Today, proper and sustainable use of water and soil resources is on the agenda of executive agencies. Therefore, optimum design of drainage systems is very important. Effective porosity and soil hydraulic conductivity coefficients should be determined with acceptable precision. In addition, it is essential to evaluate the drainage equation for the unsteady subsurface under farm conditions. This research was carried out in subsurface drainage system in Shadegan Plain. The effective porosity and soil hydraulic conductivity were determined by measuring the time, the height of water table, and the discharge outlet from the lateral drain. Moreover, through inverse solving, the soil effective porosity and hydraulic conductivity in Shadegan Plain were estimated. The mean effective porosity calculated using Taylor method was 0.0117 (dimensionless) and the mean soil hydraulic conductivity calculated by Skaggs method was 0.31 m/d. In following, using measured data of time and height of water table and also prevalent unsteady subsurface equations, the drain intervals were estimated. The calculated distance was analyzed with 50 (m) measured distance. The evaluations showed that the Glover, Dumm, Van Schilfgard, Bouwer and Van Schilfgard, modified Glover and Hammad equations have priority repectively. In using unsteady drain equations, it should be noted that it is not possible to prioritize specific equation. On the other hand, for each region, these equations must be evaluated individually and the best equation must be selected.