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Journal of Geographical Sciences >

2018 , Vol. 28 >Issue 8: 1099 - 1112

DOI: https://doi.org/10.1007/s11442-018-1544-3

研究论文

Determination of land salinization causes via land cover and hydrological process change detection in a typical part of Songnen Plain

  • WANG Zhiyong , 1, 2 ,
  • LI Lijuan , 1, *
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  • 1. Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
  • 2. University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author: Li Lijuan, Professor, E-mail:

Author: Wang Zhiyong (1988-), PhD Candidate, specialized in water and salt cycle. E-mail:

Received date: 2017-10-12

  Accepted date: 2018-01-26

  Online published: 2018-08-10

Supported by

Key Deployment Project of CAS, No.KFZD-SW-314;National Natural Science Foundation of China, No.91547114

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Journal of Geographical Sciences, All Rights Reserved
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Abstract

Causes of land salinization were determined via land cover and hydrological process change detection in a typical part of Songnen Plain. The area of saline land increased from 4627 km2 in 1980 to 5416 km2 in 2000, and then decreased to 5198 km2 in 2015. The transformation between saline land and other land covers happened mainly before 2000, and saline land had transformation relationship mainly with cropland, grassland, and water body. From 1979 to 2007, groundwater depth fluctuated to increase and was mainly deeper than 3.3 m. Spatially, the area of the region where groundwater depth was deeper than 3.3 m increased from 46.7% in 1980 to 84% in 2000, while the area of the region almost occupied the whole region after 2000. Precipitation and evaporation changed little, while runoff decreased substantially. Shallow groundwater, change of cropland, grassland, and water body induced from human activities and decrease of runoff and increase of irrigation and water transfer from outer basin were the main reasons for land salinization before 2000. After 2000, groundwater with relatively great depth could not exert great influence on land salinization. Protection of grassland and wetland prevented the increase of the area of saline land.

Key words： change detection; hydrological process; land cover; land salinization; Songnen Plain

Cite this article

WANG Zhiyong , LI Lijuan . Determination of land salinization causes via land cover and hydrological process change detection in a typical part of Songnen Plain[J]. Journal of Geographical Sciences, 2018 , 28(8) : 1099 -1112 . DOI: 10.1007/s11442-018-1544-3

1 Introduction

Land salinization is one of the most common land degradation processes (UNEP, 1991). More than 77 m ha (million hectares) of land were salt-affected and about 43 m ha were attributed to secondary salinization at global scale in 2007 (FAO, 2007). Moreover, some estimates indicated that one-third of the irrigated land in the major countries with irrigated agriculture was badly affected by salinity or expected to be salinized in the near future (Lambert et al., 2002; Akhtar et al., 2013). Land salinization often occurs particularly in arid or semiarid areas (Masoud and Koike, 2006), where low rainfall, high evapotranspiration rates or soil textural characteristics impede the washing out of the salts which subsequently built-up in the soil surface layers. Threats of land salinization on ecosystem and environment are obvious (Line et al., 2010). Accumulation of soluble salts in the soil is one of the main limiting factors for agriculture by decreasing the soil productivity (Rengasamy, 2002), limiting the growth of crops (Datta et al., 2002; Yamaguchi and Blumwald, 2005), constraining agricultural productivity (Qadir et al., 2008) and even leading to the abandonment of arable lands (Ding et al., 2011). Besides, it also poses threats on environment by increasing the salinity of water, decreasing biodiversity of ecosystem (Halse et al., 2003), and affecting other major soil degradation phenomena such as soil dispersion, soil erosion (García-Ruiz, 2010), and engineering problems (Metternicht and Zinck, 2003). Given these adverse impacts, detecting its causes is important for better management practices.
Songnen Plain, which is the largest production base of commodity grain in China, has 3.2 m ha saline land (21% of the area of the Plain) (Wang et al., 2009) and is the biggest distribution area of soda saline land in China (Gu, 2010). Many reasons resulted in the occurrence of the land salinization of Songnen Plain. Neo-plate tectonics provided the sources and transport path, and generated the accumulation environment (Wang et al., 1985). Shallow depth groundwater provided the solvent and carrier for the dissolving and rise of the soil salt (Song et al., 2000; Zhang et al., 2000). Arid or semiarid climate provided the power for the rise of the water and salt in the soil profile (Yu et al., 1993; Liu et al., 2002). In the last several decades, land salinization has been aggravated constantly. Human activities, which lead to the changes of the hydrological process, are the main driving factors to the soil salinization (Liu et al., 2005). Moreover, shallow depth groundwater and ponding rainwater lead to the primary land salinization, while the secondary land salinization was induced from the significant decrease of the chances and occurrence frequency of flooding and waterlogging in the low-lying plains (Yang et al., 2010). Even many researches had paid attention to saline land of Songnen Plain, the detailed spatio-temporal transformation relationship with other land covers and long-term hydrological process, especially, the spatio-temporal distribution of groundwater were not depicted at the same time. In this study, a typical part of Songnen Plain was chosen to analyze its causes for land salinization by revealing the spatio-temporal change of land covers and hydrological process simultaneously.

2 Study area

Land salinization, which mainly distributed in the western part, was a severe problem in Songnen Plain. This part including 5 counties where land salinization problem was typical was chosen as the study area (Figure 1). The area of the typical part was 25,664 km2. The terrain decreased from Northwest to Southeast and the altitude of the whole part was mainly from 100 to 200 m. The Taoer River flows through this part and runs toward Nenjiang River. The flat terrain made the precipitation and runoff from the upper reach accumulated in this area and evaporation was the main means of discharge. High evaporation (1194 mm, Φ20 cm pan evaporation), low precipitation (386 mm), and a large amount of irrigation created a favorable condition for the aggregation of soil salt on the land surface.
Figure 1 Location of the typical part of Songnen Plain

3 Data and methods

3.1 Land cover database

Six land cover databases, with a mapping scale of 1:1,000,000, of the years 1980, 1990, 2000, 2005, 2010, and 2015 were collected from ‘Data Center for Resources and Environmental Sciences Chinese Academy of Sciences’ (website: http://www.resdc.cn/Default.aspx). The data source for the land cover database was Landsat MSS/TM/ETM/OLI, China-Brazil Earth Resources Satellite, Small Satellite Constellation for Environment and Disaster Monitoring, and Forecasting HJ-1 satellite digital images (Liu et al., 2014; Liu et al., 2015). More detailed information about this land cover database could be found in previous reports (Liu et al., 2010).
In this paper, based on the classified 25 second-level land cover types, the land cover data were grouped into seven classes: cropland, woodland, grassland, water body (including swampland), built-up area, unused land, and saline land to reveal the spatio-temporal dynamic of the saline land.

3.2 Hydrological data

Hydrological data included groundwater depth, precipitation, evaporation, and runoff.
Groundwater depth collected from the ‘Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences’. The data included the groundwater depth every 5 days from 16 wells from 1979 to 2007, annual mean groundwater depth every 5 days from 61 wells in 1980, annual mean groundwater depth every 5 days from 98 wells in 1990, annual mean groundwater depth every 5 days from 86 wells in 2000 and 2005. The data from 1979 to 2007 were used for analyzing the temporal variation of groundwater depth. The data in 1980, 1990, 2000, and 2005 were used for analyzing the spatial distribution of groundwater depth based on the spatial interpolation by using the Kriging interpolation method. The spatial distribution of groundwater depth in 2010 and 2015 was also analyzed based on “Monthly report of groundwater dynamics”, which was opened for inquiry at website (http://www.hydroinfo.gov.cn/).
Annual precipitation and evaporation from 1960 to 2015 were collected from Baicheng Meteorological Station for the temporal variation analysis. Runoff from 1983 to 2006 at Taonan Hydrological Station was collected for the temporal variation analysis. Spatial interpolation data of precipitation in 1980, 1990, 2000, 2005, 2010, and 2015 were collected for the analysis of spatial distribution. The dataset is provided by Data Center for Resources and Environmental Sciences, Chinese Academy of Sciences (RESDC) (http://www.resdc.cn). Data on annual pan evaporation (Φ20 cm) of Songnen Plain in 1980, 1990, 2000, 2005, 2010, and 2015 at 36 meteorological stations were collected for spatial interpolation by using the Spline method. Spatial distribution in the typical area was cut out from the whole area for spatial dynamic analysis.

3.3 Transfer matrix

Land cover transfer matrix was used for the analysis of transformation between saline land and other kinds of land covers. Details about this method could be found from previous studies (Lai et al., 2014; Li et al., 2016).

3.4 Gravity

Gravity was the geographic center that was defined as a point constructed from the average longitude and latitude weighted by the area of a particular kind of land cover patch individually. Gravity represented its spatial distribution center. The change of the gravities’ location showed the change of the spatial distribution of a particular kind of land cover. Gravity was used to represent the spatial distribution center of saline land in this study.
The expression for Gravity calculation was listed as following:
${{X}_{t}}=\sum\limits_{i=1}^{N}{{{C}_{ti}}\times {{X}_{i}}}/\sum\limits_{i=1}^{N}{{{C}_{ti}}}$ ${{Y}_{t}}=\sum\limits_{i=1}^{N}{{{C}_{ti}}\times {{Y}_{i}}}/\sum\limits_{i=1}^{N}{{{C}_{ti}}}$ (1)
where Xt and Yt are the longitude and latitude of a particular land cover in the year t; Cti is the area of the number i patch of the particular land cover in the year t; Xi and Yi are the longitude and latitude of the number i patch for the particular land cover. Gravities of saline land and water body were calculated by using the spatial analysis software Arcgis 10.2.

4 Results

4.1 Spatio-temporal change of saline land area

In the typical part of Songnen Plain, cropland, grassland, saline land, and water body were the major land covers. From 1980 to 2015, the mean area of saline land was 5170 km2 (Table 1). The area of saline land increased from 4627 km2 in 1980 to 5416 km2 in 2000, and then decreased to 5198 km2 in 2015. Most of the region was occupied by saline land except for the built-up area in Taobei and the riversides of Taoer and Nenjiang rivers (Figure 2). Saline land almost occupied the southeast part of the whole area. The spatial distribution centers have the almost similar location with the spatial center of the whole area (Figure 2). All of this indicated that the saline land distributed uniformly in the study area. The distribution center moved from Northwest to Southeast when the area of saline land increased. In the 5 counties, Da’an was the major distribution region. The bigger the area of saline land, the closer to Da’an the distribution center. The change of saline land in Da’an was the major reason for the change of spatial distribution.
Table 1 Area of land covers in the typical part of Songnen Plain (km2)
Year Cropland Woodland Grassland Water body Built-up land Unused land Saline land
1980 9259 572 6447 3769 531 459 4627
1990 9382 388 5856 3845 552 504 5137
2000 11816 1205 3325 3185 567 150 5416
2005 12003 1194 3363 3067 565 150 5322
2010 11983 1193 3363 3066 586 150 5323
2015 13278 1202 3327 1896 613 150 5198
Figure 2 Land covers of the typical part of Songnen Plain

4.2 Transformation of saline land

Transformation among different land covers was calculated (Table 2). From 1980 to 1990, saline land transformed mostly from grassland (1044 km2), cropland (831 km2), and water body (408 km2) into cropland (704 km2), grassland (627 km2), and water body (477 km2). From 1990 to 2000, saline land transformed mostly from grassland (965 km2), cropland (831 km2) and water body (704 km2) into cropland (1026 km2), grassland (768 km2) and water body (379 km2). In the periods of 2000 to 2005, 2005 to 2010, and 2010 to 2015, the transformation between saline land and other land covers was slight. Generally, before 2000, saline land transformed from cropland, grassland, and water body dramatically, but after 2000, there was almost no transformation between saline land and other land covers.
Table 2 Transfer matrix of land covers in the typical part of Songnen Plain (km2)
Cropland Woodland Grassland Water
body		 Built-up
land		 Unused
land		 Saline
land		 Total
1990
1980 Cropland 6521 130 1016 419 291 51 831 9259
Woodland 203 126 135 27 6 8 67 572
Grassland 1125 69 3595 450 78 86 1044 6447
Water body 472 16 372 2439 28 34 408 3769
Built-up land 295 4 62 23 74 12 61 531
Unused land 62 7 49 10 5 247 79 459
Saline land 704 36 627 477 70 66 2647 4627
Total 9382 388 5856 3845 552 504 5137 25664
2000
1990 Cropland 7022 366 478 364 308 13 831 9382
Woodland 168 132 37 11 4 1 35 388
Grassland 2333 432 1625 380 67 54 965 5856
Water body 705 54 336 2016 26 4 704 3845
Built-up land 319 16 50 19 82 1 65 552
Unused land 243 55 31 16 12 55 92 504
Saline land 1026 150 768 379 68 22 2724 5137
Total 11816 1205 3325 3185 567 150 5416 25664
2005
2000 Cropland 11765 5 19 10 1 0 16 11816
Woodland 14 1187 3 0 0 0 1 1205
Grassland 56 1 3221 8 0 0 39 3325
Water body 121 0 19 3008 0 0 37 3185
Built-up land 3 0 0 0 564 0 0 567
Unused land 0 0 0 0 0 150 0 150
Saline land 44 1 101 41 0 0 5229 5416
Total 12003 1194 3363 3067 565 150 5322 25664
2010
2005 Cropland 11974 0 5 0 23 0 2 12003
Woodland 1 1193 0 0 0 0 0 1194
Grassland 5 0 3358 0 0 0 0 3363
Water body 1 0 0 3066 0 0 0 3067
Built-up land 2 0 0 0 563 0 0 565
Unused land 0 0 0 0 0 150 0 150
Saline land 1 0 0 0 0 0 5321 5322
Total 11983 1193 3363 3066 586 150 5323 25664
Cropland Woodland Grassland Water
body		 Built-up
land		 Unused
land		 Saline
land		 Total
2015
2010 Cropland 11874 26 21 14 23 0 25 11983
Woodland 17 1173 1 0 2 0 0 1193
Grassland 88 0 3261 4 3 0 7 3363
Water body 1162 2 11 1867 1 0 23 3066
Built-up land 20 0 1 0 565 0 0 586
Unused land 0 0 0 0 1 149 0 150
Saline land 117 1 32 11 18 1 5143 5323
Total 13278 1202 3327 1896 613 150 5198 25664

4.3 Change of hydrological process

From 1979 to 2007, groundwater depth of the 16 wells was almost greater than 1.7 m and mainly greater than 3.3 m (Table 3 and Figure 3). In the 16 wells, there were 15 wells that the ratio between the data number which was larger than 1.7 m and the whole data number was bigger than 80%. There were 8 wells that ratio between the data number which was larger than 3.3 m was bigger than 70% (Table 3). The groundwater depth had a relatively regular seasonal pattern that the groundwater rose in spring, dropped down in summer with fluctuation, and rose again in autumn and winter (Figure 3). In the long run, from 1979 to 2007, the groundwater depth of 13 wells had an increasing trend and the groundwater depth changed with fluctuation in the other 3 wells.
Figure 3 Temporal distribution of groundwater depth in the typical part of Songnen Plain
Table 3 Ratio between data number in typical groundwater depth interval and the whole data number in one well (%)
Well number Groundwater depth
<1.7 m 1.7-2.0 m 2.0-2.2 m 2.2-2.4 m 2.4-3 m 3-3.3 m >3.3 m
DA1 0 0.7 1.2 0.8 5.0 2.5 89.8
TB1 14.6 10.9 9.5 8.9 20.7 4.6 30.8
TB2 0 0 0 0 10.0 16.0 74.0
TN1 47.5 8.2 4.2 4.2 5.8 7.2 23.0
TN2 0 0 0 1.1 11.9 13.7 73.4
TN3 2.3 1.4 0 0 0 0 96.3
TN4 3.7 7.8 8.4 7.8 19.6 8.7 43.9
TN5 0 0 0.1 1.8 12.2 9.4 76.6
TY1 0 5.1 14.8 23.1 51.2 5.6 0.2
TY2 18.8 8.3 5.9 6.1 13.4 3.4 44.2
TY3 0 0 0 0 1.7 11.6 86.7
TY4 8.8 7.9 6.7 5.1 20.9 11.3 39.3
TY5 3.2 4.0 6.5 12.7 23.4 13.7 36.4
TY6 0 0 2.0 5.1 16.0 11.4 65.6
ZL1 0 0 0 0 0 0.4 99.6
ZL2 0.2 1.4 2.5 1.9 7.6 4.1 82.1
Groundwater depth was larger than 3.3 m in the most part of the whole area (Figure 5). In 1980, groundwater depth was mainly less than 1.7 m (27%) and larger than 3.3 m (46.7%). In 1990, groundwater depth was mainly 2.4-3 m (14.5%) and larger than 3.3 m (65%). In 2000 and 2005, groundwater depth was mainly larger than 3.3 m, and the area ratio was 84% and 99.9%, respectively. As shown in the “Monthly report of groundwater dynamic”, groundwater depth was mainly larger than 4 m and was 2-4 m in a relatively small part from 2010 to 2015. The area of the region where the groundwater depth was larger than 3.3 m increased to the whole region from 1980 to 2005 and the groundwater level went down consistently from 2000 to 2015.
The precipitation and evaporation at Baicheng meteorological station did not show a significant changing trend with an approximately 40-year period. The mean precipitation and evaporation was 386 mm and 1194 mm, respectively (Figure 4). From 1983 to 2006, the runoff of Taonan hydrological station fluctuated to decrease, except for a deluge in 1998 and after 2000, the runoff decreased to be nearly exhausted. Precipitation showed no typical spatial distribution pattern (Figure 5). From 1980 to 2015, the spatial distribution pattern of precipitation changed a lot. Evaporation showed two kinds of spatial distribution pattern, thus decreased from Southwest to Northeast and decreased from Southeast to Northwest. From 1980 to 2015, the change of evaporation spatial distribution pattern was not dramatic with 1700-2100 mm in domination.
Figure 4 Temporal distribution of evaporation, precipitation and runoff in the typical part of Songnen Plain
Figure 5 Spatial distribution of precipitation, evaporation and groundwater depth in the typical part of Songnen Plain

4.4 Causes of land salinization

Land salinization was the result of salt aggregation on land surface. Any factors which could exert influence on the salt aggregation in the upper soil layer, such as change of evaporation interface, accessibility to land surface of groundwater, and so on, may cause the change of land salinization. Causes of land salinization were determined from land cover change and hydrological process change.
4.4.1 Determination from land cover change
From 1980 to 2015, saline land had close transfer relationship with cropland, grassland, and water body. This meant that the factors that could exert influence on the change of cropland, grassland, and water body may exert influence on the change of saline land. Even cropland was mainly composed of dry land, the ratio between the area of paddy field and the area of cropland increased from 4.9% in 1980 to 12.7% in 2015. The increase of cropland included the increase of dry land and paddy field. From 1980 to 2015, the area of dry land increased from 8617 km2 to 10319 km2 (increased by 19.8%), while, the area of paddy field increased from 447 km2 to 1502 km2 (increased by 236%). The amount of water for irrigation mainly depended on the area of paddy field, so that the high speed increase of area of paddy field highly increased the irrigation amount which was the main reason for secondary land salinization.
Change of grassland was composed of change of high coverage grassland and middle coverage grassland. From 1980 to 2015, high coverage grassland decreased from 3672 km2 to 1519 km2 (decreased by 58.6%), and middle coverage grassland decreased from 2645 km2 to 1760 km2 (decreased by 33.5%), respectively. The change of area of grassland mainly happened in the period from 1980 to 2000. In this period, cropland, water body, and saline land were the major land cover transformation types. Grassland included high coverage grassland and middle coverage grassland. From 1980 to 2000, high coverage grassland mainly transformed into cropland and middle coverage grassland, middle coverage grassland mainly transformed into saline land, and low coverage grassland mainly transformed into saline land and dry land. Transformation from grassland to saline land was a direct reason for land salinization and transformation from grassland to cropland increased the risk of land salinization.
Change of water body referred to change of lake and flood plain. From 1980 to 2015, lake decreased from 1370 km2 to 797 km2 (decreased by 41.8%), and flood plain increased from 207 km2 to 339 km2 (increased by 63.8%), respectively. From 1980 to 2015, the areas of lake changed to paddy field, dry land, middle coverage grassland, flood plain, saline land, and swampland were 14 km2, 103 km2, 18 km2, 122 km2, 192 km2, and 182 km2, respectively, while, the area of flood plain changed to dry land was 11 km2. The transformation from lake to saline land was a direct reason and transformation from lake or flood plain to shallow water area, middle coverage grassland, and dry land increased the risk of land salinization.
4.4.2 Determination from hydrological process change
Besides interface of water movement, change of water movement process was also a vital factor for land salinization. Rise of groundwater was a major reason of land salinization, so the completeness of rise to the ground surface was a necessary condition for evaporation. Based on the previous study in Songnen Plain (GSIJP, 2006) (Table 4), 3.3 m of the groundwater depth was seen as a critical value that groundwater could cause land salinization. In the typical part of Songnen Plain, from 1979 to 2015, groundwater depth was larger than 3.3 m in most of the cases. The ratio between the area of the region where groundwater depth was larger than 3.3 m and the whole region increased from 46.7% in 1980 to 84% in 2000, and then increased from 92.6% in 2001 to 99.9% in 2005. The region where groundwater depth was larger than 3.3 m almost occupied the whole region steadily from 2005 to 2015. Before 2000, the relatively shallow groundwater was an important reason for land salinization, while, after 2000, the groundwater with relatively large depth was not a direct reason for land salinization.
Table 4 Groundwater depth threshold induced for soil salinization in Songnen Plain (Geological Survey Institute of Jilin Province, 2006)
Soil texture Soil salinization level Groundwater depth threshold (m)
Sandy loam Non >3.3
Light 2.4-3.3
Middle 2.0-2.4
Heavy <2.0
Clay loam Non >3.0
Light 2.2-3.0
Middle 1.7-2.2
Heavy <1.7
In the typical part of Songnen Plain, evaporation was bigger than precipitation. In this area, evaporation was the major discharge manner. With a large amount of cropland and insufficient precipitation and surface water runoff, irrigation, which came from deep groundwater, river, lake, and transferred water from outer basin, was an important water source for cropland. Reduction of surface water runoff and addition of irrigation produced a phenomenon that area of the region with long time waterlogging decreased and area of the region waterlogged temporarily increased. It increased the cycle of vertical leach and evaporation process, which was the key process of land salinization. The area of paddy field and flood plain increased by 1026 km2 from 1980 to 2000, and then increased by 161 km2 from 2000 to 2015 (Figure 6). The area of river, lake, water reservoir, and swampland decreased by 873 km2 from 1980 to 2000, and then decreased by 142 km2 from 2000 to 2015. Before 2000, the increase of the area of shallow waterlogging region and the decrease of the area of deep water body exerted much influence on the increase of saline land. After 2000, the area of the region covered by the water and the area of saline land both kept relatively stable.
Figure 6 Change of area of region covered by water in the typical part of Songnen Plain

5 Discussion

At different spatio-temporal scales, different factors had different influences on the occurrence and development of land salinization. Most studies focused on the influence from the shallow groundwater and the rise process of the soil salt. In natural process, shallow groundwater determined the salt aggregation of the land surface. While, when the hydrological process was interrupted by comprehensive human activities, the mechanism of salt aggregation changed into a complex pattern. Yang et al. (2010) depicted a mechanism through analyzing the transport process of soil water and salt, thus, in Songnen Plain, human activities had changed the surface hydrological process, and significantly decreased the ponding chances and occurrence frequency of rainwater and runoff. The ponding rainwater drove the salt accumulated in the upper soil from the depression to the hilltop, and formed the new distribution pattern of salt-affected soils. The shallow groundwater was not a main cause of the ongoing secondary soil salinization, although it played a significant role at early stages in the primary soil salinization. Liu et al. (2005) indicated that the hydrological change induced from the land use change was the main driving factors. The relationship between the increased saline land area and the distance to the water body edge indicated that the increase of saline land came from the dynamic of water body.
In this study, the area of saline land increased constantly from 1980 to 2000, and kept stable after 2000. The transformation among land covers showed that cropland, grassland, and water body had close transformation relationship with saline land. Moreover, there was also close transformation relationship among cropland, grassland, and water body, especially before 2000. Increase of paddy field, dry land, middle coverage grassland, low coverage grassland, and the nearly dry land surface including flood plain and swampland provided a large amount of interface for evaporation, which could carry soil salt to the ground surface. The area of the region where groundwater depth was larger than 3.3 m increased from 1980 to 2000, while, the area occupied almost the whole region after 2000. From 1980 to 2000, shallow groundwater could lead to land salinization, but was not the most important reason for the increase of saline land. Because the area of saline land increased when the area of the region where groundwater depth was larger than 3.3 m increased. Change of cropland, grassland, and water body induced from human activities and decrease of runoff and increase of irrigation and water transfer from outer basin were the main reasons for the increase of saline land from 1980 to 2000. After 2000, groundwater with relatively large depth could not exert much influence on land salinization. Protection of grassland and wetland prevented the increase of the area of saline land.

6 Conclusions

Land salinization was a severe and typical problem at the typical part of Songnen Plain. Land cover database and hydrological data were collected for analysis of spatio-temporal change of saline land and hydrological process and causes of land salinization were determined from the transformation of saline land with other kinds of land covers and the change of hydrological processes. Conclusions had been gotten as follows:
1) The area of saline land increased from 4627 km2 in 1980 to 5416 km2 in 2000, and then decreased to 5198 km2 in 2015. Saline land almost distributed in the whole region, except the northwest part of the study area. The transformation between saline land and other kinds of land covers mainly before 2000, and transformation among saline land, cropland, grassland, and water body were the major transfer types.
2) From 1979 to 2007, groundwater depth of the 16 wells was mainly larger than 3.3 m and had an increasing trend. Seasonally, the groundwater rose in spring, dropped down in summer with fluctuation, and rose again in autumn and winter. Spatially, the area of the region where the groundwater depth was larger than 3.3 m increased from 1980 to 2000, while the region almost occupied the whole region after 2000. Precipitation and evaporation changed little, while runoff decreased very much.
3) Groundwater could exert influence on land salinization before 2000. The nearly dried-up runoff, increase of paddy field (198.2%) and flood plain (67.6%), and decrease of high coverage grassland (52.3%), middle coverage grassland (36.8%), and lake (36.9%) increased the area of interface of vertical water movement, which increased the area of saline land indirectly. Change of cropland, grassland, and water body induced from human activities and the decrease of runoff and the increase of irrigation and water transfer from outer basin were the main reasons for the increase of saline land from 1980 to 2000. After 2000, groundwater with relatively large depth could not exert much influence on land salinization directly. Protection of grassland and wetland prevented the increase of the area of saline land.

The authors have declared that no competing interests exist.

References

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1
Akhtar A, Shahbaz K, Nisar Het al., 2013. Characterizing soil salinity in irrigated agriculture using a remote sensing approach. Physics and Chemistry of the Earth, 55-57: 43-52.Managing salinity in irrigated agriculture is crucial for minimising its negative environmental impacts and for ensuring the long-term sustainability of irrigated agriculture. It demands establishing rapid monitoring systems that help develop sustainable management plans. Remote sensing offers several advantages over the conventional proximal methods to map and predict areas at salinity risk. This paper presents an integrated approach to characterize soil salinity using remotely-sensed data in the District Faisalabad, Punjab, Pakistan. The IRS-1B LISS-II digital data was acquired and analysed in combination with field data and topographical maps. Remotely-sensed data based salinity indices or band combinations were developed to monitor the occurrence pattern of salt-affected soils. Using supervised maximum likelihood classification, the images were classified into eight land use classes with an overall accuracy of around 90%. The classified images showed that 22.2% of the total area was under salt-affected soils in 1992. The occurrence pattern of salt-affected soils varied with positive and negative trends during 1992 1995 to a minimum of 10.6%. The delineation analysis into levels of saline soils revealed three types based on USDA classification (USDA, 1954). The slightly saline, moderately saline and strongly saline soils during 1992 were in the order of 15%, 3%, and 1% respectively. The interactive behaviour of salinity and sodicity and their combinations showed that saline-sodic soils occurred predominantly ranging from 6.9% to 17.3% of the salt-affected soils. The shallow watertable was found to be of hazardous quality in 28% of the study area. The relationship between salt-affected soils, waterlogged soils and groundwater quality revealed that 60 70% of the salt-affected soils occurred in shallow watertable areas during 1992 1995. The reuse of poor quality groundwater for irrigation and the failure of tile drainage system in the area are likely to further increase the risk of salinisation in the Indus Basin of Pakistan.

DOI

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Datta K K, deJong C, 2002. Adverse effect of waterlogging and soil salinity on crop and land productivity in northwest region of Haryana, India.Agricultural Water Management, 57(3): 223-238.In the irrigated areas of semi-arid regions, especially in northwest India, a considerable recharge to the groundwater leads to waterlogging and secondary salinization. In several sub-areas groundwater is mined, water tables fall, and salts are added to the root zone because a high proportion of irrigation water is derived from pumped groundwater of poor quality. Out of 1 million hectares of irrigation induced waterlogged saline area in northwest India, approximately half a million hectares are in the state of Haryana. Taking a homogenous physical environment as a starting point, the way and the extent to which farmers activities will affect the salinity and sodicity situation depend on farming and irrigation practices. In the past, soil salinity was mainly associated with high groundwater tables, which bring salts into the root zone through capillary rise when water is pumped. But nowadays, increasing exploitation of groundwater for irrigation purposes has led to declining groundwater tables and a threat of sodification and salinization due to use of poor quality groundwater. Farmers in northwest India are facing a situation in which they have to deal with salt volumes that are harmful for water uptake of crops. They are also facing the problem of sodicity, which has an adverse effect on the physical structure of the soil, causing problems of water intake, transfer and aeration. To mitigate the adverse effect of soil salinity on crop yield, the farmers irrigate frequently, either mixing canal water and groundwater, or alternately using canal water and groundwater. Due to differences in environmental parameters in the farming systems, such as groundwater quality, soil types and uneven distribution of irrigation water, income losses to the farming community are not uniform. This paper highlights the economic loss due to environmental degradation through the twin problems of waterlogging and soil salinity, which threaten the sustainability of agricultural production in Haryana state. Our analysis shows that the net present value of the damage due to waterlogging and salinity in Haryana is about Rs. 23,900/ha (in 1998 1999 constant prices). The estimated potential annual loss is about Rs. 1669 million (about US$ 37 million) from the waterlogged saline area. The major finding of the paper is that intensification per se is not the root cause of land degradation, but rather the policy environment that encouraged inappropriate land use and injudicious input use, especially excessive irrigation. Trade policies, output price policies and input subsidies all have contributed to the degradation of agricultural land.

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Ding J L, Wu M C, Tashpolat T, 2011. Study on soil salinization information in arid region using remote sensing technique.Agricultural Sciences in China, 10(3): 404-411. (in Chinese)Extracting information about saline soils from remote sensing data is useful, particularly given the environmental significance and changing nature of these areas in arid environments. One interesting case study to consider is the delta oasis of the Weigan and Kuqa rivers, China, which was studied using a Landsat Enhanced Thematic Mapper Plus (ETM+) image collected in August 2001. In recent years, decision tree classifiers have been successfully used for land cover classification from remote sensing data. Principal component analysis (PCA) is a popular data reduction technique used to help build a decision tree; it reduces complexity and can help the classification precision of a decision tree to be improved. A decision tree approach was used to determine the key variables to be used for classification and ultimately extract salinized soil from other cover and soil types within the study area. According to the research, the third principal component (PC3) is an effective variable in the decision tree classification for salinized soil information extraction. The research demonstrated that the PC3 was the best band to identify areas of severely salinized soil; the blue spectral band from the ETM+ sensor (TM1) was the best band to identify salinized soil with the salt-tolerant vegetation of tamarisk (Tamarix chinensis Lour); and areas comprising mixed water bodies and vegetation can be identified using the spectral indices MNDWI (modified normalized difference water index) and NDVI (normalized difference vegetation index). Based upon this analysis, a decision tree classifier was applied to classify landcover types with different levels of soil saline. The results were checked using a statistical accuracy assessment. The overall accuracy of the classification was 94.80%, which suggested that the decision tree model is a simple and effective method with relatively high precision.

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FAO, 2007. Extent and causes of salt-affected soils in participating countries. AGL: Global Network on Integrated Soil Management for Sustainable Use of Salt-affected Soils. .

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García-Ruiz M J, 2010. The effects of land uses on soil erosion in Spain: A review.Catena, 81(1): 1-11.Soil erosion is a key factor in Mediterranean environments, and is not only closely related to geoecological factors (lithology, topography, and climatology) but also to land-use and plant cover changes. The long history of human activity in Spain explains the development of erosion landscapes and sedimentary structures (recent alluvial plains, alluvial fans, deltas and flat valleys infilled of sediment). For example, the expansion of cereal agriculture and transhumant livestock between the 16th and 19th centuries resulted in episodes of extensive soil erosion. During the 20th century farmland abandonment prevailed in mountain areas, resulting in a reduction of soil erosion due to vegetation recolonization whereas sheet-wash erosion, piping and gullying affected abandoned fields in semi-arid environments. The EU Agrarian Policy and the strengthening of national and international markets encouraged the expansion of almond and olive orchards into marginal lands, including steep, stony hill slopes. Vineyards also expanded to steep slopes, sometimes on new unstable bench terraces, thus leading to increased soil erosion particularly during intense rainstorms. The expansion of irrigated areas, partially on salty and poorly structured soils, resulted in piping development and salinization of effluents and the fluvial network. The trend towards larger fields and farms in both dry farming and irrigated systems has resulted in a relaxation of soil conservation practices.

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Geological Survey Institute of Jilin Province (GSIJP), 2006. Evaluation of groundwater resources and environmental problems in Songnen Plain. (in Chinese)

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Gu H B, Song Y, Pan J, 2010. Research progress of influencing factors on salinization of Songnen Plain.Journal of Anhui Agricultural Science, 38(30): 95-98. (in Chinese)Study on influencing factors for soil salinization formation was the basis for salt and water movement,water and salt transport modeling and control,prediction and prevention of soil salinization,and further improving,managing,developing and utilization of saline area.Based on the saline-alkali formation effect in Songnen Plain,the effect of the geological structure,the hydrogeology,the climate and human factors on the salinization were analyzed,the organization chart was given.The controllable and uncontrollable factors were determined,and some suggestions for future research were proposed.

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Ha X P, Ding J L, Tiyip Tet al., 2006. SI-albedo space-based extraction of salinization information in arid area.Acta Pedologica Sinica, 46(4): 698-703. (in Chinese)Soil salinization is getting more and more attention the world over for its adverse impact on the social economy,the environment,and the agricultural eco-system.The total area of salinized soil in Xinjiang reaches 8.476 10~6 hm~2,accounting for 31.1%of the total cultivated land.It is,therefore,necessary and important to study soil salinization in arid regions for solution to this problem.Remote sensing(RS) technology demonstrates a number of advantages in this field.But how to extract salinization information accurately from RS images is the basis of the study.In this paper a case study of Yutian County monitoring soil salinization by means of remote sensing,is carried out.Yutian County was selected for this study because of its importance as a significant site for agricultural development.Located in the south of the Keriya oasis,it has recently been exposed to severe soil salinization.Seven-spectrum-band Enhanced Thematic Mapperplus (ETM+) images dated October 7,2002 were used against the data of soil features obtained from field investigation and analysis of typical soil information,to extract Salinization Index(SI) and land surface albedo,which are very important biophysical parameters of land surface.In this paper the relationship between salinization index(SI) and albedo was analyzed quantitatively.Through experiment and theoretical reasoning,the authors proposed a conception of SI-Albedo space and discussed its biophysical characteristics.Analysis revealed that location could be used to improve the current strategies for salinization in the SI-Albedo space,and hence the strategies for salinization mapping,by defining measurements in this feature space.Therefore,the authors present a methodology to monitor severity of salinization.Field data, available data in the literature,and ancillary data were linked with land cover characteristics(salinization index,land surface albedo) derived from Landsat ETM+ multispectral images.An information extraction model,using the decisiontree classification method,was established and applied to classification of RS images.Results indicate that the classification based on SI-Albedo space has a higher classification accuracy than the one based on maximum likelihood.Its highest overall-accuracy is about 0.92%higher than the maximum likelihood.Although both techniques show some mix-class phenomena in the classification result,but the classification based on Si-Albedo space has less than the maximum likelihood, and thus a higher separability.Salinity soil distribution maps show that the soil salinization of this study area is relatively severe and varying in degree and type;The area is dominated with light salinization and moderate salinization.The former is distributed mainly in farmland,while the latter around the Bostan swamp.And based on the salinized soil map, the salinity soil early-warning line was derived for anticipating further soil degradation.Such contrasting and complementary behavior suggests a potential synergism between salinization index and land surface albedo for mapping and monitoring of a complex soil salinization environment such as Keriya oasis.

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Halse S A, Ruprecht J K, Pinder A Met al., 2003. Salinisation and prospects for biodiversity in rivers and wetlands of south-west Western Australia.Australian Journal of Botany, 51(6): 673-688.ABSTRACT Saline water was common in south-west Western Australian aquatic systems prior to land-clearing because most streams and wetlands were ephemeral and evapo-concentrated as they dried, and there were high concentrations of stored salt in groundwater and soil profiles. Nevertheless, a 1998 review of salinity trends in rivers of south-west Western Australia showed that 20-fold increases in salinity concentrations had occurred since clearing in the medium-rainfall zone (300 700 mm). More recent data confirm these trends and show that elevated salinities have already caused substantial changes to the biological communities of aquatic ecosystems. Further substantial changes will occur, despite the flora and fauna of the south-west being comparatively well adapted to the presence of salinity in the landscape. Up to one-third of wetland and river invertebrate species, large numbers of plants and a substantial proportion of the waterbird fauna will disappear from the wheatbelt, a region that has high biodiversity value and endemism. Increased salinities are not the only threat associated with salinisation: increased water volumes, longer periods of inundation and more widespread acidity are also likely to be detrimental to the biota.

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Khan N M, Sato Y, 2001. Monitoring hydro-salinity status and its impact in irrigated semi-arid areas using IRS-1B LISS-II data.Asian J. Geoinform, 1(3): 63-73.

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Lai N, Li X G, Tuerdi Aet al., 2014. Dynamic changes of the salt-affected soil in the oasis of the lower reaches of Kaidu River in the recent 50 years.Remote Sensing Information, 29(3): 35-43.Taking the Oasis of the lower reaches of Kaidu River as an example,using the map of soil-type of Xinjiang uygur autonomous region in 1958,MSS image in 1973,TM image in 1990 and ETM+image in 2010 as data sources,the classification information of soil salinization was extracted with the spectral angle method,and the spatial and temporal changes in saltaffected soil in the study area were analyzed with the land utilization transfer matrix and salinization dynamic model SSDI.The results showed that:1)the area of the salt-affected soil decreased 866.81km2 from 1958 to 2010,and SSDI was 1.09,where,the area of the moderate salinized decreased most quickly,and SSDI was 2.18.2)the salt-affected soil types converted each other,the conversion degree of severe salinization was the highest,accounted for 43.47%;the types of salt-affected soil converted to non-salt-affected soil and bare land,the conversion degree of slightly salinized soil was the highest,accounted for 80.59%.3)method of correlation analysis was applied using annual average temperature,annual evaporation,annual rainfall,population and total agricultural output as independent variables,and the area of salt-affected soil as the dependent variable.The individual variable and salinization land were negatively correlated,the correlation index between the annual average temperature and the area of salt-affected soil was the largest of all,and the value was 0.98.The correlation index about the annual evaporation was the least,and the value was 0.28.

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Lambert K S, Shiati K, 2002. Irrigation and salinity: A perspective review of the salinity hazards of irrigation development in the arid zone. Irrigation and Drainage Systems, 16(2): 161-174.

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Li X, Qiao M, Zhou S, 2016. Causes and spatial-temporal changes of soil salinization in Manasi irrigation region of Xinjiang Region during 1985-2014.Bulletin of Soil and Water Conservation, 36(3): 152-158.Objective]To investigate the spatial distribution of saline alkali land in Manasi irrigation area,in order to understand the type and the spatial and temporal changes of local saline alkali land,and find out appropriate improvement measures.[Methods]Based on the soil data in 1985,Landsat TM images in 1998 and CBERS(China-Brazil earth resource satellite)images in 2006 and 2014,the RS and GIS technology was used to extract the information of soil salinization during four periods.The land transfer matrix was used to analyze the spatial-temporal changes of salt-affected land in Manasi irrigation region during the past 30 years.[Results](1)The area of soil salinization land in Manasi irrigation region had increased from 4.27 10~4 to7.90 10~4 hm~2 during the past 30 years.(2)Soil salinization was mainly distributed in the inner area of Manasi irrigation region,represented as a block pattern.[Conclusion]Nature factors are the internal causes of the formation and change of the soil salinization,and anthropogenic influence is one of the driving factors that promoting the development of soil salinization,especially for the secondary salinization of oasis.

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Line J G, Finlaysonc C M, Falkenmark M, 2010. Managing water in agriculture for food production and other ecosystem services.Agricultural Water Management, 97(4): 512-519.Agricultural systems as well as other ecosystems generate ecosystem services, i.e., societal benefits from ecological processes. These services include, for example, nutrient reduction that leads to water quality improvements in some wetlands and climatic regulation through recycling of precipitation in rain forests. While agriculture has increased rovisioning ecosystem services, such as food, fiber and timber production, it has, through time, substantially impacted other ecosystem services. Here we review the trade-offs among ecosystem services that have been generated by agriculture-induced changes to water quality and quantity in downstream aquatic systems, wetlands and terrestrial systems. We highlight emerging issues that need urgent attention in research and policy making. We identify three main strategies by which agricultural water management can deal with these large trade-offs: (a) improving water management practices on agricultural lands, (b) better linkage with management of downstream aquatic ecosystems, and (c) paying more attention to how water can be managed to create multifunctional agro-ecosystems. This can only be done if ecological landscape processes are better understood, and the values of ecosystem services other than food production are also recognized.

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Liu G M, Yang J S, Li D S, 2002. Evaporation regularity and its relationship with soil salt. Acta Pedologica Sinica, 39(3): 384-389. (in Chinese)With a one year indoor silt loam column simulation experiment,regularity of evaporation of groundwater under different groundwater conditions was studied,and so was the relationship between soil electric conductivity (of 0～40cm layer) and total amount of evaporation.The results showed that even under different groundwater conditions total amount of evaporation was in linear relationship with duration of the experiment,that the lower the mineralization of the groundwater was,the more sensitively related the cumulative amount of evaporation was to variation of the mineralization of the groundwater,and that the deeper the groundwater table was,the more sensitively related the cumulative amount of evaporation was to variation of the depth of groundwater table.The relationship of total amount of evaporation with groundwater table as well as groundwater salinity was thus set up.After mineralized groundwater influenced 0～40cm layer soil,soil electric conductivity there was logarithmic to total amount of evaporation when the groundwater table was at 85cm or 105cm,but exponential when the groundwater table was at 155cm.

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Liu J, Zhang Z, Xu Xet al., 2010. Spatial patterns and driving forces of land use change in China during the early 21st century.Journal of Geographical Sciences, 20(4): 483-494.Land use and land cover change as the core of coupled human-environment systems has become a potential field of land change science (LCS) in the study of global environmental change. Based on remotely sensed data of land use change with a spatial resolution of 1 km × 1 km on national scale among every 5 years, this paper designed a new dynamic regionalization according to the comprehensive characteristics of land use change including regional differentiation, physical, economic, and macro-policy factors as well. Spatial pattern of land use change and its driving forces were investigated in China in the early 21st century. To sum up, land use change pattern of this period was characterized by rapid changes in the whole country. Over the agricultural zones, e.g., Huang-Huai-Hai Plain, the southeast coastal areas and Sichuan Basin, a great proportion of fine arable land were engrossed owing to considerable expansion of the built-up and residential areas, resulting in decrease of paddy land area in southern China. The development of oasis agriculture in Northwest China and the reclamation in Northeast China led to a slight increase in arable land area in northern China. Due to the “Grain for Green” policy, forest area was significantly increased in the middle and western developing regions, where the vegetation coverage was substantially enlarged, likewise. This paper argued the main driving forces as the implementation of the strategy on land use and regional development, such as policies of “Western Development”, “Revitalization of Northeast”, coupled with rapidly economic development during this period.

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Liu L, Chen X, Xu X Let al., 2014. Changes in production potential in China in response to climate change from 1960 to 2010.Advances in Meteorology, 1-10.

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Liu L, Xu X L, Liu J Yet al., 2015. Impact of farmland changes on production potential in China during 1990-2010.Journal of Geographical Sciences, 25(1): 19-34The quantity and spatial pattern of farmland has changed in China, which has led to a major change in the production potential under the influence of the national project of ecological environmental protection and rapid economic growth during 1990-2010. In this study, the production potential in China was calculated based on meteorological, terrain elevation, soil and land-use data from 1990, 2000 and 2010 using the Global Agro-ecological Zones model. Then, changes in the production potential in response to farmland changes from 1990 to 2010 were subsequently analyzed. The main conclusions were the following. First, the total production potential was 1.055 billion tons in China in 2010. Moreover, the average production potential was 7614 kg/ha and showed tremendous heterogeneity in spatial pattern. Total production in eastern China was high, whereas that in northwestern China was low. The regions with high per unit production potential were mainly distributed over southern China and the middle and lower reaches of the Yangtze River. Second, the obvious spatiotemporal heterogeneity in farmland changes from 1990 to 2010 had a significant influence on the production potential in China. The total production potential decreased in southern China and increased in northern China. Furthermore, the center of growth of the production potential moved gradually from northeastern China to northwestern China. The net decrease in the production potential was 2.97 million tons, which occupied 0.29% of the national total actual production in 2010. Third, obvious differences in the production potential in response to farmland changes from 1990 to 2000 and from 2000 to 2010 were detected. The net increase in the production potential during the first decade was 10.11 million tons and mainly distributed in the Northeast China Plain and the arid and semi-arid regions of northern China. The net decrease in the production potential during the next decade was 13.08 million tons and primarily distributed in the middle and lower reaches of the Yangtze River region and the Huang-Huai-Hai Plain. In general, the reason for the increase in the production potential during the past two decades might be due to the reclamation of grasslands, woodlands and unused land, and the reason for the decrease in the production potential might be urbanization that occupied the farmland and Green for Grain Project, which returned farmland to forests and grasslands.

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Liu Q, He Y, Deng Wet al., 2005. Study on the soil salinization process in the changeable environment: A case study in the middle and lower reaches of Taoer River.Journal of Arid Land Resources and Environment, 19(6): 113-117. (in Chinese)The mass-energy-information system contributed to the soil salinization in the changeable environment.This research analysed the climatic factors,changes of the land use/land cover and hydrological process in the drainage,which influence the soil salinization.By virtue of the ARC/INFO 8.3,the authors got the space correlation between the dynamic saline-sodicsoil cells and water area,meadow.The conclusion is that the climatic factors is not the main driving factor to the soil salinization;out-of-order human activities has strongly influenced the hydrological process,including the land use/land cover changing influenced the hydrological process,which lead to desertification.The human activities shich lead to the changes of the hydrologicalprocess are themain driving factors to the soil salinization.

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Masoud A A, Koike K, 2006. Arid land salinization detected by remotely-sensed land cover changes: A case study in the Siwa region, NW Egypt. Journal of Arid Environments, 66(1): 151-167.Siwa region, located in the north Western Desert of Egypt, has been recently subjected to severe soil salinity problems. Monitoring and analysis of the recent landcover dynamics through the integration of remote sensing and GIS could provide base information for documenting salinity change trends and for anticipating further degradation where the absence of long-term salinity records is an obstacle. Three Landsat TM/ETM+ satellite images taken over a span of 16 years (1987 2003) coupled with a 30-m DEM and field observations served as the basic sources of data. Standard image enhancements, classifications, and change detection techniques were applied to determine changes between the available images. Changes were analysed in conjunction with the land surface characteristics, such as slope, radiometric thermal temperature, vegetation indices, and tasselled cap transformations. Such analyses enabled the characterization of alterations in vegetation cover and provided evidence for locating possible future changes due to soil salinity. The results confirmed an acceleration in the rate of soil salinization and vegetation death after the year 2000. Further, this was found to be related to the relative climate warming and the improper drainage systems set up after the year 2000 in addition to the absence of an effective water resource management plan. Recommendations and measures that may prevent or ameliorate the exacerbation of these problems are proposed.

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Metternicht G I, Zinck J A, 2003. Remote sensing of soil salinity: Potentials and constraints.Remote Sensing of Environment, 85: 1-20.Soil salinity caused by natural or human-induced processes is a major environmental hazard. The global extent of primary salt-affected soils is about 955 M ha, while secondary salinization affects some 77 M ha, with 58% of these in irrigated areas. Nearly 20% of all irrigated land is salt-affected, and this proportion tends to increase in spite of considerable efforts dedicated to land reclamation. This requires careful monitoring of the soil salinity status and variation to curb degradation trends, and secure sustainable land use and management. Multitemporal optical and microwave remote sensing can significantly contribute to detecting temporal changes of salt-related surface features. Airborne geophysics and ground-based electromagnetic induction meters, combined with ground data, have shown potential for mapping depth of salinity occurrence. This paper reviews various sensors (e.g. aerial photographs, satellite- and airborne multispectral sensors, microwave sensors, video imagery, airborne geophysics, hyperspectral sensors, and electromagnetic induction meters) and approaches used for remote identification and mapping of salt-affected areas. Constraints on the use of remote sensing data for mapping salt-affected areas are shown related to the spectral behaviour of salt types, spatial distribution of salts on the terrain surface, temporal changes on salinity, interference of vegetation, and spectral confusions with other terrain surfaces. As raw remote sensing data need substantial transformation for proper feature recognition and mapping, techniques such as spectral unmixing, maximum likelihood classification, fuzzy classification, band ratioing, principal components analysis, and correlation equations are discussed. Lastly, the paper presents modelling of temporal and spatial changes of salinity using combined approaches that incorporate different data fusion and data integration techniques.

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Mulder V L, Bruin S, Schaepman M Eet al., 2011. The use of remote sensing in soil and terrain mapping: A review.Geoderma, 162(1/2): 1-9.78 Remote sensing offers possibilities for improving current soil databases. 78 Soil attribute retrievals from remote sensing should be used as covariates in DSM. 78 The gap between proximal and remote sensing has to be bridged. 78 We will be seeing future instruments launched soon enhancing the perspectives of DSM. 78 A coherent multidisciplinary method for soil and terrain mapping should be developed.

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Poenaru V, Badea A, Cimpeanu S Met al., 2015. Multi-temporal multi-spectral and radar remote sensing for agricultural monitoring in the Braila Plain.Agriculture and Agricultural Science Procedia, 6: 506-516.The objective of the paper is to investigate the sensitivity of Landsat OLI and C-band radar signals to monitor an agricultural area affected by soil salinization and land degradation. The chosen test area - Braila Plain has the special particularities such as: dry climate, high annual average temperatures (9-110C), very dry and hot summers which cause a large potential evapotranspiration and conduct to a moisture deficit in soil, alkaline soils, winter winds with an average speed of 2.7 - 3.4 m/s. The soil type and climate conditions favor the culture of maize (50%), wheat and successive crops (16%), alpha-alpha (18%), sugar beet (6%), sunflower (7%), vegetables and other crops (3%). Taking into account the soil type, climate conditions and geomorphological characteristics of the studied area, the paper focuses on evaluation of Sentinel-1 sensor capabilities to monitor soil degradation and surface soil moisture. A multi-temporal series of Sentinel-1 data gathered from October 2014 until January 2015 is used. Crop growing stages are investigated with multi-temporal Landsat OLI and MODIS data. The normalized difference vegetation index (NDVI), specific leaf area index, land thermal index, soil moisture index and soil salinity information are retrieved from Landsat data. The potential evapotranspiration is computed from MODIS data to evaluate the effects of soil salinity on growing crops. The results confirm soil degradation and the synergy of using multi-spectral and radar data for crops monitoring.

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Qadir M, Tubeileh A, Akht Jet al., 2008. Productivity enhancement of salt-affected environments through crop diversification.Land Degradation & Development, 19(4): 429-453.Recent trends and future demographic projections suggest that the need to produce more food and fibre will necessitate effective utilization of salt-affected land and saline water resources. Currently at least 20 per cent of the world's irrigated land is salt affected and/or irrigated with waters containing elevated levels of salts. Several major irrigation schemes have suffered from the problems of salinity and sodicity, reducing their agricultural productivity and sustainability. Productivity enhancement of salt-affected land and saline water resources through crop-based management has the potential to transform them from environmental burdens into economic opportunities. Research efforts have led to the identification of a number of field crops, forage grasses and shrubs, aromatic and medicinal species, bio-fuel crops, and fruit tree and agroforestry systems, which are profitable and suit a variety of salt-affected environments. Several of these species have agricultural significance in terms of their local utilization on the farm. Therefore, crop diversification systems based on salt-tolerant plant species are likely to be the key to future agricultural and economic growth in regions where salt-affected soils exist, saline drainage waters are generated, and/or saline aquifers are pumped for irrigation. However, such systems will need to consider three issues: improving the productivity per unit of salt-affected land and saline water resources, protecting the environment and involving farmers in the most suitable and sustainable crop diversifying systems to mitigate any perceived risks. This review covers different aspects of salt-affected land and saline water resources, synthesizes research knowledge on salinity/sodicity tolerances in different plant species, and highlights promising examples of crop diversification and management to improve and maximize benefits from these resources. Copyright 2008 John Wiley & Sons, Ltd.

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Rengasamy P, 2002. Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: An overview.Australian Journal of Experimental Agriculture, 42(3): 351-361.

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Song C C, Deng W, 2000. Characters of groundwater and influence on the interior salt-affected soil in the West of Jilin Province.Scientia Geographica Sinica, 20(3): 246-230. (in Chinese)

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UNEP, 1991. Status of desertification and implementation of the United Nations plan of action to combat desertification. Nairobi, Kenya.

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Wang F, Chen X, Luo Get al., 2013. Detecting soil salinity with arid fraction integrated index and salinity index in feature space using Landsat TM imagery.Journal of Arid Land, 5: 340-353.Modeling soil salinity in an arid salt-affected ecosystem is a difficult task when using remote sensing data because of the complicated soil context (vegetation cover, moisture, surface roughness, and organic matter) and the weak spectral features of salinized soil. Therefore, an index such as the salinity index (SI) that only uses soil spectra may not detect soil salinity effectively and quantitatively. The use of vegetation reflectance as an indirect indicator can avoid limitations associated with the direct use of soil reflectance. The normalized difference vegetation index (NDVI), as the most common vegetation index, was found to be responsive to salinity but may not be available for retrieving sparse vegetation due to its sensitivity to background soil in arid areas. Therefore, the arid fraction integrated index (AFII) was created as supported by the spectral mixture analysis (SMA), which is more appropriate for analyzing variations in vegetation cover (particularly halophytes) than NDVI in the study area. Using soil and vegetation separately for detecting salinity perhaps is not feasible. Then, we developed a new and opera-tional model, the soil salinity detecting model (SDM) that combines AFII and SI to quantitatively estimate the salt content in the surface soil. SDMs, including SDM1 and SDM2, were constructed through analyzing the spatial characteristics of soils with different salinization degree by integrating AFII and SI using a scatterplot. The SDMs were then compared to the combined spectral response index (COSRI) from field measurements with respect to the soil salt content. The results indicate that the SDM values are highly correlated with soil salinity, in contrast to the performance of COSRI. Strong exponential relationships were observed between soil salinity and SDMs (>0.86, RMSE=0.71, RMSE=16.21). These results suggest that the feature space related to biophysical properties combined with AFII and SI can effectively provide information on soil salinity.

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Wang L, Seki K, Miyazaki Tet al., 2009. The causes of soil alkalinization in the Songnen Plain of Northeast China.Paddy Water Environ., 7: 259-270.The causes of soil alkalinization in the Songnen Plain of Northeast China were mainly analyzed from two aspects, natural and anthropogenic. Natural factors of alkalinization are parent materials, topographic positions, freeze-thaw action, wind conveyance, water properties and semi-arid/sub-humid climate. Some of them were always being neglected, such as freeze-thaw action and wind conveyance. Anthropogenic causes are mainly population pressure, overgrazing and improper agricultural and economic policies. In recent decades, overgrazing played a main role in secondary soil alkalinization, which led to the decline of Leymus chinensis grasslands. Now, the alkalinization is very severe, and more than 3.2×10 6 ha area has been affected by salt, which becomes one of the three largest sodic–saline areas in the world.

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Wang Z X, Su Q S, Lin S Z et al., 1985. Groundwater and Quaternary Geology at Baicheng. Beijing: Geological Publishing House. (in Chinese)

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Yamaguchi T, Blumwald E, 2005. Developing salt-tolerant crop plants: Challenges and opportunities. Trends in Plant Science, 10(12): 615-620.Soil salinity, one of the major abiotic stresses reducing agricultural productivity, affects large terrestrial areas of the world; the need to produce salt-tolerant crops is evident. Two main approaches are being used to improve salt tolerance: (i) the exploitation of natural genetic variations, either through direct selection in stressful environments or through mapping quantitative trait loci and subsequent marker-assisted selection; and (ii) the generation of transgenic plants to introduce novel genes or to alter expression levels of the existing genes to affect the degree of salt stress tolerance. Here, we discuss the challenges and opportunities provided by recently developed functional tools for the development of salt-tolerant crops.

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Yang J, Zhang G, 2010. Effect of hydrological process change on soil salinization in western Songnen Plain.Journal of Arid Land Resources and Environment, 14(9): 168-172. (in Chinese)Soil salinization is a main ecological environmental problem in Songnen plain. During past decades,rainfall fluctuated without obvious trends,river runoff declined year by year,natural swamps and wetlands shrank greatly,and regional groundwater level declined gradually. At the same time,the area of salt-affect soils increased significantly and ecological environment deteriorated especially at low plain regions. Analysis of soil primary and secondary salinization processes indicated that the main cause of the ongoing soil salinization was that human activities have changed greatly the hydrological processes over the land surface in past decades,and then decreased significantly the chances and time of flooding and waterlogging in the low plains. The soil salinization was mainly caused by the shallow groundwater and the ponding rainwater. The salt accumulated in the upper soil mainly sourced from the shallow groundwater. The ponding rainwater drove the salt accumulated in the upper soil from the dishes to the rideaus,and formed the current distribution pattern of saline and sodic soils. The shallow groundwater was not a main cause of the ongoing secondary soil salinization,although it played a significant role at early stages in the primary soil salinization.

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Yu R P, You W R, 1993. Monitor and Prevention of Salt-affected Soil. Beijing: Science Press, 41-49. (in Chinese)

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Zhang D F, 2000. Study on soil salinization in the Western Plain of Jilin Province based on GIS [D]. Changchun: Changchun University of Science and Technology. (in Chinese)

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