Feasibility of Sequential and Coupled Inversion of Time Domain Reflectometry Data to Infer Soil Hydraulic Parameters under Falling Head Infiltration

Accurate estimation of soil hydraulic properties is a prerequisite for efficient soil and water management. On a small scale, time domain reflectometry (TDR) measurements obtained during an infiltration event can be used for estimating soil hydraulic properties either using a sequential or a coupled inversion approach. In the traditional sequential approach, the TDR measurements are inverted into water content averages based on travel time analysis and subsequently used for calibrating a hydrologic model. Travel time analysis has been reported to be subjective and difficult to use for analyzing TDR measurements obtained during infiltration. In this paper, we extend the sequential inversion approach by using water content profiles (WCPs) obtained via inverse modeling of TDR measurements and introduce a coupled inversion approach which directly uses the TDR measurements for constraining the inversion for hydraulic properties without first inverting them into WCPs or averages. By comparing the feasibility of these approaches to infer three Mualem-van Genuchten (MVG) hydraulic parameters (alpha, n, K-s) from TDR measurements obtained under falling head infiltration, we concluded that the coupled approach is more practical and less uncertain than the sequential approach. In particular, the coupled inversion approach allows to simultaneously monitor ponding depth and water infiltration, which avoids the laborious task of manually measuring the ponding depths and can thus enable rapid estimation of the soil hydraulic parameters for multiple locations through automatic measurements of ponded infiltration for multiple rings through TDR multiplexing.