A dripper-TDR method for in situ determination of hydraulic conductivity and chemical transport properties of surface soils

Abstract

Field determined hydraulic and chemical transport properties can be useful for the protection of groundwater resources from land-applied chemicals. Most field methods to determine flow and transport parameters are either time or energy consuming and/or they provide a single measurement for a given time period. In this study, we present a dripper-TDR field method that allows measurement of hydraulic conductivity and chemical transport parameters at multiple field locations within a short time period. Specifically, the dripper-TDR determines saturated hydraulic conductivity (Ks), macroscopic capillary length (λc), immobile water fraction (θim/θ), mass exchange coefficient (α) and dispersion coefficient (Dm). Multiple dripper lines were positioned over five crop rows in a field. Background and step solutions were applied through drippers to determine surface hydraulic conductivity parameters at 44 locations and surface transport properties at 38 locations. The hydraulic conductivity parameters (Ks, λc) were determined by application of three discharge rates from the drippers and measurements of the resultant steady-state flux densities at the soil surface beneath each dripper. Time domain reflectometry (TDR) was used to measure the bulk electrical conductivity of the soil during steady infiltration of a salt solution. Breakthrough curves (BTCs) for all sites were determined from the TDR measurements. The Ks and λcvalues were found to be lognormally distributed with average values of 31.4 cm h−1 and 6.0 cm, respectively. BTC analysis produced chemical properties, θim/θ, α, and Dm with average values of 0.23, 0.0036 h−1, and 1220 cm2 h−1, respectively. The estimated values of the flow and transport parameters were found to be within the ranges of values reported by previous studies conducted at nearby field locations. The dripper TDR method is a rapid and useful technique for in situ measurements of hydraulic conductivity and solute transport properties. The measurements reported in this study give clear evidence to the occurrence of non-equilibrium water and chemical movement in surface soil. The method allows for quantification of non-equilibrium model parameters and preferential flow. Quantifying the parameters is a necessary step toward determining the influences of surface properties on infiltration, runoff, and vadose zone transport. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. AbstractField determined hydraulic and chemical transport properties can be useful for the protection of groundwater resources from land-applied chemicals. Most field methods to determine flow and transport parameters are either time or energy consuming and/or they provide a single measurement for a given time period. In this study, we present a dripper-TDR field method that allows measurement of hydraulic conductivity and chemical transport parameters at multiple field locations within a short time ...

A review of model applications for structured soils: a) Water flow and tracer transport

“…In an application with several dripping rates, the saturated hydraulic conductivity, K s , the ratio of the immobile and the saturated water content, θ im /θ s , and λ c were similar, while α was different compared to corresponding values derived using the disk infiltrometer (Al-Jabri et al, 2002). The dripper line method was further extended by the installation of a TDR probe beneath each infiltration spot to also measure solute breakthrough to yield the dispersion coefficient in the mobile region (Al-Jabri et al, 2006). The accuracy of these methods depends on the validity of the underlying assumptions, including piston movement, a constant mobile water concentration equal to the ( Vanderborght et al, 1997;Vanderborght et al, 2002;Vanderborght and Vereecken, 2007) Scale I: soil aggregate (Ped), artificial or repacked soil column (AC), undisturbed column (UC), lysimeter (L); scale II: soil profile (S), plot (P), field (F); acronyms: computer tomography (CT), saturated hydraulic conductivity (K s ), macroscopic capillary length (lambda(c)), immobile water fraction (θ im /θ), mass exchange coefficient (α) and dispersion coefficient (D m ) or dispersivity (λ), saturated and boundary hydraulic conductivities (K s and K b ), boundary head between macropore and micropore domains (h b ), exponent (n⁎) of relation between K and water content (θ), macropore fraction (θ sma ), and half spacing (d) between equivalent parallel fractures, dispersivity (D v ), ρ bulk density, K(θ) hydraulic conductivity function.…”

Field Measurement of Soil Surface Chemical Transport Properties for Comparison of Management Zones

“…1964;van Genuchten and Wierenga, 1976) was then fi t to these BTCs to determine soil chemical transport parameters. Al-Jabri et al (2006) were able to measure chemical transport properties simultaneously at 16 fi eld locations in a few hours.…”

“…This has, in turn, limited the improvement of models to describe accurately chemical movement in and through the soil under varying conditions, such as with preferential fl ow (van Genuchten and Wierenga, 1977). Recently, Al-Jabri et al (2006) introduced a new technique to allow rapid fi eld measurement of soil chemical transport properties at multiple locations. In this method, a chemical tracer was applied through drip emitters positioned over the soil surface.…”

“…The method of Al-Jabri et al (2006) offers a promising new tool for measurement and comparison of soil chemical transport properties across varying soil conditions. Improved measurement brings the opportunity for data assimilation into agroecosystem models as well as providing direct insight into relationships between soil factors and chemical transport.…”

“…Yet this technique has not been applied to date for comparison between differing soil management conditions. In this study, we applied the technique of Al-Jabri et al (2006) for measurement of soil surface chemical transport properties in four soil management zones: No-till nontraffi cked (NTNT), no-till traffi cked (NTT), chisel plow nontraffi cked (CPNT) and chisel plow traffi cked (CPT). The purpose of this work was to demonstrate the effectiveness of the method for measuring and comparing chemical transport properties for differing soil management across a similar soil type.…”