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Comprehensive evaluation on the ecological function of groundwater in the Shiyang River watershed
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Abstract: With an arid climate and shortage of water resources, the groundwater dependent ecosystems in the oasis–desert ecotone of the Shiyang River Watershed has been extremely damaged, and the water crisis in the oasis has become a major concern in the social and the scientific community. In this study, the degeneration characteristics of the groundwater ecological function was identified and comprehensive evaluated, based on groundwater depth data, vegetation quadrat and normalized difference vegetation index (NDVI) from Landsat program. The results showed that (1) the suitable groundwater depth for sustainable ecology in the Shiyang River Watershed is about 2-4 m; (2) the terms of degenerative, qualitative and disastrous stages of the groundwater ecological function are defined with the groundwater depths of about 5 m, 7 m and 10 m; (3) generally, the groundwater ecological function in the oasis-desert ecotone of the lower reaches of Shiyang River Watershed is weak with an area of 1 397.9 km2 identified as the severe deterioration region, which accounted 74.7% of the total area. In the meantime, the percentages of the good, mild and moderate deterioration areas of groundwater ecological function are 3.5%, 5.5% and 16.3%, respectively, which were mainly distributed in the Qingtu lake area and the southeastern area of the Shoucheng town; (4) the degradation and shrinkage of natural oasis could be attributed to the dramatic groundwater decline, which is generally caused by irrational use of water and soil resources. This study could provide theoretical basis and scientific support for the decision-making in environmental management and ecological restoration of the Shiyang River Watershed.
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Figure 1. Study area and the investigation sites
Note: SRB – Shiyang River Basin; LRSRB – the Lower reaches of the Shiyang River Basin; investigation sites – investigation sites of groundwater level depth and the ecological vegetation quadrat; ①②③④⑤represent the oasis–desert transition zones of the Hongyashan reservoir area, the South Lake area, the west of the Minqin irrigation area, the east of the Minqin irrigation area and the Qingtu Lake area, respectively.
Figure 2. Schematic diagram of groundwater flow system
1-Bedrock; 2-Quaternary; 3-Fault; 4-Goudwater table; 5- Groundwater movement; 6-Spring; 7-Surface runoff flow
Figure 3. Contour map of groundwater depth in the Minqin subbasin of the lower reaches of the Shiyang River Basin (August, 2018)
Figure 4. Relationship between NDVI and groundwater level depth in the Oasis-desert ecotone (Cao et al. 2020)
Figure 5. Relationship between NDVI and groundwater level depth in the shallow groundwater area (less than 5 meters) of the Oasis-desert ecotone
Figure 6. Relationship between species richness and groundwater level depth in the Oasis-desert ecotone
Figure 7. Evaluation of groundwater ecological function in the Oasis-dessert area of the lower Shiyang River Basin
Table 1. Relationship between groundwater level depth and the index of the dessert vegetation quadrats
Groundwater level
depth/mSpecies richness/No. Total coverage/% Dominant species Dominant species
height /m0-2 1-3 75-90 Tamarix spp. 3.8 Phragmites australis 1.0 Kalidiumfoliatum 0.6 2-4 2-7 45-100 Elaeagnus angustifolia 6.0 Tamarix spp. 2.6 caragana microphylla 2.2 3-6 3-10 30-70 Reaumuriasoongorica 0.6 hedysarumscoparium 1.2 5-7 2-4 20-30 Nitraria spp. 0.5 Reaumuriasoongorica 0.5 >7 1-2 <25 Haloxylonammodendron 1.2 Nitraria spp. 0.4 
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Table 2. Grading value of comprehensive evaluation index of groundwater ecological function
Index V1 V2 V3 V4 Groundwater level depth/m 2-4 5 7 10 NDVI > 0.7 0.2 0.13 0.08 Species richness ≥ 5 4 3 2 Abundance of the dominant species ≥ 5 4 3 2 Rating value 1 0.7 0.5 0.3 Note: when groundwater table depth was less than 2 m, its score was calculated in a linearly decreasing manner. For dominant species abundance, it may be scored 7 indicating “extremely abundant”, 6 “highly abundant”, 5 “abundant”, 4 “fairly many”, 3 “not many”, 2 “sparse”, and 1 “only one plant.”
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Table 3. Grading value of vegetation type index of groundwater ecological function
Vegetation type Value Arbor > Shrub > Herb 1.00 Shrub > Arbor > Herb 0.83 Arbor > Herb 0.67 Arbor > Shrub 0.83 Shrub > Herb 0.67 Shrub only 0.57 Herb > Shrub 0.50 Herb only 0.33
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Table 4. The total score of groundwater ecological function comprehensive evaluation and its indication
Grading V1 V2 V3 V4 The total score >0.7 0.7-0.5 0.5-0.3 <0.3 Indication Good condition Mild degermation Moderate degeneration Severe degeneration
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