Non-uniform Soil Water Research Articles

Large broad-leaved canopy of banana (Musa nana Lour.) induces dramatically high spatial–temporal variability of throughfall

Abstract Throughfall (TF) is an important water input of rainfall redistribution into floor, and its spatial–temporal variability under some species' canopies has been documented to evaluate effect on splash erosion. However, the understanding of TF variability under large broad-leaved canopy remains insufficient. In this study, the spatial heterogeneity, temporal stability and drop size of TF were quantified using variogram fitting, normalised ranking and filter paper staining, respectively, under banana (Musa nana Lour.) canopy comprising long and wide leaves. Results indicated TF pattern showed strong spatial correlation at a range of 3–5 m. High spatial variability of TF was found, which was affected by rainfall event size and was accompanied by great canopy disturbance. TF plots revealed high time variability, which was mainly controlled by unstable banana canopy structure. TF drop size from leaf dripping points varied in 3–10 mm and showed significant differences (p < 0.05) among five kinds of leaf shapes, implying that concave and broken banana leaves were involved in the variability of TF drop size. Overall, results demonstrate the spatial–temporal variability of TF is dramatically induced by banana canopy with broad leaves, which may result in non-uniform soil water content and splash erosion under the canopy.

Micro-scale spatial variability in soil heat flux (SHF) in a wine-grape vineyard

In vineyards, hourly soil heat flux (SHF) may account for as much as 30% of net radiation. Therefore, inaccurate estimates of SHF may lead to non-negligible errors when quantifying the surface energy balance. The typical canopy height to width ratio of two along with widely spaced rows (row spacing exceeding canopy height), and leaf biomass concentrated in the upper half of the vine canopy result in variable shading conditions producing sharp, sometimes abrupt, differences in SHF between adjacent points within the interrow space. Drip irrigation, which is also a typical practice in many vineyards in semi-arid regions, adds an additional source of variability in the interrow soil moisture which strongly affects SHF. Lastly, the common practice in Californian wine-grape vineyards to plant cover crop in the interrow, forming two distinct areas below the canopy—bare soil and crop cover, further increases SHF variability in the interrow. This small-scale variability challenges the measurement of SHF in such agrosystems. The objective of the research was to assess the micro-scale (within the interrow between two vine rows) spatial variability in SHF, and to determine which of the three variables—non-uniform (in both space and time) shading patterns, non-uniform surface cover (bare soil vs. cover crop) and non-uniform soil water content (due to the drip irrigation)—is the most significant driver for the local heterogeneity. The variability of incoming solar radiation reaching the ground was found to be the primary source for the spatial and temporal variation of SHF once the vine canopy was fully developed. The water content distribution and the grass cover in the interrow both had minor impacts on the spatial and temporal variation in SHF. It was further found that a transect of five equally distributed sensors across the interrow accurately represented the area-average SHF given by the 11-sensor array, particularly during the growing season.

Water uptake model analysis of crop roots under non-uniform water potential

Aims Our objective is to consider a water uptake model of crop roots under non-uniform water potential. This is based on principles of water conservation in soil-plant system and water potential's driving force on water transport. Methods We introduced two expressions of non-uniform soil water potential, linear and exponential functions, and compared the analytical solutions under non-uniform and uniform water potentials. This model is used to analyze the effect of ratio of radial resistance and axial resistance on plant water uptake and to determine the optimal root length within a reasonable range of this ratio. Important findings In certain circumstances, the water uptake model for plant roots with non-uniform water potential can be simplified to a uniform water potential model, and the optimal root length is close to the actual value after the Poseuille formula is changed to fit plant vessels.

Performance of the theta probe ML2 in the presence of nonuniform soil water profiles

An attempt is made to investigate the soil permittivity regime which is predicted by ML2 theta probe in cases where a nonuniform soil moisture profile prevails. Our investigation was performed in laboratory columns, under temperature-controlled conditions for a number of different porous media. Five distinct cases, where the moisture regime could be established in a predesigned manner were tested. These are: (a) Air over a water or ethanol layer of changing thickness. (b) Air over a saturated porous material of changing thickness and the same in the reverse order. (c) Dry porous material over the same but saturated porous material of changing thickness and vice versa. (d) Gradual wetting from the top surface of initially dry soils. (e) Initially saturated vertical soil profiles, left to gradually dry by evaporation from both open surfaces (top and bottom). From our experimental investigations it is shown that the bulk dielectric constant predicted by the ML2 probe in nonuniformly wet profiles irrespective of their layer thickness is closely described by the arithmetic averaging scheme. This fact makes the water content predictions from the manufacturer calibration equations for these cases, seriously overestimated. A comparison of the actual water contents with those predicted with the calibration equations of Robinson et al. [Robinson, D.A., Jones, S.B., Blonquist, J.M., Friedman, S.P., 2005. A physical derived water content/permittivity calibration model for coarse–textured layered soils. Soil Sci. Soc. Am. J. 69 (5), 1372–1377] gives very satisfactory results.

Effect of hysteresis and rainfall intensity on finger dynamics

AbstractThe purpose of this paper is to present sand‐box experiments and a hysteresis model to explain the formation and hydraulic properties of fingered flow. Previous researches have found that a fingered path persists over long periods of constant infiltration; hysteresis could be an important factor. In this paper, a major point is to utilize the boundary and series internal scanning curves of the hysteresis model derived by Huang et al. (2005) to clearly obtain the variations between soil water content (θ) and matric suction head (ψ). The hydraulic properties of fingered flow are also acquired. For the application of the model in the field, the hysteresis curves derived by Hogarth et al. (1988) are compared. Experimental results verified that both hysteresis models and the present soil water content concept model of fingered flow can be adequately applied in field tests. We attempted to use digital image analysis to visualize and determine soil water content of fingered flow easily and quickly. The initial velocity and width of fingered flow were not constant under consistent rainfall intensity. The present concept model showed the matric suction head is uniform between different soil water content sharply in the horizontal direction. Nonuniform soil water content existed even when the potential was horizontally equalized. Copyright © 2008 John Wiley & Sons, Ltd.

Green–Ampt Model for Layered Soils with Nonuniform Initial Water Content Under Unsteady Infiltration

The Green–Ampt equation is often used to estimate infiltration in hydrologic modeling. Only a few of the currently used simulation models, however, can simulate infiltration into a heterogeneous soil with nonuniform initial soil water content by the Green–Ampt equation during nonsteady rainfall. In this study, we derived an infiltration model for layered soils under nonsteady infiltration based on the Green–Ampt method (Green–Ampt for Layered Soils, or GALS). In the GALS model, a formula for estimating average suction (S) was derived to include initial soil water effect, and the soil profile was divided into several layers to consider both soil texture and initial soil moisture effects on infiltration. The new model can predict infiltration capacity, ponding time, and cumulative infiltration. Three infiltration experiments, steady‐state infiltration into a layered soil with nonuniform initial soil water content, nonsteady infiltration into a layered soil with uniform soil water content; and nonsteady‐state infiltration into a layered soil with nonuniform soil water content, were conducted for model testing. Comparison showed that the GALS model was in good agreement with another Green–Ampt‐based model and the measured data.

Flow Regime Effects on Reactive and Non-reactive Solute Transport

The lower boundary maintained throughout leaching experiments in column and lysimeter studies influences the transport behavior of solutes. In this study we wanted to determine the effects of different flux regimes and soil organic matter treatments on reactive (pesticide) and non-reactive (bromide) solute movement in lysimeters. Four large soil monoliths, collected from the same experimental site but differing in the organic carbon content by an added manure layer, were subjected to two consecutive leaching tests. Unsaturated steady-state flow with a unit hydraulic gradient was established in Test 1 by applying a constant negative pressure at the base of the lysimeter while during Test 2 atmospheric pressure was maintained (zero-tension). Although less herbicide (isoproturon) was leached from the soil with added manure, the flux regime, as controlled by the lower boundary, dominated the adsorption and the degradation coefficients. The lower moisture content of Test 1 enhanced isoproturon binding, whilst chemical degradation was more effective under the increased but non-uniform soil water content of Test 2. This study demonstrates how solute leaching studies using zero-tension lysimeters do not represent unsaturated in-field processes; the physicochemical processes occurring in unsaturated field soils are only adequately reflected in leaching tests when a constant negative soil water pressure head is maintained throughout the soil sample.

Transport of bromide in the Bainsvlei soil: Field experiment and deterministic/stochastic model simulation. II. Intermittent water application

Despite the fact that non-uniform soil water content and variable input water fluxes are usually encountered in the field, tracer experiments have usually been carried out under steady-state conditions. The objective of this study was to analyse solute transport in a Bainsvlei soil under intermittent water application using Br– as a tracer. Sprinkler was used to apply water on a plot 200 by 200 cm. Soil core samples were taken every 20 cm to a depth of 160 cm several times during the experiment. The soil Br– concentration data were fitted to the steady-state convection–dispersion analytical model in the CXTFIT package. The average coefficients of determination yielded by this fit (r2 = 0.810) clearly support that the data could be analysed successfully with CXTFIT. The average pore-water velocity of 1.72 cm/day and average dispersion coefficient of 26.19 cm2/day determined from this fit are lower than the fitted values of the steady-state experiments. The Br– moved slower under the intermittent application of water than in the steady case, a conclusion supported by the deeper location of Br– peaks under continuous application than intermittent application after the same amount of water is applied.

Water uptake in crop growth models for land use systems analysis: II. Comparison of three simple approaches

This paper addresses water uptake by roots as calculated in summary crop growth models for use at the land use systems level. Simple comparative exercises were carried out using three models. The results show considerable differences in daily and cumulative water uptake. One model calculates moisture changes for subdivisions of the root zone, even though the procedure on which it is based was intended for use over the rooted zone as a whole. This is combined with an algorithm to allow for compensatory effects in soils with non-uniform water distribution, resulting in artefacts. The results of another model based on the electrical analogue, confirm that theoretical models help to improve understanding of complex processes and guide research, but using them for predictions may give worse results than conventional empirical methods. The third model uses a ‘soil water depletion fraction’ to indicate the limit of readily available water in the rooted soil as a whole. The procedure was originally developed for irrigated agriculture and seems inappropriate for rain fed conditions characterised by irregular wetting and drying cycles that result in non-uniform soil water distribution. Simple comparative exercises as carried out, help to reveal weak points in the models studied and indicate ranges of uncertainty. Error propagation analyses using local data must be added to indicate whether improvements at local level are necessary and to identify priorities for further research, feeding an upward spiral to enhance model accuracy.

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.

Soil water variability under subsurface drip and furrow irrigation

Non-uniformities in soil hydraulic properties and infiltration rates are considered to be major reasons for the inefficiencies of some surface irrigation systems. These non-uniformities may cause non-uniformities in soil water contents and could potentially affect plant growth. To investigate whether the non-uniformities in soil water contents can be overcome by well-managed irrigation systems, fields with clay loam soils and planted to cotton were irrigated with a continuous-flow, a surge flow, and a subsurface drip system. Measurements of water contents in each field were taken throughout the growing season at several depths. The water contents measured within the top 0–0.9 m in the three irrigations systems were evaluated in terms of their spatial and temporal variabilities. The analyses indicated that on this soil, use of the surge flow system did not lead to increased spatial uniformities of soil water contents compared with the continuous-flow system. Use of the subsurface drip system resulted in very non-uniform soil water contents above the depth of the emitters. Variability in water contents below the emitter depth was comparable to the surface irrigation systems.

Modelling non-uniform soil water uptake by a single plant root

The assumption of uniform water flow to the root or uniform water potential at the root surface was shown by Hainsworth and Aylmore (1986, 1989) to be erroneous. The present paper demonstrates how the non-uniform uptake of water by a single root can be modeled. Differential equations are numerically solved to describe simultaneous water movement in the plant and in the soil. In the plant, boundary conditions are the water potentials at the root surface (Ψs) and in the xylem at the root base (Ψb). A set of difference equations describe the flow of water radially through the cortex to the xylem and in the xylem axially upwards to the base. For calculating the water flow in the soil and the values of Ψs, i.e. the boundary conditions for flow in the root, the finite element method (FEM) is used, the boundary conditions being the flux of water into the plant root and the zero flow across the wall, bottom and surface of a hypothetical soil cylinder surrounding the root. ei]Section editor: B E Clothier

Modelling non-uniform soil water uptake by a single plant root

The assumption of uniform water flow to the root or uniform water potential at the root surface was shown by Hainsworth and Aylmore (1986, 1989) to be erroneous. The present paper demonstrates how the non-uniform uptake of water by a single root can be modeled. Differential equations are numerically solved to describe simultaneous water movement in the plant and in the soil. In the plant, boundary conditions are the water potentials at the root surface (Ψs) and in the xylem at the root base (Ψb). A set of difference equations describe the flow of water radially through the cortex to the xylem and in the xylem axially upwards to the base. For calculating the water flow in the soil and the values of Ψs, i.e. the boundary conditions for flow in the root, the finite element method (FEM) is used, the boundary conditions being the flux of water into the plant root and the zero flow across the wall, bottom and surface of a hypothetical soil cylinder surrounding the root. ei]Section editor: B E Clothier

Probability Distribution of Solute Travel Time for Convective Transport in Field‐Scale Soils Under Unsteady and Nonuniform Flows

This study addresses the development of probability distributions of travel times for one‐dimensional (vertical) solute transport in soils. The field‐scale soils are considered heterogeneous, with stationary fluctuations of soil hydraulic properties in the horizontal direction but nonstationary fluctuations of these properties in the vertical direction due to layering of the soil, which induces nonstationary heterogeneity. Approximate ensemble probability distribution functions of conservative solute travel time for vertical convective solute transport were derived directly from the convective transport stochastic partial differential equation, under both deterministic and stochastic soil surface water flux (infiltration rate) and under unsteady and nonuniform soil water flows. General depth‐varying initial and time‐varying boundary conditions were used in these derivations. The magnitude of the approximation in the theoretical probability distribution functions of travel time is quantified mathematically. Utilizing the soil water content data from a University of California, Davis, field site, it is shown that the mathematical condition for this approximation is satisfied for this field. The spatial heterogeneity is represented through a nonstationary soil water content random field which covaries both in time and in space. Dispersion emerges naturally in the derived ensemble probability distribution functions of solute travel time, owing to the stochasticity of soil water content at field scale. Then the theoretical expression for mean solute concentration over a field is derived, by means of the theoretical solute travel time distribution, as a function of time and soil depth, under vertical transport with rectangular pulse solute loading for the upper boundary condition. Comparisons of theoretical probability density functions of solute travel time against their empirical counterparts, obtained from field experimental observations under steady but nonuniform soil water flow, show good agreement. Comparisons of theoretical mean solute concentrations, as they evolve with time and soil depth, against field experimental observations also show good agreement. However, further field experiments under unsteady flow conditions are required for the comprehensive validation of the developed theory.

Non-uniform soil water extaction by plant roots

Using the technique of Computer Assisted Tomography applied to gamma ray attenuation measurement of soil water content, it has been shown that the assumption of uniform absorption of soil water along a plant root is clearly erroneous and that drawdown distance is a function of time. The results suggest that the plant sequentially removes water from the top to the bottom of the root as soil hydraulic resistance becomes a major limiting factor in the upper layers, even at the high soil water potential (−0.30 MPa) used.

Soil Water Uptake by Alfalfa1

AbstractWater uptake patterns of alfalfa (Medicago sativa L.) assist us in understanding proposed models governing plant water uptake. The data in this paper are presented to elucidate some details of passive water uptake from profiles with nonuniform soil water distributions. Soil water content under an alfalfa seed crop was monitored with a neutron moisture probe. Alfalfa roots withdraw soil water in the lower portion of the root zone (where soil matric potentials were between −7 and −10 bars), while the upper portion of the profile was above −2 bars. This indicates that for passive water uptake to occur, large water potential differences must exist between the root xylem and the soil in the upper, moist portion of the profile. Plant water potential measurements support passive uptake.