Gerard J. Kluitenberg

A small multi-needle probe for measuring soil thermal properties, water content and electrical conductivity

Abstract Ready access to reliable low cost instrumentation for use in laboratory and field experimentation remains a priority for many working in the soil and environmental sciences area. In this paper we provide an overview of the multi-needle heat-pulse probe which can now provide measurements of soil temperature, soil thermal diffusivity, volumetric heat capacity, thermal conductivity, volumetric water content, and bulk soil electrical conductivity, all at the same position and time. We show that the multi-needle probe can provide high quality measurements, in some cases as good as or better than those obtained using other current methodology. As an example, multi-needle probes have been shown to yield measurements of volumetric water content with a root mean square error of 0.01 m 3 m −3 . Because of its small size the multi-needle probe should prove particularly useful for measurements near the soil surface, near to plant roots, and in other situations requiring fine spatial resolution in measurements. These probes can also be easily automated for unattended use to facilitate data collection as a function of time and space. We also highlight issues that need further analysis and research in helping guide further development of more robust multi-needle probe designs, especially for field applications.

Multi-Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soils hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.

Crop residue effects on surface radiation and energy balance — review

SummaryCrop residues alter the surface properties of soils. Both shortwave albedo and longwave emissivity are affected. These are linked to an effect of residue on surface evaporation and water content. Water content influences soil physical properties and surface energy partitioning. In summary, crop residue acts to soil as clothing acts to skin. Compared to bare soil, crop residues can reduce extremes of heat and mass fluxes at the soil surface. Managing crop residues can result in more favorable agronomic soil conditions. This paper reviews research results of the quantity, quality, architecture, and surface distribution of crop residues on soil surface radiation and energy balances, soil water content, and soil temperature.

Patterns of Tamarix water use during a record drought

During a record drought (2006) in southwest Kansas, USA, we assessed groundwater dynamics in a shallow, unconfined aquifer, along with plant water sources and physiological responses of the invasive riparian shrub Tamarix ramosissima. In early May, diel water table fluctuations indicated evapotranspirative consumption of groundwater by vegetation. During the summer drought, the water table elevation dropped past the lowest position previously recorded. Concurrent with this drop, water table fluctuations abruptly diminished at all wells at which they had previously been observed despite increasing evapotranspirative demand. Following reductions in groundwater fluctuations, volumetric water content declined corresponding to the well-specific depths of the capillary fringe in early May, suggesting a switch from primary dependence on groundwater to vadose-zone water. In at least one well, the fluctuations appear to re-intensify in August, suggesting increased groundwater uptake by Tamarix or other non-senesced species from a deeper water table later in the growing season. Our data suggest that Tamarix can rapidly shift water sources in response to declines in the water table. The use of multiple water sources by Tamarix minimized leaf-level water stress during drought periods. This study illustrates the importance of the previous hydrologic conditions experienced by site vegetation for controlling root establishment at depth and demonstrates the utility of data from high-frequency hydrologic monitoring in the interpretation of plant water sources using isotopic methods.

Modeling the effect of mulch optical properties and mulch-soil contact resistance on soil heating under plastic mulch culture☆

Abstract A numerical model was developed to simulate the effect of plastic mulches on the field energy balance and soil temperature regime. Newton-Raphson methods were used to solve the energy balances of the soil and mulch simultaneously. The resultant temperature of the soil surface was used as a boundary condition for an implicit finite difference model of soil heat flow. The model contained a detailed radiation section to account for mulch optical properties in shortwave and longwave bands. Heat transfer between the mulch and soil surface was modeled using a thermal contact resistance, and transport in the boundary layer was quantified using flux profile theory. The effect of evaporation and condensation on the underside of the mulch was not considered. The accuracy of the model was verified by comparing simulated soil temperatures with field data collected under five commercially available mulches. Simulated average daily temperatures were within ±1.3°C of the observed values and root mean square errors were less than 2.5°C in all cases. The model showed that mulch optical properties strongly influenced the way energy was partitioned at the surface. Complete characterization of the mulches in the longwave spectrum was required for realistic simulation. Changing longwave reflectance by only 0.3, for example, caused maximum soil temperature to change by 5°C under some mulches. Thermal contact resistance between the mulch and soil affected temperatures by up to 20°C beneath mulches that strongly transmitted or absorbed radiation. However, contact resistance was not an important factor for mulches that had nearly equal shortwave transmittance and absorptance. Results demonstrate that a complex relationship exists between mulch optical properties and thermal contact resistance. Modification of mulch longwave properties during manufacturing and control of thermal contact resistance during mulch installation appear to be two promising areas for research. An alternative numerical scheme was evaluated that utilized linear forms of the energy balance equations. The linearized model produced results almost identical to the original model, but required less computing time and could be adapted easily to simulate multiple mulch layers.

Comparison of techniques for extracting soil thermal properties from dual-probe heat-pulse data

Temperature-by-time data obtained using a dual-probe heat-pulse device can be analyzed using two different approaches to determine the soil thermal diffusivity, volumetric heat capacity and thermal conductivity. One approach, referred to as the single-point method, is based on accurate identification of the peak in the temperature-by-time measurements. The second approach involves a nonlinear model fit of the appropriate temperature model to the temperature-by-time data. In this paper, we analyze dual-probe heat-pulse data and show how the soil thermal properties determined using these two approaches compare. The single-point method is easy to apply, but results are sensitive to choice of the peak value, which can be difficult to identify if the data are sparse and contain noise. The nonlinear model fit (Marquardt method) copes better with broad, flat peaks and sparse, noisy data. Soil thermal properties obtained using either approach should be checked by comparing the fitted model with the measured temperature-by-time data. By doing so, one can quickly determine the validity of the results.

Positional variation in the soil energy balance beneath a row-crop canopy☆

When crops are grown in a row configuration, differential shading of the soil, coupled with other soil-canopy micrometeorological interactions, may result in large positional variations in the soil energy balance. Experiments were conducted near Manhattan, KS, to examine positional variations in the soil energy balance beneath a soybean (Glycine max) canopy during periods of partial cover. Continuous measurements of net radiation (Rn) and soil heat flux (G) were obtained at four equally spaced positions between one pair of plant rows. Net radiation was calculated from radiation, temperature, and emissivity measurements of the soil, canopy, and sky. Soil heat flux was determined at each location from detailed heat flux and subsurface temperature measurements. When the soil surface was dry, sensible heat flux (H) was estimated as a residual of the heat balance by assuming that latent heat flux was negligible. Measurements of H and soil-air temperature gradients were used to estimate aerodynamic transport coefficients at each position. Results indicate that the magnitude and pattern of the soil heat balances are strongly dependent on location beneath the canopy. The patterns of G, Rn, and surface temperature at each measurement position were associated with the patterns of shortwave soil irradiance, which were functions of sun-canopy geometry. Soil irradiance between the plant rows sometimes exceeded global irradiance because of reflected radiation from the canopy. Daily Rn between the rows was five to ten times greater than that measured directly beneath the canopy, depending on environmental conditions. Large positional variations in G were also documented. Temperature differences between the sunlit and shaded portions of the surface exceeded 25°C when the surface was dry. Air temperatures, measured at 5 cm above each position, were also dependent on position with respect to the plant rows. When the surface was dry, the majority of sensible heat flux occurred from the soil directly between the plant rows, exceeding 400 W m−2 under certain conditions. Estimates of local aerodynamic transfer coefficients for the soil surface ranged from 1 to 50 mm s−1, but were highly variable and not correlated with above-canopy windspeeds or position beneath the canopy. Results suggest that positional variation in aerodynamic transport from the soil cannot be discerned with the methods used in this study.

Root-zone hydraulic lift evaluated with the dual-probe heat-pulse technique

Roots are movers of water in the soil. One method of movement is through hydraulic lift, which occurs when plants extract water from a moist subsoil and release it into a dry topsoil. Detection of hydraulic lift has been hampered by the lack of instruments sensitive enough to measure the small amount of water moved. Recently, the dual-probe heat-pulse (DPHP) technique has been used to monitor with fine spatial resolution the soil water content in root-zones. The objective of this research was to determine if water is released by hydraulic lift, using the DPHP technique. Sunflower (Helianthus annuus L.) was grown in a column (38 cm height; 25 cm diam.; bulk density = 1.45 Mg/m3) packed with a Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) with its roots divided between a top dry layer and a lower wet layer. Eight DPHP sensors installed in the soil column were used to monitor soil water content. During 24 measurement days, hydraulic lift was evident only when the plant was wilted. This occurred when the lower ‘wet’ layer had been allowed to dry and then it was re-watered. At this time, the roots in the upper dry layer released water, increasing the soil water content in the centre of the root mass by 0.019 m3/m3 (increase from 0.121 m3/m3 to 0.140 m3/m3). The soil-water increase was similar to other values reported in the literature and show it to be small.

Temperature dependence of nitrogen mineralization rate constant: a theoretical approach

Experimental evidence suggests that nitrogen (N) mineralization proceeds differently under fluctuating temperature conditions than it does at constant temperature. Although N mineralization is believed to follow first-order kinetics, and the mineralization rate constant is believed to follow Q10 temperature dependence, no effort has been made to use this information to predict the effect of fluctuating temperature on N mineralization. In this paper, we present solutions to the first-order N mineralization equation for a rate coefficient with Q10 temperature dependence. Solutions are presented for a number of simple patterns of temperature fluctuation in time. Example calculations show that the nonlinear temperature dependence of the Q10 relationship causes mineralization under fluctuating temperature conditions to exceed that occurring at constant temperature. Sensitivity analysis shows that the Q10 constant and the amplitude of the temperature fluctuation strongly influence the difference in mineralization obtained for the two temperature patterns. These results can be used to improve the design of experiments conducted to study the effect of-temperature fluctuations.

Canopy temperature as a measure of salinity stress on sorghum

SummaryA complete understanding of plant response to combined water and salinity stress is desirable. Previous growth chamber and greenhouse experiments with sorghum and maize indicate that soil salinity, by negatively affecting growth processes, may reduce consumptive water use, thus prolonging the supply of available soil moisture. In the present field experiment, canopy temperature measurements were used to examine the effect of soil salinity on the plant-soil water relations of sorghum (Sorghum bicolor L. cv. Northrup King 1580). An infrared thermometer was used to measure canopy temperature during a 9-day period including two irrigations in plots of various salinities. The salinity treatments were created by a dual line-source sprinkler irrigation system, which applied waters of different quality. Excess irrigation allowed soil moisture to be uniform across the salinity treatments at the beginning of the measurement period. Consumptive water use and soil salinity were measured to quantify the salinity and water treatments. Grain and dry matter yields provided measures of plant response. Canopy temperature measurements were sensitive enough to detect differences across the salinity treatments when soil moisture was uniform for several days following irrigation. However, over the 9-day measurement period, plants in the low-salt plots used more water than plants in the high-salt plots. This differential water use eventually offset the salinity-induced stress, with the result that temperature differences were eliminated. Differences in temperature were observed again following irrigation. The results demonstrate that canopy temperature can be used as a tool to detect salinity stress on sorghum. Timing of measurements with regard to irrigation is identified as a key factor in detecting temperature differences that can be attributed to the presence of soil salinity.