Sensible Heat Observations Reveal Soil-Water Evaporation Dynamics

Soil-water evaporation is important at scales ranging from microbial ecology to large-scale climate. Yet routine measurements are unable to capture rapidly shifting near-surface soil heat and water processes involved in soil-water evaporation. The objective of this study was to determine the depth and location of the evaporation zone within soil. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, and temperature below a bare soil surface in central Iowa during natural wetting/drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance, attributed to latent heat from water vaporization, provides an estimate of in situ soil-water evaporation. As the soil dried following rainfall, results show divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicates the location of a heat sink associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provide depth and time patterns consistent with observed soil-water depletion patterns. Immediately after rainfall, evaporation occurred near the soil surface. Within 6 days after rainfall, the evaporation zone proceeded \u3e 13 mm into the soil profile. Evaporation rates at the 3-mm depth reached peak values \u3e 0.25 mm h−1. Evaporation occurred simultaneously at multiple measured depth increments, but with time lag between peak evaporation rates for depths deeper below the soil surface. Implementation of finescale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation

Sensible Heat Balance Measurements of Soil Water Evaporation beneath a Maize Canopy

Soil water evaporation is an important component of the water budget in cropped fields; few methods are available for continuous and independent measurement. A sensible heat balance (SHB) approach has been demonstrated for continuously determining soil water evaporation under bare surface conditions. Applicability of SHB measurements beneath a crop canopy cover has not been evaluated. We tested SHB using heat-pulse sensors to estimate evaporation beneath a full maize (Zea mays L.) canopy. We also implemented a modified SHB approach incorporating below-canopy net radiation, which extended the range of conditions under which SHB is applicable. Evaporation was measured at three positions: row (R), interrow (I), and interrow with roots excluded (IE). Evaporation rates were generally small, averaging −1 across all dates, positions, and measurement methods during the drying 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 (r2 = 0.61) but significant (p \u3c 0.001) when compared separately at R and I positions. Correlation was improved (r2 = 0.81) when evaporation estimates were combined across positions, with differences 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

An Improved Approach for Measurement of Coupled Heat and Water Transfer in Soil Cells

Laboratory experiments on coupled heat and water transfer in soil have been limited in their measurement approaches. Inadequate temperature control creates undesired two-dimensional distributions of both temperature and moisture. Destructive sampling to determine soil volumetric water content (θ) prevents measurement of transient θ distributions and provides no direct information on soil thermal properties. The objectives of this work were to: (i) develop an instrumented closed soil cell that provides one-dimensional conditions and permits in situ measurement of temperature, θ, and thermal conductivity (λ) under transient boundary conditions, and (ii) test this cell in a series of experiments using four soil type–initial θ combinations and 10 transient boundary conditions. Experiments were conducted using soil-insulated cells instrumented with thermo-time domain reflectometry (T-TDR) sensors. Temperature distributions measured in the experiments show nonlinearity, which is consistent with nonuniform thermal properties provided by thermal moisture distribution but differs from previous studies lacking one-dimensional temperature control. The T-TDR measurements of θ based on dielectric permittivity, volumetric heat capacity, and change in volumetric heat capacity agreed well with post-experiment sampling, providing r 2 values of 0.87, 0.93, and 0.95, respectively. Measurements of θ and λ were also consistent with the shapes of the observed temperature distributions. Techniques implemented in these experiments allowed observation of transient temperature, θ, and λ distributions on the same soil sample for 10 sequentially imposed boundary conditions, including periods of rapid redistribution. This work demonstrates that, through improved measurement techniques, the study of heat and water transfer processes can be expanded in ways previously unavailable

Bare Soil Carbon Dioxide Fluxes with Time and Depth Determined by High-Resolution Gradient-Based Measurements and Surface Chambers

Soil CO2 production rates and fluxes vary with time and depth. The shallow near-surface soil layer is important for myriad soil processes, yet knowledge of dynamic CO2 concentrations and fluxes in this complex zone is limited. We used a concentration gradient method (CGM) to determine CO2 production and effluxes with depth in shallow layers of a bare soil. The CO2concentration was continuously measured at 13 depths in the 0- to 200-mm soil layer. For an 11-d period, 2% of the soil CO2 was produced below a depth of 175 mm, 8% was produced in the 50- to 175-mm soil layer, and 90% was produced in the 0- to 50-mm soil layer. Soil CO2concentration showed similar diurnal patterns with temperature in deeper soil layers and out-of-phase diurnal patterns in surface soil layers. Soil CO2 flux from most of the soil layers can be described by an exponential function of soil temperature, with temperature sensitivity (Q10) ranging from 1.40 to 2.00 (1.62 ± 0.17). The temperature-normalized CO2 fluxes are related to soil water content with a positive linear relationship in surface soil layers and a negative relationship in deep soil layers. The CO2 fluxes from CGM and chamber methods had good agreement at multiple time scales, which showed that the CGM method was able to estimate near-surface soil CO2 fluxes and production. The contrasting patterns between surface and deep layers of soil CO2 concentration and fluxes suggest the necessity of intensive CO2concentration measurements in the surface soil layer for accurate determination of soil-atmosphere CO2 flux when using the CGM

Cumulative Soil Water Evaporation as a Function of Depth and Time

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\u27 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

Latent heat in soil heat flux measurements

The surface energy balance includes a term for soil heat flux. Soil heat flux is difficult to measure because it includes conduction and convection heat transfer processes. Accurate representation of soil heat flux is an important consideration in many modeling and measurement applications. Yet, there remains uncertainty about what comprises soil heat flux and how surface and subsurface heat fluxes are linked in energy balance closure. The objective of this study is to demonstrate the presence of a subsurface latent heat sink, which must be considered in order to accurately link subsurface heat fluxes between depths near and at the soil surface. Measurements were performed under effectively bare surface conditions in a silty clay loam soil near Ames, IA. Soil heat flux was measured with heat-pulse sensors using the gradient heat flux approach at 1-, 3-, and 6-cm soil depths. Independent estimates of the daily latent heat sink were obtained by measuring the change of mass of microlysimeters. Heat flux measurements at the 1-cm depth deviated from heat flux measurements at other depths, even after calorimetric adjustment was made. This deviation was most pronounced shortly after rainfall, where the 1-cm soil heat flux measurement exceeded 400 W m−2. Cumulative soil heat flux measurements at the 1-cm depth exceeded measurements at the 3-cm depth by \u3e75% over a 7-day rain-free period, whereas calorimetric adjustment allowed 3- and 6-cm depth measurements to converge. Latent heat sink estimates from the microlysimeters accounted for nearly all of the differences between the 1- and 3-cm depth heat flux measurements, indicating that the latent heat sink was distributed between the 1- and 3-cm depths shortly after the rainfall event. Results demonstrate the importance of including latent heat when attempts are made to link or extrapolate subsurface soil heat flux measurements to the surface soil heat flux

Soil water retention and hydraulic conductivity dynamics following tillage

Soil bulk density (ρb) may be purposely reduced in agricultural fields using tillage to improve hydraulic properties. However, tillage alters the soil structure, resulting in unstable soils. As the soil stabilizes, ρb increases over time. While this is known, studies on soil hydraulic properties in tilled soils, including comparisons between tilled and non-tilled soils, commonly assume a rigid soil structure. This study presents changes in soil water retention and saturated hydraulic conductivity (Ksat) as ρb increased dynamically with time following tillage at a loam-textured field site. Over the summer of 2015, soil cores were collected at several depths below the surface following precipitation events. Soil water retention curves and Ksat were determined using pressure cells and the constant head method, respectively. Tillage reduced ρb to 0.94 g cm−3. Changes in ρb increased with depth, reaching a ρb of 1.11 g cm−3 in the 0–5 cm layer, and a ρb of 1.42 g cm−3 at the deepest tilled layer. Soil water retention curves were markedly steeper for samples with higher ρb, indicating an overall increase in water retained at a soil matric potential (Ψ) of −33 kPa. Evaluation of two modeling approaches for water retention as a function ρbindicated that changes in water retention with increases in ρb could be reasonably estimated if a matching point was used. No clear relationship between Ksat and ρbwas obvious for ρb \u3c 1.06 cm3 cm−3, but for ρb \u3e 1.06 cm3 cm−3, Ksat decreased markedly (order of magnitude) as ρb increased. Hydraulic properties varied strongly depending on time since tillage and soil depth, and results have implications for models of tilled soils, as well as for studies comparing tilled and non-tilled soils

Field Measurement of Soil Surface Chemical Transport Properties for Comparison of Management Zones

Management of chemicals in soil is important, yet the complexity of field soils limits prediction of management effects on transport. To date, few methods have been available for field measurement of chemical transport properties, but a recently developed dripper–time domain reflectometry technique allows rapid collection of data for determining these properties. The objective of this work was to apply this technique for comparison of chemical transport properties for different soil management zones. Experiments were conducted comparing four interrow management zones: no-till nontrafficked, no-till trafficked, chisel plow nontrafficked, and chisel plow trafficked. Drip emitters were positioned at 12 locations in each zone and used to apply water followed by a step input of CaCl2 tracer solution. Breakthrough curves were measured via electrical conductivity with time domain reflectometry probes. The mobile–immobile model was fit to the breakthrough curves to determine chemical transport properties. Mean chemical transport properties were 0.34, 0.11 h−1, 10 cm h−1, 164 cm2 h−1, and 5 cm, for the immobile water fraction, mass exchange coefficient, average pore-water velocity, mobile dispersion coefficient, and dispersivity, respectively. All five properties showed significant differences between management zones. Differences in mass exchange and mobile dispersion coefficients coincided with differences in tillage, while differences in mean pore water velocities coincided with differences in traffic. The immobile water fraction was largest for the no-till nontrafficked zone. These results represent one of very few reports for field measurement of chemical transport properties and the first application of this approach for comparison of chemical transport properties across management zones

Method for Maintaining One-Dimensional Temperature Gradients in Unsaturated, Closed Soil Cells

One-dimensional temperature gradients are difficult to achieve in nonisothermal laboratory studies because, in addition to desired axial temperature gradients, ambient temperature interference (ATI) creates a radial temperature distribution. Our objective was to develop a closed soil cell with limited ATI. The cell consists of a smaller soil column, the control volume, surrounded by a larger soil column, which provides radial insulation. End boundary temperatures are controlled by a new spiral-circulation heat exchanger. Four cell size configurations were tested for ATI under varying ambient temperatures. Results indicate that cells with a 9-cm inner column diameter, 5-cm concentric soil buffer, and either 10- or 20-cm length effectively achieved one-dimensional temperature conditions. At 30°C ambient temperature, and with axial temperature gradients as large as 1°C cm−1, average steady-state radial temperature gradients in the inner soil columns were−1 Thus, these cell configurations meet the goal of maintaining a one-dimensional temperature distribution. These cells provide new opportunities for improving the study of coupled heat and water movement in soil

Evaluation of a new, perforated heat flux plate design

Accurate measurement of heat flux is essential to optimize structural and process design and to improve understanding of energy transfer in natural systems. Laboratory and field experiments evaluated the performance of a new, perforated heat flux plate designed to reduce flow distortion for environmental applications. Laboratory tests involving dry and saturated sand showed that performance of the new CAPTEC plate is comparable to a solid, standard REBS plate. Very low thermal gradients may have however led to poor performance of the CAPTEC plate in saturated sand. Water infiltration and redistribution experiments using clayey and sandy soils showed an apparent reduced disruption of liquid water and vapour in the soil surrounding the CAPTEC plate as compared to solid Hukseflux and standard REBS plates. Surface area of REBS plate, though smaller than that of CAPTEC, did not lead to any significantly improved evaporation, due to perforations on CAPTEC plate. Field tests in a loam soil indicated that the CAPTEC plates were durable and produced daily total flux values within ~ 0.15 MJ m− 2 of independent estimates