Effects of land use on slope runoff and soil loss in the Loess Plateau of China: A meta-analysis
Graphical abstract
Introduction
As important processes leading to soil erosion, soil loss and runoff changes, induced by inappropriate land use, are among the most severe global environmental issues (Fu et al. 2011; Nyssen et al. 2015; Robinson et al. 2013). A comprehensive approach to these problems at a regional scale needs to involve the representative environmental conditions for the assessment of soil loss and runoff. Furthermore, through the rational allocation of land use modes, the soil structure can be improved, so do the erosion resistance and the scour resistance of the soil (Fu et al. 2011; Maetens et al. 2012; Xin et al. 2008). For instance, the Grain for Green project was launched in 1999 on the Loess Plateau, with the aims of controlling soil erosion and mitigating land degradation (Wang et al. 2007). These ecological restoration measures promoted the transformation of land type in the region, reducing the sediment discharge in the Loess Plateau by 90% (Deng et al. 2012; Vina et al. 2016; Wang et al. 2008). However, in order to gain insight into these processes and to develop strategies to mitigate their impacts, it is necessary to determine the sediment yield of the different landforms and land-use types. More detailed field measurements are needed to better quantify the soil loss.
Various methods have been employed to obtain field-measured soil loss data. The field runoff experimental plot is regarded as the most common and widely used method (Bagarello and Ferro 2010; Wei 2002). Most studies were based on the continuous observation of multiple runoff plots to analyze the relationship between runoff and sediment yield and obtain the estimation and characterization of soil erosion. A good example was the development of the universal soil loss equation (Laflen and Flanagan 2013; Renard et al. 1997; Wischmeier and Smith 1978). The studies of runoff plots in the Loess Plateau have mostly involved single runoff plots, focusing on the relationship among rainfall and different ecological measures, runoff, and soil loss (Chen et al. 2010; Kang et al. 2001; Liu et al. 2019). These case studies provide a good understanding of the impact of local erosion control factors on soil loss and runoff. However, because of the diversity and variable nature of plot-scale studies, the findings of these independent plot-scale studies are difficult to be extended to regional scales (Bagarello et al. 2012; Labriere et al. 2015; Maetens et al. 2012).
To extrapolate the plot-scale runoff and soil loss data to larger areas, all available data on soil loss and runoff at the regional scale need to be compiled. Such compilation of relevant datasets for the runoff plots has been carried out in the Mediterranean and in European regions for further analysis (Maetens et al. 2012). The compilation of field measurements in Brazil revealed that the relationship between rainfall with soil loss and runoff at the plot-scale was affected by the land-use types and the spatio-temporal patterns of land use coverage (Anache et al. 2017). In China, an extensive dataset was compiled from erosion plot measurements after applying soil and water conservation measures, which, subsequently, was used to assess the efficacy of the measures on reducing soil and water loss (Zhao et al. 2019b). These compilation shed light on the key factors that determine the rate and variation of annual soil loss and runoff on subcontinent and regional scales.
Although there are many descriptive comments and viewpoints on the relationship between land use with soil loss and runoff in the Loess Plateau, the impact assessment of slope erosion was mainly carried out at the plot-scale, which cannot be directly applied up to larger scales due to the limitation of scale effects (e.g. different measurement methods employ, experiments performed over different time periods or insufficient treatment repetitions) (Chen et al. 2018; Eshghizadeh et al. 2016; Nunes et al. 2011). Most studies have only evaluated the overall impact of governance measures on soil erosion or watershed runoff, and lack information on other important control factors on soil loss and runoff by only assessing the effect of a single control factor (e.g. soil and water conservation technique) (Zhang et al. 2010a; Zhao et al. 2013). Although quantitative analysis of the ecological rehabilitation has been carried out in the Loess Plateau (Hu et al. 2017), the impacts of the slope gradient and the plot length of the runoff plot have not been assessed. While Zhao et al. (2019a) recognized the importance of rainfall, the direct relationship between rainfall and land use was not analyzed. Furthermore, no comprehensive compilation of soil loss and runoff data at runoff plot scales exists in the Loess Plateau, hindering the intuitive analysis of the factors of erosion processes and the optimal vegetation type in the Loess Plateau.
Therefore, we integrated runoff plot-scale data to quantify the effect of land-use types on runoff and soil loss via a meta-analysis. The main objectives of this study were: (1) to compile data on the soil loss and runoff rate at the plot-scale in the Loess Plateau; (2) to analyze the relationship between runoff and soil loss rate with different land-use types and their relationships with annual precipitation; and (3) to evaluate the optimal land-use types in different climatic regions in the Loess Plateau.
Section snippets
Literature search and runoff plot selection criteria
The data for the meta-analysis were collected from peer-reviewed journals. English literature was obtained from academic databases such as Web of Science, Science Direct, and Google Scholar, while papers published in Chinese journals were retrieved from the China National Knowledge Infrastructure (CNKI) and Medalink. Keywords were employed during the search processes included “land use”, “runoff”, “erosion”, “soil loss”, “soil and water conservation”, “sediment reduction”, “plot*”, “plot data”,
The effects of plot length and slope gradient on annual runoff and soil loss
The result of correlation coefficient tests for slope gradient and plot length with runoff, annual runoff coefficients, and soil loss were summarized in Table 3. The annual runoff was positively related to the plot length for plots with Chinese pine, artificial grassland, cropland, bare soil, and fallow. A significant negative relationship between annual runoff and plot length was found for plots with natural grassland, sea-buckthorn, mixed forest, and shrubland. Concerning the relationship
Implications of the analyses of the database from the plot area for the Loess Plateau
In the runoff plot database, most of the research was conducted on cropland, which was related to the land use type of the Loess Plateau. The area of the Loess Plateau is approximately 620,000 km2, up to 32.6% of which is cultivated land (Zhou et al. 2016). Meanwhile, more than 73% of the literature considered grassland and bare soil as control groups. The runoff plot data can account for the severity of erosion in the Loess Plateau (Wu et al. 1993). However, of all the runoff plots in the
Conclusion
The ecological restoration of the Loess Plateau increases the vegetation coverage and effectively controls the soil erosion but leads to excessive land use. Because of the inappropriate vegetation restoration mode, large areas of artificially restored vegetation began to degrade, thus the optimization of vegetation allocation has become an urgent necessity. To evaluate the relationship between land use and runoff and erosion at the regional scale of the Loess Plateau, the data sets of all
CRediT authorship contribution statement
Xuexian Zhang: Investigation, Methodology, Writing- Original draft preparation.
Jinxi Song: Supervision, Conceptualization, Project administration, Funding acquisition.
Yirui Wang: Data curation, Software.
Wenjia Deng: Visualization.
Yifan Liu: Validation, Formal analysis.
Declaration of competing interest
We declare that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
Acknowledgements
This study was jointly supported by the National Natural Science Foundation of China (Grant Nos. 51679200 and 51379175), Hundred Talents Project of the Chinese Academy of Sciences (Grant No. A315021406) and Foundation Project of State Key Laboratory of Soil Erosion and Dryland Agriculture in the Loess Plateau (A314021402-2009). We are especially grateful to the Editor, Associate Editor, anonymous reviewers for their helpful comments and suggestions, which have improved the quality of the
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