Abstract: If E is the evaporation from soil,E0 the evaporative power and Q the possible maximum amount of water which may be supplied by soil for evaporation during given period,we may consider that ∂E/∂E0 and ∂E/∂Q are some founctions of Q,E0 and E viz ∂E/∂E0=F(Q,E0,E),∂E/∂Q=ФF(Q,E0,E).Assuming E=E1+E2,F(Q,E0,E)=F1(Q,E0,E1)+F2(Q,E0,E2)and Ф(Q,E0,E)=Ф1(Q,E0,E1)+Ф2(Q,E0,E2),we get Eqs.(9)and (10).By applying dimensional analysis and the theory of differential equation we have shown that the forms of Fi and Фi(i=1,2) are Eqs.(15),and hence derived formula (34).The relationship between E/E0 and w computed by formula(34) is shown in figure 2,it is in good agreement with the measurement by Popof-evaporation gauge.