A simplified mathematical model of the water transport in the soil-plant-atmosphere continuum system in unsteady state regime (transient and periodic) is built with the help of an electrical analogy which uses the quadripole method. From an electrical diagram of the system, the balance equations are solved. The first part is devoted to the study of unsteady state regime. The solution in the Laplace space leads to the flux and potential evolution for long times evolution (season). The second part treats the steady periodic regime (day) with the help of Fourier analysis. A satisfactory overall solution can be obtained by superposing both solutions.