Soil Water Content Profiles Research Articles

L'utilisation des algorithmes génétiques pour l'identification de profils hydriques de sol à partir de courbes réflectométriques

We developed a unidimensional microwave propagation model in a stratified medium. It calculates, from a given soil water content profile, the reflected signal trace one would measure by timedomain reflectometry along a transmission line inserted in the soil. We identify the soil water content profile corresponding to a measured reflected signal trace using an identification technique based on optimisation of the model parameters by a genetic algorithm.

Use of non‐contacting electromagnetic inductive method for estimating soil moisture across a landscape

There is a growing interest in real‐time estimation of soil moisture for site‐ specific crop management. Non‐contacting electromagnetic inductive (EMI) methods have potentials to provide real‐time estimate of soil profile water contents. Soil profile water contents were monitored with a neutron probe at selected sites. A Geonics LTD EM‐38 terrain meter was used to record bulk soil electrical conductivity (ECA) readings across a soil‐ landscape in West central Minnesota with variable moisture regimes. The relationships among ECA, selected soil and landscape properties were examined. Bulk soil electrical conductivity (0–1.0 and 0–0.5 m) was negatively correlated with relative elevation. It was positively correlated with soil profile (1.0 m) clay content and negatively correlated with soil profile coarse fragments (>2 mm) and sand content. There was significant linear relationship between ECA (0–1.0 and 0–0.5) and soil profile water storage. Soil water storage estimated from ECA reflected changes in landscape and soil charactenstics.

Predicted drainage for a sandy loam soil: sensitivity to hydraulic property description

Prediction of unsaturated flow phenomena at field-scale requires a set of hydraulic functions that capture local-scale variability which is present in all natural soils. Several sets of hydraulic functions measured on different core sizes collected across the field were used to predict drainage from a saturated soil profile. Simulations were carried out with a one-dimensional numerical model, based on the Richards equation. Four methods were considered: (1) Monte-Carlo simulation using 500 unimodal retention and conductivity functions representing θ(ψ) and K s data measured on 0.05 m diameter 0.051 m long core samples; (2) multiple simulations with a set of 60 multimodal retention and conductivity functions which better represented the measured θ(ψ) data of method 1; (3) one single simulation with a set of hydraulic functions obtained from a gravity-drainage experiment on 15 1 m long 0.3 m diameter soil columns collected from the same field; (4) one single simulation with a set of hydraulic functions obtained by scaling the retention and conductivity data from method 3. A perfect match of the final mean outflow was obtained when scaled retention data was used in combination with scaled K s values (method 4). All other cases underestimated the total outflow to a varying degree: 30% for method 1, 29% for method 2, and 21% for method 3. The results further revealed that none of the four methods was able to completely describe the mean observed drainage from the start until the equilibrium condition. This was further demonstrated by the disparities between the mean observed soil water content profile and the simulated values using hydraulic functions from method 4. Especially after the first day, differences were large, presumably because macropore flow could not be described using the Richards flow equation. Despite the introduction of multimodal retention and conductivity functions which better described the retention behaviour of small soil cores (method 2) in comparison with unimodal retention functions (method 1), mean predicted outflow for both methods was nearly identical.

Tillage and residue effects on root growth and yields of grain sorghum following wheat

Crop production in the Southern Great Plains of TX (USA) is limited by insufficient precipitation. Furrow diking and residue management systems in water deficit regions can influence water infiltration, root growth and water extraction and, consequently, plant growth and yield. Field studies were conducted on a clay loam soil in unfertilized rotation systems to determine the influence of tillage and residue management practices on grain sorghum [ Sorghum bicolor (L.) Moench] rooting depth and changes in soil water content and cone index (CI). Treatments imposed on a wheat-sorghum-fallow (WSF) rotation system included furrow diking (FD), conventional tillage (CT), no-tillage with wheat ( Triticum aestivum) residue maintained on the field (NT+), and no-tillage with wheat residue removed from the field (NT−). Furrow diking and NT+ management systems increased sorghum grain yields 19 and 15%, respectively, compared to CT. Grain yields from the NT− system were not significantly different from the CT system. The FD system decreased CI and increased depth of rooting, profile water use and grain yield. Conventional tillage and NT− adversely affected these variables. Tillage operations loosened the soil and reduced the CI of the tilled layer. A decrease in CI values in the NT+ treatment was related to increased root development and penetration of roots to deeper soil layers. Root length in the 40 to 60 cm depth of FD and NT+ treatments was 30 to 85% greater compared to the CT and NT− treatments. Greater rooting densities, at all three growth stages (30, 60 and 90 days after emergence of sorghum), under FD and NT+ were associated with higher soil profile water contents. Data from this study show that profile water content, root development and sorghum grain yield can be significantly increased in semiarid regions with limited rainfall by incorporating furrow diking or surface residues into the management system.

Landscape Effects on Desiccation Cracking in an Aqualf

AbstractThe rapid transport of water and solute through desiccation soil cracks can lead to crop water and nutrient stress as well as ground and surface water contamination. This study hypothesized that desiccation cracking varies with landscape positions. Cracking was quantified at three landscape positions in a Mexico silt loam (fine, montmorillonitic, mesic Mollic Endoaqualf). The elevation changes of seven disks placed at 15‐cm depth increments in the profile were measured to determine changes in soil layer thickness. Crack volume in the upper 90‐cm profile was determined from thickness changes of the six soil layers. Disk elevation and soil profile water content measurements were made weekly during the crop season (April–November) in 1992 and 1994 on summit, backslope, and footslope positions. During both years, the backslope position profile developed the largest amount of crack volume (4.3 cm3 cm−2 in 1992, 4.8 cm3 cm−2 in 1994) and had the least amount of water depletion (10.0 cm3 cm−2 in 1992, 9.7 cm3 cm−2 in 1994). At the backslope position, a thin, high‐clay‐content Ap horizon and a shallow, clay‐rich Bt horizon (both high in smectite clay) lead to these developments. Differences in shrinkage characteristics of aggregates from Ap, Bt, and C horizons across landscape positions were related to textural and mineralogical differences. Bt horizon clay contents as high as 64.4%, comprised of up to 75% smectite, produced the greatest volume reductions (41%) of aggregates from the Bt horizon compared with volume reductions of aggregates from the Ap and C horizons. Soil texture and mineralogy influenced the magnitude of desiccation soil cracking and water depletion.

A unified model for infiltration and redistribution during complex rainfall patterns

A relatively simple conceptual model for infiltration during complex rainfall sequences is presented. It is a reformulation of an analytically derived model developed earlier by Smith et al. (1993) [Water Resour. Res., 29(1): 133–144] and Corradini et al. (1994) [Water Resour. Res., 30(10): 2777–2784] in a more homogeneous version suitable for hydrologic applications. The model relies on a single ordinary differential equation through which successive cycles of infiltration-redistribution can be described. The wetting profile, σ(z), is described by a single similarity curve which is stretched in both z and σ dimensions in accordance with the curve area, I, representing infiltrated water. The actual profile of soil water content after rewetting may be compound, composed two parts, each part represented by a similarity profile shape which evolves in time. The model is calibrated using a silty loam soil and is then tested on a variety of soils, from fine-textured to sandy loam soil types, by comparison with a numerical solution of the Richards equation. Like the earlier formulation, the model simulates infiltration rate particularly well, and the greater simplicity of this model sacrifices very little accuracy of the earlier, more complex model.

Soil measurements during HAPEX-Sahel intensive observation period

This article describes measurements made at each site and for each vegetation cover as part of the soils program for the HAPEX-Sahel regional scale experiment. The measurements were based on an initial sampling scheme and included profile soil water content, surface soil water content, soil water potential, infiltration rates, additional measurements on core samples, and grain size analysis. The measurements were used to categorize the state of the surface and profile soil water regimes during the experiment and to derive functional relationships for the soil water characteristic curve, unsaturated hydraulic conductivity function, and infiltration function. Sample results for different supersites and different vegetation covers are presented showing soil water profiles and total soil water storage on days corresponding to the experimental ‘Golden Days’. Sample results are also presented for spatial and temporal distribution of surface moisture content and infiltration tests. The results demonstrate that the major experimental objective of monitoring the supersites during the most rapid vegetative growth stage with the largest change of the surface energy balance following the rainy season was very nearly achieved. Separation of the effects of probable root activity and drainage of the soil profile is possible. The potential for localized advection between the bare soil and vegetation strips of the tiger bush sites is demonstrated.

Calibration of the time‐domain reflectometer and determination of the volumetric water content of the soil profile in an ultisol of Costa Rica

Abstract An experiment was conducted to determine if time‐domain reflectometry (TDR) could be used to measure the water content at different depths in the O‐to‐75 cm soil layer. Probes of three wires (1/8 inch diameter and 30 cm exposed length) were installed in field plots differing in current crop‐fertilization history. Measurements of volumetric water content using bulk density and gravimetric water content were made to calibrate the TDR method. Comparison of water contents determined by TDR with those from gravimetric samples showed that there is a linear relationship (small offset but same slope) of water content with depth, indicating that there is little difference in volumetric water content from the 0 to 75 depth. However, the TDR method gives consistently lower water content values as compared with values obtained by gravimetric determination. Continuous measurements of profile soil water content with TDR in wet and dry periods during the year indicated that the mayor differences in volumetric w...

Determinação da condutividade hidráulica do latossolo roxo distrófico por atenuação de raios gama e tensiometria.

Results for the hydraulic conductivity of a dark red latosol (Oxisol) under laboratory and field conditions are presented. The laboratory experiments simulated field conditions through the measurement of the soil water content profiles as a function of time in soil columns. The data were obtained by the 241 Am gamma-ray transmission method, using standard gamma ray spectrometry equipament. Tensiometers at the depths of 10 and 25 cm were used to obtain the soil water content profiles as a function of time in the field experiments. The hydraulic conductivity functions were determined through internal soil drainage. The results showed higher values of the hydraulic conductivity measured in the field, compared with the laboratory values. The hydraulic conductivity determination methods presented distinct values for the field experiments as well as for the laboratory ones.

Spatial partitioning of the soil water resource between grass and shrub components in a West African humid savanna.

Most savanna water balance models assume water partitioning between grasses and shrubs in a two-layer hypothesis, but this hypothesis has not been tested for humid savanna environments. Spatial partitioning of soil water between grasses and shrubs was investigated in a West African humid savanna by comparing the isotopic composition (oxygen-18 and deuterium) of soil water and plant stem water during rainy and dry conditions. Both grass and shrub species acquire most of their water from the top soil layer during both rainy and dry periods. A shift of water uptake pattern towards deeper horizons was observed only at the end of the dry season after shrub defoliation. The mean depth of water uptake, as determined by the isotopic signature of stem water, was consistent with grass and shrub root profiles and with changes in soil water content profiles as surveyed by a neutron probe. This provides evidence for potentially strong competition between shrubs and grasses for soil water in these humid savannas. Limited nutrient availability may explain these competitive interactions. These results enhance our understanding of shrub-grass interactions, and will contribute to models of ecosystem functioning in humid savannas.

Water use efficiency of irrigated wheat in the Tarai region of India

Experiments were conducted during the winter seasons of 1983–1984 and 1984–1985 to identify suitable irrigation regimes s for wheat grown after rice in soils with naturally fluctuating shallow water table (SWT) at a depth of 0.4 to 0.9 m and medium water table (MWT) at a depth of 0.8 to 1.3 m. Based on physiological stages, the crop was subjected to six irrigation regimes viz., rainfed (I0); irrigation only at crown root initiation (I1); at only crown root initiation and milk (I2); at crown root initiation, maximum tillering and milk (I3); at crown root initiation, maximum tillering, flowering and milk (I4); and at crown root initiation, maximum tillering, flowering milk and dough (I5). Tube-well water with an EC <0.4 dsm−1 was used for irrigation. Based on 166 mm effective precipitation during the cropping season, 1983–1984 was designated as a wet year and 1984–1985 with 51 mm as a dry year. The change in profile soil water content ΔW (depletion) in the wet year was less (23%) under SWT and 10% under MWT as compared to the dry year. The ground water contribution (GWC) to evapotranspiration (ET) was 58% under SWT and 42% under MWT conditions in both the years. The GWC in the wet year was 20% under SWT and 23% under MWT. Of the total net water use (NWU), about 85% was ET and 15% drainage losses. The NWU was highest (641 and 586 mm) in I5 under SWT and MWT conditions, respectively, but not the yield (5069 kg ha−1). Compared to I5, NWU in I2 treatment decreased by 10% in the wet and 25% in the dry year. A similar trend was observed in the I3 treatment under MWT condition. However, there was no statistically significant difference between yields of the I1 to I5 treatments of either water table depth during the wet year. This was also true during the dry year for the I2 to I5 treatments. Under SWT, in I2, the grain yield was 5130 kg ha−1 and under I3 regime, 5200 kg ha−1. Under MWT in I3, the yield was 5188 kg ha−1 and under I4 regime, 5218 kg ha−1. Thus it appears that in the Tarai region where the water table remains shallow (<0.9 m) and medium (<1.3 m) for most of the wheat growing season applications of more than 120 and 180 mm irrigation under SWT and MWT conditions, respectively were not necessary. Irrigation given only at crown root initiation and milk stages under shallow water table conditions, and at crown root initiation, maximum tillering and milk stages under medium water table conditions, appears to be as effective as frequent irrigations.

Measurements of transpiration from savannah shrubs using sap flow gauges

Sap flow gauges were used to measure transpiration from the shrub component of a 7-year-old agricultural fallow in Niger, West Africa. The aim of the study was to provide the data necessary to develop and validate multi-source models of evaporation from semi-arid land surfaces by measuring transpiration from a single component of the vegetation. The vegetation at the study site was a type of savannah, consisting of scattered shrubs, with an understorey of annual grasses and herbs. Sap flow measurements were restricted to Guiera senegalensis J.F. Gmel., the dominant shrub species. Sap flows were measured throughout the wet season of 1990 (June–September), using sap flow gauges operating on a constant power heat balance principle. Sap flow rates measured in individual stems were strongly related to leaf area and net radiation. Strong positive correlations were observed between the hourly mean flow rates measured in different stems. Daily transpiration, T, averaged across a 0.25 ha sample plot was compared with daily total evaporation (measured by eddy correlation), E. T was calculated by scaling up from sap flow measurements in a sample of stems to the whole plot by using the frequency distribution of stem diameter in the plot, and assuming that sap flow in each stem was proportional to its basal cross-sectional area. E varied from about 2 mm day −1 at the start of the wet season to 4–6 mm day −1 during August and September, when maximum leaf area and soil profile water content were observed. E, a measure of evaporation from all plant and soil sources, always exceeded T. At the end of two dry periods in June and July, when soil evaporation and transpiration from the herb understorey probably totalled about 0.5 mm day −1, E was only 0.7–0.9 mm day −1 greater than T, suggesting that the scaling procedure adopted gave reasonable estimates of bush transpiration. Over the whole wet season, transpiration from G. senegalensis accounted for 35% of the total wet season rainfall of 454 mm.

Water movement and salt leaching in drained and irrigated marsh soils of southwest Spain

The Guadalquivir river marshes in southwest Spain, are situated in a former estuary that has now been drained. They cover some 140 000 ha. Soils formed in this zone are alluvial, very clayey, salinesodic, and of vertic character with a shallow, very-saline water table. An area within these marshes, reclaimed in 1979, is the object of this detailed study which looks at some physical and chemical properties of soil, and also the influences of sprinkler and furrow irrigation on drainage and salt leaching. These studies were conducted between 1988 and 1990. A 1 ha experimental plot was used for this purpose. The plot is situated within an area of the marshes being used for standard agricultural practices, equipped with a drainage system of ceramic pipes buried 1 m deep and spaced 10 m apart, ending at an open ditch collector which is perpendicular to them. The following measurements have been carried out: soil bulk density changes with water content, hydraulic conductivity and sorptivity, soil water content and water tension profiles changes, water table level, drainage outflow and salt content of soil, soil solution and drainage water. Cotton was used as the crop. The results obtained show that after ca. 10 years in reclamation the electrical conductivity and exchangeable sodium percentage decreased in the 0–90 cm layer of this soil, particularly in the top 50 cm, that reduces the adverse effects of salinity on crop development. During both sprinkling and furrow irrigation a rapid response of the drain pipes was observed because of the soil fissures and crack network that resulted from the shrinking and swelling processes. Maximum drainage outflow was reached when the irrigation stopped. Water movement in this soil is characterized by an initial rapid phase, due to the soil fissures and cracks, followed by a second slow one controlled by the soil matrix. The efficiency of irrigation in salt leaching was higher in the case of furrow irrigation (16 g of salt leached per litre of applied water) than in the case of sprinkling irrigation (10 g l −1), owing to the use of more water in the latter. The salinity of soil and soil solution decreases after irrigation starts, the time taken to reach a minimum value for the different soil layers and the irrigation method used were different. In 1989 due to water restrictions at some time of the crop period, the irrigation schedule was different to that normally used, and the crop was affected by water stress and capillary rise of saline water.

Measured and Simulated Surface Soil Drying

AbstractThe USDA initiated the Wind Erosion Prediction System (WEPS) to develop improved technology for predicting wind erosion. A HYDROLOGY submodel has been developed for WEPS to simulate the soil energy and water balances. This study was conducted to evaluate the performance of the HYDROLOGY submodel in predicting surface soil drying. Water content was measured gravimetrically in a bare 5‐ by 30‐m plot for 14 d after irrigation during July and August 1991. The plot was located 5 m directly north of a bare weighing lysimeter at the USDA‐ARS Conservation and Production Research Laboratory at Bushland, TX. Hourly samples were taken from depth increments of 0 to 2, 2 to 6, 6 to 10, 10 to 30, and 30 to 50 mm. Furthermore, soil cores were taken to 900 mm at 6‐h intervals. Water content was also measured daily at the lysimeter and between the lysimeter and gravimetric sampling plot using a neutron probe to 2.1 m. The submodel accurately predicted that no deep percolation occurred throughout the simulation period. Simulation results agreed well with the measured daily evaporation rates from the lysimeter (r2 = 0.96). Furthermore, the submodel reasonably estimated the soil water content profiles, particularly the status of soil water at the soil‐atmosphere interface. The mean absolute error, which describes the average absolute deviation between measured and simulated soil water contents, was 0.015 m3 m−3. The HYDROLOGY submodel of WEPS shows a potential to accurately simulate soil water dynamics, as needed for wind erosion modeling. The submodel successfully predicts the changes in water content at the soil surface, which relate to the susceptibility of the soil to wind erosion.

乾燥表土層(DSL)法によるHEIFE沙漠観測点の砂地面からの蒸発量評価

Most methods of measuring soil-surface evaporation originate from those of measuring water-surface evaporation, and hence they are not necessarily suitable for making measurements of evaporation from dry land surfaces. The major reason is that, as observation shows, when the dry surface layer (DSL) forms on the soil, the constant-flux layer of water vapor is barely developed in the surface air layer. However, during most of the day, the upward water vapor flux in the DSL is nearly constant with depth. The principle of the DSL method is to estimate this upward near-constant flux in the DSL from the depth profiles of temperature and soil water content in the layer. The DSL method was applied to measurements of evaporation from a sand surface at the HEIFE desert station and they were compared to the decreases in soil moisture in the top 1 m of sand, from which the following results were obtained. (a) Estimates of daily evaporation about a week after rainfall of 15 mm or so, which were made on the basis of seven measurements taken by the DSL method during the day, were in good agreement with the daily decreases in soil moisture in the top 30 cm of sand. (b) Daily evaporation from dry soil can be approximately estimated from one measurement of evaporation rate made by the DSL method in the afternoon and the time lapsed since the last rainfall.

Numerical and Field Evaluation of Soil Water Sampled by Suction Lysimeters

AbstractPorous cup suction‐lysimeters are widely used for extracting soil water in solute transport monitoring, bat it is not clear how the sampled concentration can change with the ambient solute concentration, or with applied suction inside the lysimeter. This research was designed to numerically evaluate soil water sampled by suction lysimeters and to test simulation results with field observations. An axisymmetric three‐dimensional finite element model (SWMS_2D) was used to simulate soil water sampled by suction lysimeters in a Zimmerman fine sand (mixed, frigid Argic Udipsamments). To test the simulated results, KBr was uniformly spread on the soil surface. After 183 mm rainfall, suction was applied to the lysimeters (with ceramic cups at a depth of 0.8 m) to withdraw soil water; soil column samples were also removed from the surrounding soil and sectioned to determine solute concentration in situ using a bromide electrode. Simulation showed that when there is a concentrated solute band near a porous cup, as can occur with banded fertilizers, the concentration of sampled soil water in the lysimeter is considerably higher than the resident concentration in the soil at the same depth. The influence of a concentrated solute band on the concentration of sampled soil water decreased as the distance to the cup increased. Variations in applied suction (25, 35, and 45 kPa) appear to have little influence on the amount and concentration of field lysimeter extracted soil water. The measured amount of water in the lysimeter (78.4 mL) was comparatively more than the simulated amount (53.9 mL) when the mean water content of nine soil columns was used as an initial condition for the simulation, but was within the range of simulated amounts (32.5–91.5 mL) when the lower and upper 95% limits of the measured soil‐water content profiles were used as initial conditions for the simulations. Measured Br− concentration of the sampled water (3.13 × 10−4 M) was lower than the simulated concentration (4.66 × 10−4 M) when the geometric means of each layer from the nine soil column Br− concentrations were used as the initial conditions. The difference between measured and simulated concentration may have been due to variability in soil hydraulic properties.

Drought Resistance in Centipedegrass Cultivars

In our study, we sought to determine if an experimental cultivar of centipedegrass [`TC178'; Eremochloa ophiuroides (Munro) Hack.] had superior turf characteristics under extended droughts. Common centipedegrass (CC), vegetatively propagated `TC178' (VG178), and seed-propagated (F3) `TC178' (SD178) were evaluated in a 2-year controlled watering study that compared turf characteristics and drought resistance. The grasses were established under an automated rainfall shelter and were subjected to three drought regimes: watered twice per week (no stress), 2 to 3 weeks between watering (moderate), and 4 to 6 weeks between watering (severe). Turf characteristics (visual rating and clipping biomass) were measured weekly and soil water content profiles were measured daily. Visual ratings among cultivars were similar for no-stress conditions, but visual ratings of SD178 and VG178 were 18% higher than for CC for moderate stress and 28% higher for severe stress. At the end of moderate stress periods, clipping biomass of VG178 was 24% greater than for CC, but by the end of the severe stress periods, biomass from VG178 was 22% lower than for CC. Available soil water content profiles indicated that the three cultivars extracted soil water at the same rate. Visual ratings and growth decline with survival under severe stress showed that VG178 and SD178 had significantly better drought resistance than CC. `TC178' provides a superior appearance turf that will stand up to the droughts common in its adapted region.

Soil Temperature Limitation to Water Use by Field‐Grown Winter Wheat

AbstractModification of crop water use patterns in response to the field soil temperature regime has important agronomic and environmental implications. A 2‐yr field study was conducted to ascertain how soil temperature modification influenced seasonal soil water depletion for a winter wheat (Triticum aestivum L.) crop near Bozeman, MT. Soil surface cover materials including clear plastic, white metal, and styrofoam were applied in the fall after germination and plant establishment. Soil temperature and water content profiles were monitored throughout the winter and spring growing seasons. Soil water depletion was significantly accelerated for those treatments having relatively warmer soil temperatures. Differences of up to 65 mm of cumulative soil water depletion were observed between warm and cool treatments during spring. Significant water extraction from a given depth within the soil profile did not occur until the soil temperature reached 10 to 11 °C during 1985. Repeated rainfall during 1986 obscured any relationship between minimum soil temperature and water uptake in the upper soil profile, but the same relationship as during 1985 was noted at greater depths. Although the nature of our field data does not allow determination of whether cold temperatures constrained root growth, root water uptake, or both of these processes, our field results substantiate conclusions drawn by other investigators using controlled‐environment techniques. It is clear that cultural practices or climatic changes affecting seasonal soil temperature can impact soil water utilization by winter wheat in thermally limited growing areas.

Estimating near-surface soil moisture availability using a meteorologically driven soil-water profile model

This paper presents a simple water budget model for determining the vertical profile of soil water content. The model is driven by conventional meteorological and land use data. The modeled solutions of soil water content become independent of their starting conditions after a few weeks to a few months into a simulation depending on rainfall and evapotranspiration. Once the model becomes free of its initial conditions (a state to which we refer as ‘balanced’), the model is driven almost completely by current environmental conditions. Two sets of data were used to validate the model which showed close agreement between simulations and measurements. This paper presents the details of the water budget model and verifications from two field experiments.

Testing and comparison of three unsaturated soil water flow models

The performance of three soil water flow models is compared using field data collected from a structureless sandy loam soil cropped to soybeans (located near Simcoe, Ontario, Canada), and a structured clay soil cropped to grass hay (near Ottawa, Ontario, Canada). Two of the models, Soil Water and Actual Transpiration Rate, Extended (SWATRE) and Leaching Estimation and Chemistry Model (LEACHW), are based on the one-dimensional pressure head form of Richards' equation, and the third, SWASIM, is based on the one-dimensional diffusivity-water content form of Richard's equation. The models are tested and compared on the basis of their ability to predict in situ measured soil water content profiles during the growing season at the two field sites. All three models produce acceptable predictions, as evidenced by an average error within ±0.0404 cm 3·cm −3 and a relative error within ±0.2915. None of the models is consistently more accurate than the others. Discrepancies between the models are attributed primarily to differences in how the models account for soil evaporation and plant transpiration.