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• This method estimated K s , θ s , n and α dr of a soil column by inverse analysis. • 1D infiltration followed by successive steady-states drainages were measured. • The drainage curves are generated by a differential h between the top and the bottom. • The method was tested on 2.5 cm high synthetic and experimental soil columns. • Accurate estimates of θ s , K s , α dr and n were obtained. Determination of the saturated hydraulic conductivity, K s , and the water retention curve, θ(h) , is of paramount importance to characterize the hydraulic behavior of the vadose zone. Given the van Genuchten hydraulic model, defined by the residual, θ r , and saturated, θ s , volumetric water content and the α and n parameters, this work presents a new laboratory procedure to estimate K s , θ s , n and α for a drainage process, α dr , from the inverse analysis of successive drainage steady-states curves generated by a tension-gradient between the surface and the base of a soil column. To this end, a double disc system, one connected a bubbling tower and placed at the soil surface and the second one placed under the soil core, was employed. The second disc was connected to an air-vacuum system. The experiment presented two parts: a first 1D downward infiltration at saturation on a dry soil column, followed by successive drainage steps. During the drainage process, the tension of the upper and lower discs varied between 0 and −5 cm, and from −5 to −100 cm, respectively. The soil sorptivity, S , and θ s were calculated from the 1D transient infiltration measure, K s was calculated by Darcy's law, α dr and n were optimized from the inverse analysis of the steady-state curves under tension-gradient and α for a wetting process, α w , was calculated from previously obtained S , θ s , K s and n. Once K s estimated, α dr and n were optimized by minimizing the Q = h b - h n objective function, where h b and h n are the experimental and calculated tensions at the base of the soil core. Given a α dr value, the optimimum n was computed as the value that provides a minimum Q. By repeating this process for a sequence of α dr , different Q -isolines were obtained, one for each h b value, which crossing-point corresponded to the actual α dr and n values. The method was tested on 2.5 cm high columns of four different synthetic soils. Next, it was applied on an experimental sand column of 5 cm height and on 2.5 cm high columns filled with sieved loam, clay loam and clay soil. The estimated α dr and n were compared with corresponding values measured in the same soils with the pressure plate technique and α w was contrasted with the corresponding value calculated with an empirical hysteresis model. The method, which was fast (from 1 to 2 h) and easy to implement for small-scale experiments, was successfully applied to soil samples 2.5 cm high and allowed to explore a range of soil tensions from 0 to −100 cm. Overall, accurate estimates of θ s , K s , α dr and n were obtained in both synthetic and experimental soils. A significant relationship was also obtained between α w estimated from S and the corresponding value calculated from the hysteresis model. [ABSTRACT FROM AUTHOR]

• This work presents an upward infiltration method to estimate K PF = K T - K s. • K s and K T are measured from the transient infiltration and an overpressure step. • It was satisfactorily validated on packed soil with and artificial PF channel. • It was applied on soil cores collected from a grazing and tillage experiment. • 5, 35 and 60% of soil cores presented significant, moderate and negligible PF. Soil water preferential flow, PF, is the phenomenon where water moves along certain pathways, while bypassing a fraction of the porous matrix. This work presents a laboratory procedure to estimate the hydraulic conductivity of the soil water preferential flow, K PF. Using an upward infiltration curve followed by an over-pressure step, K PF is calculated as the difference between the total hydraulic conductivity, K T , measured with the constant head method, and the soil matrix hydraulic conductivity, K s , calculated from the inverse analysis of the transient upward infiltration curve. Previous to this analysis, a new approach to detect and remove the effect of possible overpressures due to the soil core placement on the sorptivimeter on the hydraulic properties estimate was presented. This consisted on searching the best fitting between the experimental infiltration curve after removing successive early time steps and the corresponding simulated function. The method to estimate K PF was validated on four different 5-cm height soil columns packed with 2-mm sieved clay loam and loam soils traversed longitudinally by a closed/opened pipe of 1.5 mm and 2.5 mm internal diameter, respectively. This same experiment was also repeated on the same soils without artificial preferential channel. Next, the method was applied on 20 undisturbed soil cores collected from two fields with different grazing intensity and soil tillage management (conventional, CT, reduced, RT and no-tillage, NT, practices). Results showed that correction made to remove the infiltration curve jump due to an overpressure step allowed more accurate and realistic estimates of the soil matrix hydraulic properties. On the other hand, both K T and K s , which could be satisfactorily calculated from a single upward infiltration curve followed by an overpressure step, where satisfactorily employed to quantify K PF. While no significant effect of grazing intensity on K PF was observed, the CT K PF was significantly larger than those obtained in RT and NT. [ABSTRACT FROM AUTHOR]

• A method to estimate θ s , α , n and K s from an upward infiltration is presented. • The influence of θ s, θ r and h i , on the α , n and K s optimization was studied. • It was applied to 50 mm high undisturbed cores under different soil management. • θ s and h i affected α , n and K s , and h i should be located the d θ dh ≈ 0 region. • It was sensible to different texture and structure due to the soil management. Estimation of the α and n van Genuchten (1980) parameters and the saturated hydraulic conductivity, K s , of undisturbed soil samples is critical for realistic water flow simulations in the vadose zone. Given the Latorre and Moret-Fernández (2019) method, which allows estimating K s , α and n from the inverse analysis of a single upward infiltration, the objective of this work is to advance on the theoretical understanding of this method and to apply it on undisturbed soil cores. To this end, the influence of the saturated, θ s , and residual, θ r , water content and initial soil tension, h i , on the α , n and K s optimization was first studied on synthetic soils. A procedure to simultaneously estimate θ s , α , n and K s , which consisted on optimizing α , n and K s for a range of θ s values, was proposed. To this end, α , n and K s were optimized leaving each θ s as a fixed value. The method was next applied to experimental curves measured from sieved and undisturbed soil cores of 50 mm high and diameter, sampled the last ones from three fields with different management: bare vs. under plant soil; agricultural soil under conservation vs. conventional tillage; and overburden soil from mine vs. topsoil formed by the original soil. Results showed that θ s had an important effect on α , n and K s optimization, where the minimum error within the selected θ s interval corresponded to the theoretical θ s. An important effect of h i on the optimization was also observed. However, this influence was omitted by using a h i value located in the θ r region, where d θ dh ≈ 0. Under this assumption, θ r only affected the θ s optimization, which value varied with θ r. A robust relationship (R2 = 0.99; p < 0.0001) was found between theoretical and optimized θ s , K s and α and n calculated for the synthetic soils. Thus, the method allowed estimating four of the five van Genuchten (1980) parameters. An absolute minimum was also observed when the method was applied on experimental infiltration curves. The θ s measured gravimetrically on the undisturbed samples was, on average, 8.1% higher than the optimized one. Overall, the method allowed detecting significant differences (p < 0.05) in hydraulic properties between bare and under plant soils and among tillage treatments. The large variability found in the mine's soils prevented to find significant differences within this scenario. [ABSTRACT FROM AUTHOR]