Temporal changes of soil hydraulic properties under different land uses

Soil hydraulic properties impact the hydrologic cycle. Soil hydraulic conductivity (K) and the pore size distribution parameter, Gardner alpha, are important parameters for understanding some aspects of unsaturated soil water flow. They influence infiltration and runoff and the transport of nutrients in soils. K and Gardner alpha vary with time as well as with spatial position. The objective of this study was to identify whether K, Gardner alpha, and the contribution of pore size classes to flow varied through time under natural conditions in an Entisol in Shenmu County, China. Disc infiltration experiments at pressure heads of -15,-6,-3, and 0 cm were performed for four times from May to August in 2005 on four fields with different land uses (bunge needlegrass field, alfalfa field, soybean field, and korshinsk peashrub field). The sandy loam textured soil in all of the fields was classified as Ust-Sandic Entisol. The soybean field was moldboard plowed and seeded about 20 days before the commencement of the first set of infiltration measurements, but the other three fields were not subjected to recent human activities before or during the infiltration measurements. The results showed that K and Gardner alpha under the different land uses generally decreased from May to August, while no significant difference with respect to hydraulic properties was found between different land uses except for K at pressure head of -15 cm. The contribution of each pore fraction to flow also changed significantly with time, i.e., for macropores (>0.5 mm), mesopores1 (0.5-0.25 mm), and mesopores2 (0.25-0.1 mm) the contribution decreased, while it increased for micropores (<0.1 mm). Analysis of variance suggested that K and Gardner alpha were influenced more by a time factor than by land use. Our results will be helpful for describing hydraulic parameters with the aim of modeling soil water flow more accurately. (C) 2009 Elsevier B.V. All rights reserved.