Abstract: The amounts of used water by single dominant trees of Pinus tabulaeformis and of Robinia pseudoacacia,by using as the basis for evaluating the productivity power of available water of forest land,were measured by using ‘the measuring method of single-tree water-cycle balance’.The base amount of the available water of a forest land for stand normal growth and the productivity power index of runoff were set.The runoff coefficient is thought as a decisive factor to affect runoff productivity power.The productivity power of forest land was counted in the Loess Plateau.The results show that the minimums of annual precipitation for stand normal growth of the two species are all 500mm by collecting the runoff on nature slope.There will be only a stand with 0.3 crown densities in the area where the annual precipitation is 400 mm.

Key words: Runoff productivity power, Chinese pine, Black locust, Loess Plateau