Abstract: Applying the' black box' concept of modern control theory, a dynamical simulation method of directly determining the water up-take by plants was advanced,to avoid the difficulties in estimating root distributions,while trying to determine water use. And, following the law of iterative addition of linear systems, the very complex SPAC system was broken down into two much simpler systems to make solutions to the problem and the related experiments more convenient and much easier.The simulation method was applied to estimate water use by (peach) trees. A modified water up -take model (function) and its parameter identification were suggested. The validities of the simulation method and the modified water up-take model were verified.