Spatiotemporal changes of soil and water loss (2012–2019) in three government-divided regions, Chongqing Municipality, China Open Access

Soil and water loss has been the most serious environmental problem greatly threatening the water quality and agriculture productivity worldwide (Pimentel 2006; Durán Zuazo & Rodríguez Pleguezuelo 2008; Zhao et al. 2015). In China, the area of soil and water loss released by the Ministry of Water Resources is 2,710,800 km² in 2019, occupying 28.23% of the land area of China. Therein, the area of soil and water loss driven by water erosion is 1,134,700 km², accounting for 41.86% of the total area of soil and water loss. In recent years, the Ecological Civilization Construction (ECC) has risen to the core national strategy in China, with the aim of improving humankind's survival and development conditions (Zhang et al. 2016). It is bent on the coordination of the relationship between humans and nature in a comprehensive, scientific and systematic manner (UNEP 2016), and emphasizes the importance of sustainable ecological and environmental development for the human material life level (Zhang et al. 2020; Zuo et al. 2021). Therefore, soil and water loss has been a key problem to hamper the sustainable development of society, economy and ecology, which is needed to be urgently controlled in the future (Li et al. 2009; Rodrigo-Comino & Cerdà 2018).

China has attached great importance to the carrying capacity of resources and environment, and the development model has changed from a horizontal cascade development model to a regional integration in-depth development model (Zhang et al. 2021). This encourages the decision-maker to consider the soil and water loss as regional environmental problem to couple with the local development (Brazier et al. 2001). Chongqing Municipality is an important central city of China and is the economic center of the Yangtze River Economic Belt, which is experiencing the most serious soil and water loss. Soil and water loss is restricting the high quality development of society, economy and ecology of Chongqing City (Tan et al. 2020). Recently, the Chongqing government grouped its 38 counties into three regions including urban region (UR), northeast region (NER), and southeast region (SER), based on the different geographical environments and urbanization processes, to accelerate the achievement of a harmonious development in Chongqing City (He et al. 2017). These three regions play different roles in achieving the harmonious development of Chongqing City. The UR is expected to drive the economic development of other two regions, while the government carries out the demand of ‘protection on the large area but development on partial area’ in view of the function of ecological conservation and protection of NER and SER, respectively (He et al. 2017). This may present different characteristics of soil and water loss in response to the different nature and human activities in these three regions, however, such information is not available currently and further interpretation of dynamics of soil and water loss in these regions can greatly contribute to the ECC of Chongqing City.

Previous studies showed that the distributions of soil and water loss had obviously spatial and temporal characteristics (Li et al. 2009; Liu et al. 2016), and unravelling those distribution characteristics will have great significance for making optimal strategies to control soil and water loss and restore impaired ecosystems, which will greatly contribute to the ECC. Commonly, characterization of soil and water loss was conducted in terms of different spatiotemporal scales such as slope runoff plot and watershed scale (Guo et al. 2019). Also, the work of soil and water conservation is often carried out on the basis of administrative units (Tan et al. 2020). Xiao & Xiao (2018) studied the characteristics of soil erosion and conservation and assessed the ecosystem service potential in Chongqing City. Additionally, many researchers have studied the soil and water loss characteristics in the watershed of Three Gorges Reservoir, which is the largest waterbody in Chongqing City (Liu et al. 2016; Chu et al. 2020; Xiao et al. 2020). However, to the best of our knowledge, few studies are performed on the three regions in Chongqing City to provide references and guidelines for controlling regional soil and water loss.

In this study, a series of dataset on soil and water loss were compiled in detail from the annual Bulletin of Soil and Water Conservation in Chongqing City, spanning from 2012 to 2019, to further study the changes of soil and water loss in three different regions in Chongqing City. The specific objectives of this study are to (1) determine the changed area of soil and water loss for these three regions, (2) compare the change rate for the area of soil and water loss in each soil and water loss classification and in each region, and (3) examine the relationship between change rate and the restored year.

The standards for classification and gradation of soil erosion (SL-190-2007) issued by the Ministry of Water Resources of the People's Republic of China was used to determine the changes in the area of soil and water loss for different classifications. Accordingly, six classifications for the soil and water loss were grouped: < 500, 500–2,500, 2,500–5,000, 5,000–8,000, 8,000–15,000 and >15,000 t km⁻² a⁻¹, which were corresponded to the micro, light, moderate, intensive, severe and violent soil and water loss, respectively (Table S1). Generally, the soil and water loss over the light degree was adopted to assess the intensity of soil and water loss (Zhang et al. 2017). Consequently, the area of micro soil and water loss was not considered in this study.

A Shapiro-Wilk test was used to determine the normality of variables. The log₁₀-transformed was conducted when the data were not normally distributed. One-way analysis of variance (ANOVA) with Tukey's (or Tamhane's when equal variances not assumed) post hoc multiple comparison test was adopted to examine the differences in the CR in 2016, 2018 and 2019, compared to 2012 among three study regions, and those differences under each classification among three study regions, respectively. The Levene's test was used to determine the homogeneity of variances. Simple regression analysis was used to coordinate the relationships between the CR and the year under each classification in each region. A significant level of P < 0.05 was specified in this study. All the mentioned statistical analyses were conducted using SPSS 24.0 (IBM, Armonk, New York, USA).

The area of soil and water loss decreased from 9,465.91 km² to 7,355.36 km² in the UR, 15,359.59 km² to 12,482.49 km² in the NER, and 6,537.89 km² to 5,606.72 km² in the SER from 2012 to 2019, respectively (Table 2). The area of soil and water loss in the NER almost accounted for 50% of the total area of soil and water loss in Chongqing City, followed by the UR and SER.

The area of soil and water loss had a decreasing trend accompanying by the acceleration of urbanization in Chongqing City during 2012–2019, suggesting the successful ecological cultivation construction in the recent years. Similar to that, our results showed the area of soil and water loss in the three regions in Chongqing City presented a decreasing trend from 2012 to 2019, respectively, indicating the effective control of soil and water loss in the regional scale. The area of soil and water loss in the NER showed the highest area of soil and water loss relative to the UR and SER. This was attributed to the local topography with steep slope gradients and high altitudes and the agricultural activities, which collectively resulted in the soil and water loss hotspots in the NER (Sastre et al. 2017; Xiao & Xiao 2018). The NER is involved in the central area of the Three Gorges Reservoir and the soil and water loss is easily induced by different drivers, such as rainfall and gravity, leading to the wide distribution of soil and water loss and the fragile soil ecosystems (Zhao et al. 2015; Zhang et al. 2017). Additionally, the traditional agricultural activities such as conventional tillage were largely responsible for the high vulnerability in soil and water loss in the NER (Sun et al. 2010; Marques et al. 2015; Mekonnen et al. 2015). Thus, controlling for the soil and water loss in the NER would have great contribution to the soil and water conservation in Chongqing City (Xiao & Xiao 2018). Sun et al. (2010) found that the area of soil and water loss in the NER was 29,178.54 km² in 1999, and reduced by 45.81% in 2005 (15,811.06 km²). In this study, the area of soil and water loss decreased from 15,359.59 km² in 2012 to 12,482.49 km² in 2019, and the decreasing amplitude was almost 2.4 times smaller than that from 1999 to 2005. This reflected great potential in the reduction of soil and water loss in the future.

The OMD had a decreasing trend from 2012 to 2019. The soil and water loss over moderate degree of Zhongxian, Yunyang, Wanzhou, Kaizhou counties in the NER and Shizhu, Wulong counties in the SER were higher than the average in Chongqing City in 2012 and 2016, respectively, due largely to the severe soil and water loss surrounding the Three Gorges Reservoir (Sun et al. 2010). This result was in partial agreement with the findings of Li et al. (2009) who found that the Kaizhou, Yunyang, Fengjie, Wuxi, Wushan and Wanzhou counties in the NER were the most serious areas of soil and water loss in 1999 and 2004. The soil and water loss OMD of 17 counties in 2018 and 16 counties in 2019 counties for the UR was both higher than the average of Chongqing City, which was almost 2 times than that in 2016 (8 counties), indicating the increasing control of soil and water loss in the UR even though the acceleration of urbanization. This was also supported by the decreased counties in the NER and SER that had higher soil and water loss over moderate degree than the average of Chongqing City. The soil and water loss over moderate degree was converted significantly to the micro and light classifications in different regions from 2012 to 2019. The change rates for the area of moderate and intensive soil and water loss were the highest in the SER in 2016, 2018 and 2019, while the change rates for the area of severe and violent soil and water loss in the UR and NER were the highest in 2018 and 2019, respectively. The ecological restoration and Grain-for-Green Project contributed pronouncedly to the reduction in the area of over moderate soil and water loss (Li et al. 2009; Zuo & Gao 2022). Also, the urban soil and water conservation, e.g. temporal fence and film covering for construction site, are responsible for the decreasing soil and water loss (Zhou et al. 2006; Tian et al. 2020).

The CR was the highest in the UR for the pairwise comparative years, while the difference in CR in each study year was not significantly different among three regions, even though their local society, economy and ecology are radically different (He et al. 2017). Generally, the rapid urbanization in the UR, which is the populated area, may cause extensive expansion of disturbances to the native landscape and inevitably increase the area of soil and water loss in a short time due to the adoption of irrational strategies of soil and water conservation (Tian et al. 2020). However, Kong et al. (2018) suggested that urbanization could in turn reduce the area of soil and water loss via driving migration of rural labour to cities since the migration had large potential to alleviate the negative effects of human activities on the effectiveness of ecological restoration through decreasing the disturbances to soils from farming and increasing the forestry area. This was also supported by our findings showing the distinct variations on CR in each classification of each study year among UR, NER and SER. Compared to SER, the UR and NER showed relatively lower CR in the moderate degree in 2018 and 2019, but higher CR in the severe and violent degrees in 2016. This indicated that the extent of changed soil and water loss was markedly affected by the urbanization in the UR (Li et al. 2017b) and the soil conservation strategies (e.g. terrace construction) implemented recently in some districts of the NER (Strehmel et al. 2016). In the SER, the chronic rocky desertification threatens the efforts of soil and water conservation and results in subtle changes from high degree of soil and water loss to the low degree (Li et al. 2021; Zuo & Gao 2022), and therefore more strategies for reducing soil and water loss in the SER urgently need to be enhanced in the near future. The CR showed significant relationship with year in all three regions, suggesting that the soil and water conservation strategies conducted in different regions were effective to control its own local soil and water loss. This was also favored by the gradually increased soil conservation capacity from 2000 to 2015 in the most districts in Chongqing City (Kong et al. 2018). Tian et al. (2020) reported that the reduced area of soil and water loss and increased soil conservation capacity were mainly resulted from the increase in the vegetation cover from 1999 to 2015, which benefited from the continuing ecological restoration and Grain-for-Green project. Overall, our result indicated the consistent effects of government policies on the regional control of soil and water loss.

The development of soil and water conservation strategies were different due to the differences in the local population, environment, finance, agriculture, public works and so on (Graaff et al. 2013). It is necessary to devise rational management strategies controlling soil and water loss to achieve the ecological sustainability (Cerdà et al. 2018; Tan et al. 2020) according to the specific-site characteristics of soil and water loss in these regions. This will have great significance for consolidating the efforts of ecological restoration and Grain-for-Green project, and also for promoting the ECC in Chongqing City for the near future. He et al. (2017) concluded the potential strategies of soil and water conservation in terms of the roles of three regions in the development of Chongqing City, and emphasized that the principle of adaptation to local conditions was crucial to make the rational strategies. According to our results, the core ways for the specific three regions are given: For the UR, the rapid development of urbanization will launch more infrastructure construction projects, which predictably disturbs the landscape and increases the area of soil and water loss, and thus, the combination of severe supervision implemented by the government and the positive soil and water conservation strategies adopted by the construction organization will be taken to control urban soil and water loss and accelerate the construction of sponge city (Li et al. 2017a). For the NER, due to its sensitive natural topography and traditional agricultural activities in the surrounding of the Three Gorges Reservoir Area, the enhanced afforestation of steep sloping cropland and conservation agricultural managements such as terrace construction, contour ploughing and minimal tillage are still needed to be enhanced (Kassam et al. 2009, 2014; Strehmel et al. 2016). While, as the important ecological protection zone in the SER, further combination of ecological restoration and agroforestry, such as developing economic forests and intercropping systems (Huang et al. 2008; Liu et al. 2021), are expected to be conducive to controlling the soil and water loss and developing the local economy.

The area of soil and water loss decreased in the UR, NER and SER from 2012 to 2019, therein, the area of soil and water loss in NER mostly accounted for 50% of the total area of soil and water loss in Chongqing City. The OMD decreased from 2012 to 2019 across these three regions. The CR was markedly differed by those three regions in different classifications of soil and water loss, and declined significantly with the year in each region, suggesting the effectiveness of adopted soil and water conservation strategies in different regions on controlling its own local soil and water loss. The optimal combinations of strategies of soil and water conservation such as severe supervision, enhanced ecological restoration and sustainable agricultural managements, are required to achieve the maximum decrease in soil and water loss in the distinct three regions. Our results will provide valuable information for developing reasonable conservation strategies in Chongqing City and others with the similar natural environment and human activities.

This study was funded by the CRSRI Open Research Program (CKWV20221005/KY) and the Basic Scientific Research Service Funds of Central Level Public Welfare Research Institutes, China (CKSF2019413/CQ).

The authors declare that they have no conflict of interest.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.