泾河流域上游土石山区和黄土区森林覆盖率变化的水文影响模拟

泾河流域上游是黄土高原的重要水源地和退耕还林工程区,在较大空间尺度上定量评价区内森林覆盖增加的水文影响对科学指导林业生态环境建设、保障区域水安全和可持续发展均有重要意义。为了在尽量排除地形、土壤、气候等作用的基础上定量评价森林的影响,将泾河上游划分为土石山区和黄土区,分别制定了多种森林恢复情景,利用分布式流域生态水文模型(SWIM)模拟评价了森林覆盖率及其空间分布变化对流域年蒸散量、年产流量、年地下水补给量、年土壤深层渗漏量及日径流洪峰的影响。土石山区模拟结果表明,增加森林覆盖将增加流域蒸散和减少流域产流,如现有森林覆盖(占全流域面积比例为13.8%)情景与现有森林变为草地(占全流域面积比例为0)情景相比时,流域年蒸散量从445.4 mm变为427.7 mm(减少了17.4 mm和4%);年产流量从42.4 mm变为53.5 mm(增加了11.1 mm和26.3%),年地下水补给量从61.6 mm变为76.9 mm(增加了15.3 mm和24.8%),年深层渗漏量从72.9 mm变为88.3 mm(增加了17.7 mm和24.3%);平均森林覆盖率每增加10%,导致流域年蒸散量增加12.8 mm,年产流量减少8.0 mm,年地下水补给量减少11.1 mm。在比较干旱和土层深厚的黄土区,增加森林覆盖将同样增大流域蒸散和减少流域产流,但变化幅度明显小于降水相对丰富和土层浅薄的土石山区,平均森林面积增加10%导致流域年蒸散量增加9.0 mm,年产流量减少4.5 mm,年地下水补给量减少8.8 mm;此外,在较缓坡面造林的水文影响大于较陡坡面造林。从森林水文影响的年内变化来看时,森林覆盖率升高的水文影响在土石山区和黄土区也有差别,如土石山区5—7月份的蒸散显著增加,5—10月份的深层渗漏均有减少;而黄土区是蒸散量在5—10月均有增加,深层渗漏在7—10月份显著减少。另外,土石山区森林覆盖率增加对日径流峰值的影响不显著,而黄土区则能明显削弱,这可能主要是因土石山区的高石砾含量土壤的渗透性能明显高于黄土区的黄土,而黄土区的森林能够明显改善土壤入渗性能和减少地面径流形成。

A simulation study on the hydrological impacts of varying forest cover in the stony mountain area and loess area of the upper reaches of Jinghe Basin

The upper reach of Jinghe Basin is one of the main water-head areas and key afforestation areas in the Loess Plateau of China. A relatively large scaled evaluation of the forest hydrological impacts in this region is important for guiding the rational ecological afforestation, ensuring the regional water supply safety and sustainable development. In order to possibly remove the disturbances from topography, soil and climate, the upper reach of Jinghe Basin was divided into the stony mountain area and the loess area, and several scenarios were set up in each area. The distributed and eco-hydrological watershed model of SWIM was calibrated and validated by using the meteorological and hydrological data measured in 1997-1999 and 2000-2003 respectively, and then was used to simulate the impacts of varying forest cover and its spatial distribution on the annual evapotranspiration, water yield, groundwater recharge, deep soil percolation, and the peak daily runoff. The simulated results in the stony mountain area showed that increasing forest cover will increase the basin evapotranspiration and reduce the water yield. For example, when compared the current forest/vegetation scenario (13.8% of the whole basin) with the scenario of changing all forests into grassland (0% of the whole basin), the basin annual evapotanspiration is changed from 445.4 mm to 427.7 mm (a decrease of 17.4 mm and 4%), the annual water yield is changed from 42.4 mm to 53.5 mm (an increase of 11.1 mm and 26.3%), the annual groundwater recharge is changed from 61.6 mm to 76.9 mm (an increase of 15.3 mm and 24.8%), the deep soil percolation is changed from 72.9 mm to 88.3 mm (an increase of 17.7 mm and 24.3%). In average, an increase of forest cover of 10% will lead to an increase of basin annual evapotranspiration of 12.8 mm, a reduction of annual water yield of 8.0 mm, and a reduction of annual groundwater recharge of 11.1 mm in the stony mountain area. In the relatively dryer loess region with deep soil, the forest cover increase will also increase the basin evapotranspiration and decrease the water yield, but in an obviously smaller variation range compared with the stony mountain area where the annual precipitation is higher and the soil layer is thinner. In average, the basin annual evapotranspiration is increased by 9.0 mm, the annul water yield is decreased by 4.5 mm, and the annual groundwater recharge is reduced by 8.8 mm, when the basin forest cover is increased by 10%. In addition, the hydrological impacts caused by afforestation on more gentle slopes are stronger than those caused by afforestation on steeper slopes. When comparing the monthly distribution pattern of forest hydrological impacts, there is also a clear difference between the stony mountain area and the loess area. For example, the significant increase of evapotranspiration is found in the period from May to July, and the decrease of deep soil percolation is found in the period from May to October in the stony mountain area; while in the loess area it is in the periods of May-October and July-October respectively. In addition, the impact of varying forest cover on the peak daily runoff is not significant in the stony mountain area, probably because of the high infiltration ability of the soil with high stone fragment content; but significant (decrease) in the loess area, probably mainly because of the obviously increased rainwater infiltration into soil after afforestation and then the reduced surface runoff generation.