定量评价和预测西北干旱地区森林面积增加的水文影响,为基于水资源承载力的植被恢复与管理提供科学依据.以黄土高原泾河干流上游流域为例,制定森林植被恢复的多种情景,利用率定过的SWIM水文模型和1997-2003年间的气象数据,模拟森林覆盖率变化对蒸散和径流的影响.研究表明:1)草地变森林后,随森林覆盖率增加,年均总蒸散呈明显线性增加,年均径流则呈明显线性减少.在本研究范围内(森林覆盖率增加0～18.13％),森林覆盖率每增加10％平均导致年均总蒸散增加8.93 mm,年均径流减少6.04 mm;2)森林覆盖率变化对年总蒸散的影响具有年际差异,受年降水量影响,流域森林覆盖率增加10％时,对应的年蒸散增量分别为枯水年(10.1 mm)＞平水年(8.6 mm)＞丰水年(8.5 mm),而对应的年径流减少量分别为枯水年(9.0mm)＞平水年(5.6mm)＞丰水年(5.1 mm);3)森林覆盖率增加导致的蒸散增大和径流减少还具有月份差异,其中蒸散在春季(3-5月)和初夏(6月)增大,而夏季中期(7、8月)至秋季中期(9、10月)则减少,在秋末(11月)和冬季(12、1、2月)则变化不明显;径流在春季和夏季明显减少,而秋季中期(9、10月)则呈增加趋势,冬季变化不明显.总之,在该流域增加森林覆盖率会导致年蒸散增加和年径流减少,这种变化在枯水年明显大于平水年和丰水年;森林覆盖率变化对蒸散和径流的影响还存在月份差异,表现为春夏季影响大于秋冬季;增加森林具有一定增加基流和枯水期径流的调节作用.%[Background] Many activities of vegetation restoration have been implemented in the Loess Plateau of northwest China since the 1950s in order to improve the ecological environment.These have led to a significant increase in the forest coverage,which is being and going to be enhanced in the future due to its unique role in active responses to the global climate change.However,a series of recent studies confirmed that the increase of forest coverage caused a significant runoff reduction and concequently endangered the safe and sustainable usage of water resources.Therefore,based on the forest-water relationship a quantitative evaluation of regional hydrology on the impact of afforestation is urgent and benificial to the vegetation restoration and watershed management.[Methods] Collecting the measured meteorological data during the period of 1997-2003 in the upper reaches of Jinghe River in the Loess Plateau of northwest China,we used the calibrated eco-hydrological model of SWIM to evaluate the hydrological impacts of forest coverage change on the the evapotranspiration and runoff.The evaluation was based on simulation results from many scenarios of forest/vegetation restoration.[Results] 1) Under the scenarios of converting grassland to forestland,the yearly evapotranspiration increased and runoff decreased linearly with the rising forest coverage.Within the studied period of forest coverage increase (0-18.13％),the changing rate corresponding to an increase of forest area of 10％ of watershed area was 8.93 mm increase for the yearly evapotranspiration,but a 6.04 mm decrease of the yearly runoff.2) The impact on yearly evapotranspiration by forest coverage change showed a yearly difference because of the difference in annual precipitation.A 10％ increase of forest coverage led to an increase of yearly evapotranspiration of 10.1 mm in dry years,8.6 mm in normal years,and 8.5 mm in wet years;but a decrease of yearly runoff of 9.0 mm in dry years,5.6 mm in normal ears,and 5.1 mm in wet years,respectively.3) There was also a monthly difference in the responces of yearly evapotranspiration and water yield to the increase of forest coverage.The evapotranspiration was increased in the Spring (March-May) and the early Summer (June),decreased in the period from the mid Summer (July and August) to the mid Autumn (September and Octomber),and not significally changed in the late Autumn (November) and Winter (December,January and Februry);whereas the runoff decreased in the Spring and Summer,but increased in the mid Autumn (September,Octomber),and not obviously changed in Winter.[Conclusions] In summary,the increase of forest coverage in the studied watershed led to an increase of yearly evapotranspiration and a decrease of yearly runoff.These hydrological impacts were more significant in dry years than in normal years and wet years.The hydological impacts of forest coverage change on the evapotranspiration and runoff diffred among the months,much significantly in the Spring and Summer than in the Autumn and Winter.
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