A simplified water balance model in conjunction with an evapotranspiration(ET) model and cumulative forest cover data were used to quantify the changes in annual water yield in response to reforestation in a large watershed, northeast China. Cumulative forest cover increased by 22 %, leading to a significant decrease in estimated annual water yield. Reforestation increased ET(P = 0.0144), resulting in a remarkable decrease(P = 0.0001) in estimated annual water yield according to the water balance model. Reforestation increased ET by33 mm and decreased annual water yield by 38 mm per decade. The effect of reforestation on annual water yield can be quantified using a simplified water balance model in a large watershed, although our reforestation area was small(about 20 %) in relation to the total watershed area. A simplified water balance model in conjunction with an evapotranspiration (ET) model and cumulative forest cover data were used to quantify the changes in annual water yield in response to reforestation in a large watershed, northeast China. Cumulative forest cover increased by 22%, leading Reforestation increased ET (P = 0.0144), resulting in a remarkable decrease (P = 0.0001) in estimated annual water yield according to the water balance model. Reforestation increased ET by 33 mm and decreased annual water yield by 38 mm per decade. The effect of reforestation on annual water yield can be quantified using a simplified water balance model in a large watershed, though our reforestation area was small (about 20%) in relation to the total watershed area.