Optimal disc tension infiltrometer estimation techniques for hydraulic properties of soil under different land uses

Different land use, and associated variations in agricultural or silvicultural practices, can cause substantial variations in soils’ hydraulic properties. Thus, the optimal method for measuring these properties may vary, even among neighboring sites. In this study these variations were examined by comparing measurements of hydraulic properties of soil in fields used for poplar, alfalfa and wheat cultivation obtained with two steady flow methods (the White and Sully, and single disc with multiple tension methods) and two transient flow methods (the single disc with single tension, and multiple tensions with multiple disc radius methods). The fields were all located in Changwu municipality, a major grain-producing area in Shaanxi Province, China. Disc tension infiltrometers were used to measure unconfined, unsaturated infiltration over a range of supply pressure heads (h = −9 cm, −6 cm, −3 cm and 0) at the soil surface. Intact soil cores were sampled near the surface to determine bulk density and soil water retention curves at potentials ranging from −0.15 kPa to −100 kPa. Unsaturated hydraulic conductivity values over the range of supply pressure heads were estimated using Wooding’s equation for steady-state flow from a disc source. The van Genuchten water retention model was fitted to experimental data to estimate the parameter values. The results indicated that land use affects soils’ saturated and unsaturated hydraulic conductivity. Further, steady state flow methods are most appropriate for measuring hydraulic properties of soils under poplar and wheat, due to their high organic contents and saturated hydraulic conductivity. However, for soils supporting alfalfa (which are more homogeneous), instant methods provide better results, in addition to substantial time and labor savings.