Estimating hydraulic conductivity of internal drainage for layered soils in situ

摘要

The soil hydraulic conductivity (K function) of three layered soilscultivated at Paradys Experimental Farm, near Bloemfontein (South Africa),was determined from in situ drainage experiments and analytical models.Pre-ponded monoliths, isolated from weather and lateral drainage, wereprepared in triplicate on representative sites of the Tukulu, Sepane andSwartland soil forms. The first two soils are also referred to as CutanicLuvisols and the third as Cutanic Cambisol. Soil water content (SWC) wasmeasured during a 1200 h drainage experiment. In addition soil physicaland textural data as well as saturated hydraulic conductivity (K_s) werederived. Undisturbed soil core samples of 105 mm with a height of 77 mm fromsoil horizons were used to measure soil water retention curves (SWRCs).Parameterization of SWRC was through the Brooks and Corey model. Kosugi and vanGenuchten models were used to determine SWRC parameters and fitted with a RMSEof less 2%. The SWRC was also used to estimate matric suctions for insitu drainage SWC following observations that laboratory and in situ SWRCswere similar at near saturation. In situ K function for horizons and theequivalent homogeneous profiles were determined. Model predictions based onSWRC overestimated horizons K function by more than three orders ofmagnitude. The van Genuchten–Mualem model was an exception for certain soilhorizons. Overestimates were reduced by one or more orders of magnitude wheninverse parameter estimation was applied directly to drainage SWC withHYDRUS-1D code. Best fits (R2 ≥ 0.90) were from Brooks and Corey,and van Genuchten–Mualem models. The latter also predicted the profiles'effective K function for the three soils, and the in situ based function wasfitted with R2 ≥ 0.98 irrespective of soil type. It was concludedthat the inverse parameter estimation with HYDRUS-1D improved models' Kfunction estimates for the studied layered soils.