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Soil salinization in the oasis areas of downstream inland rivers —Case Study: Minqin oasis
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Soil salinization in the oasis areas of downstream inland rivers —Case Study: Minqin oasis Jianxia Yanga, Jun Zhaoa,∗, Guofeng Zhua,b, Yuchun Wanga, Xinggang Maa, Jianbang Wanga, Huiwen Guoa, Yu Zhanga a b
College of Geography and Environment Science, Northwest Normal University, Lanzhou, 730070, China State Key Laboratory of Cryosphere Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil salinization Remote sensing SDI Inland river
Soil salinization is the main resource and environmental problem in arid and semiarid regions, and it seriously restricts regional ecological security and sustainable agricultural development. Accurate monitoring of soil salinization and analysis of its main influencing factors are of great significance to the improvement and management of soil salinization. In this paper, the normalized difference vegetation index (NDVI) and salinity index (SI) were extracted from Landsat images of the Minqin oasis from 2000 to 2018, and these data were used to construct a soil salinization remote sensing monitoring index model (SDI). The index extracted by the model was used to monitor the soil salinization information, and the index values were graded to obtain different degrees of soil salinized land. The measured data were used to verify the accuracy of the SDI through trend analysis, stability analysis and other methods, and explore the characteristics and main influencing factors of soil salinization in the Minqin oasis. The results showed that the spatial distribution characteristics of the different degrees of soil salinity extracted by the SDI model were consistent with the measured results. The area of salinized soil in the Minqin oasis exceeded 80%, and the proportion of lightly salinized soil was the largest. The area ratio of salinized soil did not change much from 2000 to 2018, but the changes between different degrees of salinized soil were large.The unused land in the Minqin oasis exhibited a high degree of salinization, which was manifested spatially in the northwest of the Minqin oasis and the areas surrounding Qingtu Lake. In addition, the soil salinization of the Minqin oasis exhibited obvious seasonal differences, and the different degrees of salinization also changed with the change in seasons. The analysis of the main influencing factors of soil salinization in the Minqin oasis revealed that the large land occupation ratio of salinized land in the Minqin oasis was related to the local climatic factors. In recent years, changes in soil salinity in parts of the Minqin oasis and Qingtu Lake area were mainly the result of ecological water conveyance.
1. Introduction Soil salinization is the result of a combination of natural and human factors(Zhang et al., 2015; Hattar et al., 2010)and it mostly occurs in arid and semiarid areas with low precipitation(Tripathi et al., 2015), high evaporation, high groundwater levels, and highly soluble salts (Belghemmaz et al., 2017; Bouaziz et al., 2011). Soil salinization is an ecological and worldwide resource problem that restricts the stability of the ecological environment and the sustainability of land use (Yakup et al., 2017; Jiang et al., 2017), thus posing a serious threat to the biosphere and ecological security (Allbed et al., 2014). Soil salinization restricts the sustainable development of regional economies and agriculture mainly by accelerating desertification (Ke et al., 2015),
∗
inhibiting the growth of crops, hindering agricultural development, and reducing soil quality (Zhao et al., 2015). The prevention of secondary soil salinization and the transformation and treatment of soil issues have become the focus of land use research (Jin et al., 2012).The monitoring of soil salinity monitoring is an important basic work that is necessary to understand the distribution of salinized soil and explore the mechanism of soil salinization. Previous studies have aimed to monitor soil salinization dynamicsBrunner et al., 2007, identify the distribution of salinized soils(Scudiero et al., 2014), explore the mechanism of salinization development, and summarize the soil salinization research methods, and the results of these studies have important significance for agricultural production activities and ecological security and stability in arid areas (Chen et al., 2014; Zuo et al., 2014).At
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.quaint.2020.01.001 Received 14 January 2019; Received in revised form 2 January 2020; Accepted 6 January 2020 1040-6182/ © 2020 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article as: Jianxia Yang, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2020.01.001
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Fig. 1. Map of the study area.
studying the impacts of ecological water conveyance on the evolution of plant species, changes in vegetation cover (Zhao et al., 2018; Wang et al., 2018a,b), and changes in the groundwater level. However, as a targeted control area for ecological water transmission, there have been few studies on the changes in soil salinization around Qingtu Lake in recent years. Further research is needed to determine whether human intervention has resulted in the problem of secondary soil salinization and the trend of soil salinization changes in the Minqin oasis in recent years. In view of this, this article takes the Minqin oasis as the research area, and establishes SDI of soil salinization to explore the characteristics of soil salinization in the Minqin oasis from 2000 to 2018, focusing on the soil salinity around Qingtu Lake. The trend of soil salinization changes and analysis of the relationship between soil salinization and climate, hydrology and human factors provides a scientific reference for soil desertification management in the Minqin oasis.
present, remote sensing and geographic information system (GIS) technologies have been widely used to monitor and analyse soil salinization, and different remote sensing data (Fan et al., 2014), salinization models have been used to classify salinization areas. Various aspects of this research (Yang et al., 2014) provide a scientific basis for the extraction, management, monitoring and forecasting of saline soil patterns (Mougenot et al., 1993),and good results have been achieved in various areas of soil salinization research. However, in-depth analysis of the characteristics and influencing factors of soil salinization downstream of arid inland rivers, and the response of soil salinization to artificial intervention need to be further studied. The Minqin oasis of Gansu Province is located downstream of the Shiyang River Basin. The total area of the oasis is 15,900 km2, and the desertification area accounts for 94.5% of this area(Zhao et al., 2018). The Minqin oasis is bordered to the west by the Badain Jaran Desert and to the east by the Tengger DesertMa et al., 2018. The oasis consists of four geographical landscapes: desert, Gobi, denuded mountain and oasis plain (Zhu et al., 2018), which belong to arid and semiarid areas (Fig. 1). The oasis is located in a typical dry inland river region. Land desertification is a major ecological and environmental problem facing the region(Cemek et al., 2007), with soil salinization being a major type of soil desertification. Due to environmental characteristics such as high temperatures, low precipitation, high evaporation, and a lack of water resources in the Minqin oasis, coupled with the continuous development and utilization of groundwater resources, large amounts of natural and artificial vegetation have died, which has increased soil desertification (Niu et al., 2016). To this end, Minqin County has adopted a series of governance measures in recent years. Among these measures, projects such as “ecological water conveyance” and “sealing wells and pressing downfields” to reduce groundwater collection have improved the ecological environment. However, with the implementation of governance measures, a series of changes have taken place in the small regional environment, and these changes are mainly reflected by the groundwater levels, vegetation coverage, plant community composition, diversity, stability, and soil salinization. At present, the impacts of artificial intervention on related areas have mainly been identified by
2. Data and methods 2.1. Data sources and processing The remote sensing data used in this study were acquired from Landsat TM/OLI images from 2000, 2005, 2010, 2015 and 2018 with a resolution of 30 m, a synthetic image resolution of 15 m and a temporal resolution of 16 d. The image data are from the USGS (https://glovis. usgs.gov/) website. The preprocessing of the images included geometric correction, atmospheric correction, inlay and cropping. Landsat TM/ OLI images have a long coverage time and high resolution, making them suitable for small-scale studies with long sequence times and high accuracy requirements. The image data obtained were first preprocessed using ENVI5.1 software for geometric correction, radiation calibration, atmospheric correction, mosaic and clip. The meteorological data include the annual average temperature, precipitation, and evaporation of the year corresponding to the remote sensing data. The data comes from the China Meteorological Data 2
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To grade the extent of soil salinization, the soil is divided into severe salinization (SDI > 0.8), moderate salinization (0.6 < SDI < 0.8), mild salinization (0.4 < SDI < 0.6) and non-salted land (SDI < 0.4) (Prince et al., 2009; Scudiero et al., 2014).Due to the large difference in the salinity of the land index, SDI can well distinguish the distribution range of land types with different salinity in the study area(Wang et al., 2010).
Sharing Network (http://data.cma.cn/). Relevant data for ecological water conveyance were acquired from the 2015 Statistical Yearbook of Wuwei City. In July 2018, field observations of soil salinity in areas with significant changes in soil salinity were collected using a soil salinity meter (Shunkeda TR-6D), GPS, and canopy analyser in the areas around the lake, oasis farmland and surrounding the Hongyashan Reservoir. In view of the most obvious changes in salinization around Qingtu Lake and the core area of ecological water conveyance, the surrounding data mining points were dense. The spatial layout of the measured points is shown in Fig. 1.The figure not only indicates the specific location of the study area, but also gives a brief expression of the image coverage of the study area and the distribution of measured data points.In the figure, A, B, C, D and E are located in Qingtu lake area, Minqin oasis cultivated land area, Minqin Sandy Land, hongyashan reservoir area and alpine meadow area. In these areas, a number of experimental points were selected to directly measure the vegetation leaf area index, NDVI, soil salt content, soil moisture, and soil samples and water samples were collected.
2.2.2. Stability analysis Standard deviation is a measure of the extent of the distribution of data and can be used to assess the stability of SDI over a time series. Stability analysis was used to calculate the standard deviation of SDI according to the following formula (Shi et al., 2005): n
SD =
2.2.1. Construction of remote sensing monitoring index model for soil salinization Related studies have shown that the spectral characteristics of salinity in salinized soil have a great influence on the change in pixel brightness values in regional images (Csillag et al., 1993; Cemek et al., 2007). Although there are many soil salinity indices and salinization models, a large number of studies have shown that the salinity index (SI) extracted from the blue and red visible bands is relatively accurate in expressing soil salinity (Wang et al., 2010). Thus, the relationship between the normalized difference vegetation index (NDVI) and the SI was analysed, and the feature space concepts of the two indices was used to construct a soil salinization remote sensing monitoring index model (SDI) to extract and monitor soil salinity information with high precision. Because both NDVI and SI are relatively easy to obtain, the model construction is simple and straightforward. Therefore, this paper uses the multi-band information from Landsat remote sensing images, and uses formulas (1) and (2) (Everitt et al., 1988; Aldakheel et al., 2011) to calculate the SI and NDVI, and the extract the two index indices. The index uses the concept of feature space to establish SDI, as shown by formula (3)(Ding et al., 2010).
ρ1 × ρ3
NDVI =
SDI =
(NDVI − 1) 2 + SI 2
(4)
2.2.3. Trend analysis The least squares method can be used to fit the slope of the annual average SI for each pixel, which can be used to simulate the trend of each grid and comprehensively reflect the temporal and spatial patterns of soil salinization changes (Aldakheel, 2011; Barrioset et al., 2013). In this paper, we use the overall trend of SI from 2000 to 2018 to determine the significance of the interannual variation in SI. A negative slope indicates that soil salinization is decreasing, and a positive slope indicates that soil salinization is increasing (Jianguo et al., 2012; Sun and Liu., 2015). According to the F test, the correlation coefficient P value is used to express the significance of the change, and the change trend is divided into 5 levels: Significantly lower (slope < 0, P < 0.01), Reduce (slope < 0, 0.01 < P < 0.05), No significant changes (P > 0.05), Increased (slope > 0, 0.01 < P < 0.05), Significantly elevated(slope > 0, P < 0.01). 2.2.4. Accuracy evaluation of the SDI The points measured in the different areas are averaged to represent the soil throughout in the area. The SDI inversion in 2018 uses the SDI value at the point corresponding to the measurements in ENVI5.1. This method verifies the feasibility of using the SDI model to invert soil salinization.
(1)
ρ4 − ρ3 ρ4 + ρ3
n−1
In the formula, SD is the standard deviation; it is the SDIi is the SDI value of the i-th year; SDI is the average SDI value of n years; and n is the research time period. The smaller the SD value is, the more concentrated the data distribution and the better the stability. Stability is divided into five categories: difference (SD > 0.5), poor (0.3 < SD < 0.5), medium (0.2 < SD < 0.3), better (0.1 < SD < 0.2), and good (SD < 0.1).
2.2. Research methods
SΙ =
∑i = 1 (SDIi − SDI ) 2
(2) (3)
3. Results and analysis
Where ρ1, ρ3 and ρ4 are the reflectivities of the corresponding Landsat bands and are the image reflectances of the blue light band, red light band, and near-infrared band of the Landsat image, respectively. The components and contents of soil salinity, soil moisture, organic matter, soil quality, soil colour, surface roughness, and coverage are the main factors affecting the reflectivity and spectral characteristics of salinized soil (Wang et al., 2010). Many studies have shown that the shortwave infrared band reflects the absorption of water and anions, which can be used to distinguish between chloride and sulfate in dry soil. Due to the vibration of carbonate ions, carbonate exhibits absorption in the thermal infrared (11–12 μm) band(Cemek et al., 2007). The root ion has an absorption band at 10.2 μm, which can be used to distinguish between alkaline soil and saline soil(Juan et al., 2011). When the humidity is low, the salt exhibits higher reflection in the blue light region. According to these research bases, the SDI of the blue, red, and nearinfrared bands of the Landsat images can be used to extract soil salinity information.
3.1. Distribution and interannual changes in SDI The interannual changes in soil salinization are shown in Fig. 2. From 2000 to 2018, the spatial distribution of soil surface salinity in the Minqin oasis was unevenly distributed, and the proportion of salinized land occupied by different degrees of salinization changed significantly (Table 1). The area ratio of salinized land in 2000 was 83.4%, and the area mainly consisted of mild salinization; thus, the degree of salinization was low. The proportion of salinized land in 2005 was 91.7%, and the area was mainly characterized by moderate and mild salinization; mildly salinized land accounted for the largest proportion. Salinized land accounted for 92.9% of the area in 2010 and was mainly characterized by light salinization. In 2015, salinized land accounted for 90.6% of the area, the proportion of severe salinization decreased, the proportion of mildly salinized land increased, and the proportion of mildly salinized land was the largest. In 2018, 90% of the land was 3
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Fig. 2. Spatial distribution of interannual variations in soil salinization.
salinized land was first converted to moderate, mild and non-salinized land. After 2015, the mildly salinized land changed to moderate and severe salinized land, and the degree of salinization first increased, then decreased and increased again.
Table 1 Table of changes in specific gravity of soil salinization areas.
Severe salinization Moderate salinization Mild salinization Non-salted land
2000
2005
2010
2015
2018
17.1% 29.2% 37.4% 16.3%
15.9% 35.8% 40% 8.3%
16.6% 36.4% 39.9% 7.1%
11% 33% 46.6% 9.4%
17.8% 37.2% 35% 10%
3.2. Seasonal changes Spring and winter represent the pan-salt season, which is dry and windy, and underground salt is transported to the surface of the saline soil, which increases the surface reflectivity. The summer and autumn are rainy, and the soluble salt in the soil is dissolved. During summer and autumn, vegetation cover is present on the soil surface and is also an important indicator of the level of soil salinization. The seasonal changes in soil salinization reflect the relationship between soil salinization and climate factors, which has certain representativeness. Therefore, opposite values of SDI are observed in spring and winter versus summer and autumn, and the spatial distributions of these values are shown in Fig. 3 and Fig. 4. This figure shows that in the spring, summer, autumn, and winter, the degree of soil salinization is significantly different, and there are significant seasonal differences, which are mainly reflected by the highest degree of soil salinity in spring and the lowest degree soil salinity in autumn. The proportion of severe salinization land is relatively large in spring. In the summer, some severe salinization land on the periphery of the Minqin oasis and in the northwestern region were converted to moderately salinized land. In the autumn, some moderately salinized land was changed to mild salinized land. However, in the winter, some lightly salinized land was changed to moderate and severe salinized land, and the seasonal changes in soil salinization were obvious. Surface vegetation coverage, soil moisture, air temperature, precipitation, and evaporation all differed in the different seasons. The seasonal changes in soil salinization indicate that climate and hydrological factors have a certain effect on soil salinization.
Table 2 Ecological water conveyance information. Year
Water flow into the lake /104m3
Formed surface area /106m2
Groundwater depth /m
Annual variation of groundwater depth /m
2008 2009 2010 2011 2012 2013 2014 2015 2016
– – 1290 2160 3000 2000 3300 2833 3358
– – 3.00 10.00 15.00 15.00 22.00 22.36 25.20
3.91 3.84 3.78 3.60 3.54 3.46 3.20 3.14 2.99
– −0.07 −0.06 −0.18 −0.06 −0.08 −0.26 −0.06 −0.15
salinized. During this time, the proportions of severe, moderate and non-salinized land increased compared with those in previous years, while the proportion of mildly salinized land decreased. Although the percentage of the total area of salinized land in the Minqin oasis changed slightly from 2000 to 2018, the changes between different degrees of salinized land were obvious. From 2000 to 2005, the proportions of severe salinization and non-salinized land decreased, while the proportions of moderate and mild salinization land increased. Severe and moderate salinization land slightly increased from 2005 to 2010, while the proportions of mild and non-salinized land decreased. From 2010 to 2015, moderately and severely salinized land decreased, while mildly salinized land exhibited the greatest increased, and nonsalinized land increased slightly. From 2015 to 2018, there was an increase in both severely and moderately salinized land, and the increase was mainly due to the transformation from mildly salinized land . In summary, from 2000 to 2018, the interconversion between severe, moderate, and mildly salinized land and non-salinized land was obvious, and the overall transformation trend was that the severely
3.3. Spatial variation characteristics of SDI Fig. 2 also shows that the spatial distribution of soil salinization in the Minqin oasis from 2000 to 2018 has obvious regional differences. Severe salinization land is mainly distributed in the northwest, northeast, and oasis areas of Minqin. Among these areas, the salinization around Qingtu Lake decreased significantly in 2015, and some areas changed from severely salinized land to moderately and mildly 4
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Fig. 3. Spatial distribution characteristics of salinization in different seasons.
salinized land; later, non-salinized land appeared. To obtain accurate SDI spatial change characteristics, stability and trend analysis methods are used for further exploration.
3.3.1. Stability of the SDI The stability of the SDI can reflect the main areas of soil salinity changes. Good stability means that the salinity degree has not changed significantly. Medium stability means that the salinity degree has changed. Poor stability means that the salinity degree change is
Fig. 4. Information on the soil salinization coverage ratio in different seasons. 5
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Fig. 5. Soil salinization stability characteristics of MinQin from 2000 to 2018.
4. Discussion
obvious. Fig. 5a shows the spatial distribution characteristics of stability of SDI in minqin county from 2000 to 2018, and Fig. 5b shows the calculated coverage ratio of stability of SDI at different levels. The figure shows that the areas with poor salinity stability in the Minqin oasis occupy a large area mainly in the northwest. The areas around Qingtu Lake, Baiyan Lake, Hongyashan Reservoir and the periphery of the Minqin oasis are the main areas where the degree of salinity has changed in recent years. The stability of other regions is centred, indicating that the degree of salinization has changed to some extent.
4.1. Using the SDI to determine the feasibility of soil salinization Studies have shown that the SDI has high accuracy in the inversion of soil salinization, and the SDI can adequately distinguish the distribution ranges of different salinity types in the study area. To further determine the applicability of this study, the average values of the measured verification points in different regions are taken in this paper to represent the soil salt content in the region. The SDI extracted from the remote sensing data in 2018 was compared with the soil salinity at the corresponding point to verify the feasibility of using SDI to obtain soil salinity information. The distribution characteristics of soil salinity in the areas around the Hongyashan Reservoir, Minqin cultivated land, Minqin Sandy Land and Qingtu Lake are shown in Fig. 7. The figure shows that the soil salinity around the Hongyashan Reservoir and Qingtu Lake is relatively high. The degree of soil salinization in the cultivated land and sandy land in Minqin is relatively low. The spatial distribution characteristics of the soil salinity information extracted by the SDI are consistent with the measured data results, indicating that the SDI can be used to accurately extract different degrees of soil salinity information.
3.3.2. Trends in the SDI A positive value in the trend analysis indicates that the degree of soil salinization is increasing, and a negative value indicates a decreasing trend. The significance test results of the soil salinity trend from 2000 to 2018 are shown in Fig. 6a, and the proportions of different trends are shown in Fig. 6b. The figure clearly shows that the areas where soil salinization is increasing in the Minqin oasis are mainly distributed in the northwest of the Minqin oasis and around Baiyan Lake, Qingtu Lake, and Hongyashan Reservoir. In other areas, there are no significant changes or decreases in soil salinization. Among these areas, the proportion of the area with increasing salinization is 37.4%, and the proportion of the area with decreasing salinization is 12.4%. The proportion of the area with increasing salinization is much larger than that with decreasing salinization. The increasing trend indicates that the soil salinization in the Minqin oasis showed an increasing trend during the study period. Salinization in the Qingtu Lake area decreases with the formation of the water surface, but salinization in the surrounding area first decreases and then increases and generally increases significantly.
4.2. Impact of climate and hydrology factors on the SDI Climate factors such as air temperature, precipitation, and evaporation can not only affect land cover changes but also have a great impact on soil salinization. For hot and arid regions in most irrigation areas, every element of the climate is important for the formation of soil
Fig. 6. Trends of soil salinization. 6
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moisture content increases, and the soluble salts in the soil dissolve. As the water moves, the salt separated in the soil also moves, and when the temperature and evaporation are high, the soil is lost as the water evaporates.The salt that is segregated in the ground gathers again on the ground surface, which is one of the reasons for the spatial and seasonal differences in the distribution of soil salinization and the main reason for the large proportion of salinized soil in Minqin County.
4.3. The impact of human factors on the SDI At present, human activities have also become an important factor affecting soil salinization. Changes in land use types, land use intensity, and groundwater extraction by human activities have affected soil salinization in different aspects and to varying degrees. For example, unreasonable groundwater extraction will directly cause changes in the groundwater depth, water bodies and local microclimates and then indirectly cause changes in soil salinization. The groundwater level and its chemical components are the main causes of soil salinizationZhang et al., 2019. As the groundwater level rises, the amount of water evaporation also increases, and more salt will accumulate at the surface through capillary water. Arid and semiarid areas have limited precipitation and high evaporation, which hinders the process of salt leaching to deep soil depths. When humans unreasonably utilize land resources and water resources, soil salt leaching will be further hindered and soil salinization will increase. In recent years, the human activity that has had the greatest impact on the Minqin oasis is the implementation of the ecological water transfer project from the Hongyashan Reservoir to Qingtu Lake in September 2010. For this reason, it is extremely important to analyse the effects of ecological water transport on soil salinization as an artificial factor. The largest change caused by the implementation of ecological water transport is the change in groundwater levels in Minqin County and Qingtu LakeTable 2. In the case of no significant change in precipitation, it is clear that ecological water transport will be the main reason for the change in groundwater level. Therefore, taking the change in groundwater level caused by ecological water transport as an artificial factor, the analysis of the impact on soil salinity is representative. Fig. 9 shows the average groundwater level changes in the Minqin oasis and Qingtu Lake. The figure shows that the average groundwater depth in the Minqin oasis has increased in recent years, while the groundwater depth around Qingtu Lake has decreased significantly. Qingtu Lake is the core area for ecological water transport. Therefore, the relationship between soil salinization and ecological water transport in the Qingtu Lake region was analysed to discuss the impacts of human factors on soil salinization. shows that the degree of soil salinization around Qingtu Lake is most significant around 2010. Trend analysis shows that the overall soil salinization in Qingtu Lake shows a significant increasing trend. Among
Fig. 7. Soil salt content.
salinization. Climate change has brought many problems to the soil environment, such as a series of biological changes in the soil physical composition (water content), chemical composition (various salt ion content), and plant species. Climate warming can cause not only microorganisms to rapidly decompose soil organic matter and soil nutrients and soil fertility to rapidly decrease but also soil moisture to evaporate and accelerate the upward movement of salt, causing soil salt accumulation and soil salinization. In addition, water-salt movement is an important link in the process of soil salinization. The effects of salt and water are mutually restricted, so when studying soil salinization, the two factors cannot be separated. The factors are simply isolated. Soil salinity and water transport are closely related. The water-salt balance problem is the root cause of soil salinization. The movement of surface water and groundwater directly affects the spatial distribution of soil moisture and salinity. To this end, it is necessary to collect data on air temperature, precipitation, evaporation, and soil relative humidity in Minqin County and analyse the effects of these factors on soil salinization. Due to the limitations of data collection, only the average temperature, precipitation and evaporation of Minqin County in 2000, 2005, 2010 and 2015 are shown in Fig. 8a and Fig. 8b. The figure shows that the Minqin oasis has always had the climatic characteristics of high temperatures, limited precipitation and much more evaporation than precipitation, and the hydrological characteristics of high soil relative humidity. This finding indicates that the Minqin oasis has always been in a state where water consumption is far greater than replenishment, and the water resources obtained by replenishment have a short retention time. Soil moisture is mainly stored on the surface layer, and soil moisture is easily lost by evaporation. The evaporation of salt ion aggregates results in a high degree of salinization; thus, there the area of saline-alkali land is large. When the rainfall is abundant, the soil
Fig. 8. Changes in climate and soil relative humidity in Minqin County from 2000 to 2015. 7
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Table 3 Land use changes in the study area from 2000 to 2015. class
2000
2005
2010
2015
Agriculture Woodland Grassland Waterbodies Built-up Unused
9% 1.2% 13% 0.1% 0.4% 76.3%
10% 1.2% 12.8% 0.1% 0.4% 75.5%
10.3% 1.2% 12.9% 0.2% 0.4% 75%
10.2% 1.2% 13.2% 0.2% 0.6% 74.6%
4.4. SDI and land cover pattern As land use data were not obtained for 2018, only the land cover from 2000 to 2015 is shown in Fig. 10. The figure shows that the land cover change in the Minqin oasis was not obvious from 2000 to 2015. The statistical information on the different types of land cover change is shown in Table 3. The agricultural land and water bodies ratio increased from 2000 to 2015, and the proportion of unused land decreased slightly. The land use type in the Minqin oasis is mainly unused land. To explore the differences in soil salinization changes in the different land use types, a spatial superposition analysis of land use data and soil salinization data was performed. The statistics on the salinization change characteristics of different land use types from 2000 to 2015 are shown in Table 4. This table shows that the salinized land in the Minqin oasis is mainly unused land, and the degree of salinization is high. In addition, grassland and agricultural land also face the problem of soil salinization. In recent years, the land use type that exhibited the most significant changes in salinity was unused land. For example, in the northwest of the Minqin oasis, the degree of salinization of unused land around the Hongyashan Reservoir has changed greatly. The soil salinization changes in agricultural land and grassland areas are also obvious, and these changes are mainly reflected in the Qingtu Lake area and the outer area of the Minqin oasis. Due to the small proportion of
Fig. 9. Groundwater level change patterns.
the analysed years, the degree of soil salinization was the lowest in 2015 and increased in 2018. The implementation of ecological water transfer from the Hongyashan Reservoir to the Qingtu Lake area resulted in the large water area of Qingtu Lake, but there are some unstable water catchments in this area. Each swelling process is a process of salt accumulation in the water body. Therefore, with the annual increase in ecological water transport and the change in water area, the soil salinization in some areas of Qingtu Lake also changed. In addition, changes in groundwater levels caused by ecological water transport have improved the local environment around Qingtu Lake, and soil salinization has also improved in a short period of time; however, secondary soil salinization will occur under high temperature and high evaporation conditions. Salinization and secondary soil salinization are slow processes; thus, the characteristics of soil salinity first increase and then decrease with time, and the degree of soil salinization spatially differs with the differences in ecological water transport in some areas of Qingtu Lake. The soil salinized land changed significantly.
Fig. 10. Land cover change maps for the study area in 2000, 2005, 2010 and 2015. 8
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Table 4 Soil salinization statistics of different land use types for 2000, 2005, 2010 and 2015. class
Agriculture
Woodland
Grassland
Waterbodies
Built-up
Unused
2000
Severe salinization Moderate salinization Mild salinization Non-salted land
0.55% 0.81% 4.46% 3.32%
0.11% 0.27% 0.61% 0.15%
1.82% 5.23% 4.57% 4.51%
0 0 0 0
0 0 0.15% 0.12%
19.05% 34.65% 22.50% 0.52%
2005
Severe salinization Moderate salinization Mild salinization Non-salted land
0.61% 1.35% 2.38% 6.19%
0.14% 0.39% 0.36% 0.24%
1.55% 5.88% 3.77% 7.17%
0 0 0 0
0 0 0 0.17%
14.27% 37.92% 22.23% 1.10%
2010
Severe salinization Moderate salinization Mild salinization Non-salted land
2.77% 1.20% 3.28% 1.99%
0.25% 0.68% 0.12% 0
3.39% 7.71% 0.56% 0.31%
0 0 0 0
0 0 0 0.14%
22.67% 50.54% 1.76% 0.44%
2015
Severe salinization Moderate salinization Mild salinization Non-salted land
0.30% 1.00% 4.06% 5.26%
0 0.20% 0.64% 0.20%
1.10% 3.91% 6.29% 0.64%
0 0 0.15% 0
0 0 0.18% 0.17%
12.08 29.49% 32.78% 0.83%
Fig. 11. The sudden salinization of different land use types a) Map of agricultural land in 2000–2015; b) map of grassland areas in 2000–2015; c) map of unused land in 2000–2015.
measured data, so it is feasible to use the SDI model to invert soil salinization. The soil salinization problem in the Minqin oasis is obvious. The area of salinized land exceeds 80%, and a large proportion of the salinized land is mainly related to climate and hydrological factors. The area ratio of salinized land in the Minqin oasis did not change much from 2000 to 2018, but the different degrees of salinized land exhibited a certain level of changes.According to statistics of the characteristics of soil salinization of different land use types, it is found that the salinized land in the Minqin oasis is mainly unused land, and the degree of salinization has changed significantly in recent years. The soil salinization in the Minqin oasis exhibits obvious seasonal differences. The degree of soil salinization was the highest in spring and the lowest in autumn, and with the change in season, the degree of salinization changed. In recent years, the proportion of salinized land in the Minqin oasis
other land use types, the degree of salinization was not obvious. Therefore, the spatial distribution of soil salinization in cultivated land, grassland, and unused land is shown in Fig. 11. Fig. 11a is the spatial distribution of soil salinity in agricultural land, Fig. 11b is the spatial change in soil salinity in grassland, and Fig. 11c is the salinization of unused land. The figure shows that the degree of soil salinization in agricultural land is the lowest, and this land use type is mainly nonsalinized land. A small amount of cultivated land was severely salinized in 2000 and 2010, while the grassland was mainly mild and severe salinized land. The degree of salinization is relatively high; the salinity of unused soil is the highest, where the salinization was mainly severe, moderate and mild.
5. Conclusions The spatial distribution characteristics are consistent with the 9
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has increased, and the areas where salinization has changed significantly are mainly northwest of the Minqin oasis, around Qingtu Lake, and around the Hongyashan Reservoir. The change in soil salinization is the most obvious in the Qingtu Lake area, which is mainly caused by the change in groundwater level caused by ecological water transport and the reduction in water area. The soil salinity was the lowest in the Minqin oasis in 2015, the salinity increased in 2018, and the performance of the Qingtu Lake area was the most obvious, indicating that ecological water transport has a short-term effect on soil salinity. This finding shows that ecological water transport can improve soil salinization in the short term, and long-term soil secondary salinization may occur due to soil moisture loss. Therefore, the response and mechanism of soil salinization downstream of inland rivers to changes in ecological water transport and groundwater levels require further observation and research.
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Declaration of competing interest We declare that the contents of this manuscript have not been copyrighted or published previously.we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Soil salinization in the oasis areas of downstream inland rivers —Case Study: Minqin oasis”. Acknowledgements This work is financially supported by national Natural Science Foundation of China (41661084/41867030/41661005) and National Natural Science Foundation of the State Natural Science Foundation of China (41421061) and National key Laboratory of Cryosphere Science Autonomous Project (SKLCS-ZZ-2017). References Aldakheel, Y.Y., 2011. Assessing ndvi spatial pattern as related to irrigation and soil salinity management in al-hassa oasis, Saudi Arabia. J. Indian.Soc.Remote Sens. 39 (2), 171–180. Allbed, A., Kumar, L., Aldakheel, Y.Y., 2014. Assessing soil salinity using soil salinity and vegetation indices derived from ikonos high-spatial resolution imageries: applications in a date palm dominated region. Geoderma 230–231 (7), 1–8. Barrios, J.M., Verstraeten, W.W., Maes, P., et al., 2013. Relating land cover and spatial distribution of nephropathia epidemica and lyme borreliosis in Belgium. Int. J. Environ. Health Res. 23 (2), 132–154. Belghemmaz, S., Fenni, M., Afrasinei, G.M., et al., 2017. Assessment of land degradation related to groundwater irrigation of oasis environments: (case study: the zibans, biskra, Algeria). In: Euro-mediterranean Conference for Environmental Integration. Bouaziz, M., Matschullat, J., Gloaguen, R., 2011. Improved remote sensing detection of soil salinity from a semi-arid climate in Northeast Brazil. Compt. Rendus Geosci. 343 (11), 795–803. Brunner, P., Li, H.T., Kinzelbach, W., et al., 2007. Generating soil electrical conductivity maps at regional level by integrating measurements on the ground and remote sensing data. Int. J. Remote Sens. 28 (15), 3341–3361. Cemek, B., G〇Ler, M., KiliG, K., et al., 2007. Assessment of spatial variability in some soil properties as Klated to soil salinity and alkalinity in Bafra plain in northern Turkey. Environ. Monit. Assess. 124 (1–3), 223–234. Chen, S., Gao, C., Bin, X.U., et al., 2014. Quantitative inversion of soil salinity and
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Soil salinization in the oasis areas of downstream inland rivers —Case Study: Minqin oasis Jianxia Yanga, Jun Zhaoa,∗, Guofeng Zhua,b, Yuchun Wanga, Xinggang Maa, Jianbang Wanga, Huiwen Guoa, Yu Zhanga a b
College of Geography and Environment Science, Northwest Normal University, Lanzhou, 730070, China State Key Laboratory of Cryosphere Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil salinization Remote sensing SDI Inland river
Soil salinization is the main resource and environmental problem in arid and semiarid regions, and it seriously restricts regional ecological security and sustainable agricultural development. Accurate monitoring of soil salinization and analysis of its main influencing factors are of great significance to the improvement and management of soil salinization. In this paper, the normalized difference vegetation index (NDVI) and salinity index (SI) were extracted from Landsat images of the Minqin oasis from 2000 to 2018, and these data were used to construct a soil salinization remote sensing monitoring index model (SDI). The index extracted by the model was used to monitor the soil salinization information, and the index values were graded to obtain different degrees of soil salinized land. The measured data were used to verify the accuracy of the SDI through trend analysis, stability analysis and other methods, and explore the characteristics and main influencing factors of soil salinization in the Minqin oasis. The results showed that the spatial distribution characteristics of the different degrees of soil salinity extracted by the SDI model were consistent with the measured results. The area of salinized soil in the Minqin oasis exceeded 80%, and the proportion of lightly salinized soil was the largest. The area ratio of salinized soil did not change much from 2000 to 2018, but the changes between different degrees of salinized soil were large.The unused land in the Minqin oasis exhibited a high degree of salinization, which was manifested spatially in the northwest of the Minqin oasis and the areas surrounding Qingtu Lake. In addition, the soil salinization of the Minqin oasis exhibited obvious seasonal differences, and the different degrees of salinization also changed with the change in seasons. The analysis of the main influencing factors of soil salinization in the Minqin oasis revealed that the large land occupation ratio of salinized land in the Minqin oasis was related to the local climatic factors. In recent years, changes in soil salinity in parts of the Minqin oasis and Qingtu Lake area were mainly the result of ecological water conveyance.
1. Introduction Soil salinization is the result of a combination of natural and human factors(Zhang et al., 2015; Hattar et al., 2010)and it mostly occurs in arid and semiarid areas with low precipitation(Tripathi et al., 2015), high evaporation, high groundwater levels, and highly soluble salts (Belghemmaz et al., 2017; Bouaziz et al., 2011). Soil salinization is an ecological and worldwide resource problem that restricts the stability of the ecological environment and the sustainability of land use (Yakup et al., 2017; Jiang et al., 2017), thus posing a serious threat to the biosphere and ecological security (Allbed et al., 2014). Soil salinization restricts the sustainable development of regional economies and agriculture mainly by accelerating desertification (Ke et al., 2015),
∗
inhibiting the growth of crops, hindering agricultural development, and reducing soil quality (Zhao et al., 2015). The prevention of secondary soil salinization and the transformation and treatment of soil issues have become the focus of land use research (Jin et al., 2012).The monitoring of soil salinity monitoring is an important basic work that is necessary to understand the distribution of salinized soil and explore the mechanism of soil salinization. Previous studies have aimed to monitor soil salinization dynamicsBrunner et al., 2007, identify the distribution of salinized soils(Scudiero et al., 2014), explore the mechanism of salinization development, and summarize the soil salinization research methods, and the results of these studies have important significance for agricultural production activities and ecological security and stability in arid areas (Chen et al., 2014; Zuo et al., 2014).At
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.quaint.2020.01.001 Received 14 January 2019; Received in revised form 2 January 2020; Accepted 6 January 2020 1040-6182/ © 2020 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article as: Jianxia Yang, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2020.01.001
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Fig. 1. Map of the study area.
studying the impacts of ecological water conveyance on the evolution of plant species, changes in vegetation cover (Zhao et al., 2018; Wang et al., 2018a,b), and changes in the groundwater level. However, as a targeted control area for ecological water transmission, there have been few studies on the changes in soil salinization around Qingtu Lake in recent years. Further research is needed to determine whether human intervention has resulted in the problem of secondary soil salinization and the trend of soil salinization changes in the Minqin oasis in recent years. In view of this, this article takes the Minqin oasis as the research area, and establishes SDI of soil salinization to explore the characteristics of soil salinization in the Minqin oasis from 2000 to 2018, focusing on the soil salinity around Qingtu Lake. The trend of soil salinization changes and analysis of the relationship between soil salinization and climate, hydrology and human factors provides a scientific reference for soil desertification management in the Minqin oasis.
present, remote sensing and geographic information system (GIS) technologies have been widely used to monitor and analyse soil salinization, and different remote sensing data (Fan et al., 2014), salinization models have been used to classify salinization areas. Various aspects of this research (Yang et al., 2014) provide a scientific basis for the extraction, management, monitoring and forecasting of saline soil patterns (Mougenot et al., 1993),and good results have been achieved in various areas of soil salinization research. However, in-depth analysis of the characteristics and influencing factors of soil salinization downstream of arid inland rivers, and the response of soil salinization to artificial intervention need to be further studied. The Minqin oasis of Gansu Province is located downstream of the Shiyang River Basin. The total area of the oasis is 15,900 km2, and the desertification area accounts for 94.5% of this area(Zhao et al., 2018). The Minqin oasis is bordered to the west by the Badain Jaran Desert and to the east by the Tengger DesertMa et al., 2018. The oasis consists of four geographical landscapes: desert, Gobi, denuded mountain and oasis plain (Zhu et al., 2018), which belong to arid and semiarid areas (Fig. 1). The oasis is located in a typical dry inland river region. Land desertification is a major ecological and environmental problem facing the region(Cemek et al., 2007), with soil salinization being a major type of soil desertification. Due to environmental characteristics such as high temperatures, low precipitation, high evaporation, and a lack of water resources in the Minqin oasis, coupled with the continuous development and utilization of groundwater resources, large amounts of natural and artificial vegetation have died, which has increased soil desertification (Niu et al., 2016). To this end, Minqin County has adopted a series of governance measures in recent years. Among these measures, projects such as “ecological water conveyance” and “sealing wells and pressing downfields” to reduce groundwater collection have improved the ecological environment. However, with the implementation of governance measures, a series of changes have taken place in the small regional environment, and these changes are mainly reflected by the groundwater levels, vegetation coverage, plant community composition, diversity, stability, and soil salinization. At present, the impacts of artificial intervention on related areas have mainly been identified by
2. Data and methods 2.1. Data sources and processing The remote sensing data used in this study were acquired from Landsat TM/OLI images from 2000, 2005, 2010, 2015 and 2018 with a resolution of 30 m, a synthetic image resolution of 15 m and a temporal resolution of 16 d. The image data are from the USGS (https://glovis. usgs.gov/) website. The preprocessing of the images included geometric correction, atmospheric correction, inlay and cropping. Landsat TM/ OLI images have a long coverage time and high resolution, making them suitable for small-scale studies with long sequence times and high accuracy requirements. The image data obtained were first preprocessed using ENVI5.1 software for geometric correction, radiation calibration, atmospheric correction, mosaic and clip. The meteorological data include the annual average temperature, precipitation, and evaporation of the year corresponding to the remote sensing data. The data comes from the China Meteorological Data 2
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To grade the extent of soil salinization, the soil is divided into severe salinization (SDI > 0.8), moderate salinization (0.6 < SDI < 0.8), mild salinization (0.4 < SDI < 0.6) and non-salted land (SDI < 0.4) (Prince et al., 2009; Scudiero et al., 2014).Due to the large difference in the salinity of the land index, SDI can well distinguish the distribution range of land types with different salinity in the study area(Wang et al., 2010).
Sharing Network (http://data.cma.cn/). Relevant data for ecological water conveyance were acquired from the 2015 Statistical Yearbook of Wuwei City. In July 2018, field observations of soil salinity in areas with significant changes in soil salinity were collected using a soil salinity meter (Shunkeda TR-6D), GPS, and canopy analyser in the areas around the lake, oasis farmland and surrounding the Hongyashan Reservoir. In view of the most obvious changes in salinization around Qingtu Lake and the core area of ecological water conveyance, the surrounding data mining points were dense. The spatial layout of the measured points is shown in Fig. 1.The figure not only indicates the specific location of the study area, but also gives a brief expression of the image coverage of the study area and the distribution of measured data points.In the figure, A, B, C, D and E are located in Qingtu lake area, Minqin oasis cultivated land area, Minqin Sandy Land, hongyashan reservoir area and alpine meadow area. In these areas, a number of experimental points were selected to directly measure the vegetation leaf area index, NDVI, soil salt content, soil moisture, and soil samples and water samples were collected.
2.2.2. Stability analysis Standard deviation is a measure of the extent of the distribution of data and can be used to assess the stability of SDI over a time series. Stability analysis was used to calculate the standard deviation of SDI according to the following formula (Shi et al., 2005): n
SD =
2.2.1. Construction of remote sensing monitoring index model for soil salinization Related studies have shown that the spectral characteristics of salinity in salinized soil have a great influence on the change in pixel brightness values in regional images (Csillag et al., 1993; Cemek et al., 2007). Although there are many soil salinity indices and salinization models, a large number of studies have shown that the salinity index (SI) extracted from the blue and red visible bands is relatively accurate in expressing soil salinity (Wang et al., 2010). Thus, the relationship between the normalized difference vegetation index (NDVI) and the SI was analysed, and the feature space concepts of the two indices was used to construct a soil salinization remote sensing monitoring index model (SDI) to extract and monitor soil salinity information with high precision. Because both NDVI and SI are relatively easy to obtain, the model construction is simple and straightforward. Therefore, this paper uses the multi-band information from Landsat remote sensing images, and uses formulas (1) and (2) (Everitt et al., 1988; Aldakheel et al., 2011) to calculate the SI and NDVI, and the extract the two index indices. The index uses the concept of feature space to establish SDI, as shown by formula (3)(Ding et al., 2010).
ρ1 × ρ3
NDVI =
SDI =
(NDVI − 1) 2 + SI 2
(4)
2.2.3. Trend analysis The least squares method can be used to fit the slope of the annual average SI for each pixel, which can be used to simulate the trend of each grid and comprehensively reflect the temporal and spatial patterns of soil salinization changes (Aldakheel, 2011; Barrioset et al., 2013). In this paper, we use the overall trend of SI from 2000 to 2018 to determine the significance of the interannual variation in SI. A negative slope indicates that soil salinization is decreasing, and a positive slope indicates that soil salinization is increasing (Jianguo et al., 2012; Sun and Liu., 2015). According to the F test, the correlation coefficient P value is used to express the significance of the change, and the change trend is divided into 5 levels: Significantly lower (slope < 0, P < 0.01), Reduce (slope < 0, 0.01 < P < 0.05), No significant changes (P > 0.05), Increased (slope > 0, 0.01 < P < 0.05), Significantly elevated(slope > 0, P < 0.01). 2.2.4. Accuracy evaluation of the SDI The points measured in the different areas are averaged to represent the soil throughout in the area. The SDI inversion in 2018 uses the SDI value at the point corresponding to the measurements in ENVI5.1. This method verifies the feasibility of using the SDI model to invert soil salinization.
(1)
ρ4 − ρ3 ρ4 + ρ3
n−1
In the formula, SD is the standard deviation; it is the SDIi is the SDI value of the i-th year; SDI is the average SDI value of n years; and n is the research time period. The smaller the SD value is, the more concentrated the data distribution and the better the stability. Stability is divided into five categories: difference (SD > 0.5), poor (0.3 < SD < 0.5), medium (0.2 < SD < 0.3), better (0.1 < SD < 0.2), and good (SD < 0.1).
2.2. Research methods
SΙ =
∑i = 1 (SDIi − SDI ) 2
(2) (3)
3. Results and analysis
Where ρ1, ρ3 and ρ4 are the reflectivities of the corresponding Landsat bands and are the image reflectances of the blue light band, red light band, and near-infrared band of the Landsat image, respectively. The components and contents of soil salinity, soil moisture, organic matter, soil quality, soil colour, surface roughness, and coverage are the main factors affecting the reflectivity and spectral characteristics of salinized soil (Wang et al., 2010). Many studies have shown that the shortwave infrared band reflects the absorption of water and anions, which can be used to distinguish between chloride and sulfate in dry soil. Due to the vibration of carbonate ions, carbonate exhibits absorption in the thermal infrared (11–12 μm) band(Cemek et al., 2007). The root ion has an absorption band at 10.2 μm, which can be used to distinguish between alkaline soil and saline soil(Juan et al., 2011). When the humidity is low, the salt exhibits higher reflection in the blue light region. According to these research bases, the SDI of the blue, red, and nearinfrared bands of the Landsat images can be used to extract soil salinity information.
3.1. Distribution and interannual changes in SDI The interannual changes in soil salinization are shown in Fig. 2. From 2000 to 2018, the spatial distribution of soil surface salinity in the Minqin oasis was unevenly distributed, and the proportion of salinized land occupied by different degrees of salinization changed significantly (Table 1). The area ratio of salinized land in 2000 was 83.4%, and the area mainly consisted of mild salinization; thus, the degree of salinization was low. The proportion of salinized land in 2005 was 91.7%, and the area was mainly characterized by moderate and mild salinization; mildly salinized land accounted for the largest proportion. Salinized land accounted for 92.9% of the area in 2010 and was mainly characterized by light salinization. In 2015, salinized land accounted for 90.6% of the area, the proportion of severe salinization decreased, the proportion of mildly salinized land increased, and the proportion of mildly salinized land was the largest. In 2018, 90% of the land was 3
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Fig. 2. Spatial distribution of interannual variations in soil salinization.
salinized land was first converted to moderate, mild and non-salinized land. After 2015, the mildly salinized land changed to moderate and severe salinized land, and the degree of salinization first increased, then decreased and increased again.
Table 1 Table of changes in specific gravity of soil salinization areas.
Severe salinization Moderate salinization Mild salinization Non-salted land
2000
2005
2010
2015
2018
17.1% 29.2% 37.4% 16.3%
15.9% 35.8% 40% 8.3%
16.6% 36.4% 39.9% 7.1%
11% 33% 46.6% 9.4%
17.8% 37.2% 35% 10%
3.2. Seasonal changes Spring and winter represent the pan-salt season, which is dry and windy, and underground salt is transported to the surface of the saline soil, which increases the surface reflectivity. The summer and autumn are rainy, and the soluble salt in the soil is dissolved. During summer and autumn, vegetation cover is present on the soil surface and is also an important indicator of the level of soil salinization. The seasonal changes in soil salinization reflect the relationship between soil salinization and climate factors, which has certain representativeness. Therefore, opposite values of SDI are observed in spring and winter versus summer and autumn, and the spatial distributions of these values are shown in Fig. 3 and Fig. 4. This figure shows that in the spring, summer, autumn, and winter, the degree of soil salinization is significantly different, and there are significant seasonal differences, which are mainly reflected by the highest degree of soil salinity in spring and the lowest degree soil salinity in autumn. The proportion of severe salinization land is relatively large in spring. In the summer, some severe salinization land on the periphery of the Minqin oasis and in the northwestern region were converted to moderately salinized land. In the autumn, some moderately salinized land was changed to mild salinized land. However, in the winter, some lightly salinized land was changed to moderate and severe salinized land, and the seasonal changes in soil salinization were obvious. Surface vegetation coverage, soil moisture, air temperature, precipitation, and evaporation all differed in the different seasons. The seasonal changes in soil salinization indicate that climate and hydrological factors have a certain effect on soil salinization.
Table 2 Ecological water conveyance information. Year
Water flow into the lake /104m3
Formed surface area /106m2
Groundwater depth /m
Annual variation of groundwater depth /m
2008 2009 2010 2011 2012 2013 2014 2015 2016
– – 1290 2160 3000 2000 3300 2833 3358
– – 3.00 10.00 15.00 15.00 22.00 22.36 25.20
3.91 3.84 3.78 3.60 3.54 3.46 3.20 3.14 2.99
– −0.07 −0.06 −0.18 −0.06 −0.08 −0.26 −0.06 −0.15
salinized. During this time, the proportions of severe, moderate and non-salinized land increased compared with those in previous years, while the proportion of mildly salinized land decreased. Although the percentage of the total area of salinized land in the Minqin oasis changed slightly from 2000 to 2018, the changes between different degrees of salinized land were obvious. From 2000 to 2005, the proportions of severe salinization and non-salinized land decreased, while the proportions of moderate and mild salinization land increased. Severe and moderate salinization land slightly increased from 2005 to 2010, while the proportions of mild and non-salinized land decreased. From 2010 to 2015, moderately and severely salinized land decreased, while mildly salinized land exhibited the greatest increased, and nonsalinized land increased slightly. From 2015 to 2018, there was an increase in both severely and moderately salinized land, and the increase was mainly due to the transformation from mildly salinized land . In summary, from 2000 to 2018, the interconversion between severe, moderate, and mildly salinized land and non-salinized land was obvious, and the overall transformation trend was that the severely
3.3. Spatial variation characteristics of SDI Fig. 2 also shows that the spatial distribution of soil salinization in the Minqin oasis from 2000 to 2018 has obvious regional differences. Severe salinization land is mainly distributed in the northwest, northeast, and oasis areas of Minqin. Among these areas, the salinization around Qingtu Lake decreased significantly in 2015, and some areas changed from severely salinized land to moderately and mildly 4
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Fig. 3. Spatial distribution characteristics of salinization in different seasons.
salinized land; later, non-salinized land appeared. To obtain accurate SDI spatial change characteristics, stability and trend analysis methods are used for further exploration.
3.3.1. Stability of the SDI The stability of the SDI can reflect the main areas of soil salinity changes. Good stability means that the salinity degree has not changed significantly. Medium stability means that the salinity degree has changed. Poor stability means that the salinity degree change is
Fig. 4. Information on the soil salinization coverage ratio in different seasons. 5
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Fig. 5. Soil salinization stability characteristics of MinQin from 2000 to 2018.
4. Discussion
obvious. Fig. 5a shows the spatial distribution characteristics of stability of SDI in minqin county from 2000 to 2018, and Fig. 5b shows the calculated coverage ratio of stability of SDI at different levels. The figure shows that the areas with poor salinity stability in the Minqin oasis occupy a large area mainly in the northwest. The areas around Qingtu Lake, Baiyan Lake, Hongyashan Reservoir and the periphery of the Minqin oasis are the main areas where the degree of salinity has changed in recent years. The stability of other regions is centred, indicating that the degree of salinization has changed to some extent.
4.1. Using the SDI to determine the feasibility of soil salinization Studies have shown that the SDI has high accuracy in the inversion of soil salinization, and the SDI can adequately distinguish the distribution ranges of different salinity types in the study area. To further determine the applicability of this study, the average values of the measured verification points in different regions are taken in this paper to represent the soil salt content in the region. The SDI extracted from the remote sensing data in 2018 was compared with the soil salinity at the corresponding point to verify the feasibility of using SDI to obtain soil salinity information. The distribution characteristics of soil salinity in the areas around the Hongyashan Reservoir, Minqin cultivated land, Minqin Sandy Land and Qingtu Lake are shown in Fig. 7. The figure shows that the soil salinity around the Hongyashan Reservoir and Qingtu Lake is relatively high. The degree of soil salinization in the cultivated land and sandy land in Minqin is relatively low. The spatial distribution characteristics of the soil salinity information extracted by the SDI are consistent with the measured data results, indicating that the SDI can be used to accurately extract different degrees of soil salinity information.
3.3.2. Trends in the SDI A positive value in the trend analysis indicates that the degree of soil salinization is increasing, and a negative value indicates a decreasing trend. The significance test results of the soil salinity trend from 2000 to 2018 are shown in Fig. 6a, and the proportions of different trends are shown in Fig. 6b. The figure clearly shows that the areas where soil salinization is increasing in the Minqin oasis are mainly distributed in the northwest of the Minqin oasis and around Baiyan Lake, Qingtu Lake, and Hongyashan Reservoir. In other areas, there are no significant changes or decreases in soil salinization. Among these areas, the proportion of the area with increasing salinization is 37.4%, and the proportion of the area with decreasing salinization is 12.4%. The proportion of the area with increasing salinization is much larger than that with decreasing salinization. The increasing trend indicates that the soil salinization in the Minqin oasis showed an increasing trend during the study period. Salinization in the Qingtu Lake area decreases with the formation of the water surface, but salinization in the surrounding area first decreases and then increases and generally increases significantly.
4.2. Impact of climate and hydrology factors on the SDI Climate factors such as air temperature, precipitation, and evaporation can not only affect land cover changes but also have a great impact on soil salinization. For hot and arid regions in most irrigation areas, every element of the climate is important for the formation of soil
Fig. 6. Trends of soil salinization. 6
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moisture content increases, and the soluble salts in the soil dissolve. As the water moves, the salt separated in the soil also moves, and when the temperature and evaporation are high, the soil is lost as the water evaporates.The salt that is segregated in the ground gathers again on the ground surface, which is one of the reasons for the spatial and seasonal differences in the distribution of soil salinization and the main reason for the large proportion of salinized soil in Minqin County.
4.3. The impact of human factors on the SDI At present, human activities have also become an important factor affecting soil salinization. Changes in land use types, land use intensity, and groundwater extraction by human activities have affected soil salinization in different aspects and to varying degrees. For example, unreasonable groundwater extraction will directly cause changes in the groundwater depth, water bodies and local microclimates and then indirectly cause changes in soil salinization. The groundwater level and its chemical components are the main causes of soil salinizationZhang et al., 2019. As the groundwater level rises, the amount of water evaporation also increases, and more salt will accumulate at the surface through capillary water. Arid and semiarid areas have limited precipitation and high evaporation, which hinders the process of salt leaching to deep soil depths. When humans unreasonably utilize land resources and water resources, soil salt leaching will be further hindered and soil salinization will increase. In recent years, the human activity that has had the greatest impact on the Minqin oasis is the implementation of the ecological water transfer project from the Hongyashan Reservoir to Qingtu Lake in September 2010. For this reason, it is extremely important to analyse the effects of ecological water transport on soil salinization as an artificial factor. The largest change caused by the implementation of ecological water transport is the change in groundwater levels in Minqin County and Qingtu LakeTable 2. In the case of no significant change in precipitation, it is clear that ecological water transport will be the main reason for the change in groundwater level. Therefore, taking the change in groundwater level caused by ecological water transport as an artificial factor, the analysis of the impact on soil salinity is representative. Fig. 9 shows the average groundwater level changes in the Minqin oasis and Qingtu Lake. The figure shows that the average groundwater depth in the Minqin oasis has increased in recent years, while the groundwater depth around Qingtu Lake has decreased significantly. Qingtu Lake is the core area for ecological water transport. Therefore, the relationship between soil salinization and ecological water transport in the Qingtu Lake region was analysed to discuss the impacts of human factors on soil salinization. shows that the degree of soil salinization around Qingtu Lake is most significant around 2010. Trend analysis shows that the overall soil salinization in Qingtu Lake shows a significant increasing trend. Among
Fig. 7. Soil salt content.
salinization. Climate change has brought many problems to the soil environment, such as a series of biological changes in the soil physical composition (water content), chemical composition (various salt ion content), and plant species. Climate warming can cause not only microorganisms to rapidly decompose soil organic matter and soil nutrients and soil fertility to rapidly decrease but also soil moisture to evaporate and accelerate the upward movement of salt, causing soil salt accumulation and soil salinization. In addition, water-salt movement is an important link in the process of soil salinization. The effects of salt and water are mutually restricted, so when studying soil salinization, the two factors cannot be separated. The factors are simply isolated. Soil salinity and water transport are closely related. The water-salt balance problem is the root cause of soil salinization. The movement of surface water and groundwater directly affects the spatial distribution of soil moisture and salinity. To this end, it is necessary to collect data on air temperature, precipitation, evaporation, and soil relative humidity in Minqin County and analyse the effects of these factors on soil salinization. Due to the limitations of data collection, only the average temperature, precipitation and evaporation of Minqin County in 2000, 2005, 2010 and 2015 are shown in Fig. 8a and Fig. 8b. The figure shows that the Minqin oasis has always had the climatic characteristics of high temperatures, limited precipitation and much more evaporation than precipitation, and the hydrological characteristics of high soil relative humidity. This finding indicates that the Minqin oasis has always been in a state where water consumption is far greater than replenishment, and the water resources obtained by replenishment have a short retention time. Soil moisture is mainly stored on the surface layer, and soil moisture is easily lost by evaporation. The evaporation of salt ion aggregates results in a high degree of salinization; thus, there the area of saline-alkali land is large. When the rainfall is abundant, the soil
Fig. 8. Changes in climate and soil relative humidity in Minqin County from 2000 to 2015. 7
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Table 3 Land use changes in the study area from 2000 to 2015. class
2000
2005
2010
2015
Agriculture Woodland Grassland Waterbodies Built-up Unused
9% 1.2% 13% 0.1% 0.4% 76.3%
10% 1.2% 12.8% 0.1% 0.4% 75.5%
10.3% 1.2% 12.9% 0.2% 0.4% 75%
10.2% 1.2% 13.2% 0.2% 0.6% 74.6%
4.4. SDI and land cover pattern As land use data were not obtained for 2018, only the land cover from 2000 to 2015 is shown in Fig. 10. The figure shows that the land cover change in the Minqin oasis was not obvious from 2000 to 2015. The statistical information on the different types of land cover change is shown in Table 3. The agricultural land and water bodies ratio increased from 2000 to 2015, and the proportion of unused land decreased slightly. The land use type in the Minqin oasis is mainly unused land. To explore the differences in soil salinization changes in the different land use types, a spatial superposition analysis of land use data and soil salinization data was performed. The statistics on the salinization change characteristics of different land use types from 2000 to 2015 are shown in Table 4. This table shows that the salinized land in the Minqin oasis is mainly unused land, and the degree of salinization is high. In addition, grassland and agricultural land also face the problem of soil salinization. In recent years, the land use type that exhibited the most significant changes in salinity was unused land. For example, in the northwest of the Minqin oasis, the degree of salinization of unused land around the Hongyashan Reservoir has changed greatly. The soil salinization changes in agricultural land and grassland areas are also obvious, and these changes are mainly reflected in the Qingtu Lake area and the outer area of the Minqin oasis. Due to the small proportion of
Fig. 9. Groundwater level change patterns.
the analysed years, the degree of soil salinization was the lowest in 2015 and increased in 2018. The implementation of ecological water transfer from the Hongyashan Reservoir to the Qingtu Lake area resulted in the large water area of Qingtu Lake, but there are some unstable water catchments in this area. Each swelling process is a process of salt accumulation in the water body. Therefore, with the annual increase in ecological water transport and the change in water area, the soil salinization in some areas of Qingtu Lake also changed. In addition, changes in groundwater levels caused by ecological water transport have improved the local environment around Qingtu Lake, and soil salinization has also improved in a short period of time; however, secondary soil salinization will occur under high temperature and high evaporation conditions. Salinization and secondary soil salinization are slow processes; thus, the characteristics of soil salinity first increase and then decrease with time, and the degree of soil salinization spatially differs with the differences in ecological water transport in some areas of Qingtu Lake. The soil salinized land changed significantly.
Fig. 10. Land cover change maps for the study area in 2000, 2005, 2010 and 2015. 8
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Table 4 Soil salinization statistics of different land use types for 2000, 2005, 2010 and 2015. class
Agriculture
Woodland
Grassland
Waterbodies
Built-up
Unused
2000
Severe salinization Moderate salinization Mild salinization Non-salted land
0.55% 0.81% 4.46% 3.32%
0.11% 0.27% 0.61% 0.15%
1.82% 5.23% 4.57% 4.51%
0 0 0 0
0 0 0.15% 0.12%
19.05% 34.65% 22.50% 0.52%
2005
Severe salinization Moderate salinization Mild salinization Non-salted land
0.61% 1.35% 2.38% 6.19%
0.14% 0.39% 0.36% 0.24%
1.55% 5.88% 3.77% 7.17%
0 0 0 0
0 0 0 0.17%
14.27% 37.92% 22.23% 1.10%
2010
Severe salinization Moderate salinization Mild salinization Non-salted land
2.77% 1.20% 3.28% 1.99%
0.25% 0.68% 0.12% 0
3.39% 7.71% 0.56% 0.31%
0 0 0 0
0 0 0 0.14%
22.67% 50.54% 1.76% 0.44%
2015
Severe salinization Moderate salinization Mild salinization Non-salted land
0.30% 1.00% 4.06% 5.26%
0 0.20% 0.64% 0.20%
1.10% 3.91% 6.29% 0.64%
0 0 0.15% 0
0 0 0.18% 0.17%
12.08 29.49% 32.78% 0.83%
Fig. 11. The sudden salinization of different land use types a) Map of agricultural land in 2000–2015; b) map of grassland areas in 2000–2015; c) map of unused land in 2000–2015.
measured data, so it is feasible to use the SDI model to invert soil salinization. The soil salinization problem in the Minqin oasis is obvious. The area of salinized land exceeds 80%, and a large proportion of the salinized land is mainly related to climate and hydrological factors. The area ratio of salinized land in the Minqin oasis did not change much from 2000 to 2018, but the different degrees of salinized land exhibited a certain level of changes.According to statistics of the characteristics of soil salinization of different land use types, it is found that the salinized land in the Minqin oasis is mainly unused land, and the degree of salinization has changed significantly in recent years. The soil salinization in the Minqin oasis exhibits obvious seasonal differences. The degree of soil salinization was the highest in spring and the lowest in autumn, and with the change in season, the degree of salinization changed. In recent years, the proportion of salinized land in the Minqin oasis
other land use types, the degree of salinization was not obvious. Therefore, the spatial distribution of soil salinization in cultivated land, grassland, and unused land is shown in Fig. 11. Fig. 11a is the spatial distribution of soil salinity in agricultural land, Fig. 11b is the spatial change in soil salinity in grassland, and Fig. 11c is the salinization of unused land. The figure shows that the degree of soil salinization in agricultural land is the lowest, and this land use type is mainly nonsalinized land. A small amount of cultivated land was severely salinized in 2000 and 2010, while the grassland was mainly mild and severe salinized land. The degree of salinization is relatively high; the salinity of unused soil is the highest, where the salinization was mainly severe, moderate and mild.
5. Conclusions The spatial distribution characteristics are consistent with the 9
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has increased, and the areas where salinization has changed significantly are mainly northwest of the Minqin oasis, around Qingtu Lake, and around the Hongyashan Reservoir. The change in soil salinization is the most obvious in the Qingtu Lake area, which is mainly caused by the change in groundwater level caused by ecological water transport and the reduction in water area. The soil salinity was the lowest in the Minqin oasis in 2015, the salinity increased in 2018, and the performance of the Qingtu Lake area was the most obvious, indicating that ecological water transport has a short-term effect on soil salinity. This finding shows that ecological water transport can improve soil salinization in the short term, and long-term soil secondary salinization may occur due to soil moisture loss. Therefore, the response and mechanism of soil salinization downstream of inland rivers to changes in ecological water transport and groundwater levels require further observation and research.
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Declaration of competing interest We declare that the contents of this manuscript have not been copyrighted or published previously.we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Soil salinization in the oasis areas of downstream inland rivers —Case Study: Minqin oasis”. Acknowledgements This work is financially supported by national Natural Science Foundation of China (41661084/41867030/41661005) and National Natural Science Foundation of the State Natural Science Foundation of China (41421061) and National key Laboratory of Cryosphere Science Autonomous Project (SKLCS-ZZ-2017). References Aldakheel, Y.Y., 2011. Assessing ndvi spatial pattern as related to irrigation and soil salinity management in al-hassa oasis, Saudi Arabia. J. Indian.Soc.Remote Sens. 39 (2), 171–180. Allbed, A., Kumar, L., Aldakheel, Y.Y., 2014. Assessing soil salinity using soil salinity and vegetation indices derived from ikonos high-spatial resolution imageries: applications in a date palm dominated region. Geoderma 230–231 (7), 1–8. Barrios, J.M., Verstraeten, W.W., Maes, P., et al., 2013. Relating land cover and spatial distribution of nephropathia epidemica and lyme borreliosis in Belgium. Int. J. Environ. Health Res. 23 (2), 132–154. Belghemmaz, S., Fenni, M., Afrasinei, G.M., et al., 2017. Assessment of land degradation related to groundwater irrigation of oasis environments: (case study: the zibans, biskra, Algeria). In: Euro-mediterranean Conference for Environmental Integration. Bouaziz, M., Matschullat, J., Gloaguen, R., 2011. Improved remote sensing detection of soil salinity from a semi-arid climate in Northeast Brazil. Compt. Rendus Geosci. 343 (11), 795–803. Brunner, P., Li, H.T., Kinzelbach, W., et al., 2007. Generating soil electrical conductivity maps at regional level by integrating measurements on the ground and remote sensing data. Int. J. Remote Sens. 28 (15), 3341–3361. Cemek, B., G〇Ler, M., KiliG, K., et al., 2007. Assessment of spatial variability in some soil properties as Klated to soil salinity and alkalinity in Bafra plain in northern Turkey. Environ. Monit. Assess. 124 (1–3), 223–234. Chen, S., Gao, C., Bin, X.U., et al., 2014. Quantitative inversion of soil salinity and
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