1. Introduction
Soil is a natural history complex formed with the combined action of certain natural factors, such as climate, biology, water and human activities. Because of the complexity of soil-forming factors, soils exhibit spatial variation. The spatial distribution of soil salinization and the temporal and spatial changes of groundwater depth reflect the degree and state of salinization in the soil cultivated layer [
1,
2]. Regional soil-type distribution maps can reflect the characteristics and trends of soil spatial variation, and constitute basic data for research on soil-water and salt transport, saline–alkali land improvement, soil fertility assessment, irrigation engineering planning and rational use of soil resources [
3]. In traditional soil-type distribution maps, based on limited soil profile points, the soil-type boundaries are delineated, according to certain factors, such as landforms, topography and the phenological landscape. This method lacks a quantitative theoretical basis, and it is still difficult to describe soil characteristics and soil-type boundaries [
4,
5].
A Geographical Information System (GIS) is a computer system that collects, stores, manages, analyzes and expresses spatial information. This type of system can be applied to efficiently manage and process original data on soil types and information on the regional distribution of soil attributes [
6,
7]. Of the available methods, the Thiessen polygon is embedded in GIS as a mapping tool by algorithms. It can segment physical quantities with spatial attributes according to limited known points, and provide a good technical platform and a means to draw soil-type maps [
8]. In recent years, GIS spatial interpolation has been applied to analyze soil data with continuous spatial coverage. For example, Panagopoulos et al. used Kriging interpolation to study soil salinity variability in an area of 2208 m
2 in the Mediterranean region [
9]. Gumiere et al. used the inverse distance weighting (IDW) method to predict soil physical properties and moisture content in a study area in Quebec, Canada [
10]. Bogunovic et al. studied the effect of different interpolation methods on the prediction accuracy of the spatial variation of soil nutrients, and found that the radial basis function interpolation of soil-available potassium and pH generated smaller prediction errors, whereas IDW interpolation had higher prediction accuracy for soil-available phosphorus [
11].
The research on saline–alkali land has mainly focused on the control and improvement of soil salinization [
12,
13,
14], the water and salt transportation mechanism [
15], the effect of salinization on crop growth [
16,
17] and the relationship between groundwater depth and mineralization [
18]. There have been few studies on the spatial distribution of soil salinity and groundwater depth [
19],and how groundwater depth affects salinity. Farmland soil-moisture transport is affected by topography, meteorological conditions, irrigation conditions, crop types and distribution, soil types and distribution, temporal and spatial changes of groundwater and soil salinity [
20,
21,
22]. For small irrigation areas below 667 hm
2, some influencing factors, such as meteorological conditions, can be approximately considered to be consistent over the region; however, spatial variation cannot be ignored for factors with strong spatial variability, such as crops, soil and groundwater [
23,
24,
25]. A reasonable grasp of the spatial distribution of soil salinization and groundwater depth can provide a scientific basis for improvement of saline–alkali soil along the Yellow River, thereby increasing grain yield and achieving the sustainable development of agriculture and the soil–water ecological environment. The Hetao irrigation area, Inner Mongolia, is one of the three major artesian water-diversion irrigation areas in China, and is also an important grain-producing area. The average annual Yellow River diversion is planned to be 5.2 billion m
3 [
15]; however, 23.3% of the total land area has been damaged by salinization, which seriously restricts the grain yield [
26].
Shahao is the trunk irrigation area of the Hetao irrigation area, which is a typical area along the Yellow River. Therefore, in this study, Shahao was selected for the purpose of the research. A map of the regional soil types was drawn by GIS on the basis of field-soil surveys, sampling and experimental data from 2007–2010. The spatial interpolation of the soil salinization index and groundwater depth in the study area were conducted. There are three main aims of the present study. The first aim is a quantitative analysis of the temporal and spatial variations of the groundwater depth in observation wells throughout one hydrological year. The second aim is a quantitative analysis of the spatial distribution of soil salinization at different soil depths at sampling points. The third aim is the assessment of the temporal and spatial changes of groundwater depth and soil salinization, and the identification of the main influencing factors, with the intention of improving the agricultural ecological environment and providing reference information to achieve sustainable agriculture.
4. Discussion
Shahao has certain unique natural conditions, such as climate, topography, soil and groundwater. The groundwater depth varies with autumn irrigation, the freezing period and the crop-growth period during the year [
34] (
Figure 4). Spatially, the groundwater depth is higher in the upstream area and low in the downstream area (
Figure 4). The topography of Shahao is high in the south and low in the north (
Figure 1c), which influences the regional pattern of groundwater depth. The groundwater depth in turn indirectly affects soil-water transport, resulting in the accumulation of salt in the soil and increased relative concentrations of Na
+ in the soil solution [
35,
36], meaning that the native soil was influenced by salinization prior to development.
The crop emergence rate of heavily salinized cropland is only 30–50%, and almost no grass grows on the saline soil, which cannot be cultivated without irrigation (
Figure 5 and
Figure 6). Irrigation from the diversion of the Yellow River ensures a sufficient water supply for agriculture while leaching the soil salinity. This irrigation enables the development and utilization of this area, but is also an important factor causing the high soil salinity. First, because of the high salt content of the Yellow River itself and extensive irrigation and farming techniques, salt accumulates in the soil in the root layer of crops, which in turn leads to the occurrence of secondary soil salinization and deterioration of the water–soil environment. Second, autumn irrigation is perfumed before the soil freezes in autumn and winter every year. The autumn irrigation is the largest input of water during the year. The irrigation water carries a large amount of soluble salts and freezes in the soil. When the soil melts in spring, the exposed soil surface and strong evaporation lead to salt enrichment in the surface soil [
37,
38,
39]. Enrichment is greater toward the surface [
40,
41] (
Figure 5), resulting in stronger soil alkalinity in the surface and middle layers (
Figure 6), which is the local famous “spring soil salt-return phenomenon”. Finally, although some soluble salts and Na
+ in the soil can reach the deep soil or groundwater through leaching by irrigation water and deep seepage, the salt does not flow out of the soil because of poor drainage [
42,
43]. When the groundwater level rises seasonally and with irrigation (
Figure 4a,b,i–l), under the action of strong evaporation, ions return to the surface soil [
44]; thus, irrigation water leaching cannot solve the problem of soil salinization and alkalinization by itself.
Therefore, to control soil salinization and alkalinization in the Hetao irrigation area, multi-factor linkage control must be conducted. First, irrigation must be improved; specifically, more water should be introduced in areas with severe salinization, and less non-salinized and mildly salinized areas should be irrigated. Second, farming techniques must be improved, and salt- and alkaline-tolerant crops should be selected. Third, in areas with severe salinization and alkalinization, chemical amendments and other improvement measures should be applied in combination with irrigation and leaching to control salt and alkalinity. Overcoming the difficulties, speeding up the construction of drainage systems and improving the drainage environment are also important measures. Only by adhering to the principle of multi-party governance can the problem of salinization in the Hetao irrigation area be solved, the agricultural water–soil environment be improved and productivity be increased.