Comparison of laboratory methods for determining water retention curves in purple soil

Four laboratory methods, namely, the sand box, the HYPROP system, pressure plates and the dewpoint potential meter, were used to determine soil water retention curves (SWRC) for samples of farmland purple soil. SWRC were determined over the whole entire moisture range for samples obtained from two horizons (i. e., 2-7 cm, 7-12cm). Measured data was compared to identify the most reliable method. The root-mean-square errors of the soil water content (E_RMS(θ)), established under low suctions (h>-100 cm), the sand box and HYPROP system were small (0.026-0.082 cm³/cm³) with the coefficient of determination (R²) between measured water retention curves by the two methods being greater than 0.962. Contrastingly, the E_RMS(θ) of soil water content, established under high suctions (h<-330 cm), using pressure plates and the dewpoint potential meter were larger (0.062-0.097 cm³/cm³), with lower R² values (0.775-0.952). In conclusion, the selection and combined use of appropriate methods for SWRC determination, over the entire soil moisture range, should be decided upon according to the soil's pore size distribution and the application objective.