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A water budget model is proposed to estimate the infiltration, runoff, evapotranspiration and recharge in vadose zone and apply to a case study. The instantaneous redistribution of infiltrated water is assumed to be uniform and a linear relationship between evapotranspiration and effective saturation is imposed. Infiltration is described by Philip's solution in conjunction with the time compression approximation method during rainfall. Runoff occurred when rainfall rate exceeds soil-infiltrating rate. The soil profile drainage was determined by evapotranspiration and recharge. Cumulative infiltration, runoff, evapotranspiration and recharge are estimated with different climate conditions and different soil hydraulic properties during simulating period. Analysis shows that initial effective saturation affects the estimated results in this water budget model in the short or mid-term simulations while not in long-term simulations. The climatic conditions of Yun-Lin plain area, Taiwan from 1991 to 1997 are used by referring to hydrological and hydrogeological parameters to provide the computational procedures of this study for estimating recharge. Results showed that the amount of annual recharge was affected by the amount of annual rainfall and soil properties. Nomenclature A = Parameter related to sorptivity [L 1/2 ] a = Infiltration constant [−] D = Diffusivity [L 2 T] d r = Depth of water reservoir [L] E = Evapotranspiration flux [L T −1 ] E p = Potential evapotranspiration rate [L T −1 ] i = Infiltration rate [L T −1 ] k = Unsaturated hydraulic conductivity [L T −1 ] k s = Saturated hydraulic conductivity [L T −1 ] P = Rainfall intensity [L T −1 ] P cum (t) = Cumulative rainfall [L] S = Sorptivity [L T −1/2 ] t = time [T] t e = Equivalent time to ponding [T] t p = Ponding time [T] 484 J.-F. CHEN ET AL. q = Unsaturated water flux [L T −1 ] q cum ie (t) = Amount of infiltration excess runoff [L T −1 ] q cum se (t) = Amount of saturation excess runoff [L T −1 ] q r = Recharge flux [L T −1 ] φ = Effective saturation [−] φ 0 = Initial effective saturation [−] λ = Pore size distribution index [−] θ = Volumetric water content [−] θ 0 = Initial volumetric water content [−] θ r = Residual water content [−] θ s = Saturated water content [−] h = Soil pressure head (<0 if soil unsaturated) [L] h s = Air entry value [L] h 0 = Initial soil pressure head [L]

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