Characteristics of an in-situ unsaturated hydraulic conductivity of soil

A soil hydraulic conductivity (K) indicates the ability of a soil to conduct water and is often assumed to be in one-to-one correspondence with matric head (h) or volumetric water content (θ) for mathematically analyzing field soil water regimes. However, to what extent this assumption represents K in a field is rarely questioned, and how h-K and θ-K relations actually behave under natural field conditions is seldom investigated sufficiently. Therefore, this study aimed at clarifying actual behaviors of h-K and θ-K relations under natural field conditions (in-situ K), and evaluated the degree to which the in-situ K(h) diverted from its equivalent h-K relations that were measured through drainage processes starting from water-saturated conditions (primary-drying K(h)). The in-situ K values were obtained in the range of 4×10E-8 cm /s < K < 8×10E-6 cm /s with -55 cm < h < -5 cm during a four-years monitoring in a field. And the variation among the observed in-situ K values for a given h was one to two orders of magnitude, implying that the amount and pattern of water-conducting pores for a given h had changed frequently through years of wetting-drying cycles of the field soil, and that the application of a single-valued K(h) function to quantifying in-situ soil water flow can cause errors in these orders of magnitude. Also the in-situ K was 10 to 100 times lower than the primary-drying K in any h. The in-situ K fluctuated even for a given θ. Thus, it was suggested that neither an h-K relation nor a θ-K relation can sufficiently work for understanding a field soil water regime if it is regarded as merely a single-valued function. And more knowledge about the temporal variability in in-situ K is required for understanding soil water regimes in a field.