Soil Drying Process Research Articles

NMR-based pore water distribution characteristics of silty clay during the soil compaction, saturation, and drying processes

The pore water distribution (PWD) of soil provides key information about soil shear strength, hydraulic conductivity, and water-retention ability. The PWD characteristics of the soil depend on the specimen preparation conditions (i.e., the variations in initial water content or compaction degree) and drying-wetting processes, which were investigated by using the nuclear magnetic resonance (NMR) technique. The NMR T2 spectrum is mathematically transformed into the PWD curve to more accurately quantify the variations in pore water content stored with pores of different radii. The pore water in silty clay can be categorized into strongly bound water, intra-aggregate pore water, and inter-aggregate pore water, which are identified by the pore radius of <0.05 μm, 0.05-1.0 μm and >1.0 μm, respectively. The effects of compaction degree and initial water content on the PWD of the soil exhibit distinctions. For unsaturated compacted specimens, the PWDs remain unaffected by changes in compaction degree, while an increase in initial water content promotes the formation of clay aggregates, thereby increasing the maximum water-holding pore radius and the intra-aggregate pore water content. For saturated compacted specimens, the inter-aggregate and intra-aggregate pore water content increases with a decrease in the compaction degree, while the changes in the PWD shape from unimodal to bimodal with an increase in the initial water content. During the soil drying process, inter-aggregate pore water is rapidly drained, the intra-aggregate dominant pore water content increases first and then decreases, and the strongly bound water content remains constant. The relationship between the cumulative pore water distribution curve and the soil-water characteristic curve (SWCC) can be established through the Young-Laplace theory. The SWCC can be well predicted by using the envelope of the cumulative pore water distribution curves during soil drying.

A nuclear magnetic resonance based quantification of pore water distribution in unsaturated soils

This study presents a novel approach to quantify the distribution characteristics of pore water in unsaturated soil by using the nuclear magnetic resonance (NMR) technology. The SWCC and NMR T2 curves under different matric suction were measured during the drying process of soil specimen, and a conversion relationship between T2 and pore radius r of was determined. The method using capillary drainage model to simulate soil drying process was feasible, and pores of unsaturated soil were divided into saturated pore, unsaturated pore, and dry pore when the suction exceeded the air entry value. Pore water distribution curve based on T2 well described the distribution characteristics of pore water in unsaturated soil; the quantification of pore water distribution was realized through a defined index named pore water damage potential (Mcp), and a good mathematical relationship was observed between the relative damage degree and the matric suction. By introducing the Mcp, a modified VG model that reflected the characteristics of pore water distribution was in a good quantitative agreement with the experimental observations. The results achieved provided a valuable reference for revealing the internal mechanism of SWCC and establishing mechanical model of unsaturated soil based on pore water distribution.

The Interrelationship Between Water Use Efficiency and Radiation Use Efficiency Under Progressive Soil Drying in Maize.

The maximizing of water use efficiency (WUE) and radiation use efficiency (RUE) is vital to improving crop production in dryland farming systems. However, the fundamental question as to the association of WUE with RUE and its underlying mechanism under limited-water availability remains contentious. Here, a two-year field trial for maize designed with five progressive soil drying regimes applied at two different growth stages (three-leaf stage and seven-leaf stage) was conducted during the 2013–2014 growing seasons. Both environmental variables and maize growth traits at the leaf and canopy levels were measured during the soil drying process. The results showed that leaf WUE increased with irrigation reduction at the early stage, while it decreased with irrigation reduction at the later stage. Leaf RUE thoroughly decreased with irrigation reduction during the progressive soil drying process. Aboveground biomass (AGB), leaf area index (LAI), a fraction of absorbed photosynthetically active radiation (fAPAR), and light extinction coefficient (k) of the maize canopy were significantly decreased by water deficits regardless of the growth stages when soil drying applied. The interrelationships between WUE and RUE were linear across the leaf and canopy scales under different soil drying patterns. Specifically, a positive linear relationship between WUE and RUE are unexpectedly found when soil drying was applied at the three-leaf stage, while it turned out to be negative when soil drying was applied at the seven-leaf stage. Moreover, the interaction between canopy WUE and RUE was more regulated by fAPAR than LAI under soil drying. Our findings suggest that more attention must be paid to fAPAR in evaluating the effect of drought on crops and may bring new insights into the interrelationships of water and radiation use processes in dryland agricultural ecosystems.

The Generation of Soil Spectral Dynamic Feedback Using Landsat 8 Data for Digital Soil Mapping

The soil spectral dynamic feedback captured from high temporal resolution remote sensing data, such as MODIS, during the soil drying process after a rainfall could assist with digital soil mapping. However, this method is ineffective in utilizing the images with a relatively high spatial resolution. There are an insufficient number of images in the soil drying process since those high spatial resolution images tend to have a low temporal resolution. This study is aimed at generating soil spectral dynamic feedback by integrating the feedback captured from the images with a high spatial resolution during the process of multiple drying after different rainfall events. The Landsat 8 data with a temporal resolution of 16 day was exemplified. Each single spectral feedback obtained from Landsat 8 was first adjusted to eliminate the impact of different rainfall magnitudes. Then, the soil spectral dynamic feedback was reorganized and generated based on the adjusted feedback. Finally, the soil spectral dynamic feedback generated based on Landsat 8 was used for mapping topsoil texture and compared with the mapping results based on the MODIS data and the fusion data of MODIS and Landsat 8. As revealed by the results, not only could the generated soil spectral dynamic feedback based on Landsat 8 data improve the details of the spatial distribution of soil texture, but it also enhances the accuracy of mapping. The mapping accuracy based on Landsat 8 data is higher than that based on the MODIS data and fusion data. The improvements of accuracy are more obvious in the areas with more complex surface conditions. This study widens the scope of application for soil spectral dynamic feedback and provides support for large-scale and high-precision digital soil mapping.

Construction of land surface dynamic feedback for digital soil mapping considering the spatial heterogeneity of rainfall magnitude

The land surface dynamic feedback (LSDF) information captured by time-series remote sensing data during the soil-drying process after a rainfall event provides effective covariates for digital soil mapping over low-relief areas. However, current methods used to capture LSDF require a uniform rainfall magnitude in the geographic space; a condition that is not often met for large areas. Here, we propose a LSDF construction method considering the spatial heterogeneity of rainfall magnitudes by adjusting the evaporation variables in the LSDF. For this, the relationships between evaporation and rainfall magnitudes were first established. The LSDFs from various locations for rainfall events with different magnitudes were then adjusted based on these relationships. Using a case study, the adjusted LSDFs after two rainfall events were then used to predict soil texture over a low-relief area. The results showed that the cubic polynomial model performed best when constructing the relationship between evaporation adjustment and rainfall magnitude, giving the highest R2 value and a low Akaike information criterion. Adjustment to the LSDF decreases with increasing rainfall and the rate of change in the adjustment also decreases with increasing rainfall. For both rainfall events, prediction accuracies with the adjusted LSDFs were higher than those based on the original LSDFs. Furthermore, the greater the adjustment, the greater the improvement in the accuracy. We conclude that the proposed construction method for LSDF, accounting for the spatial heterogeneity of rainfall magnitudes, offers improved predictive power for digital soil mapping over large areas.

A Physically Based Method for Soil Evaporation Estimation by Revisiting the Soil Drying Process

AbstractWhile numerous models exist for soil evaporation estimation, they are more or less empirically based either in the model structure or in the determination of introduced parameters. The main difficulty lies in representing the water stress factor, which is usually thought to be limited by capillarity‐supported water supply or by vapor diffusion flux. Recent progress in understanding soil hydraulic properties, however, have found that the film flow, which is often neglected, is the dominant process under low moisture conditions. By including the impact of film flow, a reexamination on the typical evaporation process found that this usually neglected film flow might be the dominant process for supporting the Stage II evaporation (i.e., the fast falling rate stage), besides the generally accepted capillary flow‐supported Stage I evaporation and the vapor diffusion‐controlled Stage III evaporation. A physically based model for estimating the evaporation rate was then developed by parameterizing the Buckingham‐Darcy's law. Interestingly, the empirical Bucket model was found to be a specific form of the proposed model. The proposed model requires the in‐equilibrium relative humidity as the sole input for representing water stress and introduces no adjustable parameter in relation to soil texture. The impact of vapor diffusion was also discussed. Model testing with laboratory data yielded an excellent agreement with observations for both thin soil and thick soil column evaporation experiments. Model evaluation at 15 field sites generally showed a close agreement with observations, with a great improvement in the lower range of evaporation rates in comparison with the widely applied Priestley and Taylor Jet Propulsion Laboratory model.

PRACTICAL APPLICATION OF THE MICROWAVE OVEN IN THE GEOTECHNICAL LABORATORY

Abstract The main purpose of the paper is to present the usefulness of microwave ovens in the everyday work of geotechnical laboratories. Microwave ovens are commonly used to determine soil moisture and to dry soils that are too moist for other laboratory tests. The activities related to the above processes are not standarized in the majority of countries. The only available guidelines are based on the research projects. For the purpose of this particular research 345 tests have been conducted for moisture content determination for 3 soil types: fine sand (FSa; SP – poorly graded sand), sandy gravel (saGr; GW – well graded gravel-sand mixtures) and silty clay (siCl; CH – inorganic clay of high plasticity), using both – standard and microwave methods. The research cycle consisted of 3 different variants which includes soil moisture content, specimen initial mass, number of specimens in microwave oven and their placing. It was established that different factors have impact on the length of the process and it lasted between 2 to 17 minutes. During the heating different undesirable phenomena were observed, e.g., soil explosions or melting and glowing of clay. Based on the above research results this paper introduces guidelines for the soil drying process in the microwave oven (the optimization of the procedure and handling of the dried samples).

Numerical Modeling of Soil Evaporation Process and Its Stages Dividing during a Drying Cycle

The soil water evaporation is a critical component of both the surface energy balance and water balance, affecting the mass and energy exchange between the land and the atmosphere. Evaporation process is involved in the highly complex interactions between media properties, transport processes, and boundary conditions. So, it is difficult to accurately determine these near-surface highly dynamic processes based only on the sparse field data and on the measurement-based methods. The objective of this paper was to obtain a detailed description of the soil water evaporation process and to better understand the evolutions of variables involved in the evaporation process during different stages of evaporation. To do this, a numerical simulation experiment in a bare silt soil was conducted to reproduce the soil drying process during a 20-d period after a 2-cm rainfall event. According to simulation results, the whole 20-d simulation period was divided into two main stages as well as a transient period from stage 1 to stage 2. Diurnal patterns of energy and water balance components, soil moisture, soil temperature, water fluxes, evaporation rate, dry surface layer (DSL), and evaporation zone during this drying process were fully described, which, in turn, could be as the possible indicators for judging the shift of stages of evaporation.

Data-Gap Filling to Understand the Dynamic Feedback Pattern of Soil

Detailed and accurate information on the spatial variation of soil over low-relief areas is a critical component of environmental studies and agricultural management. Early studies show that the pattern of soil dynamics provides comprehensive information about soil and can be used as a new environmental covariate to indicate spatial variation in soil in low relief areas. In practice, however, data gaps caused by cloud cover can lead to incomplete patterns over a large area. Missing data reduce the accuracy of soil information and make it hard to compare two patterns from different locations. In this study, we introduced a new method to fill data gaps based on historical data. A strong correlation between MODIS band 7 and cumulated reference evapotranspiration has been confirmed by theoretical derivation and by the real data. Based on this correlation, data gaps in MODIS band 7 can be predicted by daily evaporation data. Furthermore, correlations among bands are used to predict soil reflectance in MODIS bands 1–6 from MODIS band 7. A location in northeastern Illinois with a large area of low relief farmland was selected to examine this idea. The results show a good exponential relationship between MODIS band 7 and CET00.5 in most locations of the study area (with average R2 = 0.55, p &lt; 0.001, and average NRMSE 10.40%). A five-fold cross validation shows that the approach proposed in this study captures the regular pattern of soil surface reflectance change in bands 6 and 7 during the soil drying process, with a Normalized Root Mean Square Error (NRMSE) of prediction of 13.04% and 10.40%, respectively. Average NRMSE of bands 1–5 is less than 20%. This suggests that the proposed approach is effective for filling the data gaps from cloud cover and that the method reduces the data collection requirement for understanding the dynamic feedback pattern of soil, making it easier to apply to larger areas for soil mapping.

Coupled Liquid Water, Water Vapor, and Heat Transport Simulations in an Unsaturated Zone of a Sandy Loam Field

Information on the coupled liquid water, water vapor, and heat transport under arable field conditions is still limited, particularly for unsaturated soils of semi-arid and arid regions such as New Mexico. HYDRUS-1D model was applied to evaluate various transport mechanisms associated with temporal variations in water content and soil temperature in the unsaturated zone of a sandy loam furrow-irrigated field located at Leyendecker Plant Science Research Center, Las Cruces, New Mexico. The model was calibrated using measured soil water content and soil temperature at 5-, 10-, 20-, and 50-cm depths during a 19-day period (day of the year [DOY] 85 to DOY 103, 2009) and validated for a 31-day period (DOY 104 to DOY 134, 2009). Measured and optimized soil hydraulic and thermal properties and hourly meteorological data were used in model simulations. HYDRUS-1D simulated water contents and soil temperatures correlated well with the measured data at each depth. The total liquid water flux, composed of isothermal and thermal liquid water flux, dominated the soil water movement during early periods after irrigation, whereas the contribution of total water vapor flux, composed of primarily thermal and much smaller isothermal water vapor flux, increased with increasing soil drying. During the soil drying process, the total liquid flux within 15-cm depth eventually changed to water vapor flux near the surface. The upward total liquid and vapor fluxes decreased from 5 cm, indicating that vapor flux was much higher in the layer near the soil surface. The total vapor flux in this unsaturated soil layer was approximately 10.4% of the total liquid and vapor fluxes during the simulation period.

Indicadores da condição hídrica do solo com soja em plantio direto e preparo convencional

Indicadores da condição hídrica do solo foram avaliados em um experimento de campo, em Eldorado do Sul, RS, Brasil. Utilizou-se um Argissolo Vermelho Distrófico típico, utilizado durante oito anos em sistema plantio direto e preparo convencional. A cultivar de soja Fepagro Rs(-1)0, foi semeada em 20/11/2003, com 0,40 m entre fileiras e 300 mil plantas por hectare, em tratamentos irrigados e não irrigados. Variáveis do sistema solo-planta-atmosfera foram monitoradas e a ênfase neste trabalho visou aos períodos secos; monitoraram-se, também, variações no potencial matricial da água no solo, entre 0,075 e 1,20 m de profundidade. Verificou-se que o tempo de secagem do solo foi mais prolongado nas parcelas sob plantio direto indicando, em períodos de secagem, potenciais matriciais menos negativos, menores temperaturas máximas e menor amplitude térmica que em preparo convencional; também, a altura de plantas e o índice de área foliar apontaram que maiores estoques de água em plantio direto podem reduzir efeitos do déficit hídrico em soja cultivada neste sistema de manejo. Esses indicadores reforçam a importância da análise integrada de respostas das culturas em um enfoque sistêmico de manejo de solo e água.

Avaliação do tensiômetro eletrônico no monitoramento do potencial matricial de água no solo

O presente trabalho apresenta o monitoramento do potencial de água no solo obtido por meio do uso de um tensiômetro eletrônico. Os testes foram conduzidos no Laboratório de Mecânica e Eletrônica do Departamento de Engenharia Agrícola da UFC, tendo sido utilizados nove exemplares de tensiômetro eletrônico e três exemplares de tensiômetro convencional de mercúrio. Durante o período de um mês, os dados foram armazenados a uma freqüência de 2 Hz, sendo possível obter detalhes quanto ao monitoramento do potencial da água do solo, o efeito das flutuações de temperatura nas leituras do tensiômetro e a realização de testes de tempo de resposta e de sensibilidade. O efeito das flutuações de temperatura acentuou-se à medida que o potencial matricial diminuiu, sendo o uso de sensores de temperatura em transdutores de pressão de fundamental importância para corrigir o erro de leitura, principalmente para leituras coletadas em horário em que ocorrem as maiores temperaturas do dia. O tempo de resposta do equipamento aumenta à medida que o potencial matricial diminui. Os dispositivos testados apresentaram sensibilidades, em média, de 42,833 mV (kPa)-1 e precisão média de 0,02334 kPa ou 0,2380 cm c H2O.

Critical growth of a semi-linear process

This paper is motivated by the modelling of leaching of bacteria through soil. A semi-linear process X t − may be used to describe the soil-drying process between rain showers. This is a backward recurrence time process that corresponds to the renewal process of instances of rain. If a bacterium moves according to another process h, then the fact that h(t) stays above X t − means that the bacterium never hits a dry patch of soil and so survives. We describe a critical behaviour of h that separates the cases when survival is possible with a positive probability from the cases when this probability vanishes. An explicit formula for the survival probability is obtained in case h is linear and rain showers follow a Poisson process.

Critical growth of a semi-linear process

This paper is motivated by the modelling of leaching of bacteria through soil. A semi-linear process Xt− may be used to describe the soil-drying process between rain showers. This is a backward recurrence time process that corresponds to the renewal process of instances of rain. If a bacterium moves according to another process h, then the fact that h(t) stays above Xt− means that the bacterium never hits a dry patch of soil and so survives. We describe a critical behaviour of h that separates the cases when survival is possible with a positive probability from the cases when this probability vanishes. An explicit formula for the survival probability is obtained in case h is linear and rain showers follow a Poisson process.

Evaluation of methods for soil surface moisture estimation from reflectance data

The estimation of soil moisture from reflectance measurements in the solar spectral domain (400-2500 nm) was investigated. For this purpose, 18 soils representing a large range of permanent characteristics was considered. Reflectance data were measured in the laboratory during the soil drying process with a high spectral resolution spectroradiometer. Five approaches were compared. The first one was based on single-band reflectance and on the normalization of reflectance data by the reflectance of the corresponding soil under dry conditions. The second and the third approaches were based on either reflectance derivatives or absorbance derivatives. The fourth and fifth approaches were based on the differences of reflectance and absorbance observed in two non-consecutive bands. In the first step, the relationships were calibrated over half the dataset (nine soils) with emphasis on the selection of the most pertinent spectral bands. Results showed that, for the first approach, the bands corresponding to the highest water absorption capacities (1944 nm) yielded the best soil moisture retrieval performances. For the second and third approaches, the bands corresponding to sharp edges of the water absorption features performed better (1834 nm for the reflectance derivatives and 1622 nm for the absorbance derivatives). The fourth and fifth approaches that can be considered as a generalization of the derivative approach when bands are no longer consecutive, the best performances were achieved when the bands were not separated too much. The best overall retrieval performances were achieved with the absorbance derivatives and the difference of absorbance, confirming the non-linear character of the relationship between soil moisture and reflectance. The previously calibrated relations were tested over the evaluation dataset made of the nine remaining soils. It showed additionally that the normalization of reflectance values by that observed under dry conditions was only partly minimizing soil type effects. The best performances for the lowest soil moisture values (<0.20 m 3 /m 3 ) were obtained with the reflectance derivatives. However, because of the non-linear behaviour for the highest soil moisture values, the derivatives of absorbance and difference of absorbance provided the best estimates for these moisture levels. Results were discussed in view of potential applications of high spectral resolution sensors.

Chemistry of Soil Profiles Indicates Recurring Patterns and Modes of Moisture Migration

The hypothesis that patterns and modes of soil moisture migration can be interpreted from relationships between soluble sodium percentage (SSP) and total soluble cations (TSC) with depth in soil profiles is developed. Capillary migration of moisture during the soil drying process is more apt to leave a chemical imprint on soil profiles than the initial wetting process since it occurs last, and cations move in closer proximity to exchange surfaces. Soil profiles must be sampled in contiguous increments small enough to differentiate important variables. Each soil sample must be wetted exactly to saturation prior to extraction of soluble Ca, Mg, and Na for analysis. Both cation exchange and salt precipitation cause progressive losses of multivalent cations from solution as moisture moves through soil. Resulting increases in SSP indicate directions of moisture migration. Decreases in TSC accompanied by increases in SSP apparently result from movement in thin films in response to capillary tensions > 1 bar (salt sieving). Moisture migration in extremely thin films in response to high tensions at forward extremities of capillary migrations apparently produces increases in TSC values accompanied by increases in SSP values (salt precipitation).