1. Conclusions The states of the surface and root zoon soil moisture are considered as key variables controlling surface water and energy balances. Force-restore method (FRM) is widely used to estimate the soil moisture variations near the surface based on meteorological data for the prediction of climate changes. The applicability of the method seems to be insufficient from a soil physical view. The difficulty to estimate the variations could be partly ascribed to the hardness of accurate evaluation of soil water flux through the bottom of a root zone under various conditions. We investigated the soil water flux during drainage based on numerical solutions and developed approximate solutions. The variation of the flux in a root zone was approximately described by a product of two functions: one is a time function expressing the flux through a deep reference layer where soil moisture substantially remains constant, and the other is a depth function. Matric flux potential, which was convertible into water content, was also approximated in a similar manner. On these processes, we found a stationary flux could be treated separately, and simply be added to an unsteady one even for the soil with nonlinear hydraulic properties (like a Brooks and Corey type). Evapotranspiration influenced the moisture distribution, especially in an upper layer of a root zone. According to the numerical solutions, however, the estimation of gravity drainage using the average water content of the root zone seemed to be rational except in an advanced dry phase, when the flux at the bottom of a root zone was upward. Incorporating the stationary flux, we could expand the application of FRM to more dry area. For further improvement of FRM, precipitation effects in a root zone have to be examined. In addition, the investigation in the thin surface layer are important to predict the moisture variation near the soil surface. Efficient prediction of root zone soil moisture content with improved force-restore method Hideki KIYOSAWA Graduate school of Bioresources, Mie University, Tsu, Japan kiyosawa@bio.mie-u.ac.jp Bibliography Bakker, M., and J.L. Nieber. 2009. Damping of sinusoidal surface flux fluctuations with soil depth. Vadose Zone J. 8:119-126. Kiyosawa, H. 1996. Approximate solutions for the soil water movement during drainage under a high groundwater table condition. Trans. JSIDRE 185, 755-765 (in Japanese). Mathias, S.A., and A.P. Butler. 2006. Linearized Richards’ equation approach to pumping test analysis in compressible aquifers. Water Resour. Res. 42:W06408, doi:10.1029/ 2005WR004680. Montaldo, N., J.D. Albertson, M.Mancini, and G. Kiely. 2001. Robust simulation of root zone soil moisture with assimilation of surface soil moisture. Water Resour. Res. 37, 2889-2900. Abstract Objectives To analyze drainage processes in a vadose zone to clarify the structure of flux profiles and the characteristics of the flux through the bottom of a root zone. To seek a method to estimate the soil water content and the flux at the bottom of a root zone. Installation of stationary flux Approximate solutions for drainage Problem preparation Example (Brooks & Corey type) Drainage rate from a root zone The conventional force-restore method considers the near-surface and total root zone soil layers. The water content of the total root zone  2 evolves according to the water balance of this zone (depth d 2 ), P g : Precipitation E g : Evaporation E t : Transpiration q 2 is the drainage rate from the root zone, and often evaluated using the unit gradient assumption of gravity drainage and the hydraulic conductivity for the average water content (Montaldo, et al. 2001). Using numerical solutions, we found the assumption applicable in many cases with the slight correction of water content, except in an advanced evapotranspiration case when the flux at the bottom of the root zone was upward. The soil water flux for drainage was described as the sum of a steady term and an unsteady one. The unsteady term approximated as the product of a time function which indicates the drainage rate and a function of depth. Incorporating the stationary upward flux, FRM could be applicable to more dry situation. Groundwater level 150cm Totally saturated at t=0 1: Drainage only (D) 2: D+E (Ep = 6mm/day) 3: D+ET (ETp = 6mm/day) 4: D+ET (ETp = 12mm/day) 5: Hydraulic conductivity Using eqs. 5,3, we can estimate the flux and the water content at the bottom of the root zone. The availability of these equations to FRM depends on the fitness of the linearization and the induced parameters. Eqs. 3 and 5 show the independence of the stationary flux and the unsteady one. If the hydraulic conductivity approximates mainly the unsteady one, the total flux q 2 is, q2=qs+K(q2=qs+K( When evapotranspiration is dominant, q 2 become negative and the variation of upward flux could be approximated. Case 3 (D+ET) Drainage rate (cm/s)