Soil,Water,Atrnosphere and plant (SWAP)has been used in this study due to its versatility.lnitially.SWATR a onedimensional model was developed by Faddes et al. (1978)to descride transient water flow in the heterogeneous soil root systern under the influence of groundwater. Subsequently.several researchers improved the model(Belmans et al.,1983Kabat et al.,1992;Van Dam et al.,1997).lt is designed to simulate unsaturated flow ,soil-plant transport,heat flow and crop growth in the soil-plant- atmosphere environment at the field scale.The model applies Richard's equation for soil water flow in the soil matrix described as below