Using the simplified falling head technique to detect temporal changes in field-saturated hydraulic conductivity at the surface of a sandy loam soil

Determining temporal changes in field-saturated hydraulic conductivity (Kfs) is important for understanding and modeling hydrological phenomena at the field scale. Little is known about temporal variability of Kfs values measured at permanent sampling points. In this investigation, the Simplified Falling Head (SFH) technique was used for an approximately two-year period to determine temporal changes in Kfs at 11 permanent sampling points established at the surface of a sandy loam soil. Additional Kfs measurements were obtained by the single-ring Pressure Infiltrometer (PI) technique to also compare the SFH and PI techniques. The lowest mean values of Kfs, M(Kfs), were detected in December and January (20.5 < M(Kfs) < 146.2 mm h-1) whereas higher results (190.5 < M(Kfs) < 951.9 mm h-1) were obtained in the other months of the year. The Kfs values were higher and less variable in the dry soil (i < 0.21 m3 m-3, M(Kfs) = 340.6 mm h-1, CV(Kfs) = 106%) than in the wet one ((i > 0.21 m3 m-3, M(Kfs) = 78.4 mm h-1, CV(Kfs) = 185%). Both wet and dry soil were less conductive at the end of the study period than at the beginning one but a more appreciable change was detected for the dry soil (Kfs decreasing by 83.4%) than for the wet one (Kfs decreasing by 63.0%). The simple SFH technique yielded Kfs results similar to the more laborious and time-consuming PI technique (i.e., mean values differing at the most by a factor of two). It was concluded that i) the soil water content was an important factor affecting the Kfs results obtained in a relatively coarse-textured soil, ii) the impact of time from the beginning of the experiment on the saturated hydraulic conductivity was larger for a repeated sampling of dry soil than of wet soil, and iii) the SFH technique yielded reliable Kfs results in a relatively short period of time without the need for extensive instrumentation or analytical methodology.