The Characteristics of Spatial Distribution of Saline-alkali Wasteland in Datong Basin,Northern China

Chuxin ZHU,Rutian BI

College of Resources and Environment,Shanxi Agricultural University,Taigu 030801,China

Responsible editor:Xiaoxue WANG Responsible proofreader:Xiaoyan WU

Saline-alkali wasteland results from soil salinization and high salinity in soil inhibits the growth and development of most plants[1-2].If soil salinization is aggra vated,soil fertility will be severely reduced,which could lead to environmental degradation.However,salinealkali wasteland could become valuable reserved land resources with great potential if proper amelioration measures are performed.

Saline-alkali land has reached 950 000 000 hm2on the earth.Thus soil salinization becomes one of the most serious environment issues.China occupies a large area of 36 000 000 hm2of saline-alkali land[3].In Shanxi Province,saline-alkali land area is 300 000 hm2,68% of which was distributed in semi-arid area of Datong basin[4].There are a lot of researches on salinity spatial distribution.Geostatistical method is a common method to analyze and predict salinity distribution.It was used to predict the spatial dynamics of electrical conductivity,pH and extractable sodium in southeastern Spain[5].And also used to model spatial variability of soil salinity in coastal low lands in west Africa to determine the significant effects of climate,river hydrology,topography and tidal flooding[6].The geostatistical method and GIS technique were used to analyze the spatial distribution of saline soil in Yinchuan plain of China[7].There was temporal analysis of salinity on catchments in Western Australia to predict the future salinization[8].Digital mapping was used to model vertical and lateral variation of soil salinity[9].From the scale aspect,such as field-scale,to find out that soil salinity and alkalinity in salinesodic soil had very high variability[10].Regional topographic factors had impacts on spatial distribution of soil salinization as well[11].

Researches of Datong basin mainly focused on the groundwater environment and its chemical properties[12-15].And specific improvement measures of saline land[16-18].However,there has been little spatial distribution research on saline-alkali wasteland of Datong basin.Both spatial and temporal patterns,together with local factors such as land-use,groundwater trends,variations in micro-relief,and local hydrological processes,will dictate the choice of the most appropriate salinity management response[19].In this paper,we analyzed the spatial distribution and causes of saline-alkali wasteland in Datong basin from four factors:topography,geology,hydrology and soil.The results could be used as reference and guidance for the amelioration of saline-alkali wasteland in Datong basin.

Materials and Methods

Study area

Datong basin (112°10′-114°19′E,39°5′-40°35′ N) is located in an arid and semi-arid region of northern Shanxi Province,northern China.It has continental monsoon climate in temperate zone with an average annual temperature of 6-7 ℃,an average annual rainfall of 300-400 mm and a very high average evaporation of about 2 000 mm per year[14].Average altitude of Datong basin is 1 614 m.There are two subbasins in Datong basin,one is Sanggan subbasin,and the other is Yang subbasin[20].

Datong basin is an alluvial-pluvial plain formed by water flow.Terrain inside the basin is flat and with gentle undulations.The soil in Datong basin mainly consists of castano cinnamon soil and castanozems.Because of the limitations such as high altitude,low accumulated temperature,short freefrost period and large area of salinealkali wasteland,numerous of money,manpower and resources are needed to increase the crop yield.Crops in Datong basin are millet,maize,potato,wheat,naked oat and flax.Yield of crop in Datong basin is lower than those in central and southern Shanxi Province.

Study area of this paper is the saline-alkali wasteland in Datong basin,and is approximately 34 433 hm2(Fig.1).

Sampling and data processing

Sampling and laboratory analysis were undertaken from April 17thto May 21stin 2013.One sampling point was taken in every 20 hm2in study area.A total of 1729 soil samples were obtained at 0-20 cm depth.Elevation,location,slope and soil type were obtained from the soil sampling.Posterior to sampling,TS and pH were measured in the laboratory.In addition,we overall obtained the information of cropping system,water resources,land use condition,crop yield and economic development from relevant agricultural department and native farmers of some counties in Datong basin,specifically including Shuocheng District,Shanyin County,Hunyuan County,Datong County,Yanggao County and Tianzhen County.

The total salinity (TS) and pH in the whole saline-alkali wasteland were determined by method of inverse distance weighted (IDW) interpolation in ArcGIS 10.0.IDW determined cell values using a linearly weighted combination of a set of saline-alkali wasteland sample points.The weight was a function of inverse distance.There were surfaces of TS distribution and pH distribution built by IDWinterpolation at last.TS value was classified to≥10,4-＜10,2-＜4,1-＜2 and ＜1 g/kg,and pH value was classified to 7.5-8.5,8.5-9.0 and ＞9.0[7].

The rivers were extracted from DEM using Hydrology tool in ArcGIS 10.0.1500 was taken as flow accumulation value to extract streamnet.The streams were classified into 6 orders according to STRAHLER system.Only stream orders of 3 to 6 were used in this paper.Extraction of boundary of Datong basin was based on DEM of WGS84 coordinate system.Ten administrative counties(Shuocheng District,Shanyin County,Ying County,Huairen County,Nanjiao District,Xinrong County,Datong County,Hunyuan County,Tianzhen County and Yanggao County) and river basin boundary were extracted from DEM to obtain two DEM layers.The boundary of Datong basin was determined by overlapping the two DEM layers.DEM data was from SRTM dataset with the resolution of 90 m,the column number was 59 and row numbers were 4 and 5.It was provided by International Scientific&Technical Data Mirror Site,Computer Network Information Center,Chinese Academy of Sciences(http://www.gscloud.cn).

Results and Analysis

Topography

Distribution index of topographic grade[21-22]was used to characterize the topography of different land use types and quantify the topographic distribution of saline-alkali wasteland.The index was calculated using the following equations:

where T is the topographic index.E and S are elevation and slope of a random point.EAand SAare average elevation and average slope of this random point in its zonal area.P stands for the distribution index of each topographic class.

where Sijis the area of land use type of i in topography grade of j.Siis land use type of i and Sjis the topography grade of j.S is the area of Datong basin.

The major land use types in Datong basin were farmland,forest,grassland,water,construction land,and saline-alkali wasteland.Land use types except those mentioned above were of very small proportion so they could be negligible.Meanwhile,topographic characteristic of water was not comparable with that of other land use types,so water was eliminated.Farmland,forest,grassland and construction land were finally selected to compare with saline-alkali wasteland.

Topographic index (T) was divided into 10 topographic grades.Curves of distribution index (P) of farmland,forest,grassland,and construction land are of small difference.Compared with those four types mentioned above,curve of saline-alkali wasteland showed the greatest fluctuation with an obvious trough.It indicated that saline-alkali wasteland developed preferably on some particular terrains.In addition,the distribution index of saline-alkali wasteland became much higher than other land use types where the topographic grade reached its minimum and maximum.It meant that terrain of minimum and maximum topographic grades were not suitable for farmland,forest,grassland and construction land,but were advantageous to saline-alkali wasteland development(Fig.2).

With the help of slope and terrain information,it was clear that low-lying and flat were the characteristics of regions with minimum and maximum topographic grades.Floodplain and fluvial terrace were the main landforms of these regions.Because of the flat terrain,the drainage of Sanggan River and Yang River was poor.Meanwhile,groundwater ascending from underground to surface hardly drained away.As groundwater evaporating,salt began to precipitate and gather on surface.As a result,the saline-alkali wasteland was formed.

Geology

Topography of Datong basin showed apparent transition from mountains to flat basin.Flat basin is surrounded by the tilt alluvial plain on piedmonts,which is composed of a series interconnected alluvial fans.The alluvial fans are developed because water of river flowing from the mountains suddenly reduced or dispersed.The central part of basin is alluvialpluvial plain.It is formed by water erosion and stack.Area of the plain is large and with very gentle undulation.

Surface of the basin contains large amount of alluvial and pluvial sediment and a thin layer of loess.In the middle and lower stratum contains lacustrine sediment.From piedmonts to the central part of basin,the alluvial and lacustrine sediment changes from coarse to fine.Pluvial plain containing fine sand,silt and sandy loam mainly is distributed on terraces of both sides of Sanggan River.It is formed in the Late Pleistocene to Holocene.Pores of silt and sandy loam are easy for groundwater to move in the soil,so that salt dissolved in water accumulate on the surface.The Quaternary sediment containing soluble salts is the parent material of contemporary soil,and has great influence on the formation of current soil texture.

Outcrop distribution of saline-alkali wasteland is Quaternary Holocene alluvium and it accounts for 40%of the total area of Datong basin.Located at the lowest part of central basin,the alluvium is distributed on terraces of Sanggan River and its tributaries Yu River,Hun River,as well as Yang River.87% of the saline-alkali wasteland is distributed on the riverbed containing gravel,silt and sandy loam,and the rest is distributed on river terrace.Formation of saline-alkali wasteland is germane to the geomorphic units.Between the levees and alluvial fans are some intersecting concave grounds.These concave grounds contain soil layer of alternate sandy loam and clay due to staggered sedimentation and groundwater flow,which leads to poor performance of soil hydraulic conductivity.So most of the concave grounds are closed or semi-closed salt accumulation center with high groundwater salinity and severe salinization[23].

Hydrology

Sanggan River and Yang River are the two major rivers cross through Datong basin.Sanggan River flows through Shanyin County,Ying County,Huairen County,Datong County and Yanggao County.Yu River and Hun River are its tributaries.Yu River flows from north to south and Hun River flows from east to west.Yang River flows from west to east.

Along the river,Sanggan River was divided into three regions according to hydrology and distribution of the tributaries.Node A was the outlet of Dongyulin reservoir.Node B was the place where Yu River met Sanggan River.In this paper,we defined west of node A was the upstream region of Sanggan River,midstream region was between A and B and east of node B was downstream region (Fig.3 (a)).Yang River was divided into two regions by Node C,which was the maximum elevation gap of Yang River(Fig.3(b)).

Analyzing TS by using the interpolation method of inverse distance weighted (IDW).In midstream region,area of saline-alkali wasteland was the largest among those three regions.TS value distributed in this region was the highest.TS value in upstream region was less than that in midstream region,and TS value in downstream region was the least.The area proportion of TS value in each region is presented in Table1.

In Sanggan subbasin,TS value in upstream region was from 4 to 10g/kg and mainly gathered in Shuocheng County.Saline-alkali wasteland in this region was distributed near node A,the area accounted for 20%of the total area of saline-alkali wasteland.TS value in this region was modest.The Dongyulin reservoir was the reason of high groundwater table in this region leading to the distribution of saline-alkali wasteland.The area of saline-alkali wasteland in midstream region was 71% of the total.TS value in Shanyin County and Ying County was greater than 10 g/kg,and the rest of midstream region was from 4 to 10 g/kg.The gradient of Sanggan River in this region was 0.15‰ and the elevation difference was smaller than 10m.So the river flow was gentle and slow.Large amount of saline-alkali wasteland was distributed here due to the high groundwater table.It was caused by small elevation difference of river level and groundwater table so the groundwater table was difficult to decrease.TS value in downstream region was less than 4g/kg.The salinealkali wasteland scattered in Datong County and Yanggao County with small area.The gradient of Sanggan River in this region was 2.6‰.Great amount of groundwater exploitation caused descending of the groundwater table and the ascending of soil aeration.

In Yang subbasin,most TS value in west region of node C was greater than 4 g/kg,and that in east was less than 4 g/kg.The distribution coincided with that in Sanggan subbasin.

In order to find out the distribution of TS value in the lateral sides of rivers,we made the buffers of river centerline.Setting the buffer radius needed to be considered river level and terrain[30].We made the buffer of 4 km and 2 km radius respectively on both sides of the river and its tributaries.The area proportion of TS value in each buffer zone is showed in Table2.

In Sanggan subbasin,86% of the total saline-alkali wasteland was distributed inside the 4 km buffer zone.TS value was greater than 2 g/kg inside and less than 2 g/kg outside the zone.TS value of most saline-alkali wasteland inside 4 km buffer zone was from 4 to 10 g/kg.When further reduced the buffer radius to 2 km,TS value of saline-alkali wasteland in 2 km buffer zone was the highest in the whole basin.

In Yang subbasin,all the salinealkali wasteland was distributed in buffer zone of 4 km radius.The highest TS value was distributed in buffer zone of 2km radius.The distribution coincided with that in Sanggan sub-basin.

Table1 Area proportion of TS value in each region

Table2 Area proportion of TS value in each buffer zone

The reason is that infiltration of river keeps the groundwater table from decreasing,which leads to salt accumulation.Therefore the saline wasteland was formed.The farther away from the river,the less the river affects groundwater table and the lower the groundwater table is.Salt accumulation becomes less along with the decrease of the groundwater table.

The average groundwater table throughout Datong basin was about 50 m,the highest groundwater table was distributed in two regions.One region is Shanyin County and Ying County,the other region is the interconnected area of Yanggao County and Tianzhen County.The highest groundwater table is 3 m and groundwater salinity is above 3 g/L.The two regions are two salt accumulation centers in Datong basin.Groundwater table and groundwater salinity gradually decline from the center to periphery.

Soil

Parent material was the basis for soil formation and has profound impact on soil properties.Fluvial and alluvial sediment was the main parent material in Datong basin,which led to the development of fluvo-aquic soil and saline soil.Area of fluvo-aquic soil accounted for 77%of the total saline-alkali wasteland area and saline soil accounted for 13%.Area of the rest of soil types accounted for approximate 10%of total area.

Fig.4 (a) showed TS value of 60.55% of fluvo-aquic soil and nearly 50% of saline soil ranged from 4 to 10 g/kg.These two soil types were heavily salinized.TS value of meadow soil,castano cinnamon soil and castanozems were apparently weak.Fig.4(b)reflected the alkali condition by using pH value.The pH value of fluvial soil,saline soil and meadow soil were greater than 9.0 and strongly alkalized.Among which the pH value of 67.50%of meadow soil was 9.5 or more.But the area of meadow soil was so small that it could be negligible.The pH value of castano cinnamon soil and castanozems was low and they were weakly alkalized.

The change of salt accumulation of fluvo-aquic soil was seasonal.Peak period of salt accumulation on the surface happened in spring and autumn.The reason was that salt gathers on the surface because of great evaporation.Salt leaching happened in summer because salt descended to subsoil layer and bottom soil layer during the rainy period.Furthermore,soil aeration was poor since capillaries were filled with water.

Saline soil had low soil nutrients and productivity.Salinity in subsoil layer and bottom soil layer was relatively lower than that on surface.There were grey salt crust and salt efflorescence on surface of saline soil.The reason of this phenomenon was that soda content of saline soil was higher than fluvo-aquic soil.

Discussion and Conclusions

Discussions

Hydrologic factor is the main reason that forms the saline-alkali wasteland among 4 factors of topography,geology,hydrology and soil.Three nodes were set to analyze the TS distribution along the rivers.TS value in midstream region was the highest,and some of the TS value was higher than 10 g/kg.TS value in upstream region was lower than that in midstream,which mainly was between 4 to 10 g/kg.TS value was the least in downstream region.There were two reasons,first was that lateral seepage of rivers affected the groundwater.Second was that river gradient,runoff and water velocity affected the groundwater.So changes of rivers deeply affected changes of groundwater table,and distribution of saline-alkali wasteland was affected by both of them.Annual runoff of Sanggan River was gradually decreasing by years[24].After the 1970s,especially in the 1990s,the years of dry season clearly increased[25].Groundwater in Yang subbasin mainly distributed in Quaternary and near Yang River,the runoff was weak[26].Rivers in dry season led to effective irrigation area diminishing and large evaporation led to salt accumulation on the surface.Consequently,the salinization aggravated[27].From the results of TS distribution in lateral sides of rivers,saline-alkali wasteland mostly distributed on flat terrace that near the rivers such as floodplain and fluvial terrace.Fluvial terrace on both sides of the rivers was 1 to 2 m higher than the riverbed[25].With the terrain gradually steepening,the farther away from the river,the less salinization there was.

TS distribution was described along the rivers and in the lateral sides of rivers from the view of the whole basin in this paper,by dividing the rivers with nodes and calculating the gradient of rivers.The relationship between TS and water velocity or runoff could be analyzed in further studies.

Saline-alkali wasteland had a major impact on crop yield and regional agricultural production[28].The salinealkali wasteland in Yang subbasin directly caused the pecuniary loss of about 5 million yuan every year[26].During our investigation in the towns of Shanyin County and Shuocheng District in Shuozhou City,the research found out that if the soil was not fertilized and tilled after harvest,salt would be accumulated soon on the surface and saline-alkali land would be formed again.What is more,the salinization was too severe to alleviate in some area of Heshengbu Town,Shanyin County and even could not be reclaimed and was barren for a long time in.

Overall,salinization is still severe in Datong basin.Each amelioration measure has its advantage and disadvantage and the applicable region and environment is not exactly the same[29].The key to ameliorate salinealkali wasteland is to descend the groundwater table in the two salt accumulation centers of Datong basin.We were acknowledged that subsurface pipe drainage and other measures are currently used in Datong basin during our investigation,and some achievements had been reached in limited region.In order to alleviate the salinization to the greatest extent,water resource facilities are still required to regulate the groundwater effectively.

Conclusions

Topographic distribution curve of saline-alkali wasteland was different from curves of farmland,forest,grassland,and construction land.Saline-alkali wasteland was distributed on flat and low-lying terrains with poor drainage,which led to salt accumulation.

Topography of Datong basin showed apparent transition from mountains to flat basin and a plain which was slanted to the center was formed.87%of saline-alkali wasteland was distributed on Quaternary Holocene alluvium.There were intersecting concave grounds between the levees and alluvial fans that contain soil layer of alternate sandy loam and clay.Therefore,performance of soil hydraulic conductivity was poor.

One of TS value distribution is TS value in midstream region was the highest,less in upstream region and the least in downstream region along the rivers.The other is TS value of saline-alkali wasteland decreased when distance between saline-alkali wasteland and rivers increased.

Fluvo-aquic soil and saline soil were two major soil types in Datong basin.Area of fluvo-aquic soil accounted for 77% and saline soil accounted for 13%of total saline-alkali wasteland area.Both of the two soils are heavily salinized and strongly alkalized.

With the combination factors of topography,geology,hydrology and soil,we found out that there were two main reasons caused the formation of saline-alkali wasteland in Datong basin.One was small difference in elevation between river level and groundwater table which led to high level of groundwater table.The other was the existence of two salt accumulation centers.Altitude of salt accumulation center located in Shanyin County and Ying County is 999 m,the other located in Yanggao County and Tianzhen County is 1 035 m.Altitudes of those two centers were lower than the average altitude of Datong basin.Drainage performance in the centers was weak,resulting in concentrated saline-alkali wasteland.

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