Effects of vegetation construction on soil and water conservation in small watershed of purplish soil region, northern Sichuan

Purple soil is thin in depth and its corrosion resistance is poor. Coupled with heavy and concentrated rainfall and severe human disturbance, soil erosion is very serious. In recent decades, especially later 1980s, projects of forestry ecological engineering have been implemented in the area, which has brought remarkable ecological benefit. It is important for watershed manager to quantitatively evaluate soil and water conservation function of vegetation construction. Based on daily rainfall, daily runoff and daily sediment yield during 1985-2001 in Hemingguan watershed II of northern Sichuan, the change trends of runoff and sediment were analyzed by Spearman's correlation analysis. Moreover, the effect of soil and water conservation of vegetation construction was evaluated quantitatively using the methods of double mass curve and flow duration curve. The results showed that the reference period (1985-1990) and the evaluation period (1991-2001) were separated from the hydrological sequences according to the result of double mass curve and the implement time of vegetation construction. Compared with the reference period, annual runoff depth and annual sediment delivery modulus reduced by 34.6% and 89.9% in the evaluation period, whereas annual rainfall only reduced by 8.1%. It was illustrated that vegetation construction, as the main part of human activity, played an important role for runoff and sediment reduction. With forest vegetation restoring, more and more rainfall was intercepted by canopy and litter of forest. Then the intercepted water was translated into vapor by evapotranspiration of forestland or groundwater through infiltration. If annual rainfall remained unchanged, surface runoff turned smaller as evapotranspiration grew up. Simultaneously, with forest vegetation construction, forest vegetation reduced soil erosion with the improvement of root fixation, soil structure and infiltration rate. Terrain roughness was another cause to reduce soil erosion accompanied by surface runoff. The forest vegetation might control surface runoff by lush foliage and huge root system. High flow and normal flow during the evaluation period reduced by 84.2% and 76.3% compared to those during the reference period. Nevertheless, low flow increased by 650.0%. Vegetation construction obviously reduced high flow and increased low flow. The rainfall infiltration capacity of watershed underlying surface was enhanced by the vegetation construction, which increased the rainfall infiltration content and runoff interception, and delayed the formation of runoff. Therefore, high flow turned to smaller during the period of vegetation construction. In drought period, the water stored in soil and groundwater began to be released to feed low flow. For forest watershed, a large amount of water was restored under the surface, which might provide more water than bare watershed. So, in the period of vegetation construction, low flow showed an increasing trend instead of decreasing. Combining double mass curve with separation and discrimination method, the vegetation construction's contribution to the reduction of runoff and sediment was 92.9% and 94.3% during the evaluation period, respectively. In the watershed of 42 hm2, from 1987 to 1991, a total of 53 thousand Alnus cremastogyne Burk., 37 thousand Cupressus funebris Endl., 31 thousand Morus alba L., 800 Citrus reticulata Blanco, 10 hm2 Coriaria nepalensis Wall. and Vitex negundo Linn were planted. Forestland was increased from 9.72 to 20.13 hm2, which made coverage increased by over 1 time. It was seen that vegetation construction has exerted a leading role on the decrement of runoff and sediment. Furthermore, the effect of soil conservation is better than that of water conservation. When reducing the sediment, runoff is also reduced. Therefore, water safety should be taken into account besides controlling soil erosion. A reasonable vegetation construction plan is needed in the future. © 2017, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.