Distribution Of Soil Water Content Research Articles

Effects of tractor traffic on spatial variability of soil strength and water content in grass covered and cultivated sloping vineyard

Frequent machinery traffic on sloping vineyard influences spatial distribution of soil physical properties. Our objective was to assess the effects of crawler tractor traffic across the slope (20%) on spatial distribution of soil strength and water content of silt loam soil under controlled grass cover and conventionally cultivated vineyard. The experiment was situated on hillside vineyard (NW, Italy) arranged with rows crosswise the slope. The grass covered treatment included periodical mowing and chopping of herbs and the cultivated treatment—autumn ploughing (18 cm) and spring and summer rotary-hoeing in the vineyard inter-rows (2.7 m). A crawler tractor (2.82 Mg) was used at the same locations across the slope for all tillage and chemical operations. The measurements of soil bulk density, penetration resistance and volumetric water content were done in autumn (after vintage) within the sloping inter-row. The results were analyzed using classic statistics and geostatistics with and without trend. The highest variability was obtained for penetration resistance (CV 56.6%) and the lowest for bulk density (9.6%). In most cases, the semivariograms of the soil parameters were well described by spherical models. The semivariance parameters of all properties measured were influenced by trend. Three-dimensional (3D) maps well identified areas with the highest soil strength in lower crawler ruts being positioned in the upper side of vine row and successively lower strength in upper ruts situated on other side of the same row and inter-rut area. Higher strength in lower than upper ruts was induced by tractor's tilt and resulting higher ground contact pressure. Soil water content in both treatments was the lowest below the upper rut and increased in inter-rut and lower rut areas. The differences in the soil properties between the places within the inter-row were more pronounced in grass covered than in cultivated soil.

SIMULATING SOIL WATER FLOW WITH ROOT-WATER-UPTAKE APPLYING AN INVERSE METHOD

Modeling soil water flow and distribution in soil-plant systems is challenging because it is difficult and time-consuming to measure and determine the transient root distributions accurately. In this study, an inverse method by Zuo and Zhang (2002), which was utilized to estimate average root-water-uptake rate distributions using two measured soil water content profiles, was applied to simulate soil water flow in a soil-wheat system. A field experiment was conducted to measure distributions of soil water content, soil-plant osmotic potential, and root length density of winter wheat (Triticum aestivum L. cv. Nongda 186). The experiment was performed using clean water and reclaimed wastewater for irrigation. Considering uncertainties of hydraulic parameters and measured soil water content, we conducted a detailed sensitivity analysis of the inverse method. Based on the field experimental data and the inverse method, distributions of normalized root length density (Lnrd) were estimated, thereby simulating soil water flow in the soil-wheat system with water and salinity stresses. The estimated Lnrd and the simulated soil water content profiles compared well with the measured values. The results show that the inverse method was useful for estimating Lnrd distributions and simulating soil water flow with root-water-uptake continuously and effectively in field settings without requiring any measured root density data.

Spatial distribution of root length density and soil water of linear agroforestry systems in sub-humid Kenya: implications for agroforestry models

In simultaneous agroforestry systems trees can compete with crops for water, especially in semi-arid areas. However, in the (sub)humid tropics, on P-fixing Oxisols/Ferralsols small decreases in soil water content caused a decrease in P-transport to roots and therewith a soil-drying induced P-deficiency. The aim of this study was to assess the spatial distribution of soil water content in crop fields bordering tree lines and its relation with root length density distribution of the trees throughout the soil profile. To achieve this, soil water content and tree root length densities throughout the soil profile were measured over a period of 2 years in an experiment with lines of four tree species in the middle of maize fields in sub-humid western Kenya. Soil water content was significantly reduced (2–7 vol.%) near two of the three fast-growing tree species, Eucalyptus grandis and Grevillea robusta, but not near Cedrella serrata and the slower growing Markhamia lutea. These differences were related to differences in water use. Eucalyptus and Grevillea showed high water use and Cedrella and Markhamia low water use. However, soil water content distribution was not related to root length density distribution. Root length densities hardly decreased with distance to Grevillea and clearly decreased with distance to Cedrella. Most water-uptake models, including those of agroforestry models, assume that root length density distribution throughout the profile is proportional to water extraction throughout the profile. The absence of a clear relation between root length density and water extraction near Grevillea tree lines opposed this view. It can be explained by a decrease in water-potential gradient between root and soil at increasing distance from the tree base. If the change in root length density is similar or smaller than the change in water-potential gradient between root and soil, the decrease in water-potential gradient between root and soil is of similar or larger importance for determining tree-water extraction distribution throughout the profile than root length density. Thus, modeling of spatial agroforestry systems cannot assume a direct relation between tree-water extraction and root length density, but needs to include decreasing water-potential gradient between root and soil along roots with increasing distance to the stem base, especially over the horizontal dimension.

ALGEBRAIC MODEL FOR ONE-DIMENSIONAL INFILTRATION AND SOIL WATER DISTRIBUTION

Soil water infiltration is an important surface hydrological process. Most mathematical models describing soil water infiltration and soil water distribution are complicated. The objective of this paper was to present an algebraic model for one-dimensional transient soil water infiltration. The algebraic model contains three parameters, and the influence of the parameters on the algebraic models was analyzed. A one- dimensional infiltration experiment was performed. Model parameters were determined from the experimental data, and the transient soil water content distributions and the infiltration times were calculated with the algebraic model. The calculated soil water content and infiltration time values were compared with observed data. The results indicate that the model reflects the features of the transient soil water infiltration process.

Effects of Water Applications and Soil Tillage on Water and Salt Distribution in a Vertisol

Under limited water resources, modern irrigation methods tend to save water and improve water and salt regimes within the root zone. This study deals with the combined effects of water application methods and tillage practices on water and salt distributions, runoff production, and soil loss in a field irrigated with moving irrigation systems (MIS). An experiment was conducted in a cotton (Gossypium hirsutum L. cv. Sivon) field at Hazorea, Israel, where the main soil type is vertisol (Typic Chromoxerets). Sprinkling (SP) and flooding (FL) MIS, and conventional (CT) and microbasin (MB) tillage, were compared in terms of runoff and soil loss from runoff microplots (5 m 2 ), soil water content and salinity distribution with depth, yield, and plant height. Under SP conditions, no runoff and soil loss were obtained for either tillage practice. In the FL/CT treatment, the mean runoff and soil loss were about 25% of the irrigation water and 0.59 kg m -2 . The FL/MB treatment reduced the runoff and soil loss to 5.8% and 0.02 kg m -2 , respectively. The soil water contents in the SP treatments were generally lower than in the FL treatments, especially in the 0.1-to 0.6-m soil layer. No significant differences in the soil salinity, plant height, and seed-cotton yield were observed between the treatments. Microbasins tillage reduces water losses under flooding MIS to a point where they become practically similar to those obtained under sprinkler MIS. It can potentially lead to lower water losses if the microbasins storage capacity is matched to the water application rate, to avoid runoff.

Modeling root growth and the soil–plant–atmosphere continuum of cotton crops

A simple and functional model to simulate cotton root growth is coupled to models of water balance and leaf water potential to predict the behavior of the soil–plant–atmosphere continuum (SPAC) of cotton crops. The root-growth model takes into account the dry matter partitioning to roots, root depth increase as a function of thermal time, soil mechanical resistance and soil water stress, and allows the inclusion of spatial variability of soil physical properties. The water balance model uses a cascade approach to simulate drainage while soil and plant evaporation are calculated separately following the model of Ritchie. The leaf water potential model (LWPM) is based on Ohm’s Law analogy, where water flow resistance to the roots is calculated using an empirical equation and the resistance within the plant is calculated as a function of a threshold leaf water potential. The model simulated correctly the soil resistance and the temporal and spatial distribution of roots in the soil profile. The model simulated total soil water content and midday leaf water potential well, even when a compacted soil layer was present, but the agreement to observed data was poor for the vertical distribution of soil water content, mainly in the zone of greatest root concentration.

Spatial and Temporal Distribution of Soil Water Content in Fields under Different Vegetation Conditions Based on TDR Measurements

TDR (Time-domain reflectometry) is an advanced method for monitoring the water content in fields. TDR measurements can provide accurate information about vegetation effects on both spatial and temporal soil water distribution. The objective of this study was to examine the effects of vegetation conditions on the distribution of the soil water content. Soil water content was measured to express the spatial distribution in the surface layer (0-0.2 m) in grass, corn, and non-planted fields. Additionally, using the TDR method with multiple probes with different lengths, the measurements were conducted on a temporal basis at 4 depths (0--0.1, 0.1-0.2, 0.2-0.3 and 0.3-0.5 m) to determine the soil-water content profiles in the 3 fields. A significantly higher water content in both the grass and cornfields was found, compared to the non-planted field. Moreover, in the cornfield, we observed that microtopographic features significantly affected the spatial water distribution. The lower the soil water content, the higher the coefficient of variation of the soil moisture content. These results indicated that the effects of the vegetation conditions on the soil water distribution in the fields were significant. From the observation of the temporal soil water profiles in the 3 fields, it was found that soil management, i.e. tillage or no-tillage, was a major factor affecting the profiles.

Developmental changes in plant resistance to water flow in Pisum sativum (L.)

Soil and plant resistance to water flow under field conditions in pea (Pisum sativum L.) plants were measured at six ages. Transpiration flux, leaf and soil water potentials were used to calculate the total resistance to water flow using the Ohm's law analogy. Plant resistance was estimated from the slope of the water potential difference (ΔΨ) vs. transpiration (Q) relationship. Plant growth, root density and soil water content distribution were measured. Leaf area and root length both increased until the end of seed filling and decreased during seed maturation. Total resistance decreased with the transpiration flux in a non-linear relationship. Plant resistance estimated as the slope of the ΔΨ vs. Q regression line increased until pod filling and then decreased. The increased resistance to water flow during pod filling was associated with a 10% increase in cell wall thickness.

Effects of Water Applications and Soil Tillage on Water and Salt Distribution in a Vertisol

Under limited water resources, modern irrigation methods tend to save water and improve water and salt regimes within the root zone. This study deals with the combined effects of water application methods and tillage practices on water and salt distributions, runoff production, and soil loss in a field irrigated with moving irrigation systems (MIS). An experiment was conducted in a cotton (Gossypium hirsutum L. cv. Sivon) field at Hazorea, Israel, where the main soil type is vertisol (Typic Chromoxerets). Sprinkling (SP) and flooding (FL) MIS, and conventional (CT) and microbasin (MB) tillage, were compared in terms of runoff and soil loss from runoff microplots (5 m2), soil water content and salinity distribution with depth, yield, and plant height. Under SP conditions, no runoff and soil loss were obtained for either tillage practice. In the FL/CT treatment, the mean runoff and soil loss were about 25% of the irrigation water and 0.59 kg m−2 The FL/MB treatment reduced the runoff and soil loss to 5.8% and 0.02 kg m−2, respectively. The soil water contents in the SP treatments were generally lower than in the FL treatments, especially in the 0.1- to 0.6-m soil layer. No significant differences in the soil salinity, plant height, and seed-cotton yield were observed between the treatments. Microbasins tillage reduces water losses under flooding MIS to a point where they become practically similar to those obtained under sprinkler MIS. It can potentially lead to lower water losses if the microbasins storage capacity is matched to the water application rate, to avoid runoff.

Effect of Hydrotropism on Root System Development in Soybean (Glycine max): Growth Experiments and a Model Simulation

To observe root system development, soybean plants (Glycine max) were grown in root boxes that were set horizontally to reduce the effect of gravity. Along with the root system development, the two-dimensional distribution of soil water content in the root boxes was measured continuously by the time domain reflectometry (TDR) method. Root system development and its morphological architecture were strongly affected by the positions of the water supply. It is suggested that root hydrotropism plays the dominant role in root system development. In addition to root hydrotropism, the importance of root compensatory growth is suggested. A combined model of root system development and soil water flow considering root hydrotropism and compensatory growth was used to simulate root system development and soil water flow. The morphological architecture of root systems and the distribution of soil water content obtained in the experiment were successfully explained by the model simulation. These results confirmed that root hydrotropism and compensatory growth are dominant factors in root system development under a reduced effect of gravity. The validity of the model was confirmed, and its applications for various purposes were suggested.

Multi-wire time domain reflectometry (TDR) probe with electrical impedance discontinuities for measuring water content distribution

The adequate estimation of water content distribution in wetted volume is fundamental in determining the number of drippers per plant and their location below the plant canopy in drip irrigation. Measurements of water content distribution are usually made by opening trenches, which is a time-consuming method and sometimes imprecise. Recent scientific developments have created the possibility of monitoring the soil moisture content using electronic sensors. The objective of this research was to develop and test two multi-wire time domain reflectometry (TDR) probes with electrical impedance discontinuities (referred to as the multi-wire probe) for sensing soil profile water content distribution. The experiment was divided in two parts. In part one, the laboratory performance of two multi-wire probe designs was studied and their reliability to monitor the water content variation in a porous media profile was evaluated. The second part was conducted in a 250 l bucket and the soil water content distribution, for an application depth of 15 mm, was evaluated by monitoring over 6 days at discharge rates of 2 and 4 l h −1. The results demonstrated the viability of using multi-wire probes to estimate soil water content distribution with different probe designs and to consistently obtain water content measurement in water dynamic processes. The following conclusion may drawn from the main results: (1) The measured characteristic impedance of the multi-wire probe for different designs was not the same as that geometrically calculated. This was due to the non-ideal probe geometry which provoked signal loss, thus, hindering peak impedance interpretation, mainly for probe 1 design. (2) The use of multi-wire probes in the TDR equipment showed a speedy determination of soil profile water content distribution using a single measurement.

A Macroscopic Water Extraction Model for Nonuniform Transient Salinity and Water Stress

Quantitative description of root‐water uptake under combined salinity and water stress is needed to optimize crop yields and water management in arid and semiarid regions. This study was conducted to develop a simple macroscopic root‐water uptake model for nonuniform transient soil water content and salinity conditions in the root zone. This new model and previous models were tested against detailed experimental data obtained with Alfalfa (Medicago sativa L.) grown in the greenhouse in packed sandy loam (Typic Haplaquent) columns. Soil water content, pressure head, and osmotic head distributions in the root zone were varied by means of the amounts, application intervals, and salinities of the irrigation water. Experimental data under separate and combined stresses were used to test the various models using mean values of soil solution osmotic and pressure heads. The simple additive reduction function provided the worst agreement with the experimental data, while for most cases the multiplicative reduction functions could not adequately account for both water and salinity stress conditions. The newly proposed linear reduction function is neither additive nor multiplicative, but was assumed that both the intersect and slope of the reduction function increased with salinity. This model provided excellent agreement with the experimental data, particularly at higher soil solution salinities. The new reduction function could be used with any other nonlinear salinity reduction function.

ESTIMATING ROOT-WATER-UPTAKE USING AN INVERSE METHOD

It is extremely challenging, if not impossible, to measure distribution of root-water-uptake or root density directly, either in the laboratory or in the field. To estimate root-water-uptake distribution in soils, the authors of this paper developed an inverse method, whereby measured values of water content in soil profiles with root-water-uptake were used as input information, which can be acquired relatively easily and accurately. Then a soil water flow equation incorporating an unknown sink term was solved inversely for the sink term, which characterizes the root-water-uptake distribution in the root zone. The estimated distribution of root-water-uptake rate was used to calculate the normalized distribution of relative root density, from which the root-water-uptake model was established. Based on the model, soil water flow with root-water-uptake was simulated successfully. Examples of numerical experiments were designed to examine the accuracy and stability of the proposed approach for estimating the average root-water-uptake rate, considering different time intervals between successive measurements of the input distributions of soil water content, spatial intervals of the measurement points along the soil profile, simulating depth, different soils, variation of the saturated hydraulic conductivity, and the stochastic nature of the measured soil water content. Examples were also used to demonstrate applications of the method. The results showed that the inverse method is reliable for estimation of distribution of the root-water-uptake rate, compared with the theoretical values. The proposed method is useful for estimation of the normalized distribution of the relative root density, from which the root-water-uptake model can be determined and the soil water flow with root-water-uptake can be simulated continuously.

Factors regulating spatial distribution of soil water content in small semiarid catchments

The factors which control soil moisture patterns in a semiarid area are determined in two different locations: a burnt zone with little vegetation and an unburnt zone with a greater vegetal cover. In the burnt area, the factors affecting the spatial variability of the soil water content are those considered as local controls such as soil texture and slope. These factors are able to explain a significant part of the spatial distribution of soil moisture in this zone independently of the soil moisture state. In the unburnt area, the factors affecting soil moisture were those related with the presence or absence of vegetation in semiarid environments. The upslope contributing area, aspect, soil profile curvature and soil depth best explained the spatial variability of the soil moisture content in the vegetated zone. The actual influence of these factors showed marked seasonal variations due to changes in the physiological activity of the vegetal cover.These factors, which are in part responsible for the distribution of soil moisture, are not commonly integrated in traditional topographic wetness indices. The topographic index of Beven and Kirkby (Beven, K.J., Kirkby, M., 1979. Hydrological Sciences Bulletin 24, 43–69) is extended to account for the interstorm soil water losses. The new indices incorporating the hillslope aspect improve the prediction power of soil water content patterns in semiarid areas.

Isothermal and Nonisothermal Evaporation from Four Sandy Soils of Different Water Repellency

Soil water evaporation is an important component of the surface energy balance. Evaporation also affects plant available water content of soil. Soil wettability is known to affect water‐holding capacity and water flow, but the impact of soil water repellency on evaporation has not been thoroughly investigated. The objective of the study was to measure and analyze isothermal and nonisothermal evaporation rates for four sandy soils showing different degrees of water repellency. Eight vertical soil columns (two of each soil) were exposed to isothermal conditions (20 ± 1°C), and another eight soil columns were exposed to nonisothermal conditions. During the nonisothermal experiment, the top boundary temperature was held constant at 21°C, and the bottom boundary temperature was constant at 55°C, thus maintaining a constant thermal gradient. After 195 d of evaporation, each column was sectioned to determine the soil water content distribution along its axis. A numerical model based on the Philip‐de Vries theory was used to predict soil water flow. Deviations between predicted and measured values increased with increasing contact angle. Isothermal cumulative evaporation was 25% lower for water repellent soil than for the most wettable soil. Nonisothermal cumulative evaporation was 75% larger than isothermal evaporation for the wettable soil but only 14% larger than isothermal evaporation of the most water repellent soil. Evaporation and residual water contents, especially in the surface layer, were found to be strongly affected by soil wettability.

Three‐dimensional spatial and temporal monitoring of soil water content using electrical resistivity tomography

In this paper, we propose a noninvasive method for monitoring three‐dimensional (3‐D) spatial and temporal variations of soil water content in the field, soil moisture tomography. The basic idea of the method originates from Archie's relationship between soil resistivity and water content. Initially, 88 electrodes were densely buried within a 3.5 m×3.5 m square area, and potentials at the electrodes were measured by pole‐pole and Wenner array methods at given time intervals. An inversion calculation of the 3‐D soil resistivity was then conducted based on these potential data. Next, 46 soil samples were taken at representative positions in the square, and the parameters in the Archie's relationship were measured in the laboratory. Then, the 3‐D distributions of the parameters were obtained by a distance weight interpolation method. Finally, based on Archie's relationship and the 3‐D distribution of the soil resistivity and the related parameters, 3‐D distributions of soil water content were calculated. To evaluate the obtained water content, the calculated water contents were compared with those measured by heat‐probe‐type soil moisture sensors, and a comparison between the spatial distribution patterns of calculated water content and soil bulk dry density was conducted. The 3‐D variations of the calculated water content during a rainfall event were also analyzed. The results show that there are ±0.10 cm3/cm3 errors in the calculated water content, but between the calculated and the measured water content there exists a good linear relationship. It is possible to use the calculated water content to analyze the very general 3‐D distribution characteristics of the soil moisture and investigate the 3‐D rainfall infiltration process, the redistribution of soil water after rain, and other hydrological processes in the field. The proposed method is preferred for porous media where the water resistivity is relatively stable.

Study of time dependency of factors affecting the spatial distribution of soil water content in a field-plot

Temporal and spatial variability of water content in soil results from a complex interaction of different factors such as duration and frequency of rainfall, soil layering, vegetation, and topography. The objectives of this study were (i) to use a resistant median-polishing scheme to quantify the temporal variability of a depth and a horizontal location factor in an additive model, and (ii) to investigate the time stability of those two factors at a detailed temporal scale during different infiltration and redistributions cycles. Time series of water content were measured at 5 depths and 12 locations along a transect of 6 m using Time Domain Reflectometry (TDR). Measurements were repeated every 2-hours for 168 days under natural boundary conditions. At each time step, the mean water content of the soil profile, 5 depth factors and 12 location factors were estimated. The time series of these factors were qualitatively interpreted and related to the atmospheric and prevailing soil conditions. It was found that micro-heterogeneity plays an important role, even at this small plot-scale. The relative contributions of the factors were dependent on the antecedent soil moisture conditions. Also, the ratio of the deterministic variance, i.e., variance explained by the deterministic factors, of water content to the observed variance is variable in time.

Measurement of soil water content below a wastewater trench using ground‐penetrating radar

Ground‐penetrating radar (GPR) was used to measure the distribution of soil water content below a wastewater trench. Two household septic system trenches were installed in a sandy soil. Vertical time domain reflectometry (TDR) probes were installed through the bottom of one of the trenches. The TDR probes were used to measure soil water content for comparison to the GPR data. The GPR measurements were performed in zero‐offset gather (ZOG) and multiple‐offset gather (MOG) survey modes with the antennas in horizontal boreholes located on opposite sides and below the wastewater trench. The ZOG survey mode gave an estimate of the average water content in the horizontal plane below the trench at a prescribed interval along the trench. The MOG survey mode produced a tomographic image of the distribution of soil water content in the same horizontal plane below the trench. A similar range of values of water contents was measured by TDR and GPR methods.

Pesticide transport in field soils

Pesticide contamination of water supplies has become a national issue. However, it is very challenging to quantitatively describe pesticide transport in field settings. The objective of this study was to quantify pesticide transport processes in field soils through modeling and field experiments. A transfer function model was modified to describe pesticide transport in the field. The model can be used to characterize the stochastic convective transport process with input chemical concentration changing with time, non-uniform soil water content distributions, adsorption, and degradation. A field experiment was conducted to assess the fate and mobility of two commonly used herbicides, dicamba and picloram. The herbicides were applied at two application rates to four plots (two replicates). The general transfer function was useful in fitting all the herbicide data within the different plots and depths, and was also utilized to predict herbicide concentration distributions with time and depth. The predicted results had reasonably good agreement with the field experimental data. For all comparisons, the predicted results matched the trend of the concentration distributions extremely well. By considering spatial and temporal variabilities, and other factors of the field soils, the modified transfer function proved to be a comprehensive and useful tool to model pesticide transport processes in field settings.

Preferential flow in water repellent sandy soils: principles and modeling implications

Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a two-dimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.