in Ejina are significantly affected by water conditions, and areas with high Normalized Difference Vegetation Index (NDVI) values mainly distributed along the riverbanks.

Comments 2: While the introduction provides background on groundwater's importance to arid ecosystems, discussion of previous studies in similar arid zones would significantly add to the manuscript.

Response 2: Thanks for the suggestion. We add the main findings of the research on arid zones in the introduction, further illustrating the very important impact of groundwater on NDVI in arid zones. Moreover, the previous research results are basically consistent with the conclusions of this paper, which further supports the reliability of the research results in this paper.

Lines 68-72:

When the groundwater level is too shallow, strong evapotranspiration will cause a large amount of submersible evaporation, soil salinity will accumulate in the topsoil, and vegetation growth will be inhibited, which will lead to a decrease in vegetation coverage. When the groundwater level is too deep, the plant roots do not get enough water to affect their growth.

Comments 3: Lines 32-343: “When the depth of groundwater… vegetation growth.” The authors here and elsewhere in the manuscript state that shallow water depth is correlated with decreased NDVI values. Unfortunately this argument isn’t supported enough in the manuscript. In section 3.3.2 the discussion that correlates the water depths and NDVI index doesn’t present significant statistical values that would suggest that for depths lower that 4.8m the NDVI index decreases.   Furthermore Figure 5b shows a strong variation of NDVI index for lower depths, and doesn’t indicate low NDVI values. The authors are advised to either remove the argument or provide stronger support.

Response 3: Thank you very much for your professional advice. The argument is mainly based on the statistical results of Figure 6 and has certain limitations, as you have expressed. In order to make the argument rigorous, we have deleted this expression in accordance with your suggestion.

Lines 33-36.

Comments 4: Figure 5b. There is no label for the horizontal axis (NDVI)

Response 4: Figure 5 has been modified and improved in accordance with your suggestion.

Comments 5: Including comparisons of current NVDI application with other cases in literature could increase the applicability of this research.

Response : We really appreciate your suggestion and fully endorse your point of view. Through comparative analysis with the conclusions of relevant literature, we show that the statistical results are consistent with the conclusions of related studies, but the statistical suitable water level interval is slightly different. This may be due to the source of the data and the difference in accuracy.

Lines 278-281：

The statistical results are consistent with the conclusions of related studies [23], but the statistical suitable water level interval is slightly different. This may be due to the source of the data and the difference in accuracy.

Author Response File: Author Response.pdf