[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 19972003 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.