An improved combined system with constant flow method and additional equipment to scour and quantify the air bubbles is introduced in detail. In order to make the measurement closer to the true value

we can correct the overflow of water by measuring the volume of the air bubbles. The curves of outflow with time are observed from one-step drying and wetting outflow experiment. The parameters which are evaluated using the HYDRUS-1D software are used to determine the drying and wetting soil-water characteristic curve (SWCC) and the hydraulic conductivity function. By comparing the measured data

it is shown that the method can fit the observed curves of outflow with time correctly so as to verity the reliability of the method. The difference of SWCCS between the estimated and measured is small. In addition

comparing with the traditional methods

the method proposed enables experiments to complete in a short period of time. Therefore

the method proposed for determining hydraulic parameters of unsaturated soils by one-step outflow test is feasible.