Abstract
At present the large scale vegetation restoration and intensive oil exploiting had brought huge influence on local environment in Yan'an region. the sediment yield data form series experiment plots and hydrological monitoring station in the Yan'an watershed after one rainfall event on July 2, 2005, which included sediment from different land uses (crop-land plot, vegetation plots, hard road surface) and 3 types roads(mountain-road brunches, mountain-road, and mountain-transport way) has been analyzed. Results showed that the erosion intensity of the 3 type roads was respectively 500 t/km2, 3163 t/km2, and 13500 t/km2. The sediment from cropland and grass, shrub land was within 6-184 t/km2. It stated that sediment from road area which only covered 1% of total area accounted for 42.3% of the total sediment yield, far beyond that from other uses of land. Sediment from grass-land and shrub-land, which covered 70.5% of watershed area, shared 26.7% of the total sediment. The further analysis showed that the 41.2% of total sediment could be detained by re-vegetation. On the contrary, that road constructing brought heavy sediment which offset the benefit of vegetation restore by 58.4%. the suggestion were to adjust our strategy from slope management to the road erosion mitigationMany studies have confirmed it is an important measure to return the steep slope farming land to green, and to restore vegetation in line with local conditions to prevent soil erosion in the Loess Plateau [1, 2]. To implement the measure in western region, the researches on "Grain for Green", returning farmland to forest and grassland, has become popular [3, 4]. Many scholars studied the effects of farming land and trees and grass land on soil water storage [2, 5].Tang Keli stated that the slope land for farming was the main source of the sediment in Yellow river, and the maximum gradient slope for farming land use was 25° [6]. Many authorities not only pointed out that a far-reaching influence of land-use changes on the distribution of sediment source area but also put forward some new ideas about returning farming to green in Loess Plateau [7, 8].However, we were still not sure the contribution of returning farmland to forest and grassland on reducing sediment yield of the valley and known it was difficult to identify its contribution to the total sediment yield.Analysis on the contribution of the stream channel and slope sediment yield had some results already [9, 10]. It was still too early to make clear the relationship between the sediment sources changes of the valley and the management.At present, Ecological restoration in the Loess Plateau caused the sediment form the slope land declining [3]. Due to human economic activities, the mountain road developed rapidly, it is inevitable that road erosion has been intensified [11]. A. Rijsdijk and LA Bruijnzeel (1991), [12] based the valley Konto observation, pointed out that although the rural road in the area accounts for only 3% of the area, but the impact on the sediment of this area was tremendous. Nyssen J, Moneryersons J. et al (2002) [13] also think that road without protection is one of the main sources of sediment. Many kinds of protective measures have great importance to the road erosion control. So attentions were paid to the study on the protection all kind of roads.Then what will happen to the soil erosion of the watershed, driven by the vegetation restoration and new road construction? What will happen to the proportion of sediment quality from slope land, road area and gully?A correct understanding of the sediment sources pattern of the typical watershed is of great significance on assessment the roles of vegetation to slope management and the roles of prevention the linear path erosion.
Highlights
The study was conducted in Yan’gou valley of the southern, Yan’an, a hilly and gully region in the Loess Plateau
Returning farmland to green and mountain roads construction have made road erosion become the dominant factor of water loss and soil erosion in the loess hills area
In an once in every 5-year frequency rainstorm event, sediment coming from road erosion accounted for 42.3% of the total sediment of the watershed, while the forest and grass mixed land which covered 70.5% of the entire watershed only contributed 26.7% of the total
Summary
The study was conducted in Yan’gou valley of the southern, Yan’an, a hilly and gully region in the Loess Plateau. The rainfall from June to September accounted for 75% of the whole year. The valley covers 46.88 km with a gully density of 4.8 km/km, and main stream gradient of 2.41 ‰. The ecological environment has changed dramatically due to the intensive returning farmland to forest and grassland, and by the end of 2000 year, 70% of the area has been covered by forests and grasses. The most erosion-prone slope of arable land in 1997 was 1617.6 hm, and the number had been reduced to 130.7 hm in 2003. The rate of farmland reforestation had been up to 91.9%, and soil erosion intensity reduced to about 1000 t/km2.a [3]
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