Matric Head Research Articles

An analytic description of field capacity and its application in crop production

It has been established for decades that soil water field capacity (FC) includes four main components: drainage rate or flux (q), drainage time from saturation (t), soil water content in the drained zone (θ), and drainage depth (Z). Despite this, rather little work has been done to incorporate these components into analytic FC expressions, or to determine how such expressions are best used in applications of the FC concept. The primary objectives of this study were: i) develop analytic FC expressions that account for the four components of FC; ii) determine the accuracy of the FC expressions using numerical simulations of soil profile drainage from saturation; and iii) illustrate how analytic FC expressions might be used to optimize air, water and nutrient dynamics in field crop production.Using a range of representative soil types, the analytic FC expressions predicted q and θ within 36% of numerically determined values for t ≥ 0.01 days and Z = 0.3 m or 1.0 m, which is considered sufficiently accurate for most FC applications. The FC expressions showed that all four FC components are important, and that most FC applications have both agronomic and environmental implications. Static FC definitions based on specified matric head or retention curve shape were frequently inaccurate. It was concluded that the FC concept is most appropriately applied in crop production studies by using integrative FC expressions to optimize air and water storage, drainage flux, drainage time, and potential nutrient leaching rate in the crop root zone.

A Comprehensive Model for Single Ring Infiltration I: Initial Water Content and Soil Hydraulic Properties

Core Ideas New single ring infiltration model accounts for a wide range of experimental conditions. Soil capillarity as function of initial water content was explored. Model closely matched HYDRUS‐3D simulations for thirty infiltration scenarios. Model was tested with both Brooks and Corey and van Genuchten–Mualem hydraulic parameters. Brooks and Corey parameters gave more consistent predictions for cumulative infiltration. Single ring infiltration tests are commonly used to understand soil hydraulic properties. While a number of models have been developed to describe three‐dimensional infiltration from single ring sources, these expressions are often restricted to cases where the soil is initially dry and the source pressure head is negligible. These conditions may restrict the use of these expressions in real field settings. In this study we modified a set of infiltration models to explicitly account for variations in soil hydraulic properties, initial conditions, and experimental setups. The resultant expressions allowed us to explore how soil capillary length (a measure of the capillary force acting through the wetted zone) and three‐dimensional wetting profiles vary under different initial matric heads. Specifically, as initial matric head increases, infiltration becomes increasingly one‐dimensional due to a decrease of the soil capillary force. However, for most realistic situations (i.e., moderately wet to very dry initial conditions) the model can be simplified by assuming a constant capillary length. The resulting expressions compared well with numerical results from HYDRUS‐3D for five different soils, including both dry and wet initial conditions, two ring insertion depths, and two ponding depths. Overall, the model provides further insight on the variations in soil capillarity (as sorptivity) and infiltration across a range of soil types and initial conditions, and can be used to describe single ring infiltration under nearly every scenario.

Effect of soil pore size distribution on plant-available water and least limiting water range as soil physical quality indicators

Soil pore size distribution (SPSD) is one of the most important soil physical properties. This research investigated the relationships of location and shape parameters of the SPSD curves with plant-available water (PAW) and least limiting water range (LLWR) of the light-textured soils at the Torogh Agricultural Research Station in north-eastern Iran. Soil moisture release curve (SMRC), PAW and LLWR in matric heads of 100 and 330 hPa for the field capacity and location and shape parameters of the SPSD curves of 30 soils with different texture and organic carbon contents were determined, and the variable relationships were statistically analyzed. The results showed that the median equivalent pore diameter (de), mean de, standard deviation (SD*), and skewness of the SPSD curves were significantly correlated with PAW (PAW330) and LLWR (LLWR330) measured in a matric head of 330 hPa. Decrease in de and increase in the diversity of soil pore size (SD*) increased PAW330 and LLWR330. The SD* values of all the soil samples were lower than the optimal ranges suggested in literature. Neither PAW nor LLWR values were significantly different in the soils with the optimal modal de and those with non-optimal modal de. Optimal values of median and mean equivalent pore diameters and kurtosis of SPSD curves led to a significant improvement of PAW330 and LLWR330 as soil physical quality indicators. It was recommended to revise the optimal ranges for SD* and modal de for future studies.

The impact of mucilage on root water uptake—A numerical study

AbstractThe flow of water between soil and plants follows the gradient in water potential and depends on the hydraulic properties of the soil and the root. In models for root water uptake (RWU), it is usually assumed that the hydraulic properties near the plant root (i.e., in the rhizosphere) and in the bulk soil are identical. Yet a growing body of evidence has shown that the hydraulic properties of the rhizosphere are affected by root exudates (specifically, mucilage) and markedly differ from those of the bulk soil. In this work, we couple a 3‐D detailed description of RWU with a model that accounts for the rhizosphere‐specific properties (i.e., rhizosphere hydraulic properties and a nonequilibrium relation between water content and matric head). We show that as the soil dries out (due to water uptake), the higher water holding capacity of the rhizosphere results in a delay of the stress onset. During rewetting, nonequilibrium results in a slower increase of the rhizosphere water content. Furthermore, the inverse relation between water content and relaxation time implies that the drier is the rhizosphere the longer it takes to rewet. Another outcome of nonequilibrium is the small fluctuation of the rhizosphere water content compared to the bulk soil. Overall, our numerical results are in agreement with recent experimental data and provide a tool to further examine the impact of various rhizosphere processes on RWU and water dynamics.

The influence of water uptake on matric head variability in a drip-irrigated root zone

Irrigation is a service provided to agricultural crops in order to improve their yields. This is also a rare situation where the location of the costumers (roots) is not known and the heterogeneity in the water delivery system (soil) effects the water distribution. This non-uniformity of soil-plant conditions in the agricultural field complicates decision-making parameters regarding irrigation management (where to place sensors, how many sensors are needed, when and how much to irrigate). Many aspects of this non-uniformity have been studied in the past decades, including soil hydraulic properties, tillage history, irrigation method, sensor location, measured volume, and more. One aspect that has been relatively neglected is how plant roots affect variability. In this work, we focus on the effect that growing roots have on the variability of tensiometer measurements.Corn seeds were sown in a pre-designed experimental plot that consisted of three very distinct soils (sand, sandy loam, clay) to create high variation in investigated conditions. Subsurface drip irrigation was installed in the plot, and the drippers of the center line were wrapped with geotextile to create a geotextile-drip interface (GDI), i.e., a volume which has preferred conditions for root growth. Tensiometers were installed in the GDI (GDI treatment) and 10cm away from the drip lines (S10 treatment); the matric head was monitored throughout the season.The weekly averaged coefficient of variation (CV) of the GDI treatment decreased as the corn plants grew, for each soil type and the entire plot. As time passed and the corn plants grew, CV peaks, due to irrigation, and the duration of high CV decreased because of the increasing influence of root water uptake on fluxes in the measured volume. After harvest, the CV returned to initial values. S10 tensiometers showed higher CV values than the GDI tensiometers during the last week of growth. After harvest, the CV of the S10 tensiometer measurements decreased, contrary to the trend in the GDI tensiometers. The results point to the major influence of roots on matric head variability, first, by decreasing variability with time and second, through the effect of tensiometer location since the GDI tensiometers were located in volumes with high root density. This influence has major consequences regarding the methodology of tensiometer measurements to represent an agricultural field, and might provide means for global guidelines for tensiometer placement and irrigation management.

Application of the Green–Ampt model for infiltration into layered soils

Summary Using the Green–Ampt model for layered soils, a theoretical method is proposed to estimate the layers’ hydraulic parameters based on infiltration test data. To evaluate the proposed method and to investigate the effect of the layers’ permeability on infiltration, constant head infiltration tests were conducted on six different two-layered soil columns. When the upper layer was more permeable than the lower layer, piston flow was observed in both layers, and the air-filled volume fraction of the lower layer was nearly equal to the observed values in homogenous soils. In this situation, the layers’ hydraulic parameters can be estimated from the proposed method. When the lower layer was more permeable than the upper layer, piston flow with a large air-filled volume fraction was observed in the lower layer. When the lower layer was significantly more permeable with an approximately zero wetting front matric head, preferential flows or instability in the wetting front was observed in the lower layer. In two-layered soils with a more permeable layer below a low permeable layer, the hydraulic conductivity of the lower layer cannot be estimated based on infiltration test data.

Optimal tensiometer placement for high-frequency subsurface drip irrigation management in heterogeneous soils

Efficient control of irrigation systems depends on attaining representative water status data for an irrigated field. The spatial variability of soil hydraulic properties and root growth patterns, hamper the use of single-value representations. This work proposes a two-pronged approach designed for determining optimal sensor location for irrigation water management. It combines experimental results, which offer a method of pre-determining root growth patterns, with modeling analysis in which the effect of tensiometer location on coefficient of variation (CV) of matric head measurements and irrigation system operation was investigated.In the experimental part, the effect of a geotextile material, wrapped around the drippers along the drip line to create a “Geotextile Drip Interface” (GDI), on root growth patterns in the field was evaluated. The results showed higher root density around the GDI vs. regular buried drippers, with well-defined peak root density in the former vs. no one location with peak root density in the latter. Genetic root architecture had smaller effect on root distribution under the GDI treatment.The modeling part consisted of HYDRUS 2D/3D simulations of high-frequency subsurface drip irrigation in heterogeneous soils with GDI root distribution as observed in the experiment. The simulations explored different locations for tensiometer placement which will result in low variability of system operation. HYDRUS 2D/3D simulations showed that the optimal location for tensiometer placement is near the subsurface dripper, resulting in low CV of matric head measurements and applied irrigation water, high sensitivity to irrigation, and lower drainage below the root zone.Overall, the results show that measuring soil water status with tensiometers located close to drippers in GDI volume, improve control of high frequency irrigation systems and allow greater water application efficiency in drip irrigated fields.

Weighting the differential water capacity to account for declining hydraulic conductivity in a drying coarse-textured soil

Water availability to plants growing in coarse-textured soils during a drying cycle relies on the declining abilities of the soil to release water (differential water capacity) and to deliver it to the plant (unsaturated hydraulic conductivity) under varying evaporative demand. In this context, the availability of water can be quantified using the concept of the integral water capacity, IWC, in which the differential water capacity is weighted by means of a restrictive hydraulic function before integrating. We argue here that the diffusivity is an appropriate component of the restrictive hydraulic function, which leads to the employment of the so-called ‘matric flux potential’ (which we propose to re-name as the ‘matric flux transform’). As the starting point to apply the diffusivity function, we choose the inflection point of the water-retention curve drawn on semi-log paper, which, for the Groenevelt–Grant equation, occurs at a matric head, h, of precisely k0 metres. An illustrative example of the procedures is provided for a coarse-textured soil, which reveals that the restrictive function may not be sufficiently restrictive for all cases. We therefore apply an additional weighting coefficient to account for varying sensitivity of different plants to hydraulic restrictions.

Revisiting the pseudo continuous pedotransfer function concept: Impact of data quality and data mining method

The pedotransfer function (PTF) concept has been widely used in recent years as an indirect way to predict soil hydraulic properties, particularly the water retention curve (WRC). The pseudo continuous (PC) approach allows us to predict water content at any predefined matric head, resulting in an almost continuous WRC. When combined with powerful pattern recognition approaches, a PC-PTF can be trained to learn the shape of WRC from a discrete set of measured points, unlike traditional parametric PTFs which follow a predefined WRC shape dictated by the selected soil hydraulic equations. The purpose of this study was to investigate the impact of two elements on the performance of a PC-PTF: (i) data mining method (neural network, NN, versus support vector machine, SVM) and (ii) distribution and density of the provided water retention data in the training phase. Two datasets from Turkey and Belgium, consisting of mainly fine and coarse-textured soils, respectively, were employed. Multiple scenarios containing different combinations of measured water retention points in the training phase were defined. The lower root mean square error (RMSE) on average (0.044cm3cm−3) attained with the NN-based PTF shows that it is a better option than SVM (RMSE of 0.052cm3cm−3) for deriving PC-PTFs. The accuracy of PC-PTF was firmly dependent on the presence of measured water retention points in the entire range of WRC. Applying different scenarios revealed that a well distributed set of measured water retention points in the training phase could result in up to 0.03cm3cm−3 reduction in RMSE values.

Texture-depending performance of an in situ method assessing deep seepage

Summary Deep seepage estimation is important for water balance investigations of groundwater and the vadose zone. A simplified Buckingham-Darcy method to assess time series of deep seepage fluxes was proposed by Schindler and Muller (1998) . In the method dynamics of water fluxes are calculated by a soil hydraulic conductivity function. Measured soil moistures and matric heads are used as input data. Resulting time series of flux dynamics are scaled to realistic absolute levels by calibrating the method with the areal water balance. An assumption of the method is that water fluxes at different positions exhibit identical dynamics although their absolute values can differ. The aim of this study was to investigate uncertainties of that method depending on the particle size distribution and textural heterogeneity in non-layered soils. We performed a numerical experiment using the two-dimensional Richards Equation. A basic model of transient water fluxes beneath the root and capillary zone was setup and used to simulate time series of soil moisture, matric head, and seepage fluxes for 4221 different cases of particle size distribution and intensities of textural heterogeneity. Soil hydraulic parameters were predicted by the pedotransfer function Rosetta. Textural heterogeneity was modeled with Miller and Miller scaling factors arranged in spatial random fields. Seepage fluxes were calculated with the Buckingham-Darcy method from simulated soil moisture and matric head time series and compared with simulated reference fluxes. The median of Root Mean Square Error was about 0.026 cm d −1 and the median of maximum cross correlation was 0.96 when the method was calibrated adequately. The method’s performance was mainly influenced by (i) the soil textural class and (ii) the time period used for flux calibration. It performed best in sandy loam while hotspots of errors occurred in sand and silty texture. Calibrating the method with time periods that exhibit high variance of seepage fluxes yielded the best performance. The geostatistical properties of the Miller and Miller scaling field influenced the performance only slightly. However, the Miller and Miller scaling procedure generated heterogeneous flow fields that were addressed as main reason for mismatches of simulated reference fluxes and fluxes obtained with the Buckingham-Darcy method.

A Channel Network Model as a Framework for Characterizing Variably Saturated Flow in Biofilm‐Affected Soils

Understanding and predicting the hydraulic properties of biofilm‐affected porous media is of high importance in bioremediation, filtration, and bioreactors. In this study, a channel network model was applied for characterizing the variably saturated flow in biofilm‐affected soils. The soil pores are represented by a network of interconnected channels having a triangular cross‐section in which the shape of the water surface is determined by the matric head. The channel network model provides a better representation of the pore space than the capillary bundle model by accounting for pore connectivity and allowing for dual occupancy at individual pores. The effect of the biofilm on the network hydraulic properties was analyzed by considering three synthetic scenarios for the biofilm spatial distribution. The first scenario assumed that the biofilms fully clogged the smallest pores; the second scenario assumed that the biofilm covered all pore walls in a layer of uniform thickness; and the third scenario assumed that the biofilm covered a uniform fraction of the pore cross‐sectional area. We showed that the biofilm spatial distribution has a significant effect on the flow and hydraulic properties. Moreover, pore connectivity plays a significant role when considering flow in biofilm‐affected soils and therefore must be taken into account. Finally, the simulations demonstrated that the effect of biofilms on the hydraulic properties of the network is a complicated and nonlinear function that depends not only on the biofilm scenario but also on the saturation.

On Evaluating Characteristics of the Solute Transport in the Arid Vadose Zone

The transport of bromide (Br) under matric heads of 0, -2, -5, and -10 cm using undisturbed soil columns was investigated for understanding the solute transport in arid soils. Undisturbed soil cores were collected at ground surface, directly below where tension infiltrometer measurements were made in the Amargosa Desert, Nevada, United States. Laboratory experiments were conducted by introducing water containing Br tracer into a soil column maintained at steady-state conditions. The observed data of breakthrough curves (BTC) were well fitted to an one-region model, except for the cores at saturation, and a core at the matric head of -5 cm, from which the observed data were better fitted to a two-region model. Fitted pore water velocities with the one-region model ranged from 1.2 to 56.6 cm/h, and fitted dispersion coefficients (D) ranged from 2.2 to 100 cm² /h. Results for the core analyzed with the two-region model indicated that D ranged from 27.6 to 70.9 cm² /h at saturation, and 25.7 cm² /h at the matric head of -5 cm; fraction of mobile water (β) ranged from 0.18 to 0.65, and mass transfer coefficient (ω) ranged from 0.006 to 0.03. In summary, the water fluxes and Br dispersion coefficients at investigated matric heads were very high due to the coarseness of the soils and possibly due to preferential flow pathways. These high water fluxes and Br dispersion coefficients would lead to a higher risk of deeper leaching accumulating nitrate nitrogen to the groundwater, and have significant effects on the desert ecosystem.

Characterizing Disturbed Desert Soils Using Multiobjective Parameter Optimization

Simulating soil moisture dynamics in coarse‐textured alluvial soils under high evaporative demand and infrequent rainfall challenges both characterization and numerical methods. For this study, three geomorphically young disturbed soils in the central Mojave Desert were characterized using direct, indirect, and inverse parameter estimation methods for numerical simulations using HYDRUS. Observations of soil water content (θ) and matric head (h) were used to formulate our multiobjective framework optimized using the AMALGAM algorithm. Both the direct and indirect parameterization methods reproduced θ but proved inadequate for h data. Sensitivity analysis found the parameters related to the dry end of the soil hydraulic function—including residual water content (θr) and pore‐size distribution (n)—most sensitive but often conflicting between the two criteria while soil temperature was effectively simulated but insensitive. At the sites, a large dynamic range of h (1–400 m) was accompanied by a relatively small range of θ (0.04–0.10 m3 m−3), limiting our ability to resolve the wet‐end of the hydraulic functions. Direct parameterization from infiltrometer data and indirect pedotransfer functions had good agreement with θ but largely underestimated h. Pareto analysis determined the trade‐off between fitting both criteria and revealed model structure inadequacies as well as bifurcations into parameter populations, favoring either θ or h error minimization. The Pareto solutions compensated for the underestimation of h by forcing the α and n parameters to uncharacteristically low values for such coarse textures. This compensation kept the water capacity sufficiently high at lower θ to maintain conductivity toward the surface. These results are likely a consequence of either the disturbed soil system or hysteresis between measurements of θ and h.

Water and Bromide Dynamics in a Soil Amended with Different Urban Composts

Urban waste compost additions to soil can increase soil organic matter content and improve soil physical conditions, leading to agronomic and environmental benefits. The need for information still exists to evaluate more precisely the effects of urban waste compost on soil physical properties. Three types of urban waste composts, a biowaste compost (BIO), a municipal solid waste compost (MSW), and a co‐compost of green waste and sewage sludge (GWS), were applied once every 2 yr on a loamy soil for 10 yr. The effects of the three composts on soil water and solute transport dynamics were tested. Soil water matric head and water content were monitored using tensiometers and time domain reflectometry probes, respectively. A Br− tracer experiment was also conducted to evaluate the effect of compost application on nonsorbing solute transport. Water content measurements showed that the application of composts significantly (P < 0.05) affected soil water content in the plow layer, with average increases of 0.03 cm3 cm−3 for the GWS and MSW compost, and 0.015 cm3 cm−3 for the BIO compost compared with a control without organic amendment. Bromide tracing during the wetting period showed that the application of urban waste composts did not affect the soil's potential for leaching. The application of composts did reduce soil evaporation during the spring, however, which in turn favored downward Br− migration in the soil.

Soil‐Based Vegetation Technique to Quantify Effects of Rhizospheric Soil Osmotic and Matric Water Potentials on Crop Salt Tolerance

AbstractIn saline soils, plant water supply is the most critical growth factor. To better understand water supply and growth of soil‐grown crops, research should focus on root water uptake in saline soils. Plant water supply and growth is complex. One has to consider, simultaneously, soil and plant parameters: (i) the soil physical parameters texture; pF curve; osmotic, matric and total water potential; salinity at the soil/root interface; and bulk soil salinity; and (ii) the plant parameters root mass and rooting density; root morphology; transpiration; and shoot growth. Technical devices for direct and simultaneous measurement of all parameters are not yet available. This study presents a vegetation technique (VeTe) that permits to determinate required data from continuous measurement of pot water losses and by indirect calculation. The VeTe was tested using young rape (Brassica napus, cv. Lingot) as the model plant, growing in a silty soil. Rape was selected for its efficient root system to explore soil determined growth factors. Basically, the VeTe requires two vegetation phases: a pre‐cultivation phase, and an experimental phase. The objective of the first phase is to grow young plants that are homogenous in their shoot and root development through well‐controlled water management. Varying rooting densities of soils are performed when plants are pre‐cultivated in different soil volumes. The experimental phase starts when plants are irrigated with water of different salt concentrations up to soil water contents of 30 vol.%. During the experiment, plants were grown under well‐controlled, climatic conditions, and pot water losses were measured bi‐hourly. Measurement of continuous water losses serves to calculate soil moisture contents, derive osmotic and matric heads and their impact on plant transpiration. The proposed technique provides a means for quantitatively studying the combined impacts of soil osmotic and matric stresses on water uptake by crops differing in their root morphologic traits at different rooting densities.

Coupled Hydrogeophysical Inversion of Streaming Potential Signals for Unsaturated Soil Hydraulic Properties

Streaming potential (SPs) is the electric potential generated by fluid flow in a charged porous medium. The SPs signals are related to pore water velocity, bulk electrical conductivity, pore water charge excess, and soil porosity. While several studies have estimated hydraulic properties of the saturated zone from SPs, there have been fewer attempts to infer unsaturated hydraulic properties from SPs. From numerical and laboratory experiments in which infiltration and subsequent drainage was monitored with nonpolarizable Ag/AgCl electrodes and tensiometers, we showed that it is feasible to estimate three key Mualem–van Genuchten hydraulic parameters (fitting parameters α and n and saturated hydraulic conductivity Ks) and Archie's saturation exponent (na) using a coupled hydrogeophysical inversion approach. In addition to a reasonably good estimate of na, coupled hydrogeophysical inversion of actual SPs measurements during drainage provided estimates of α, n, and Ks that were comparable to those obtained from an independent inversion of the tensiometric data (matric heads). We concluded that coupled hydrogeophysical inversion of time‐lapse SPs signals is a promising method for hydraulic characterization of the vadose zone. Accurate modeling of SPs signals is essential for reliable inversion results, but there is still debate about the appropriate model for the voltage coupling coefficient at partial saturation. Our experimental data showed a nonlinear and monotonic decrease in the absolute voltage coupling coefficient with decreasing saturation. A comparison of several available models with our experimental data showed that models that consider the relative permeability and the relative electrical conductivity in addition to the saturated coupling coefficient and water saturation were most appropriate.

An artificial capillary barrier to improve root-zone conditions for horticultural crops: Response of pepper plants to matric head and irrigation water salinity

Abstract Reduced water availability and increased salinity result in similar plant responses including reduced root mass and conductivity with consequential reduced transpiration and biomass production. We hypothesized that the increased soil matric head created by a capillary barrier (CB) positioned at the bottom of the root-zone would improve plant production, especially when irrigated with brackish water. Field and lysimeter studies were conducted with bell pepper ( Capsicum annuum ) plants, comparing root-zones with and without an underlying CB, irrigated either with desalinated (DW, EC = 0.2 dS m −1 ) or brackish (SW, EC = 3.8 dS m −1 ) water, at various rates. When irrigated with SW, plants grown above a CB yielded 24% higher biomass than control plants (without CB) for all irrigation applications. But, when DW was applied, only a 6% advantage for the CB root-zone was observed. Biomass yield with the CB treatment was only slightly affected by water salinity while without CB, yields significantly decreased when irrigated with SW. The extent of evapotranspiration, plant growth and yield responses to the presence of a CB appeared to be climate dependent. When vapor pressure deficit (VPD) was low (i.e.

Water Retention Curves of Biofilm‐Affected Soils using Xanthan as an Analogue

This study investigated the effect of biofilms and, in particular, that of extracellular polymeric substances (EPS) on the hydraulic properties of porous media under unsaturated conditions. The quantitative understanding of the way biological activity alters hydraulic properties is a major key in understanding and engineering relevant systems such as soil aquifer treatment, bioremediation, and wastewater irrigation. Using an EPS analog (xanthan) we explored the effect of EPS on the water retention function of two sandy soils. The result was a significant increase in the water content at any given matric head that could reach 270% of its value within pure soil. For most of the water content range, we successfully modeled the effect of the EPS as a linear superposition of the original pure soil and the xanthan retention curves. Finally, we examined two mechanisms that can attribute to modification of the water retention curve: the EPS holding capacity and alteration of the soils' pore‐size distribution; in our case, it appears that the first mechanism was dominant.

Numerical investigation of irrigation scheduling based on soil water status

Improving the sustainability of irrigation systems requires the optimization of operational parameters such as irrigation threshold and irrigation amount. Numerical modeling is a fast and accurate means to optimize such operational parameters. However, little work has been carried out to investigate the relationship between irrigation scheduling, irrigation threshold, and irrigation amount. Herein, we compare the results of HYDRUS 2D/3D simulations with experimental data from triggered drip irrigation, and optimize operational parameters. Two field experiments were conducted, one on loamy sand soil and one on sandy loam soil, to evaluate the overall effects of different potential transpiration rates and irrigation management strategies, on the triggered irrigation system. In both experiments, irrigation was controlled by a closed loop irrigation system linked to tensiometers. Collected experimental data were analyzed and compared with HYDRUS 2D/3D simulations. A system-dependant boundary condition, which initiates irrigation whenever the matric head at a predetermined location drops below a certain threshold, was implemented into the code. The experimental model was used to evaluate collected experimental data, and then to optimize the operational parameters for two hypothetical soils. The results show that HYDRUS 2D/3D predictions of irrigation events and matric heads are in good agreement with experimental data, and that the code can be used to optimize irrigation thresholds and water amounts applied in an irrigation episode to increase the efficiency of water use.

An artificial capillary barrier to improve root-zone conditions for horticultural crops: response of pepper, lettuce, melon, and tomato

Capillary barriers (CBs) occur at the interface between two soil layers having distinct differences in hydraulic characteristics. In preliminary work without growing crops, it was demonstrated that CBs implemented in sandy soils increased hydrostatic volumetric water content by 20–70%, depending on soil texture and depth of barrier insertion. We hypothesized that the introduction of an artificial CB at the lower root-zone boundary of horticultural crops can increase yields as a result of increased water content and uptake efficiency. The effects of introduced CBs on soil water content, plant growth, and yields of bell peppers (Capsicum annum L), lettuce (Lactuca sativa L), tomatoes (Lycopersicon esculantum L.), and melons (Cucumis melo L.) were studied in a desert environment in southern Israel. Inclusion of a CB increased soil water content by 60% and biomass and fruit yields by 25% for pepper, and increased matric head and biomass yield by 80 and 36%, respectively, for lettuce. Neither tomatoes nor melons reacted significantly to the presence of CBs, in spite of increased soil moisture. Daily soil matric head amplitude was reduced fivefold when lettuce was grown with a CB. Spatial variability was highly reduced when a CB was present. When peppers were grown with a CB, the standard deviations of water content and biomass yield were reduced by 40% relative to control.