Soil erosion and surface runoff on different vegetation covers and slope gradients: A field experiment in Southern Shaanxi Province, China
Hany El Kateba, Haifeng Zhanga, , , Pingcang Zhangb, Reinhard Mosandla
a Lehrstuhl für Waldbau, Technische Universität München, Germany
b Changjiang River Scientific Research Institute, China
Received 16 August 2012, Revised 28 November 2012, Accepted 29 December 2012, Available online 31 January 2013
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doi:10.1016/j.catena.2012.12.012
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Abstract
The southern of the Shaanxi Province in central China is a region of great magnitude for water conservation. Long term anthropogenic interference in terms of deforestation and inappropriate land use has dramatically accelerated soil erosion in this region. A field experiment in the Shangnan County using 33 small erosion plots of 7 m2 in size was carried out to determine and compare the soil loss and surface runoff from five vegetation covers and three levels of slope gradient (> 10°–≤ 20°, > 20°–≤ 30°, and > 30°). The five vegetation covers embraced the most frequent rural land-use forms in the study area: farmlands including horticulture (tea plantation with peanut as an intercrop) and agriculture (maize in a winter-wheat–summer-maize rotation) activities, grasslands that have developed on abandoned farmlands, and forestlands including low and high forests (Chinese cork-oak coppices and pine plantations, respectively). The change in the runoff among the vegetation covers and slope gradients was high but not as significantly pronounced as for the change in the soil loss. Results showed that the slope gradient has an impact on the runoff and soil loss: the greater the slope gradient the higher the potential for runoff and soil loss. In addition, results exhibited that the rate of erosion is substantially affected by changes in vegetation cover. Farmlands generated the highest runoff and soil loss, whereas the tea plantations at slopes > 30° were most susceptible to erosion. Grasslands had less runoff and soil loss than farmlands. Forestlands provided evidence for their suitability for soil and water conservation in the study area, as negligible soil-losses in comparison to the other vegetation covers were generated.
Highlights
► A field experiment using small erosion plots was conducted in the Shaanxi Province. ► Five vegetation covers and three levels of slope gradient were investigated. ► Soil loss and surface runoff were determined and compared at different rainfalls. ► Erosion depends on vegetation covers, slope gradients, and rainfall intensities. ► Forestlands are most suitable for soil and water conservation in the study area.
Keywords
The Qinling mountains; Soil erosion; Surface runoff; Field erosion plots; Farmland; Forestland
1. Introduction
According to the Chinese Ministry of Environment Protection and Chinese Academy of Science (MEP and CAS, 2008), the mountainous area of southern Shaanxi Province is considered as an important ecological region in central China. Qinling Mountain in this region is a natural physical boundary that subdivides the territory into different climatic zones. The northern part of the Qinling Mountain is a warm temperate region, while the southern has a humid sub-tropical climate. Being a transitional area, southern Shaanxi Province is characterised by diverse and abundant flora and fauna, and, hence, is a hot spot for biodiversity and natural resources. However, the natural vegetation in this region has been strongly degraded in particular over the last century (Jia et al., 2002). Forestlands were converted to agriculture or horticulture lands. In addition, excessive logging was practiced to satisfy the increasing demand of the growing population for fuel wood. So, over the last five decades, the total forest area declined from 64% to 46% of mostly unmanaged forests of low quality (Li and Li, 2007 and Qin, 2009). Hu and Zhang (2003) estimated the average rate of soil erosion in southern Shaanxi Province at 34 t ha− 1a− 1, which is ten times more than the overall natural soil erosion in the upstream of the Changjiang River (Jing and Zhang, 2007).
It can be said, that the land ecosystems in southern Shaanxi Province are degraded and have lost their natural potential functions, both protective and productive. However, the Qinling mountainous region in southern Shaanxi province has been considered as soil water conservation area since 1981 (Wang, 1981). With the approval of the plan for “South to North Water Diversion Project” by the Chinese central government in the mid 1990s, southern Shaanxi Province became the headstream area for supplying water to the capital city Beijing and its surrounding industrialised regions (Zhu et al., 2008). Therefore, highest priority was given to improve the situation in southern Shaanxi Province, e.g., rehabilitation of degraded land ecosystem including restoration of the natural vegetation covers and optimisation of land use, enhancement of soil and water conservation measures, and improvement of the surface water quality.
Since 1999, the Chinese government has exerted numerous efforts to combat degradation of land ecosystems. These efforts include measures such as banning tillage on steep slope lands, protecting natural forest, and even relocating residents to localities out of the protected areas (Hu, 2004 and Wang and Ma, 2008). To support policy-making processes related to rehabilitation and restoration of degraded land ecosystems, evidence based on scientific research is required. Therefore, a Sino-German research project on the rehabilitation of degraded land ecosystems was conducted in southern Shaanxi Province (El Kateb et al., 2009). The research is an integrative approach, which includes three field experiments on erosion, reforestation, and assessment of forests potential, in addition to a socio-economic survey. The results of these studies will be integrated in a system-analysis component on modelling and development of a decision support system for a sustainable land use planning. The final outcomes of the project will be provided to local and regional administrative authorities as a planning tool to rehabilitate degraded land-ecosystems and, thus, improving surface water-quality.
The present study deals with the erosion research. Its objective is to determine the degree of soil erosion and surface runoff on different vegetation covers and slope gradients.
2. Methodology
2.1. Study area
The field experiment was carried out in Suo Yu He valley, Shangnan County (33°06′–33°44′ N and 110°24′–111°01′ E), which is located in the Qinling mountainous region at the eastern part of southern Shaanxi Province (Fig. 1). The area is representative for the Shangnan County and characterised by hilly topography at low altitudes (350 m to 700 m above sea level). According to the Agricultural Planning Committee of Shangnan County (1984) annual total sunshine is 1900 h, frostless period is 199 to 230 days, annual temperature ranges between 12.7 °C and 15.0 °C, and the dominated soil type in the study area is slight acidic, yellow-brown soil developed from granitic gneiss. The study area is at the edge of the subtropical humid climatic zone with 700 mm to 850 mm annual precipitation, which mainly falls during June to September, mostly as storm with high intensity. According to Zhang et al. (2000) the rainy season in the study area lasts from May to October. The summer precipitation in the mountainous region in southern Shaanxi Province is mainly convective rainfall (Bi et al., 2006).
Fig. 1.
Location of the study area.
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The landscape in the study area is highly fragmented as a result of the practiced traditional land-use over many decades. The major vegetation covers are forestlands, farmlands (agriculture and horticulture), and abandoned farmlands developed to grasslands. All vegetation types, except the forestlands which reach few hectares in size, have very small area size, which mostly does not exceed a quarter hectare.
2.2. Layout of the experiment
The study deals with the dominating vegetation covers and slope gradients in the study area. Table 1 shows the investigated levels of the study factors, for both the vegetation cover and slope gradient. The experiment was carried out on the dominating yellow-brown soil (a sandy-loam soil with 60, 25 and 15% sand, silt and clay, respectively) in the study area. Based on land use and contour maps, a map was constructed to facilitate the identification of the location of the different vegetation covers and their slope gradient. In the field, a re-check of the location and slope gradient of the identified vegetation covers were made. Only those vegetation covers which had had uniform slope-form were determined in the map. From the combinations of the vegetation cover and slope gradient, a selection of the experimental units was made at random. As the dominant slope class was the moderate, three experimental units from each vegetation cover level were selected at random. For the other two slope classes (slight and steep), only two units were randomly selected from each level of the vegetation cover (Fig. 2). However, as agriculture activities had been ceased at steep slopes, agriculture lands with slopes exceeding 30° could not be found. In total, 33 experimental units were selected. The underlying experimental design is a Completely Randomised Factorial Design with unequal number of replicates.
Table 1.
Description of the levels of the study factors.
Study factor Description
Vegetation cover
Agriculture land (summer maize) Farmlands with winter wheat and maize during summer. The vegetation coverage was 80% during the survey.
Horticulture land (tea plantation) Farmlands under tea cultivation in rows having a distance of 40 cm between each two rows. The age of the tea plantations varied between 8 and 10 years. Under the tea, peanut was cultivated as