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Soil thermal properties play important roles in dynamic heat and mass transfer processes, and they vary with soil water content (θ) and bulk density (ρ b). Both θ and ρ b change with time, particularly in recently tilled soil. However, few studies have addressed the full extent of soil thermal property changes with θ and ρ b. The objective of this study is to examine how changes in ρ b with time after tillage impact soil thermal properties (volumetric heat capacity, C ᵥ, thermal diffusivity, k, and thermal conductivity, λ). The study provides thermal property values as functions of θ and ρ b and of air content (n ₐᵢᵣ) on undisturbed soil cores obtained at selected times following tillage. Heat pulse probe measurements of thermal properties were obtained on each soil core at saturated, partially saturated (θ at pressure head of −50 kPa) and oven‐dry conditions. Generally, k and λ increased with increasing ρ b at the three water conditions. The C ᵥ increased as ρ b increased in the oven‐dry and unsaturated conditions and decreased as ρ b increased in the saturated condition. For a given θ, a larger ρ b was associated with larger thermal property values, especially for λ. The figures of C ᵥ, k and λ versus θ and ρ b, as well as C ᵥ, k and λ versus n ₐᵢᵣ, represented the range of soil conditions following tillage. Trends in the relationships of thermal property values with θ and ρ b were described by 3‐D surfaces, whereas each thermal property had a linear relationship with n ₐᵢᵣ. Clearly, recently tilled soil thermal property values were quite dynamic temporally due to varying θ and ρ b. The dynamic soil thermal property values should be considered in soil heat and mass transfer models either as 3‐D functions of θ and ρ b or as linear functions of n ₐᵢᵣ. HIGHLIGHTS: Thermal property values for a range of θ and ρ b were measured on undisturbed soil cores. Freshly tilled soil thermal property values were quite dynamic temporally. The thermal property values of a tilled soil were described as 3‐D surfaces with θ and ρ b. The thermal property values of a tilled soil varied linearly with n ₐᵢᵣ.

Current research on bioenergy crops shows that perennial grasses can yield substantial amounts of dry biomass with relatively low inputs of water and fertilizer. In order to minimize competition with food production, it has been suggested that bioenergy crops could be directed to land areas less suitable for commodity crops, commonly referred to as “marginal” lands. These are land units with inherent limitations to vegetative growth and production, which may be due to several factors (soil physical and chemical properties, climatic conditions, etc.). However the term “marginal” is an adjective with imprecise meaning, and objective criteria for determining “marginal” lands for siting bioenergy crops are necessary. Here we propose that such criteria may be based on soil survey classifications and realistic yield estimates, and we show an example of its use to justify site selection for bioenergy crops in different regions of North Carolina.

Soil structure-dependent properties are subject to changes with time and depth under the influences of agronomic practices and environmental factors. Wetting and drying (W/D) cycles following tillage alter the structure and pore-related functions of field soils. This study investigates the effects of W/D cycles on temporal changes of soil porosity and water retention curves (SWRCs) of a tilled layer (0- to 15-cm) during a post-tillage period. Soil volumetric water content (θ) and matric potential (ψₘ) dynamics at three depths were monitored continuously, and the total porosity (Pₜ) was determined gravimetrically. Nine W/D cycles were identified in the experimental period. Soil Pₜ decreased gradually with time and depth and became relatively stable after four W/D cycles, and the SWRCs shifted toward higher θ values at a specific ψₘ. The magnitude of SWRC change was relatively small during the first to third W/D cycles when the degrees of saturation were relatively low, reached the maximum after the fourth W/D cycle during which the soil was nearly saturated, and became less significant thereafter. Soil water holding capacity was improved during the W/D processes mainly due to the reduction of effective porosity and development of residual porosity. During the earlier four W/D cycles, a higher initial θ also contributed to the increase of the residual porosity, and thus enhanced the soil water holding capacity. The difference, however, tended to disappear in later W/D cycles. The data show clearly that the θ–ψₘ relationships following tillage are dynamic.

The heat-pulse technique, which is based on the sensible heat balance, has been developed for measuring soil-water evaporation (E). The accuracy and precision of the technique has been previously tested with the Bowen ratio method and micro-lysimeters, which are themselves subject to considerable uncertainties. This study evaluated the accuracy and precision of the heat-pulse technique using direct, continuous, and nondestructive E measurements from a weighing lysimeter. Multi-needle heat-pulse probes were installed in a silt loam soil at four locations in a 22.5-m³ (3-m × 3-m × 2.5-m) weighing lysimeter to measure soil thermal conductivity, heat capacity, and temperature gradients in the 0- to 5-cm layer. Daily E rates were estimated with sensible heat balance theory. Independent daily E rates obtained directly from a weighing lysimeter were used to evaluate the accuracy of heat-pulse E estimates during a 10-d drying period. Potential daily evaporation (E₀) was also computed using the Penman–Monteith equation. The E/E₀ ratio was >0.9 for the first 4 d but fell sharply thereafter and approached a stable value of 0.4. There was a linear relationship between heat-pulse values and weighing lysimeter values, with a slope of 0.95 and an offset of 0.09 mm d⁻¹. The absolute error of the heat-pulse E ranged from −0.02 to 0.29 mm d⁻¹ (or −1 to 8% of the true value), and the RMSE was 0.15 mm d⁻¹. The SD of the heat-pulse E data ranged from 0.07 to 0.51 mm d⁻¹, and the coefficients of variation ranged from 3 to 14%. We concluded that the heat-pulse technique can provide daily E measurements with good accuracy and precision.

Bioenergy crops are potential alternatives to traditional row-crop and pasture/hay systems. A trial comparing effects of bioenergy to traditional production on soil properties was established in 2012 under no-till in the North Carolina Piedmont. Five cropping systems included: giant miscanthus (Miscanthus × giganteus), switchgrass (Panicum virgatum L.), biomass sorghum (Sorghum bicolor spp.), tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.], and corn (Zea mays L.)/wheat (Triticum aestivum L.)/soybean (Glycine max L.) rotation. Soil samples were collected before and 3 yr after trial establishment. Sorghum produced average yield of 21.5 Mg ha–¹ in 2012 to 2015. Miscanthus and switchgrass reached yield plateaus of 21 and 15 Mg ha–¹, respectively, and removed significantly less N, P, and K than other crops, due to their rhizome systems and lower fertilizer requirements. They did not, however, demonstrate advantages over annual crops in soil physical properties. Soils under miscanthus and switchgrass had the least macropores and lowest saturated hydraulic conductivity. Summed to 30-cm soil depth, miscanthus maintained similar soil organic C as with tall fescue (58.6 vs. 55.0 Mg C ha–¹), whereas soil organic C under sorghum and switchgrass were lowest (average of 49.5 Mg ha–¹). Microbial biomass soil C under miscanthus (0–12-cm depth) was significantly greater than under annual crops. Negative effects of switchgrass on soil physical properties and organic C and N might have been due to tillage required for establishment. Typical bioenergy crops do not appear to have major negative or positive effects on soil properties in the North Carolina Piedmont.

Recent soil tests evaluating “soil health” on a broad scale may not properly consider the intrinsic limitations of soil properties, and have not been assessed in regionally unique soil conditions. To evaluate three soil tests in North Carolina, we used long-term agronomic management trials from three distinct physiographic regions: mountain (22 yr), piedmont (32 yr), and coastal plain (17 yr). Mountain and coastal plain trials included combinations of organic or chemical management with or without tillage; the piedmont trial included nine different tillage treatments. Soil samples were collected and submitted for analysis as recommended by the North Carolina Department of Agriculture and Consumer Services, Haney soil health test (HSHT), and Cornell comprehensive assessment of soil health (CASH). Plant nutrient concentrations varied but were still sufficient for crops. The CASH physical soil indicators, such as surface hardness and aggregate stability, were not statistically different, regardless of tillage intensity or management. Biological soil indicators (e.g., CO₂ respiration) responded differently to management, but this differentiation was inconsistent among locations and tests. Despite many years of conservation management, the CASH results described mountain soils as “low” or “very low” soil health for all but no-till organic management, which received a “medium” score. The HSHT results considered soil from all but moldboard plowing (piedmont) to be in good health. Finally, there was no correlation between soil health tests and crop yields from North Carolina soils. Soil health tests should be calibrated to better differentiate among soil management effects that vary depending on intrinsic soil limitations.

Summary Soil-surface heat flux (G0), an important component of the surface energy balance, is often determined by summing soil heat flux (Gz) at a depth (z) below the surface and the rate of change in soil heat storage (ΔS) in the layer above z. The soil heat flux Gz is commonly measured with passive heat flux plates, but self-calibrating plates or additional corrections are required to obtain accurate data. In some cases, ΔS is neglected because of the difficulty of monitoring the dynamics of volumetric heat capacity (C), which might lead to erroneous estimates of G0. To overcome these limitations, we introduce the heat-pulse method for measuring G0 with a multi-needle heat-pulse probe (HPP). Soil temperature (T) distribution, thermal conductivity (λ) and C of the 0–52-mm layer were measured hourly on five consecutive days with an 11-needle HPP, and Gz at 50-mm depth (G50) and ΔS of the 0–50-mm layer (ΔS0–50) were determined by the gradient and calorimetric methods, respectively. Independent measurements of G50 with a self-calibrating heat flux plate and ΔS0–50 calculated with the de Vries model C were used to evaluate the HPP data. With reliable G50 and ΔS0–50 measurements, the HPP-based G0 data agreed well with those estimated from the independent method (with a mean absolute difference of 4.5 W m−2). Supporting measurements showed that determining Gz at the 50-mm depth minimized the likelihood of errors from evaporation below the measurement depth. The multi-needle HPP provides a reliable way to determine G0 in situ. Additional analysis demonstrated that by reducing the number of needles from 11 to 5, the datalogging requirement was reduced by half and G0 was still determined with acceptable accuracy. Highlights Heat flux at the soil surface (G0) was monitored by the heat-pulse technique. A multi-needle heat-pulse probe (HPP) was used to measure subsurface soil heat flux (Gz) and heat storage concurrently. Appropriate measurement depths of Gz were determined to minimize the effects of subsurface latent heat sink on G0. A simplified calculation reduced the datalogging requirement of the multi-needle HPP.

Accurate measurement of soil temperature gradients is important for the estimation of soil heat flux and latent heat flux, both major components of the surface energy balance. Soil temperature gradients are commonly measured using heat-pulse sensors equipped with thermistors. In this study, individual thermistors showed absolute temperature differences on the order of 0.2°C when placed under uniform temperature conditions. These differences compromised measurement of soil temperature gradients over small depth increments and/or conditions with relatively minor variation in temperatures. An in situ calibration approach was found to reduce the uncertainty between thermistors to about 0.05°C in a vineyard under arid conditions. In situ calibration results were similar to laboratory results before and after field deployment for temperatures ranging between 4 and 60°C. Thermistor offsets were found to change very little over a 5-yr period, indicating that pre- or post-laboratory calibration could be sufficient. The in situ approach can be useful when calibration prior to field deployment is unavailable and/or sensor failure prevents post-field calibration.

Soil ice content is an important component for winter soil hydrology. The sensible heat balance (SHB) method using measurements from heat pulse probes (HPPs) is a possible way to determine transient soil ice content. In a previous study, in situ soil ice content estimates with the SHB method were inaccurate, due to thermal conductivity errors and the use of relatively long time steps for calculations. The objective of this study is to reexamine the SHB method for soil ice content determination. A soil freezing and thawing laboratory experiment was performed with soil columns and heat exchangers. Transient soil ice contents in the soil columns during soil freezing and thawing were determined with the SHB method. The SHB method was able to determine dynamic changes in soil ice contents during initial freezing and final thawing for soil temperatures between −5 and 0°C when latent heat values associated with ice formation or with thawing were relatively large. During an extended freezing period, when soil temperatures were below −5°C, the small associated latent heat fluxes were below the sensitivity of the SHB method, and the SHB method did not provide accurate estimates of ice contents with time. However, the soil ice contents during the extended freezing period could be estimated well from changes in volumetric heat capacity ( C ) determined with HPP. Thus, combining the SHB method for initial freezing and final thawing, with a change in C method for extended freezing periods, allowed determination of dynamic soil ice contents for the entire range of freezing and thawing soil temperatures investigated. HPPs were able to measure soil ice contents.

The bulk density (ρb) of a tilled soil layer usually varies with depth and time. It is difficult to measure in situ ρb with available instruments. Here we present a new approach that is capable of monitoring in situ ρb based on thermal conductivity (λ) measurements. The proposed approach relies on an empirical model that relates λ to soil particle-size distribution, ρb, and volumetric water content (θ). The thermo-time domain reflectometry (thermo-TDR) sensor measures λ and θ simultaneously, and knowing soil texture a priori, ρb is estimated using the empirical model. The approach was tested using datasets obtained on repacked and intact soil samples with a wide range of ρb and θ. Further evaluations were made using in situ ρb measurements with thermo-TDR sensors in four tillage treatments. The laboratory results showed that the λ-based thermo-TDR method was able to provide reliable ρb with root mean square errors (RMSE) within 0.17 g cm⁻³. Under field conditions, the λ-based thermo-TDR method also provided accurate ρb estimates with relative error generally within 10%. Under both laboratory and field conditions, the λ-based thermo-TDR method performed better than the heat capacity (C)-based thermo-TDR method. Differences were attributed mainly to C errors resulting from probe deflections, while λ measurements with the thermo-TDR method were not affected by probe deflections. The λ-based approach provides a way to obtain nondestructive, repeated, and in situ ρb estimates with the thermo-TDR method both in laboratory and field settings.

Heat-pulse sensor measurements analyzed with pulsed infinite line source (PILS) theory have been widely used to measure soil properties. The PILS theory assumes that the measured soil medium is uniform and infinite. When the sensors are positioned near the soil surface, the effects of the heterogeneity associated with the soil–air interface should not be ignored. In 1999, Philip and Kluitenberg (PK99) proposed an analytical solution using an instantaneous heating model to analyze the effects of the soil–air interface on soil thermal property measurements with heat-pulse sensors. The purpose of this study is to test the PK99 instantaneous heat source solution under controlled laboratory conditions. Soil thermal properties including volumetric heat capacity, thermal diffusivity, and thermal conductivity were measured with a commercially available dual-needle heat-pulse sensor buried at different depths beneath the soil surface. Three soil materials, sand, loamy sand, and sandy clay loam, were tested at both air-dry and saturated moisture conditions. With shallow sensor burial, measured thermal properties were underestimated by up to 50%, similar to the predicted thermal properties from the PK99 analytical solution, due to the effects of the soil–air interface. Using PK99 to adjust thermal property values obtained from shallow sensors has potential to improve estimates of water content, evaporation, and other soil measurements derived from heat-pulse sensors.

Large temperature gradients and proximity to the soil–air interface affect the measurement accuracy of thermal properties with the heat-pulse probe (HPP). The objective of this study is to improve the HPP methodology for measuring soil heat capacity (C) and thermal conductivity (l) in the surface soil layer by reducing the effects of ambient temperature variation and the soil–air interface. Thermal properties of eight soil layers (0–6, 2–6, 6–12, 12–18, 18–24, 24–30, 30–36, and 36–42 mm) were measured with the HPP in a loamy sand soil in two experiments with varying moisture conditions. Results were compared with values estimated with the de Vries models. Temperature drift caused by natural warming and cooling was removed from the observed trend of ambient temperature change with time. The influence of the soil–air interface was taken into account with a new solution using pulsed infinite line source theory with adiabatic boundary conditions. When ambient temperature change was considered, the HPP were capable of providing reasonable thermal property results at soil depths greater than 6 mm. For the 0- to 6-mm depth layer, the HPP signals were affected simultaneously by ambient temperature drift and the soil-air interface, and mixed scenarios were produced. Around midday, the HPP temperature by time curves were distorted so seriously that the effect of ambient temperature drift could not be removed. At other times, large errors were observed in the estimated C and l data, even when ambient temperature drift was removed. When the effect of the soil–air interface was accounted for, however, C and l measurement accuracies in the 0- to 6-mm layer were improved significantly. The RMSE of C was reduced from 0.43 MJ m −3 °C −1 to 0.29 MJ m −3 °C −1 in Exp. 1, and from 0.52 MJ m −3 °C −1 to 0.34 MJ m −3 °C −1 in Exp. 2. Significant error reduction was also observed in l measurements. Thus, to obtain accurate field measurements of shallow soil thermal properties, the ambient temperature drift and the impact of the soil–air interface must be properly taken into account.

Soil ice content impacts winter vadose zone hydrology. It may be possible to estimate changes in soil ice content with a sensible heat balance (SHB) method, using measurements from heat pulse (HP) sensors. Feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties in partially frozen soils. The objectives of this study were (i) to examine the SHB method for determining in situ ice content, and (ii) to evaluate the required accuracy of HP sensors for use in the SHB method. Heat pulse sensors were installed in a bare field to measure soil temperatures and thermal properties during freezing and thawing events. In situ soil ice contents were determined at 60-min intervals with SHB theory. Sensitivity of the SHB method to temperature, heat capacity, thermal conductivity, and time step size was analyzed based on numerically produced soil freezing and thawing events. The in situ ice contents determined with the SHB method were sometimes unrealistically large or even negative. Thermal conductivity accuracy and time step size were the key factors contributing to SHB errors, while temperature and heat capacity accuracy had less influence. Ice content estimated with a 15-min SHB time step was more accurate than that estimated with a 60-min time step. Sensitivity analysis indicated that measurement errors in soil temperature and thermal conductivity should be less than ±0.05°C and ±20%, respectively, but the error in the soil heat capacity could vary by ±50%. Thus, improving the accuracy of thermal conductivity measurements and using short time steps are required to accurately estimate soil ice contents with the SHB method.

Published in Agron. J. 106:1267–1274 (2014) doi:10.2134/agronj13.0488 Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. AbstrACt

ing period. The SHB evaporation estimates varied among R, I, and IE, with cumulative totals of 4.4, 7.4, and 7.9 mm, respectively, during a 12-d drying period. Lower soil water contents from plant water uptake reduced evaporation rates at R more appreciably with time than at the other positions; I and IE provided similar evaporation patterns. The SHB evaporation estimates at R and I were compared with microlysimeter data on 8 d. Correlation between approaches was modest (r 2 = 0.61) but significant (p < 0.001) when compared separately at R and I positions. Correlation was improved (r 2 = 0.81) when evaporation estimates were combined across positions, with differ ences between SHB and microlysimeters typically within the range of values obtained from microlysimeter replicates. Overall, the results suggest good potential for using SHB and modified SHB approaches to determine soil water evaporation in a cropped field. The SHB approach allowed continuous daily estimates of evaporation, separate from evapotranspiration and without destructive sampling.

The effect of soil wettability on coupled heat and water transfer in soil is not well understood. The objective of this work was to determine the effect of soil wettability on coupled heat and water transfer for two wettable soils and their artificially hydrophobized counterparts. Closed soil cells instrumented with heat-pulse sensors provided in situ measurements of soil temperature, soil volumetric water content (q), soil thermal conductivity (l), and soil thermal diffusivity (a). Nonlinear temperature distributions developed in response to the 150°C m −1 temperature gradients applied to the soil cells for both the wettable and hydrophobized soils due to soil moisture redistribution. Hydrophobized sand and silt loam soils had different responses relative to their wettable counterparts. Soil moisture redistribution was similar in the wettable and hydrophobized sand soils. Net water transfer was reduced by 56% in the hydrophobized silt loam compared with the wettable silt loam. Reduced net water transfer in the hydrophobized silt loam indicated that water vapor transfer must have been lower in the hydrophobized silt loam than in the wettable silt loam. Diffusive pathways for water vapor may increase due to hydrophobicity. In both the wettable and hydrophobized soils, l and a decreased in the warm regions and increased in the cold regions of the soil cells due to soil moisture redistribution. Wettability is a function of water content, and relative to wettable soil, water redistribution in hydrophobized soil is reduced only when the water content is small enough for the soil to behave as hydrophobic. If the water content is large enough for a hydrophobized soil to behave as wettable, water redistribution is similar to that in a wettable soil.

Concerns over runoff water quality from agricultural lands and construction sites have led to the development of improved erosion control practices, including application of polyacrylamide (PAM). We developed a quick and reliable method for quantifying PAM in soil extracts at low carbon content by using a turbidimetric reagent, Hyamine 1622. Three high-molecular weight anionic PAMs differing in charge density (7, 20, and 50 mol%) and five water matrices, deionized (DI) water and extracts from four different soils, were used to construct PAM calibration curves by reacting PAM solutions with hyamine and measuring turbidity development from the PAM–hyamine complex. The PAM calibration curve with DI water showed a strong linear relationship (r 2 = 0.99), and the sensitivity (slope) of calibration curves increased with increasing PAM charge density with a detection limit of 0.4 to 0.9 mg L -1 . Identical tests with soil extracts showed the sensitivity of the hyamine method was dependent on the properties of the soil extract, primarily organic carbon concentration. Although the method was effective in mineral soils, the highest charge density PAM yielded a more reliable linear relationship (r 2 > 0.97) and lowest detection limit (0.3 to 1.2 mg L -1 ), compared with those of the lower charge density PAMs (0.7 to 23 mg L -1 ). Our results suggest that the hyamine test could be an efficient method for quantifying PAM in environmental soil water samples as long as the organic carbon in the sample is low, such as in subsurface soil material often exposed at construction sites.

In situ determination of soil freezing and thawing is difficult despite its importance for many environmental processes. A sensible heat balance (SHB) method using a sequence of heat pulse probes has been shown to accurately measure water evaporation in subsurface soil, and it has the potential to measure soil freezing and thawing. Determination of soil freezing and thawing may be more challenging than evaporation, however, because the latent heat of fusion is smaller than the latent heat of vaporization. Furthermore, convective heat flow associated with liquid water flow and occurrence of evaporation or condensation during freezing and thawing may cause inaccurate estimation of freezing and thawing with the SHB method. The objective of this study was to examine the applicability of the SHB concept to soil freezing and thawing. Soil freezing and thawing events were simulated with the simultaneous heat and water (SHAW) model. Ice contents were estimated by applying the SHB concept to numerical data produced by the SHAW model. Close agreement between the SHB-estimated and the SHAW-simulated ice contents were observed at depths below 24 mm. The main cause of inaccuracies with the SHB method was poor estimation of heat conduction at the 12-mm depth, possibly due to simplifications of temporal or vertical distributions of temperature and thermal conductivity. The effects of convective heat flow and concurrent evaporation or condensation and freezing or thawing on the SHB method were small. The results indicate that the SHB method is conceptually suitable for estimating soil freezing and thawing. Independent, accurate estimates of thermal properties must be available to effectively use the SHB method to determine in situ soil freezing and thawing.

Given the ever-present threat of drought and the knowledge that water availability is the strongest limiting factor in vegetation growth, it is important to characterize the effect of water limitations on agricultural production. In this study, a small field plot technique for controlling soil moisture content suitable for physiological research in moist, humid areas was tested. We characterized the effect of water stress on total leaf conductance (gₗ) for two distinct determinate soybean [Glycine max (L.) Merr.] genotypes. Based on these findings, a model of gₗ as a function of plant age and soil moisture content was formulated and validated. The dependency of gₗ on plant age was well represented by a parabolic function that increased throughout the vegetative period, peaked around anthesis, and decreased throughout the reproductive period and senescence. A sigmoidal function explained the relation of gₗ to plant-available soil water content. This new empirical model effectively quantifies the response of gₗ to plant-available soil water and plant age with a functional form similar to the abscisic acid related Tardieu–Davies model.

Spence, Porche L., Deanna L. Osmond, Wesley Childres, Joshua L. Heitman, and Wayne P. Robarge, 2012. Effects of Lawn Maintenance on Nutrient Losses Via Overland Flow During Natural Rainfall Events. Journal of the American Water Resources Association (JAWRA) 48(5): 909-924. DOI: 10.1111/j.1752-1688.2012.00658.x Abstract: A sampling system was used to evaluate the effect of residential lawn management on nutrient losses via overland flow generated during natural rainfall events from three residential landscapes: a high maintenance fescue lawn (HMFL), a low maintenance fescue lawn (LMFL), and a mixed forested residential landscape (FRL). A sampling system was located in designated areas within each landscape such that 100% of the runoff follows natural flow paths to the outlet ports and collects in sterile Nalgene® B3 media bags (Thermo Fisher Scientific, Rochester, NY). A rainfall event was defined as producing ≥2.54 mm of water. A total of 87 rainfall events occurred during a 20-month monitoring period. The total runoff volume collected from the LMFL was higher than from the HMFL and FRL, but on average

Soil water evaporation is an important component of the surface water balance and the surface energy balance. Accurate and dynamic measurements of soil water evaporation enhance the understanding of water and energy partitioning at the land–atmosphere interface. The objective of this study was to measure the cumulative soil water evaporation with time and depth in a bare field. Cumulative water evaporation at the soil surface was measured by the Bowen ratio method. Subsurface cumulative soil water evaporation was determined with the heat pulse method at fine-scale depth increments. Following rainfall, the subsurface cumulative evaporation curves followed a pattern similar to the surface cumulative evaporation curve, with approximately a 2-d lag before evaporation was indicated at the 3- and 9-mm soil depths, and several more days' delay in deeper soil layers. For a 21-d period in 2007, the cumulative evaporation totals at soil depths of 0, 3, 9, 15, and 21 mm were 60, 44, 29, 13, and 8 mm, respectively. For a 16-d period in 2008, the cumulative evaporation totals at soil depths of 0, 3, 9, 15, and 21 mm were 32, 25, 16, 10, and 5 mm, respectively. Cumulative evaporation results from the Bowen ratio and heat pulse methods indicated a consistent dynamic pattern for surface and subsurface water evaporation with both time and depth. These findings suggest that heat pulse sensors can accurately measure subsurface soil water evaporation during several wetting–drying cycles.

Sod-based production systems have been successful in the southeastern and mid-Atlantic regions of the United States as an alternative to conventional tillage systems. However, research comparing these systems in North Carolina is limited. Therefore, research was conducted at four locations in North Carolina to compare corn (Zea mays L.), cotton (Gossypium hirsutum L.), peanut (Arachis bypogaea L.), and soybean [Glycine max (L.) Merr.] yield when these crops were strip tilled following 4 yr of continuous tall fescue (Shedonorus phoenix Scop.) vs. 4 yr of either corn or cotton grown in no tillage or strip tillage. Cotton yield was higher following tall fescue compared with yield following agronomic crops. In contrast, yield of corn was lower following tall fescue compared with agronomic crops while peanut and soybean yields were not affected by previous cropping history. Additional treatments in peanut included conventional tillage following both cropping systems, and pod yield was lower when peanut was strip tilled into either tall fescue or residue from corn or cotton compared with conventional tillage systems. No major differences in soil bulk density at depths of 0 to 8 cm or 8 to 16 cm were noted when comparing tall fescue or agronomic crops either in strip tillage or nontilled zones. Populations of soil parasitic nematodes were often lower in peanut following tall fescue than when following agronomic crops. These experiments indicate that sod-based systems may be an effective alternative to reduced tillage systems, especially for cotton. However, yield benefits were not observed for peanut or soybean and corn was negatively affected by tall fescue.

It is difficult to demonstrate the soil water retention relationship and related concepts because the specialized equipment required for performing these measurements is unavailable in most classrooms. This article outlines a low-cost, easily visualized method by which these concepts can be demonstrated in most any classroom. Columns (62.5 cm tall) were constructed using 25, 2.5 cm tall sections of 7.62-cm (3-inch) i.d. polyvinyl chloride pipe, which were connected using transparent tape. Three different soil materials were packed to specified bulk densities in the columns, and saturated with water. These vertical columns were then allowed to drain into a simulated water table 2.5 cm above the bottom of the soil volume until drainage ceased. After drainage, columns were sectioned to determine water content distribution with depth along the column. It was assumed that matric potential was inversely related to height above the water table. Therefore, water content measurements and assumed potentials for each section provided data for a water retention curve with minimum potential of approximately –60 cm. During drainage, measurements of soil matric potential were taken at regular intervals using tensiometers installed within the column, validating assumptions about matric potential. Among soil materials tested, those with narrow particle-size distributions, ∼100% sand, gave the widest distribution of water contents in the observed matric potential ranges. This method, with proper explanation and execution, may be a valuable learning tool by which visual, auditory, and kinesthetic learners may be better able to understand the concepts pertaining to soil–water retention relationships.

This research developed a model of saturated hydraulic conductivity for golf course and athletic field media. The model was developed from saturated flow data in packed sand cores, for which a pore-size distribution was derived from particle-size distribution, bulk density, and measured soil water characteristic data. The pores were first assumed to form an idealized structure, consisting of non-tortuous capillary tubes of uniform shape and size, and the Hagen-Poiseuille flow equation was applied to compute idealized saturated flow. The idealized saturated flows were compared with saturated flows derived from the measured saturated hydraulic conductivity data. Subsequently, an empirical relationship was established between the two in the form: Q t(m) = c + d Qt(h-p) , where Q t(m) is the saturated flow through the natural-structure sand cores and Q t(-p) is the saturated flow through the idealized pore structure for the same core. In our study, parameters c and d had values of -1.675 and 0.308, respectively, and the r 2 of the regression had a value of 0.871. The model was applied to 14 golf course sands and produced excellent results with minor anomalies.

The soil water retention curve (SWRC) is a key tool for understanding the fundamental relationship between soil moisture content and its associated energy. The objective of this study was to measure soil water retention including hysteresis at the dry end of the SWRC and to examine the effect of wettability on the SWRCs of two wettable soils and their hydrophobized counterparts. Two conditions, wettable and hydrophobic, were measured for each soil. The method used to measure the SWRCs was vapor equilibration over salt solutions of known osmotic potentials. Free water in the form of individual droplets was found to be present at the surface of the unwashed hydrophobic soils due to decreases in the osmotic potential during the hydrophobizing process. Water droplets did not form on the hydrophilic and washed hydrophobic soils. Soil wettability was found to affect soil water retention in relatively dry soil. The hydrophilic soils used in this study exhibited significant hysteresis in the water potential range of ―2.3 to ―19.2 MPa. Soil wettability and hysteresis should be considered when studying water sorption and desorption in relatively dry soil.