6.1. Characteristics of Hydro-Chemical Characteristics
Given seasonal differences, the unbalanced distribution of precipitation results in an apparent variation in surface runoff and further imbalance in the spatial distribution of hydro-chemical characteristics in the research zone [27
]. Overall, hydro-chemical characteristic parameters changed significantly during rainy and dry seasons (Table 2
). In the rainy season, the coefficient of variation (CV) of EC, mineralization, HCO3−
exceeded 50%. In the dry season, the CV of Cl−
exceeded 50%. The results showed that the ion concentration distributes unevenly in the Watershed. During dry and rainy seasons, the results of ANOVA (Table 3
) showed that the significant (Sig.) value of pH was greater than the significance level of 0.05, so the distribution of pH was uniform in each zone. pH range was mainly from 7.8 to 8.1. Overall, most of the variables showed significant spatial differences (p
< 0.05) among different zones in rainy season and less significant differences in dry season. Samples and their physicochemical parameters during dry and rainy seasons in each zone of the Watershed are presented in Table 4
and Table 5
and Figure 3
. The values of EC, HCO3−
were higher in Zone 3, which mainly included Ebinur Lake, a national ecological zone in Ebinur Lake Bird Isle and the Ganjia Lake Haloxylon Forest National Nature Reserve. Zone 3 was a seriously salinized area in the Watershed. The surrounding area of Ebinur Lake was particularly affected by human activities. As a result, the hydro-chemical characteristics in this region were more seriously affected. In addition, the highest value of mineralization, SO42−
were located in Zone 2, which mainly included the farmland of Jinghe oasis. The low concentration of EC, SO42−
were recorded in Zones 1 and 5. The value of mineralization in Zone 5 was higher than that in Zone 4. The low concentration of HCO3−
was found in Zone 5, which mainly included the farmland in Wusu County.
Through the factor analysis (FA), we have identified the changes of hydro-chemical characteristic factors that affect hydro-chemical characteristics during dry and rainy seasons. During the rainy season, for instance, the KMO and Bartlett’s sphericity test results were 0.803 and 434.174 (df = 45, Sig. < 0.001), respectively. During the dry season, they became 0.818 and 973.172 (df = 45, Sig. < 0.001), respectively. The results show that the water sample data during rainy and dry seasons are suitable for factor analysis.
During the rainy season, the first two rotated factors with eigenvalue of 1 or greater were extracted using Varimax with Kaiser Normalization, explaining 78.9% of the total variance in the hydro-chemical characteristics data set (Table 6
). For Factor 1 in the FA study, the important variables were EC, mineralization, SO42−
. The moderately important variable was Cl−
. Similarly, for Factor 2 in the FA study, the results showed that the Watershed was to be of alkaline hydro-chemical characteristics with high loadings on pH, Cl−
. During the dry season, the same two rotated factors with eigenvalue of 1 or greater were extracted using Varimax with Kaiser Normalization, explaining 86.5% of the total variance in the hydro-chemical characteristics data set. For Factor 1 in the FA study, the results showed that, except the pH, the other water parameters were important variables. In Factor 2, the results showed that the Watershed was with high loadings on pH. Overall, the majority of water parameters were moderately important variables in FA during rainy and dry seasons. In recent years, with increasing cultivation in the upstream of Ebinur Lake Watershed, irrigation water usage increased, causing the lake water levels to fall and the lake area to shrink. At the same time, the salt flowing with the river water into Ebinur Lake exacerbated the latter’s salinization, resulting in the increase of ion content. Especially, Factor 1 in the FA study during rainy and dry seasons showed that the Ca2+
were higher than other cationic contents. Ca2+
is easy to form into CaCO3
and then precipitate. With increasing salinization of water, the content of Mg2+
greatly exceeds the Ca2+
. With the increase of salt content, the ratio of Ca2+
becomes smaller and smaller, eventually causing the imbalance of Ca2+
, which is harmful to fishery in the Watershed [52
6.2. LULC patterns in Different Zones
According to the classification results (Figure 4
), we can clearly see that the farmland area increased from May to September as the crops grow in rainy season and become ripe for harvest in dry season. Meanwhile, with a shrinking water area, the salinized land increased in September. Since Ebinur Lake Watershed was characterized by little precipitation, strong evaporation, and much wind [26
], higher temperatures cause more evaporation and worsen the arid conditions in the Watershed. Thus, in dry season, combined with a large evaporation, the salt was brought to the surface, causing salinization in the surroundings of Ebinur Lake.
Based on classification, the area of various LULC patterns showed obvious differences in six zones. In Figure 5
and Table 7
, compared with the rainy season, the area of farmland obviously increased in each zone in the dry season; the area of forest-grass land and water body in each zone decreased. While the salinized land showed an increasing trend in each zone, the area of bare ground and bare rock in rainy season was less than that in the dry season in Zones 1, 2 and 6, and the area of others was greater than that in the dry season in Zones 3–5. Overall, the area of bare ground and bare rock changed little.
Overall, LULC patterns are conspicuously different in the six zones (Figure 6
). In the rainy season, large forest-grassland areas, over 50% of total land area, are distributed in Zone 4. Water areas have extensive distributions from 3.9% (Zone 5) to 8.1% (Zone 3). Around Ebinur Lake and in the Bohe and Jinghe tributaries, salinized land area ranges from 8.1% (Zone 2) to 15.7% (Zone 3). Gravel, bare ground and bare rock also represent a big proportion in Zones 1, 4 and 6, ranging from 53.1% (Zone 6) to 66.5% (Zone 4) of their respective drainage areas.
In dry season, farmland area ranges from 8.1% (Zone 2) to 22.9% (Zone 4). Most forest-grassland areas, over 20% of the total land area, are distributed in Zones 1, 2, 4 and 5. Water body is mainly found in Zones 2 and 3. Due to human activities such as farming irrigation, salinized land was concentrated in large farmland areas. Meanwhile, reduced precipitation in dry season leads to the contraction of Ebinur Lake and expansion of the salinized land around it. Large areas of salinized land are mainly found in Zone 2 and 3. In dry season, the phenomenon of salinization was more serious than in rainy season. Salinized land mainly contained the sodium, calcium, magnesium carbonate, sulfate and chloride. The proportion of salinized land is the primary predictor for mineralization values during the dry season. Bare ground and bare rock are at a high percentage and over 50% are distributed in Zones 1 and 6.
6.3. Linkage between LULC Patterns and Hydro-Chemical Characteristics
First, we extracted the ground reflectance value of various LULC patterns and analyzed the correlation between the ground reflectance values and hydro-chemical characteristics (Table 8
The result showed that the ground reflectance values of various LULC patterns had good correlation with hydro-chemical characteristic parameters. Especially in rainy season, the result showed that the ground reflectance value of farmland had was negatively correlated with Cl−
. Usually, Cl−
was affected by fertilizer and organic fertilizer around the farmland. The soil salinity is higher in Xinjiang, chlorine salt in soil salt is greater. In recent years, with the development of the soil experiments on fertilizer formula, the chloride ion content in the fertilizer is generally high [53
]. In rainy season, the crops in the study area had not yet started to grow, so the fertilizer had less influence on Cl−
. At the same time, the ground reflectance value of water body is related to pH under the significance level of 0.05. Meanwhile, the relation between the ground reflectance value of salinized land and hydro-chemical characteristics parameters were not significant, because the salinized land was not the main factor affecting hydro-chemical characteristics parameters in rainy season. In dry season, the ground reflectance values of salinized land had great impact on hydro-chemical characteristic parameters. With the decrease of the water area, the salinized land increased in dry season. Because it had higher reflectivity, the ground reflectance values of salinized land had great influence on hydro-chemical characteristic parameters.
Then, we discussed and analyzed the correlation between hydro-chemical characteristics and LULC patterns at different zones during rainy and dry seasons. The results are presented in Table 9
. During the rainy season, the water body and salinized land were significantly related to the majority of water parameters under the significance level of 0.01 and 0.05. Due to the precipitation and melt water, surface runoff was relatively rich lead to some ion as rivers into reservoir and Ebinur Lake, placing the area of water body at high relevance with hydro-chemical characteristics parameters. Due to salinized land mainly contained the sodium, calcium, magnesium carbonate, sulfate and chloride, it had a positive correlation with hydro-chemical characteristics parameters.
During the dry season, forest-grassland displayed a negative correlation with EC under the significance level of 0.01. At a significance level of 0.05, water body exhibited an obvious positive correlation with HCO3−. The water body exhibited an obvious positive correlation with EC, Ca2+ and K+ under the significance level of 0.01. Due to decrease of rainfall, irrigation, surface runoff, the area of lake, etc. largely led to the increase in ion content, which had great influence on hydro-chemical characteristics parameters. At a significance level of 0.05, salinized land exhibited a significant positive correlation with mineralization, HCO3− and Cl−. At a significance level of 0.01, salinized land exhibited a significant positive correlation with SO42−, Ca2+, Mg2+ and Na+. In dry season, the phenomenon of salinization was more serious than in rainy season. Thus, the salinized land had greater influence on hydro-chemical characteristic parameters. The bare ground and bare rock had significant positive correlations with the EC and K+ under the significance level of 0.05. Overall, the area of water body (lake, reservoir, pond, etc.) and salinized land had great influence on hydro-chemical characteristics parameters, followed by bare ground and bare rock.
Then, the buffer zones were analyzed in terms of LULC patterns and their influences on hydro-chemical characteristics during dry and rainy seasons. The results showed significant correlations between LULC patterns and hydro-chemical characteristics during the dry and rainy seasons in Ebinur Lake Watershed (Table 10
). Forest-grassland, salinized land and water body are associated with most hydro-chemical characteristic variables during both rainy and dry seasons in the Watershed. During the rainy season, because the crops in the study area had not yet started to grow, Haloxylon forest, Natural Populus euphratica and grassland more obviously influence hydro-chemical characteristics than farmland. The proportions of forest-grassland are positively correlated with pH (p
< 0.01), EC (p
< 0.01), Mg2+
< 0.05) and Na+
< 0.01) and negatively related with mineralization, Cl−
< 0.01). The water body is significantly related to most physicochemical variables (p
< 0.01 or p
< 0.05) except Ca2+
. Except the HCO3−
, the salinized land shows significant positive correlations with the other hydro-chemical characteristics parameters.
During the dry season, farmland predicts more hydro-chemical characteristic variables than during the rainy season because of intensive fertilization and agricultural runoff [10
]. The results show that the proportion of farmland is positively correlated with pH (p
< 0.01) and negatively related with EC, mineralization, Cl−
< 0.01). Forest-grassland displays significant positive correlations with pH, EC, mineralization, Cl−
< 0.01), and negatively correlations with Ca2+
< 0.01). Water body in different zones is positively correlated with EC, mineralization, SO42−
< 0.01), and negatively related with pH. Except the HCO3−
, the salinized land shows positive correlations with other hydro-chemical characteristics parameters.
Stepwise linear multiple regressions models create a “goodness of fit” (R2
values > 0.50) [55
]. EC, HCO3−
, and SO42−
during the dry and rainy seasons are predicted based on the proportions of water body (Table 11
). During the rainy season, pH, mineralization and Ca2+
are defined by forest-grassland, while the Cl−
are defined by salinized land. During the dry season, pH and Na+
are estimated based on farmland, whereas Cl−
are estimated based on forest-grassland. Specifically, mineralization is explained by salinity during the dry season.
Yan et al. [56
] discussed the formation of Ebinur Lake and its evolution process from the point of farmland, livestock, population and industry. His study gave preliminary analysis on the shrinking of the area of Ebinur Lake and the changing of watershed ecological environment over the recent 40 years. The results showed that the water consumption in Ebinur Lake area was negatively related to human activities, increasing vegetation damage caused by human activities, grassland degradation, desertification and salinization. Compared with those a decade ago, the mineralization and the ion content of Ebinur Lake both increased greatly [52
]. The phenomenon of water salinization should not be ignored.
Hydro-chemical characteristics is generally linked to LULC in the watershed [57
]. Human activities on LULC influence the types and degree of pollution. Therefore, measuring the proportions of certain LULC patterns in a watershed might enable us to conveniently predict hydro-chemical characteristics.