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Soil bulk density (ρb) is a critical parameter for describing soil structure and physical processes, yet traditional methods are unable to capture spatial and temporal changes in ρb. Recently the thermo–time domain reflectometry (thermo-TDR) technique has been applied to determine in situ ρb on the basis of soil thermal property and water content measurements. Here, we present theory, instrumentation, procedures, and comments for monitoring in situ ρb with the thermo-TDR sensor. We conclude that the thermo-TDR sensor offers a useful tool for determining ρb continuously and nondestructively.
Soil water evaporation is an important process in hydrology, engineering, and agriculture. Few techniques are capable of measuring soil water evaporation in situ. An approach has been developed to measure in situ subsurface soil water evaporation using a soil sensible heat balance (SHB) with measurement data obtained from multi-needle heat-pulse sensors. Terms in the SHB (i.e., sensible heat flux and change in sensible heat storage) are calculated from heat-pulse sensor derived soil temperature and thermal property (i.e., thermal conductivity and heat capacity) measurements for a thin soil layer with thickness corresponding to the sensor geometry. The quantity of latent heat required for soil water vaporization can be determined as the residual to the SHB (i.e., change in heat flux with depth minus change in sensible heat storage with time) for the soil layer. Dividing latent heat (per unit time) by heat of vaporization for water allows data to be converted to evaporation rate. Numerical analysis indicates that the SHB approach is most sensitive to the heat flux component of the SHB. Laboratory and field tests indicate that SHB results compare favorably with mass-balance and micrometeorologic approaches for evaporation measurement, with SHB typically differing by <0.2 mm d−1 from the reference methods.
Soil water content (θ) influences physical, chemical, and biological processes in the soil. Near-surface θ (<1-cm depth) is particularly important for surface energy partitioning, but few techniques are available for near-surface in situ θ measurements. Heat-pulse sensors can be used to determine the soil volumetric heat capacity, which is linearly related to θ. Here we describe the principles and procedures of determining near-surface in situ θ with a heat pulse sensor. The main limitations and potential errors associated with the method are also presented. When ambient soil temperature drift and the soil–air interface effects are addressed, the error in the heat-pulse-determined θ is greatly reduced. For an example, data series with θ data determined by gravimetric initial θ and heat-pulse-based change in θ (Δθ), results agree well with gravimetric θ values, yielding a coefficient of determination of 0.95. We conclude that heat-pulse sensors are useful tools for continuously and nondestructively determining near-surface θ of non-shrink–swell soils.
Soil thermal properties, including thermal diffusivity (α), thermal conductivity (λ), and volumetric heat capacity (C), are basic parameters describing the ability and efficiency of a soil to store and transfer heat. Soil thermal properties influence heat and mass transfer in soils and therefore have fundamental effects on the energy balance at the ground surface, water exchange between the soil and atmosphere, and engineered structures. Accurate determination of soil thermal properties is essential for describing thermal regimes that influence the rates of physical, chemical, and biological reactions and processes occurring in soil.
Shrink–swell soils, often classified as Vertisols or vertic intergrades, are found worldwide and are a leading cause of damage to infrastructure such as buildings, roads, and pipelines. Crack networks act as dominant environmental controls on the movement of water, contaminants, and gases. Numerous methods have been proposed to quantify the size (e.g., width, depth, volume) and connectivity of individual cracks and of larger crack networks. To measure and quantify the size and variability of cracks, we focus on two nondestructive methods, called here the tape and rod and displacement approaches, and one destructive method, called here the cast and excavate protocol. The nondestructive methods are relatively inexpensive and can allow repeated measurements, which makes them conducive to use in larger environmental studies such as observing hydrological partitioning between infiltration and surface runoff. However, the nondestructive methods are often biased toward larger cracks (due to physical limitations on the crack sizes that can be measured), require assumptions of crack geometry to determine crack volumes, and typically do not provide information on subsurface connections between cracks. The destructive cast and excavate method is better suited to sample a range of crack sizes and can be used to better understand subsurface connectivity, although it oftentimes can only be used once (precluding repeated measurements) and is labor intensive. A combination of measurements may therefore be required to best understand crack dynamics in both time and space. Altogether, the methods surveyed here enable accurate measurement and quantification of soil crack characteristics.
The measurement of soil salinity is a quantification of the total salts present in the liquid portion of the soil. Soil salinity is important in agriculture because salinity reduces crop yields by reducing the osmotic potential, making it more difficult for the plant to extract water, by causing specific-ion toxicity, by upsetting the nutritional balance of plants, and by affecting the tilth and permeability of a soil. A discussion of the principles, methods, and equipment for measuring soil salinity is presented. The discussion provides a basic knowledge of the background, principles, equipment, and current accepted procedures and methodology for measuring soil salinity in the laboratory using electrical conductivity of aqueous extracts from soil samples and measurement of total dissolved solids in the saturated soil extract. Attention is also given to the use of suction cup extractors, porous matrix or salinity sensors, electrical resistivity, and electromagnetic induction to measure salinity in soil lysimeter columns and small field plots (<10 by 10 m). Land resource managers, producers, extension specialists, Natural Resource Conservation Service field staff, undergraduate and graduate students, and university, federal, and state researchers are the beneficiaries of the information provided.