Abstract: On the basis of analysing various correlation equations of heat and water balance up to now, according to the connection of runoff coefficient a and drought index β and a characteristic parameter θ to reflect the effect of climate and underlying surface, the following equation was proposed, α=(1+β^θ)^1/θ-β, in which f=runoff, r=precipitation, E₀=evaporation capacity. They are all in millimeter per year. The proposed equation in the paper is feasible and has a fairy precision after it was verified by the data from every continent all over the world, the Huanghe River basin, Nei Monggol Autonomous Region, the East China, and Hainan Island in our country.