生态恢复对流域水沙演变趋势的影响——以北洛河上游为例

黄土高原生态环境恶劣,水土流失严重。选择退耕还林(草)程度非常显著的陕北吴旗县所在北洛河上游为研究区,探讨黄土高原丘陵沟壑区水土流失治理及大幅度退耕背景下流域水、沙等生态要素的演变规律,分析人类活动的影响贡献程度,为黄土高原生态治理及环境效应分析提供理论依据。结果表明:在1963—2009年期间年降雨量没有显著变化背景下,同时期流域年径流量和年输沙量均呈现极显著减少趋势,年均减少率分别为0.28 mm/a和180 t km-2a-1,其突变时间均发生在1979和2002年,具有很好的同步性。与1979年前相比,20世纪70—80年代水土流失综合治理以及1999年后退耕还林(草)的事件背景,使汛期和平水期径流量逐时段减少,而枯水期径流量反而持续增加。输沙量呈持续性大幅度减少态势,且其减少程度远大于径流量的变化程度。水土流失综合治理和退耕还林(草)工程实施等人类活动,对流域径流量减少的影响贡献程度分别为38.2%和51.4%,对输沙量减少的影响程度分别为74.7%和86.7%。研究结果提示,黄土高原生态环境的大幅度改善在区域尺度上已经表现出一定程度上削洪补枯的水文效应特征,以及林草措施减水更减沙的良好生物治理效果。

Hydrologic responses to vegetation restoration and their driving forces in a catchment in the Loess hilly-gully area:a case study in the upper Beiluo River

The Loess Plateau in China is characterized by a heavily dissected landscape and severe soil erosion. To fully understand the impacts of soil conservation measures and vegetation restoration on streamflow, sediment load and runoff-sediment behaviors, hydrologic data over 47 years (1963 to 2009) in the upper reaches of the Beiluo River (drainage area of 3408 km²) are assessed. The Beiluo is a secondary tributary of the Yellow River, and has had substantial vegetation restorative effects on the Loess Plateau. Based on the Mann-Kendall trend test, Pettit statistical test and flow duration curve (FDC), the daily streamflow and sediment load at Wuqi hydrological station, which is the maximum control station in the upper reaches of the Beiluo River, was chosen to investigate temporal change and evolution characteristics. Precipitation data and rainfall erosion force were interpolated using tessellation polygons from five representative meteorological stations on the Loess Plateau. A dual mass curve and empirically-based hydrological modeling method were also used to estimate the response of streamflow and sediment load to climate change and human activities. The results indicate that: (1) In the past 50 years, under a background of precipitation with no significant trend, both streamflow and sediment load had significant negative trends (P<0.01), with average annual rates of -0.28 mm km^-2 a^-1 and -180 t km^-2 a^-1, respectively. Under the influence of soil conservation measures that have been applied on the Loess Plateau since the 1960s, there were change points in 1979 and 2002. Thus, the entire study period was divided into a period 1 of base (1963-1979), period 2 of comprehensive harness of soil and water conservation (1980-2002), and period 3 of the Grain for Green project (2003-2009). (2) The FDC showed that, compared with period 1, streamflow in period 2 for 5% and 50% decreased by 17.0% and 5.2%, respectively, while it increased by 94.2% for 95%. Suspended sediment load decreased more significantly, with 34.6% and 40.1% for 5% and 50%, respectively, and no suspended sediment days remained almost constant for 95%. Period 3 saw more remarkable changes in both streamflow and suspended sediment load. For 5% and 50%, streamflow decreased by 40.2% and 26.8%, respectively, while suspended sediment load decreased by 95.7% and 96.8%, respectively. For 95%, streamflow rose by 128.1%, and no sediment days increased. The reduction of sediment load was greater than that of streamflow, with the periods altered and negative changes of sediment load. (3) Compared with period 1, streamflow and sediment load decreased by 0.10×10⁸ m³(3 mm) and 0.04×10⁸ t (1408 t km^-2 a^-1) in period 2. It was estimated that human activities accounted for 38.2% and 51.4% of the changes of streamflow and sediment load, respectively. In period 3, there was a greater reduction in both streamflow and sediment load, with 0.52×10⁸ m³(15.2 mm) and 0.33×10⁸ t (9683.1t km^-2 a^-1), respectively, with effects reaching 74.7% and 86.7%. Great improvement in the ecological environment of the Loess Plateau has already demonstrated the advantages of biological treatments on a regional scale in reducing flood volumes, and compensating streamflow.