Although soil thermal properties are required in many areas of engineering, micrometeorology, agronomy, and soil science, they are seldom measured on a routine basis. Reasons for this are unclear, but may be related to a lack of suitable instrumentation and appropriate theory. We developed a theory for the radial conduction of a short‐duration heat pulse away from an infinite line source, and compared it with the theory for an instantaneously heated line source. By measuring the temperature response at a short distance from the line source, and applying short‐duration heat‐pulse theory, we can extract all the soil thermal properties, the thermal diffusivity, heat capacity, and conductivity, from a single heat‐pulse measurement. Results of initial experiments carried out on air‐dry sand and clay materials indicate that this heat‐pulse method yields soil thermal properties that compare well with thermal properties measured by independent methods.

[1] Hydrographs from shallow wells in vegetated riparian zones frequently display a distinctive pattern of diurnal water table fluctuations produced by variations in plant water use. A multisite investigation assessed the major controls on these fluctuations and the ecohydrologic insights that can be gleaned from them. Spatial and temporal variations in the amplitude of the fluctuations are primarily a function of variations in (1) the meteorological drivers of plant water use, (2) vegetation density, type, and vitality, and (3) the specific yield of sediments in the vicinity of the water table. Past hydrologic conditions experienced by the riparian zone vegetation, either in previous years or earlier within the same growing season, are also an important control. Diurnal water table fluctuations can be considered a diagnostic indicator of groundwater consumption by phreatophytes at most sites, so the information embedded within these fluctuations should be more widely exploited in ecohydrologic studies.

A recently developed heat pulse method allows rapid, automated measurements of the volumetric heat capacity (ρc) of soil. Estimation of ρc is accomplished by using a model for the conduction of heat away from an instantaneously heated infinite line source (IHILS). This study was conducted to examine possible errors in the use of the IHILS theory by comparing the IHILS model with three other models that account for the following characteristics of the heat pulse apparatus: finite probe length, cylindrical heater geometry, and short‐duration (noninstantaneous) heating. For typical probe geometry and heating times, estimates of ρc obtained from the IHILS theory were within 1% of the estimates obtained by using the more rigorous models. The most significant error resulted from approximating short‐duration heating with instantaneous heating in the IHILS model. A generalized error analysis is presented that permits direct graphical estimation of errors for different probe geometries, different heating times, and different soil thermal properties. First‐order error analysis was also used to examine potential error in ρc as a result of errors in probe spacing (r), temperature maximum (Tm), and heat input (q), the measured quantities that are used in the IHILS model to estimate ρc. Relative errors of 1% in measuring q and Tm lead to relative errors of 1% in ρc, whereas a relative error of 1% in r resulted in a 2% error. Measurement error in r and Tm limits the precision achievable with this method.

Research was conducted to determine the optical properties of eight plastic mulches and evaluate their effects on soil, mulch, and air temperatures in the field. Optical properties of the mulches were measured in the laboratory in the shortwave (0.3 to 1.1 μm) and longwave (2.5 to 25 μm) wavebands using a spectroradiometer and Fourier transform infrared spectrophotometer, respectively. Additionally, each mulch was installed on a fine sandy loam soil near Manhattan, Kan. Air and soil temperatures were measured 5 cm above and 10 cm below the surface, respectively. Measurements of longwave radiation emitted and reflected from the surface were used to approximate the apparent temperature of the surface. Shortwave transmittance of the mulches ranged from 0.01 to 0.84, and shortwave reflectance ranged from 0.01 to 0.48, with the greatest reflectance from white and aluminized mulches. Infrared transmittance ranged from 0.87 for a black photodegradable mulch to 0.09 for aluminized material. Air temperatures at 5 cm were similar for all mulch treatments, but were typically 3 to 5C higher than the air at 1.5 m during the day. Midday soil temperatures were highest beneath mulches with high shortwave absorptance (black plastics) or those with high shortwave transmittance coupled with low longwave transmittance. Apparent surface temperatures approached 70 to 80C during midday, with the highest temperatures occurring on mulches with high shortwave absorptance. For some mulches, both, shortwave and longwave optical properties of the plastic governed the level of radiative heating. Our results suggest that conduction of heat between the plastic and the soil surface also affects the extent of soil heating in a mulched field.

The dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties. Measurements are made with a sensor that has two parallel cylindrical probes: one for introducing a pulse of heat into the soil (heater probe) and one for measuring change in temperature (temperature probe). We present a semianalytical solution that accounts for the finite radius and finite heat capacity of the heater and temperature probes. A closed‐form expression for the Laplace transform of the solution is obtained by considering the probes to be cylindrical perfect conductors. The Laplace‐domain solution is inverted numerically. For the case where both probes have the same radius and heat capacity, we show that their finite properties have equal influence on the heat‐pulse signal received by the temperature probe. The finite radius of the probes causes the heat‐pulse signal to arrive earlier in time. This time shift increases in magnitude as the probe radius increases. The effect of the finite heat capacity of the probes depends on the ratio of the heat capacity of the probes (C0) and the heat capacity of the soil (C). Compared with the case where C0/C = 1, the magnitude of the heat‐pulse signal decreases (i.e., smaller change in temperature) and the maximum temperature rise occurs later when C0/C > 1. When C0/C < 1, the magnitude of the signal increases and the maximum temperature rise occurs earlier. The semianalytical solution is appropriate for use in DPHP applications where the ratio of probe radius (a0) and probe spacing (L) satisfies the condition that a0/L ≤ 0.11.