Heterogeneity Of Water Content Research Articles

Plot Layout Method of Field Experiment for Wheat with Border Irrigation Based on Soil Water Content Heterogeneity

The objective of this research was to improve the accuracy and representativeness of experimental plot studies by determining the optimum plot area and replication number for winter wheat with border irrigation. Considering the spatial distribution of soil water content, the border effect in relation to crop growth, and the lateral seepage of soil water, we sought to study and optimize the area and specifications of irrigation experiment plots with different levels and replicates. The results show that the experimental irrigation plot consisted of two parts—the core area and the guard area. The most suitable area for the experiment plot core area, with a single level and without replicates, was 60–80 m2. The core experimental area can be arranged with two replicates per 40 m2, with differences in soil moisture content between the treatments reaching more than 15% at the two experiment levels. Each plot comprised two replicates, or if they were 20 m2, then they contained three replicates; when the soil moisture contents differed between 10% and 15%, the area of each replicate plot was 80 m2, comprising two replicates, or 30 m2 with three replicates. When the difference in soil moisture content between the treatments exceeded 15% with the three experimental levels, the area of each plot was 30 m2 and they contained two replicates, or 20 m2 containing three replicates; at differences of 10% to 15%, each replicate plot was 50 m2 containing two replicates, or 30 m2 with three replicates. The experimental plots were rectangular, with irrigation furrows dug lengthwise; therefore, the plots had aspect ratios between 7:1 and 5:1. The width of the buffer area was over 60 cm. The effect of the border on plant height and LAI for winter wheat primarily emerged with one to three rows (20–60 cm) at the jointing stage, while the effect on grain yield and biomass in winter wheat mainly emerged with one to two rows (20–40 cm). The conclusions of this research will inform the development of surface irrigation methods for silt loam in northern Henan, as a reference for optimizing experiment plots employing border irrigation with different soil textures.

Rhizosphere water content drives hydraulic redistribution: Implications of pore-scale heterogeneity to modeling diurnal transpiration in water-limited ecosystems

Trees typically survive prolonged droughts by absorbing water from deeper layers. Where soils are shallow, roots may be extract water from the underlying fractured bedrocks. In dry seasons, surface-soil moisture dynamics reflect hydraulic redistribution (HR). HR is usually estimated based on the gradient of mean, or bulk, soil water potential among layers in the rooting zone (HRB). This approach neglects the potential effect of spatial heterogeneity of water content at the millimeter scale between the rhizosphere and bulk soil. We proposed to account for the rhizosphere water balance, estimating HR to the rhizosphere (HRR) of the dry surface soil from the underlying fractured rock. The model was evaluated using a 15-year dataset collected in Sardinia. When the typical approach, based on moisture gradients among bulk soil layers, was used for estimating HRB, tree transpiration was underpredicted in all seasons, especially in spring and summer. Forcing the model with measured tree transpiration, HRB decreased during spring and summer, while the contribution of the underlying rock layer to tree transpiration was threefold that estimated using HRR-based model. The average water content of the bulk surface soil layer was very low, reaching 0.06 in the driest summers while showing little diurnal dynamics; however, concentrating water in roughly estimated rhizosphere volume, produced rhizosphere water content appreciably higher (≈0.16), and much more dynamic. Predicted HRR dominated evapotranspiration (60% - 65%) in dry springs and summers reaching 80% of tree transpiration. Most importantly, the proposed rhizosphere-HR model correctly predicts the diurnal dynamics of tree transpiration year-round, and the grass transpiration in its active spring period. Eco-hydrological models operating at sub-daily scale should consider partitioning the soil to rhizosphere volume, thus allowing both diagnostic and prognostic estimates of diurnal biosphere-atmosphere mass and energy exchanges.

Measurement of the relative air permeability of compacted earth in the hygroscopic regime of saturation

The hygroscopic behavior of earthen materials has been extensively studied in the past decades. However, while the air flow within their porous network may significantly affect the kinetics of vapor transfer and thus their hygroscopic performances, few studies have focused on its assessment. For that purpose, a key parameter would be the gas permeability of the material, and its evolution with the relative humidity of the air. Indeed, due to the sorption properties of earthen material, an evolution of the water content, and thus of relative permeability, are foreseeable if the humidity of in-pore air changes. To fill this gap, this paper presents the measurement of relative permeabilities of a compacted earth sample with a new experimental set-up. The air flow through the sample is induced with an air generator at controlled flow rate, temperature, and humidity. The sample geometry was chosen in order to reduce, as much as possible, its heterogeneity in water content, and the tests were realized for several flow rates. The results, which show the evolution of gas permeability with the relative humidity of the injected air and with the water content of the material, either in adsorption or in desorption, were eventually successfully compared to predictions of the well-known Corey's law.

Accounting for Vertical Subgrid‐Scale Heterogeneity in Low‐Level Cloud Fraction Parameterizations

AbstractMany general circulation models (GCMs) assume some heterogeneity of water amounts in their grid boxes and use probability density functions to parameterize cloud fractions CF and amounts of condensed water qc. Most GCM cloud schemes calculate the CF as the volume of the grid box that contains clouds (CFvol), whereas radiative fluxes primarily depend on the CF by surface (CFsurf), that is, the surface of the grid box covered by clouds when looking from above. This discrepancy matters as previous findings suggest that CFsurf is typically greater than CFvol by about 30%. In this paper we modify the single column model version of the LMDz GCM cloud scheme by introducing the vertical subgrid‐scale heterogeneity of water content. This allows to distinctly compute the two fractions, CFvol and CFsurf, as well as the amount of condensed water qc. This study is one of the first to take into account such vertical subgrid‐scale heterogeneity in a GCM cloud scheme. Three large eddy simulation cases of cumuliform boundary layer clouds are used to test and calibrate two different parameterizations. These new developments increase cloud cover by about 10% for the oceanic cases RICO and Barbados Oceanographic Meteorological Experiment and by up to 50% for the continental case ARM. The change in condensed water reduces the liquid water path by 10–20% and therefore the cloud opacity by 5–50%. These results show the potential of the new scheme to reduce the too few, too bright bias by increasing low‐level CF and decreasing cloud reflectance.

Vegetation impacts soil water content patterns by shaping canopy water fluxes and soil properties

AbstractSoil water content is a key variable for biogeochemical and atmospheric coupled processes. Its small‐scale heterogeneity impacts the partitioning of precipitation (e.g., deep percolation or transpiration) by triggering threshold processes and connecting flow paths. Forest hydrologists frequently hypothesized that throughfall and stemflow patterns induce soil water content heterogeneity, yet experimental validation is limited. Here, we pursued a pattern‐oriented approach to explore the relationship between net precipitation and soil water content. Both were measured in independent high‐resolution stratified random designs on a 1‐ha temperate mixed beech forest plot in Germany. We recorded throughfall (350 locations) and stemflow (65 trees) for 16 precipitation events in 2015. Soil water content was measured continuously in topsoil and subsoil (210 profiles). Soil wetting was only weakly related to net precipitation patterns. The precipitation‐induced pattern quickly dissipates and returns to a basic pattern, which is temporally stable. Instead, soil hydraulic properties (by the proxy of field capacity) were significantly correlated with this stable soil water content pattern, indicating that soil structure more than net precipitation drives soil water content heterogeneity. Also, both field capacity and soil water content were lower in the immediate vicinity of tree stems compared to further away at all times, including winter, despite stemflow occurrence. Thus, soil structure varies systematically according to vegetation in our site. We conclude that enhanced macroporosity increases gravity‐driven flow in stem proximal areas. Therefore, although soil water content patterns are little affected by net precipitation, the resulting soil water fluxes may strongly be affected. Specifically, this may further enhance the channelling of stemflow to greater depth and beyond the rooting zone.

Rock outcrops redistribute water to nearby soil patches in karst landscapes.

The emergence of rock outcrops is very common in terrestrial ecosystems. However, few studies have paid attention to their hydrological role in the redistribution of precipitation, especially in karst ecosystems, in which a large proportion of the surface is occupied by carbonate outcrops. We collected and measured water received by outcrops and its subsequent export to the soil in a rock desertification ecosystem, an anthropogenic forest ecosystem, and a secondary forest ecosystem in Shilin, China. The results indicated that outcrops received a large amount of water and delivered nearly half of it to nearby soil patches by means of runoff. No significant difference was found in the ratio of water received to that exported to the soil by outcrops among the three ecosystems annually. When the outcrop area reaches 70% of the ground surface, the amount of water received by soil patches from rock runoff will equal that received by precipitation, which means that the soil is exposed to twice as much precipitation. This quantity of water can increase water input to nearby soil patches and create water content heterogeneity among areas with differing rock emergence.

Interactions of soil water content heterogeneity and species diversity patterns in semi-arid steppes on the Loess Plateau of China

• Coverage, biomass were significantly increased with surface soil water contents. • Plant diversity were closely correlated to soil water content of 0–10 cm soil layer. • Soil water content affect species richness and then affect belowground biomass. • Species diversity is an indicator of community responses to soil water content. Soil water is a major driving force for plant community succession in semi-arid area. Many studies have focused on the relationships of species diversity–productivity, but few studies have paid attentions to the effects of soil water content heterogeneity on the plant species diversity in the semi-arid loess regions. To determine relationship of soil water content heterogeneity and plant community structure properties a semi-arid steppe on the Loess Plateau, we conducted a gradient analysis of soil water content variation and above- and below-ground properties of plant communities. Results showed that community coverage, above- and below-ground biomass were significantly and positively related to the surface soil water contents (0–5 cm). Plant diversity (Shannon index and Richness index) were closely correlated to soil water content at the soil depth of 0–10 cm. But plant height, litter biomass and root/shoot ratio were not related to soil water content. These results showed that there is an positive interaction effects for plant diversity and soil water content in the semi-arid grassland communities. Our observations indicate that change of plant species diversity is also an important community responses to soil water content heterogeneity in the semi-arid grassland ecosystem.

A Procedure to Measure the in-Situ Hygrothermal Behavior of Earth Walls.

Rammed earth is a sustainable material with low embodied energy. However, its development as a building material requires a better evaluation of its moisture-thermal buffering abilities and its mechanical behavior. Both of these properties are known to strongly depend on the amount of water contained in wall pores and its evolution. Thus the aim of this paper is to present a procedure to measure this key parameter in rammed earth or cob walls by using two types of probes operating on the Time Domain Reflectometry (TDR) principle. A calibration procedure for the probes requiring solely four parameters is described. This calibration procedure is then used to monitor the hygrothermal behavior of a rammed earth wall (1.5 m × 1 m × 0.5 m), instrumented by six probes during its manufacture, and submitted to insulated, natural convection and forced convection conditions. These measurements underline the robustness of the calibration procedure over a large range of water content, even if the wall is submitted to quite important temperature variations. They also emphasize the importance of gravity on water content heterogeneity when the saturation is high, as well as the role of liquid-to-vapor phase change on the thermal behavior.

Water content of the mantle xenoliths from Kimberley and implications for explaining textural variations in cratonic roots

AbstractWater contents estimates based on infrared spectroscopy and deformation microstructures are used to constrain the origin of textural variations observed in mantle xenoliths from Kimberley, South Africa. Infrared spectra indicate water contents of 10–370 ppm (H2O by weight) in olivine, 20–370 ppm in orthopyroxene, up to 340 ppm in garnet and 30–550 ppm in clinopyroxene, although no systematic differences are detected among texturally distinct xenoliths (i.e. granular and sheared peridotites). In contrast, active slip systems in olivine, as inferred from tilt boundaries, are different between the granular and sheared peridotites; the former deformed via the [100](010) slip system, which is commonly active under hot, dry conditions, whereas the latter deformed via the [001](100) slip system, which is commonly activated in water‐rich environments. The discrepancy in water contents estimates based on infrared spectroscopy and deformation microstructures may reflect different stages of the evolution of water content: the present water contents, as analysed by infrared spectroscopy, may have been modified by the most recent events (e.g. eruption by kimberlite magma), whereas the water contents inferred from tilt boundaries may have resulted from the various deformation processes in the deep cratonic mantle. Accordingly, we suggest that the observed heterogeneity in water content within cratonic roots facilitates local deformation, and gives rise to the range of textures seen in cratonic mantle xenoliths. Copyright © 2010 John Wiley & Sons, Ltd.

Interpretation and evaluation of combined measurement techniques for soil CO2 efflux: Discrete surface chambers and continuous soil CO2 concentration probes

Soil CO2 efflux is a large respiratory flux from terrestrial ecosystems and a critical component of the global carbon (C) cycle. Lack of process understanding of the spatiotemporal controls on soil CO2 efflux limits our ability to extrapolate from fluxes measured at point scales to scales useful for corroboration with other ecosystem level measures of C exchange. Additional complications are introduced by the effects of soil water content seasonality and rainfall on the performance of measurement techniques. In this paper we present measurements of soil CO2 efflux made at two contrasting sites within a characteristic subalpine forest of the northern Rocky Mountains. Comparison of measurements between the soil respiration chamber technique and the soil CO2 profile technique over daily and seasonal time scales indicated that soil water content plays a major role in the magnitude and seasonality of soil CO2 efflux, especially after snowmelt or following summer rainfall. Agreement between both techniques was limited during high soil water content conditions and after summer rainfall. Differences in diel hysteresis patterns of soil CO2 efflux between sites were controlled by the effects of canopy cover and temporal differences in photosynthetic activity of vegetation. Our results indicate that an accurate parameterization of soil water content heterogeneity in space and time must be a critical component of realistic model representations of soil CO2 efflux from heterogeneous landscapes.

Measurements and Modeling of Variable Gravity Effects on Water Distribution and Flow in Unsaturated Porous Media

Liquid behavior under reduced gravity conditions is of considerable interest for various components of life‐support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast‐responding tensiometers measured changes in water content and matric potential. For near‐saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20‐s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity.

The influence of time domain reflectometry rod induced flow disruption on measured water content during steady state unit gradient flow

Intrusive measurement techniques require placing a sensor within the sample, possibly changing the conditions under which the measurement is collected and thereby affecting the quality of the measurement. In this study we consider time domain reflectometry (TDR), which is an intrusive water content measurement method. TDR rods are impermeable, and thus water is forced to flow around the rods. In an unsaturated medium this changes the water content distribution in the vicinity of the rods, with the water content increased at the tops of the rods and decreased at the bottoms of the rods. TDR has nonuniform spatial sensitivity, with much higher sensitivity immediately adjacent to rods, in the regions that experience the greatest change in water content due to this flow disruption. Furthermore, the spatial sensitivity of TDR depends on the water content distribution within the sample volume. This raises the possibility that flow disruptions caused by TDR rods may affect the TDR‐measured water content. In this study we are specifically interested in the effects of flow disruption due to TDR rods. Therefore we consider steady state unit gradient unsaturated flow in a homogeneous medium to eliminate spatial heterogeneity of water content due to soil heterogeneity and transient flow conditions. For common TDR probe designs in the wide range of soils examined, flow disruption gives rise to a water content measurement error that is less than 0.005 cm3 cm−3. This is smaller than the reported accuracy of the TDR method of 0.02 cm3 cm−3. As a result, we conclude that it is appropriate to ignore flow disruption caused by commonly used TDR probes when assessing sources of TDR measurement error.

Obtaining the Spatial Distribution of Water Content along a TDR Probe Using the SCEM‐UA Bayesian Inverse Modeling Scheme

Time domain reflectometry (TDR) has become one of the standard methods for the measurement of the temporal and spatial distribution of water saturation in soils. Current waveform analysis methodology gives a measurement of the average water content along the length of the TDR probe. Close inspection of TDR waveforms shows that heterogeneity in water content along the probe can be seen in the TDR waveform. We present a comprehensive approach to TDR waveform analysis that gives a quantitative estimate of the dielectric permittivity profile along the length of the probe and, therefore, the distribution of water content. The approach is based on the combination of a multisection scatter function model for the TDR measurement system with the shuffled complex evolution Metropolis algorithm (SCEM‐UA). This combined approach allows for the estimation of the 40 parameters in the transmission line model using a series of simple calibration measurements. The proof of concept is given with measurements in a layered system consisting of air and water. Finally, TDR waveforms from layered soil samples were analyzed to estimate the distribution of the water content along the length of the probe. Results show that the proposed method provides much more reproducible results than obtained with the traditional travel time method. Because the proposed method can be fully automated, it increases the applicability of the TDR method, especially in applications where detailed (real‐time) data are required on heterogeneous infiltration.

Obtaining the Spatial Distribution of Water Content along a TDR Probe Using the SCEM-UA Bayesian Inverse Modeling Scheme

Time domain reflectometry (TDR) has become one of the standard methods for the measurement of the temporal and spatial distribution of water saturation in soils. Current waveform analysis methodology gives a measurement of the average water content along the length of the TDR probe. Close inspection of TDR waveforms shows that heterogeneity in water content along the probe can be seen in the TDR waveform. We present a comprehensive approach to TDR waveform analysis that gives a quantitative estimate of the dielectric permittivity profile along the length of the probe and, therefore, the distribution of water content. The approach is based on the combination of a multisection scatter function model for the TDR measurement system with the shuffled complex evolution Metropolis algorithm (SCEM-UA). This combined approach allows for the estimation of the 40 parameters in the transmission line model using a series of simple calibration measurements. The proof of concept is given with measurements in a layered system consisting of air and water. Finally, TDR waveforms from layered soil samples were analyzed to estimate the distribution of the water content along the length of the probe. Results show that the proposed method provides much more reproducible results than obtained with the traditional travel time method. Because the proposed method can be fully automated, it increases the applicability of the TDR method, especially in applications where detailed (real-time) data are required on heterogeneous infiltration.

Accuracy and Reproducibility of Mapping Surface Soil Water Content with the Ground Wave of Ground-Penetrating Radar

Ground-penetrating radar (GPR) is an attractive technique for the non-destructive measurement of soil water content. In this paper we investigated the accuracy of volumetric water content measurements with the ground wave of GPR. In particular, we 1) examined the influence of equipment in the vicinity of the GPR antennas on the data quality and 2) compared two methods to calculate soil permittivity from the arrival time of the air and ground wave. The first method assumes that the air wave and ground wave arrive simultaneously at zero antenna offset, whereas the second method includes a correction for a time shift in arrival time at zero antenna offset. We tested the influence of the equipment on the radar by introducing the different components (e.g., sleds, odometer and vehicle to pull equipment) stepwise. The results showed that the use of sleds caused a decrease in the amplitude of the ground wave. The presence of the sleds also changed the pulse shape of the air and ground wave and caused disturbances in the air wave. However, the data quality after the addition of all components was sufficient to allow automatic picking of arrival times by standard GPR software. The importance of the time shift at zero antenna offset was assessed with 20 duplicate WARR measurements measured along a [Formula: see text] transect and a sensitivity analysis based on GPR simulations. The results showed that: 1) there was a mean time shift at zero offset of [Formula: see text] that could not be explained from the GPR simulations and 2) there was a large variation in time shift at zero offset that could be explained by considering time picking error, positional error and soil water content heterogeneity in the sensitivity analysis. We conclude that a time shift correction in the calculation of soil permittivity would improve absolute soil water content measurements with GPR, but it should be noted that the large variation in time shifts makes it difficult to accurately estimate the mean time shift at zero offset.

Analyses of soil water content variations and GPR attribute distributions

Water flux was investigated in the frame of the project ‘preferential flow paths—3D water and solute dynamic in heterogeneous media’. The objective of the study was the non-destructive three-dimensional monitoring and description of heterogeneous flux fields with hydrological and geophysical methods. A large tank filled with homogeneous sand was set up to realize infiltration experiments. We compared the parameter distribution calculated from measurements of a ground penetrating radar system (GPR) with a simulated water content distribution using a two-dimensional numerical model based on the Van Genuchten–Mualem approach in order to assess the effectiveness of the geophysical measure for the characterization of soil water content variations. A statistical examination of both simulated water contents based on independent measured soil properties and reflection amplitudes from radargrams indicated a better conformity between geophysical data and simulated water contents assuming a heterogeneous hydraulic parameter distribution. The heterogeneous nature of the sand body could be confirmed by dye tracer experiments. The analyzed GPR attribute, the distribution of the maximum reflection amplitudes, may serve in future studies as an indicator for the expected water content heterogeneity in sandy soils.

Spatial heterogeneity of soil water around single roots: use of CT-scanning to predict fungal growth in the rhizosphere.

Whole-body computed tomography X-ray scanning (X-ray CT) of wheat, cotton and radish growing in pots containing 350 g of a white sand: clay mixture produced images from which digital data of the volumetric water content were obtained. The volumetric water contents were spatially heterogeneous around the root and altered with the depth at which the image was taken. Data represent a 'snapshot in time' of the soil around single roots in a living soil-plant system. The volumetric water contents of soil around seedlings were assessed for the growth of two soil-borne fungi, Gaeumannomyces graminis var. tritici and Rhizoctonia solani. The heterogeneity of water content was biologically significant: water contents around the roots of wheat, radish and cotton were of a great enough range to suggest the existence of heterogeneous sites in the rhizosphere which might favour or restrict fungal growth. In soil surrounding wheat roots, the most favourable regions for the growth of G. graminis were nearest the root, suggesting that the inner rhizosphere might be more amenable to the growth of this pathogenic fungus than the outer region of the rhizosphere, and the bulk soil. The technique of X-ray CT enables spatial examination of the response of soil water to environmental treatments of the soil-root system, and the non-destructive observation of the effect of the spatial heterogeneity of volumetric water content upon fungal growth in the soil, over time and at a range of depths.