Abstract: Two typically empirical root water uptake functions proposed by Feddes et al and van Genuchten were tested and evaluated through a numerical simulation to actual water management practice under a full irrigation regime. Simulations, moreover, were performed to test the sensitivity of the two functions to soil moisture situation and distribution of root mass density, and to compare the details of their respective form and function. It is concluded from results above that the Feddes water uptake function tested in this study is shown to have a broad range of sensitivity to fluctuations and changes in both the matric pressure of the soil and the distribution of root mass density along the profile of root zone. Even though the two functions were empirically derived, the Feddes function tends to provide reasonable calculations of the crop transpiration rate and to imitate the natural system better than the van Genuchten function.