Determining in-situ unsaturated soil hydraulic conductivity at a fine depth scale with heat pulse and water potential sensors

Abstract

Unsaturated hydraulic conductivity (K) of surface soil changes substantially with space and time, and it is of great importance for many ecological, agricultural, and hydrological applications. In general, K is measured in the laboratory, or more commonly, predicted using soil water retention curve and saturated hydraulic conductivity. In the field, K can be determined through infiltration experiments. However, none of these approaches are capable of continuously monitoring K insitu at fine depth scales. In this study, we propose and investigate an approach to continuously estimate fine depth-scale K dynamics under field conditions. Evaporation rate and change in water storage in a near-surface soil layer are measured with the heat pulse method. Then, water flux density at the lower boundary of the soil layer is estimated from evaporation rate, change in water storage, and rainfall or irrigation rate using a simple water balance approach. Finally, K values at different soil depths are derived using the Buckingham-Darcy equation from water flux densities and measured water potential gradients. A field experiment is performed to evaluate the performance of the proposed approach. K values at 2-, 4-, 7.5-, and 12.5-cm depths are estimated with the new approach. The results show that in-situ K estimates vary with time following changes in soil water content, and the K-water content relationship changes with depth due to the difference in bulk density. In-situ estimated K-matric potential curves agree well with those measured in the laboratory. In-situ K estimates also show good agreement with the Mualem-van Genuchten model predictions, with an average root mean square error in log10 (K, mm h-1) of 0.54 and an average bias of 0.17. The new approach provides reasonable in-situ K estimates and has potential to reveal the influences of natural soil conditions on hydraulic properties as they change with depth and time.

Advances in thermo‐time domain reflectometry technique: Measuring ice content in partially frozen soils

“…The θ i is estimated with the thermo‐TDR technique, which provides θ w using the TDR method and volumetric heat capacity ( C ) or thermal conductivity (λ) with the heat pulse method. Fine‐scale soil thermal properties and θ w are obtained simultaneously and nondestructively within similar sensing volumes (Ren, Ju, Gong, & Horton, 2005; Tian, Kool, Ren, Horton, & Heitman, 2018a). Here we present the principles, procedures, and an example dataset for θ i determination with the thermo‐TDR technique.…”

Approaches for estimating unsaturated soil hydraulic conductivities at various bulk densities with the extended Mualem-van Genuchten model

“…Variable soil bulk density (ρ b , Mg m -3 ) due to human disturbances and environmental effects is an important factor causing temporal and spatial variations in soil hydraulic properties (Sillon et al, 2003;Osunbitan et al, 2005;Zhang et al, 2017;Tian et al, 2018a). A decrease in ρ b caused by tillage can enhance soil infiltration capacity (Kribaa et al, 2001).…”

“…In the field, Ankeny et al (1990) introduced a simple method to determine K u through infiltration experiments. In addition, in-situ K u dynamics can be estimated by combined use of heat pulse and water potential sensors (Tian et al, 2018a). More commonly, K u is estimated using soil water retention parameters and K s measurement, based on relative hydraulic conductivity models (Burdine, 1953;Mualem, 1976;van Genuchten, 1980;Assouline, 2001).…”

Development and Pilot Installation of a Scalable Environmental Sensor Monitoring System for Freeze–Thaw Monitoring under Granular-Surfaced Roadways

“…For example, one of the primary techniques used in freeze–thaw investigations is to monitor a particular test site for a sufficient period of time using various sensors to measure significant hydraulic and/or thermal property parameters and inspect the freeze–thaw characteristics ( 2 – 4 ). Climatic conditions and subgrade properties can vary substantially for different test sites, however, requiring different sensor systems.…”

“…Climatic conditions and subgrade properties can vary substantially for different test sites, however, requiring different sensor systems. For instance, Tian et al ( 2 ) installed heat pulse, time domain reflectometry, and matric potential sensors at a field site in North Carolina at depths up to 15 cm (5.9 in.) to investigate unsaturated hydraulic conductivity properties of the soil.…”