Influence of saturated water content on estimating soil hydraulic properties from cumulative disc infiltrometer measurements

Abstract

The soil sorptivity, S, and saturated hydraulic conductivity, Ks, are fundamental soil hydraulic properties that can be estimated from the cumulative infiltration curve measured with a disc infiltrometer. The Haverkamp infiltration model is widely used to estimate S and Ks. This model includes as inputs the constants β and γ and the difference between the initial, θi, and final, θs, volumetric water contents, Δθ. Since Δθ would be expressive of the possible measurement errors, and assuming β, γ, and θi as known values, the first objective of this work is to analyze the influence of θs on the optimization of Ks and S. To this end, a sensitivity analysis, which consists of estimating Ks and S for a range of θs was applied on synthetic infiltration curves simulated for homogeneous columns of sand and loam soil. Then, and working on real soils under different tillage management, we evaluated different procedures to measure θs and analyzed its impact on Ks and S estimation. Four different techniques were compared: the gravimetric-core method and two TDR invasive (3 and 5 cm) and a non-invasive (NiP) probes. All TDR probes were connected to a low-cost NanoVNA. The sensitivity analysis showed that θs, Ks and S can be optimized simultaneously from the inverse analysis of an infiltration curve when β and γ are known values and the infiltration curve is near the steady-state zone. However, due to the intrinsic complexities of real soils and the fact that β and γ are unknown variables, we recommended to optimize Ks and S using measured θs. The NiP sensor connected to a NanoVNA provided a fast, inexpensive, clean, accurate and robust alternative to measure θs at the end of the infiltration experiments.

Keywords

• Sorptivity
• Hydraulic conductivity
• Infiltration curve
• Inverse analysis
• Saturated water content