Water resources have been influenced by the uneven distribution of rainfall temporally and spatially.How much quantity of water resources can be used for human life has aroused the social at-tention.The concept of usable quantity of water resources and its meaning as well as the influence factors can not be neglected while the calculation method is sought for.It will be of great significance to develop a feasible estimation method with regard to the use of water resources,regional project planning and economic development planning etc.Based on the analysis for the degree and status for water resources,the methods under different condition have been developed.