Variability Of Soil Hydraulic Properties Research Articles

Assessment of pedotransfer functions for estimating soil water retention curves for the amazon region

Knowledge of the soil water retention curve (SWRC) is essential for understanding and modeling hydraulic processes in the soil. However, direct determination of the SWRC is time consuming and costly. In addition, it requires a large number of samples, due to the high spatial and temporal variability of soil hydraulic properties. An alternative is the use of models, called pedotransfer functions (PTFs), which estimate the SWRC from easy-to-measure properties. The aim of this paper was to test the accuracy of 16 point or parametric PTFs reported in the literature on different soils from the south and southeast of the State of Pará, Brazil. The PTFs tested were proposed by Pidgeon (1972), Lal (1979), Aina & Periaswamy (1985), Arruda et al. (1987), Dijkerman (1988), Vereecken et al. (1989), Batjes (1996), van den Berg et al. (1997), Tomasella et al. (2000), Hodnett & Tomasella (2002), Oliveira et al. (2002), and Barros (2010). We used a database that includes soil texture (sand, silt, and clay), bulk density, soil organic carbon, soil pH, cation exchange capacity, and the SWRC. Most of the PTFs tested did not show good performance in estimating the SWRC. The parametric PTFs, however, performed better than the point PTFs in assessing the SWRC in the tested region. Among the parametric PTFs, those proposed by Tomasella et al. (2000) achieved the best accuracy in estimating the empirical parameters of the van Genuchten (1980) model, especially when tested in the top soil layer.

Scaling Approaches to Evaluating Spatial Variability of Saturated Hydraulic Conductivity and Cumulative Infiltration of an Acrisol

Spatial variability of soil properties has been frequently assessed using classical statistics and geostatistics. However, the scaling theory approach has also proven to be an effective method to describe the variation of soil hydraulic properties. The objective of this study was to evaluate the structure of spatial variability in soil hydraulic and hydrologic processes using scaling techniques in the Plantations Section of the Department of Crops and Soil Sciences, KNUST, Kumasi. Field infiltration studies were conducted using the single ring infiltrometer. Saturated hydraulic conductivity (Ks) was determined in the laboratory by the falling head permeameter. The similar media theory approach was employed in the scaling of Ks, while cumulative infiltration amount (I) was scaled using the linear variability theory. Scaling factors and parameters of the reference curves were computed directly from the parameters of individual soil hydraulic and hydrologic parameters and reference curves were obtained to represent Ks and I in the field. Scaling factors ranged from 0.23 to 2.66 with a mean of 1.00 for Ks, I and cumulative time. The composite Ks and I for the study area calculated by using different, but related, scaling factors was successful, though the distribution and range of the parameters were highly variable.

Hydrological and erosive response of a small catchment under olive cultivation in a vertic soil during a five-year period: Implications for sustainability

This study presented monitoring results during five hydrological years representative of the typical climate conditions in the region, in a commercial olive farm on a vertic soil with a soil management based on limited tillage and growing of natural weed vegetation along the traffic lanes. A significant sediment delivery to streams equal to 16.1 t ha−1 year−1 in average was observed, whereas especially high values during years with rainfall depth above the average annual rainfall, up to 52 t ha−1 year−1 were recorded. Our observations within the catchment indicate that rill and gully erosion were major erosion processes which require specific control measures such as the modification of soil management for reducing soil compaction along the tractor tramlines and the use of vegetation buffers to prevent rill erosion. The analysis of the hydrological variables suggests that an adequate prediction of runoff generation within this kind of catchment should take into account the complex response of the vertic soils, causing a large variability in soil hydraulic properties associated to changes in soil moisture. To model sediment loads, in addition to the need of an accurate prediction of runoff volume and peak flow, the dominance and contribution of gully and rill erosion have to be considered.

The role of macropores and multi-resolution soil survey datasets for distributed surface–subsurface flow modeling

Distributed watershed-scale modeling is often used as a framework for exploring the heterogeneity of runoff response and hydrologic performance of the catchment. The objective of this study is to apply this framework to characterizing the impacts of soil hydraulic properties at multiple scales on moisture storage and distributed runoff generation in a forested catchment. The physics-based and fully-coupled Penn State Integrated Hydrologic Model (PIHM) is employed to test a priori and field-measured properties in the modeling of watershed hydrology. PIHM includes an approximate representation of macropore flow that preserves the water holding capacity of the soil matrix while still allowing rapid flow through the macroporous soil under wet conditions. Both phenomena are critical to the overall hydrologic performance of the catchment. Soils data at different scales were identified: Case I STATSGO soils data (uniform or single soil type), Case II STATSGO soils data with macropore effect, and Case III field-based hydropedologic experiment revised distributed soil hydraulic properties and macropore property estimation. Our results showed that the Case I had difficulties in simulating the timing and peakflow of the runoff responses. Case II performed satisfactorily for peakflow at the outlet and internal weir locations. The distributed soils data in Case III demonstrated the model ability of predicting groundwater levels. The analysis suggests the important role of macropore flow to setting the threshold for recharge and runoff response, while still preserving the water holding capability of the soil and plant water availability. The spatial variability in soil hydraulic properties represented by Case III introduces an additional improvement in distributed catchment flow modeling, especially as it relates to subsurface lateral flow. Comparison of the three cases suggests the value of high-resolution soil survey mapping combined with a macropore parameterization can improve distributed watershed models.

Vertical variations of soil hydraulic properties within two soil profiles and its relevance for soil water simulations

Numerical simulations of soil water dynamics can be valuable tools for the assessment of different soil and land management practices. For accurate simulations, the soil hydraulic properties (SHP), i.e. the hydraulic conductivity and water retention function have to be properly known. They can be either estimated from physical soil properties by pedotransfer functions (PTF) or measured. In most studies, soil profiles are analyzed and sampled with respect to their pedogenic horizons. While considerable effort has been put on horizontal spatial SHP variations, vertical changes within soil profiles have not been analyzed in detail. Therefore, the objectives of this study were (i) the SHP measurement along vertical transects within two soil profiles, (ii) to evaluate their spatial variation and correlation with physical soil properties, and (iii) to assess the impact of the SHP determination method and its spatial discretization on simulated soil water balance components. Two soils, an agriculturally used silty-loam Chernozem and a forested sandy Cambisol were sampled in 0.05 m increments along vertical transects. The parameters of a dual porosity model were derived using the evaporation method and scaling was applied to derive representative mean SHP parameters and scaling factors as a measure of spatial variability. State-space models described spatial variations of the scaling factors by physical soil properties. Simulations with HYDRUS 1D delivered the soil water balance for different climatic conditions with the SHP being estimated from horizon-wise PTFs, or discretized either sample-wise, according to the pedogenic horizons, or as hydrologically relevant units (hydropedological approach). Considerable SHP variations were found for both soil profiles. In the Chernozem, variations of the hydraulic conductivity were largest within the ploughed Ap-horizon and could be attributed to variations in soil structure (macropores). In the subsoil, soil water retention showed a gradual decrease within each horizon. The observed water retention variations could be described by state-space models that comprised the contents of clay and total carbon, whereas variations of the hydraulic conductivity were described by clay content and total porosity. The hydraulic conductivity in the Cambisol was slightly undulating throughout the profile. Here, water retention was largest in the upper part of the profile and considerably decreased within the lower part of the Bhs-horizon. Simulated soil water balance components differed distinctly between the SHP discretizations. Compared to observed soil water contents, the simulations where the SHP were given by small-scale layers or hydropedological units performed best for both experimental sites. The different SHP discretizations mainly affected the estimated drainage losses and the simulated crop transpiration under medium to dry climatic conditions. The study confirmed the importance of an adequate spatial SHP discretization. The results indicate that SHP estimations by PTFs or the standard horizon-mean sampling strategy might fail to parameterize soil water simulations, especially in structured soils. The presented hydropedological approach showed a way to receive good simulation results by reducing the SHP observation density.

Anisotropic statistical scaling of vadose zone hydraulic property estimates near Maricopa, Arizona

[1] Fluid flow and mass transport in the vadose zone are strongly influenced by spatial variability in soil hydraulic properties. It has become common to characterize spatial variability geostatistically and to solve corresponding flow and/or transport problems stochastically. The typical (though not only) approach is to treat log saturated hydraulic conductivity, Y = log10 Ks, and perhaps other medium properties as statistically homogeneous, isotropic, or anisotropic multivariate-Gaussian random fields with unique variance and autocorrelation scale. A growing body of literature suggests that Y as well as many other variables may possess heavy-tailed, non-Gaussian distributions, and/or scale-dependent statistical parameters representing a multiscale, hierarchical structure. Elsewhere, we have demonstrated that these important phenomena are difficult to detect, and are not fully tractable, with standard geostatistical methods. They are however detectable and tractable with a novel geostatistical method of analysis which treats such variables as samples from sub-Gaussian random fields subordinated to truncated fractional Brownian motion (tfBm) or truncated fractional Gaussian noise (tfGn). Each such field is a mixture of Gaussian components having random variances. The purpose of this paper is to explore the extent to which hydraulic parameters of unsaturated soils exhibit heavy-tailed distributions and statistical scaling of the above type. As a test bed we have selected an experimental site near Maricopa, Arizona, at which soil texture data have been collected in boreholes and trenches to a depth of 15 m over an area of 3600 m2. Since hydraulic parameters are difficult to measure to such depths we estimate them using a neural network pedotransfer model. Input data include percent sand, silt, and clay. Outputs include estimates of saturated and residual volumetric water content, saturated hydraulic conductivity, and (due to their wide applicability) parameters of the van Genuchten-Mualem constitutive model of unsaturated soil property variations with capillary pressure head. We find indeed that vertical and horizontal increments of our hydraulic parameter estimates exhibit heavy-tailed frequency distributions and statistical scaling which conform closely to our novel geostatistical framework. Its generality and applicability to the Maricopa site has far reaching implications vis-à-vis the geostatistical characterization of vadose zone properties and the stochastic modeling of flow and transport in unsaturated media at other sites.

Functional evaluation of a simplified scaling method for assessing the spatial variability of soil hydraulic properties at the hillslope scale

Mapping soil hydraulic parameters with traditional scaling methods that use laboratory-determined hydraulic characteristics (the LAB method) is not always feasible as it involves expensive, time-consuming and sophisticated measurements on soil samples collected in several locations of the study area. An alternative scaling method (the AP method) has been recently proposed to indirectly retrieve the soil hydraulic properties following the Arya-Paris physico-empirical pedotransfer function, which makes use of particle-size distribution and bulk density values. In this synthetic study we verify the performance of the AP method from a functional perspective, by evaluating the differences in the simulated soil water budget through a Monte Carlo approach. Notwithstanding that the AP method can provide soil hydraulic property patterns with faster experimental procedures and minor costs, we observe significant bias in the predicted spatially-averaged soil water budget due to a poor parametric calibration of the AP method and an imprecise identification of the spatial correlation structure of the AP-estimated scaling factors. Citation Nasta, P., Romano, N., and Chirico, G.B., 2013. Functional evaluation of a simplified scaling method for assessing the spatial variability of soil hydraulic properties at the hillslope scale. Hydrological Sciences Journal, 58 (5), 1059–1071.

Prediction of spatially variable unsaturated hydraulic conductivity using scaled particle‐size distribution functions

Simultaneous scaling of soil water retention and hydraulic conductivity functions provides an effective means to characterize the heterogeneity and spatial variability of soil hydraulic properties in a given study area. The statistical significance of this approach largely depends on the number of soil samples collected. Unfortunately, direct measurement of the soil hydraulic functions is tedious, expensive and time consuming. Here we present a simple and cost‐effective hybrid scaling approach that combines the use of ancillary information (e.g., particle‐size distribution and soil bulk density) with direct measurements of saturated soil water content and saturated hydraulic conductivity. Our results demonstrate that the presented approach requires far fewer laboratory measurements than conventional scaling methods to adequately capture the spatial variability of soil hydraulic properties.

Environmental and management influences on temporal variability of near saturated soil hydraulic properties

Structural porosity is a decisive property for soil productivity and soil environmental functions. Hydraulic properties in the structural range vary over time in response to management and environmental influences. Although this is widely recognized, there are few field studies that determine dominant driving forces underlying hydraulic property dynamics. During a three year field experiment we measured temporal variability of soil hydraulic properties by tension infiltrometry. Soil properties were characterized by hydraulic conductivity, effective macroporosity and Kosugi's lognormal pore size distribution model. Management related influences comprised three soil cover treatment (mustard and rye vs. fallow) and an initial mechanical soil disturbance with a rotary harrow. Environmental driving forces were derived from meteorological and soil moisture data. Soil hydraulic parameters varied over time by around one order of magnitude. The coefficient of variation of soil hydraulic conductivity K(h) decreased from 69.5% at saturation to 42.1% in the more unsaturated range (−10cm pressure head). A slight increase in the Kosugi parameter showing pore heterogeneity was observed under the rye cover crop, reflecting an enhanced structural porosity. The other hydraulic parameters were not significantly influenced by the soil cover treatments. Seedbed preparation with a rotary harrow resulted in a fourfold increase in macroporosity and hydraulic conductivity next to saturation, and homogenized the pore radius distribution. Re-consolidation after mechanical loosening lasted over 18months until the soil returned to its initial state. The post-tillage trend of soil settlement could be approximated by an exponential decay function. Among environmental factors, wetting-drying cycles were identified as dominant driving force explaining short term hydraulic property changes within the season (r2=0.43 to 0.59). Our results suggested that beside considering average management induced changes in soil properties (e.g. cover crop introduction), a dynamic approach to hydrological modeling is required to capture over-seasonal (tillage driven) and short term (environmental driven) variability in hydraulic parameters.

Temporal variability of structure and hydraulic properties of topsoil of three soil types

The soil structure and hydraulic properties of arable soils considerably vary during the year due to the periodical tillage, fertilization, plant and root grow, climate impact etc. The knowledge of these soil properties is essential when assessing water regime and associated dissolved substance transport in soils. Temporal variability of soil properties measured in surface horizons of three soil types (Haplic Cambisol, Greyic Phaeozem, Haplic Luvisol) was studied in years from 2007 to 2010. Undisturbed soil samples were taken every month to evaluate the actual field soil-water content, bulk density, porosity and hydraulic properties. The grab soil samples were taken every month to evaluate aggregate stability using the WSA (water stable aggregates) index, pHH2O and pHKCl, soil organic matter content and quality. Unsaturated hydraulic conductivity for pressure head of −2cm was measured directly in the field using the minidisk tension infiltrometer. In addition soil structure was documented on micromorphological images.In some cases, the similar seasonal trends of the soil pHH2O, pHKCl, organic matter quality, bulk density, porosity or aggregate stability were observed in different soils. Parameters characterizing soil hydraulic properties were highly variable and did not show similar trends for different soils. This study showed different trends during different years. Thus data, which were obtained during one year period, could not be used to generalize soil properties development in particular soil and crop. The soil structure, aggregate stability and soil hydraulic properties were interrelated and depended on plant growth, rainfall and tillage. The drier conditions in some soils positively influenced the soil aggregate stability, slope of the retention curve at the inflection point and hydraulic conductivity. Probably due to the high variation of soil hydraulic properties no closer correlation between them and other properties was detected. The slope of the retention curve at the inflection point (e.g. indicator of soil physical quality) in many cases increased (decreased) when also the soil aggregate stability and hydraulic conductivity values increased (decreased). No closer correlation was revealed when analyzing these relationships for the entire observation period.

Soil water status and water table depth modelling using electromagnetic surveys for precision irrigation scheduling

Electromagnetic surveys have been used to quantify soil variability with respect to soil water storage in an irrigated maize field. A fluctuating water table sub-irrigates the crop in some places, and a wireless sensor network simultaneously monitors real-time depth of water table and soil moisture content, with large differences in soil moisture measured at any one time in these uniformly textured sands. These large differences justify assessment of the spatio-temporal variability of soil hydraulic properties when aiming for precision management of the resource. Regression models were used to spatially predict water table depth and moisture content at 50cm using EM38 survey data, a rainfall time series and a wetness index extracted from a digital elevation model. A multiple linear regression modelling (MLM) approach was compared with a data‐mining approach using a random forest model (RF). The RF model implements a more thorough interrogation of the data using classification trees with subsequent regression of the data and provided the best prediction of soil moisture (R2=0.94; RMSE=0.03m3m−3 using RF; R2=0.77; RMSE=0.06m3m−3 using MLM) and water table depth (R2=0.91; RMSE=7.17cm using RF; R2=0.71; RMSE=12.48cm using MLM).

Influence of soil structure and root water uptake strategy on unsaturated flow in heterogeneous media

We analyze the combined effects of the spatial variability of soil hydraulic properties and the water uptake by plant roots on unsaturated water flow. For this analysis, we use a simplified macroscopic root water uptake model which is usually applied only for homogeneous or layered soil and therefore we also determine whether it is applicable for multidimensional heterogeneous media. Analytical solutions for mean and variance of pressure head (first‐order second‐moment approximations) in layered media and numerical solutions of two‐dimensional (2‐D) autocorrelated multi‐Gaussian and non multi‐Gaussian parameter fields are analyzed for steady state and transient flow conditions. For non‐Gaussian topological features, that have little influence on the mean and the variance of the pressure field if root water uptake is ignored, we test whether the influence is significant if root water uptake is accounted for. The results reveal that, in field structures with large patches of coarse material, local regions with pressure head values at the wilting point develop; these are surrounded by wet material. Without a compensation mechanism for local stress, the global transpiration demand is not met if local wilting occurs. Various compensation mechanisms are tested that depend, respectively, on the saturation, the relative conductivity or a strategy where the deficit in the global uptake rate is equally distributed to unstressed locations. The strategies lead to a global actual transpiration rate at the potential value and attenuate the formation of locally wilted areas. Wilted regions can, however, still occur, and may be an artifact of the simplified model concept as root‐soil interactions are neglected. Therefore simplified macroscopic models for root water uptake should be used with caution in heterogeneous media.

Event-based estimation of water budget components using a network of multi-sensor capacitance probes

A new approach was developed for estimating vertical soil water fluxes using soil water content time series data. Instead of a traditional fixed time interval, this approach utilizes the time interval between two sequential minima of the soil water storage time series to identify groundwater recharge events and calculate components of the soil water budget. We calculated water budget components: surface-water excess (Sw), infiltration less evapotranspiration (I – ET) and groundwater recharge (R) from May 2001 to January 2003 at eight locations at the USDA Agricultural Research Center, Beltsville, Maryland, USA. High uncertainty was observed for all budget components. This uncertainty was attributed to spatial and temporal variation in Sw, I – ET and R, and was caused by nonuniform rainfall distributions during recharge events, variability in the profile water content, and spatial variability in soil hydraulic properties. The proposed event-based approach allows estimating water budget components when profile water content monitoring data are available. Citation Guber, A., Gish, T., Pachepsky, Y., McKee, L., Nicholson, T. & Cady, R. (2011) Event-based estimation of water budget components using a network of multi-sensor capacitance probes. Hydrol. Sci. J. 56(7), 1227–1241.

Stochastic Finite-Element Approach to Quantify and Reduce Uncertainty in Pollutant Transport Modeling

One of the important issues in simulation of contaminant transport in the subsurface is how to quantify the hydraulic properties of soil that are randomly variable in space because of soil heterogeneity. Stochastic approaches have the potential to represent spatially variable parameters, making them an appropriate tool to incorporate the effects of the spatial variability of soil hydraulic properties on contaminant fate. This paper presents development and application of a numerical model for simulation of advective and diffusive-dispersive contaminant transport using a stochastic finite-element approach. Employing the stochastic finite-element method proposed in this study, the response variability is reproduced with a high accuracy. Comparison of the results of the proposed method with the results obtained using the Monte Carlo approach yields a pronounced reduction in the computation cost while resulting in virtually the same response variability as the Monte Carlo technique.

Modeling of contaminant transport in soils considering the effects of micro- and macro-heterogeneity

This paper presents development and application of a numerical model for simulation of contaminant transport considering macro- and micro-heterogeneity in soil. Stochastic finite element approach is used to incorporate the effects of the spatial variability of soil hydraulic properties on contaminant fate. The soil micro-heterogeneity effects are modeled with a dual domain concept in which a first order kinetic expression is used to describe the transfer of the solute between the two domains. The model is used to simulate transport of non-reactive solute in a lysimeter under steady-state water flux. Numerical simulation of the problem is carried out for four different scenarios assumed for structure and formation of the soil: a single domain having uniform hydraulic properties (SDU), a dual-domain system with uniform hydraulic properties (DDU), a single domain with spatially variable hydraulic properties (SDV) and a dual-domain system with spatially variable hydraulic properties (DDV). The numerical results are compared with experimental data. The results obtained based on SDU are not in agreement with the measured data. Assuming a dual domain system (DDU) yields satisfactory results but higher accuracy is achieved using DDV scenario. The results show that the combination of the dual domain approach with the stochastic approach improves the numerical predictions. Also, using stochastic finite element approach makes it possible to evaluate variance of concentration as an index of reliability of the result.

Influence of Soil Heterogeneity and Spatial Discretization on Catchment Water Balance Modeling

This study investigated the impacts of the spatial variability of soil hydraulic properties and the effects of spatial discretization on the water balance in a fully coupled system. The integrated surface–subsurface, three‐dimensional, finite element model HydroGeoSphere was applied to the forested Wüstebach basin (27 ha) to simulate water fluxes. The fully coupled flow simulation model was applied to the headwater catchment at two different spatial resolutions (25 and 100 m). The change in spatial resolution required an aggregation of the soil map, which influenced the water fluxes and the spatial patterns of soil moisture. The nonlinear relationship between soil moisture and transpiration caused the spatial aggregation of soil moisture to have a larger effect on the water balance than did aggregating the soil hydraulic properties. In addition to the total discharge, the effects on the spatial patterns of the simulated soil moisture were also investigated. The results show that aggregating soil hydraulic properties results in lower uncertainties than does using a coarser discretization. This can be explained by the nonlinearity of the relationship between soil moisture and evapotranspiration.

Impact of land use on the hydraulic properties of the topsoil in a small French catchment

AbstractThe hydraulic properties of the topsoil control the partition of rainfall into infiltration and runoff at the soil surface. They must be characterized for distributed hydrological modelling. This study presents the results of a field campaign documenting topsoil hydraulic properties in a small French suburban catchment (7 km2) located near Lyon, France. Two types of infiltration tests were performed: single ring infiltration tests under positive head and tension‐disk infiltration using a mini‐disk. Both categories were processed using the BEST—Beerkan Estimation of Soil Transfer parameters—method to derive parameters describing the retention and hydraulic conductivity curves. Dry bulk density and particle size data were also sampled. Almost all the topsoils were found to belong to the sandy loam soil class. No significant differences in hydraulic properties were found in terms of pedologic units, but the results showed a high impact of land use on these properties. The lowest dry bulk density values were obtained in forested soils with the highest organic matter content. Permanent pasture soils showed intermediate values, whereas the highest values were encountered in cultivated lands. For saturated hydraulic conductivity, the highest values were found in broad‐leaved forests and small woods. The complementary use of tension‐disk and positive head infiltration tests highlighted a sharp increase of hydraulic conductivity between near saturation and saturated conditions, attributed to macroporosity effect. The ratio of median saturated hydraulic conductivity to median hydraulic conductivity at a pressure of − 20 mm of water was about 50. The study suggests that soil texture, such as used in most pedo‐transfer functions, might not be sufficient to properly map the variability of soil hydraulic properties. Land use information should be considered in the parameterizations of topsoil within hydrological models to better represent in situ conditions, as illustrated in the paper. Copyright © 2010 John Wiley & Sons, Ltd.

Numerically predicting seepage gradient forces and erosion: Sensitivity to soil hydraulic properties

summary Research has suggested that streambank seepage can be an important mechanism of bank instability; however, limited information is available on the level of soil characterization necessary to accurately predict seepage gradient forces and erosion. The objective of this research was to quantify the expected range of predicted seepage gradients for various degrees of site characterization. Uncertainty analysis on seepage gradient predictions was performed relative to variability in soil hydraulic properties. A two-dimensional unsaturated/saturated groundwater flow model was used to simulate a homogeneous soil layer for sand and loamy sand soils packed at various bulk densities, qb. A pedotransfer function (ROSETTA), designed to estimate unsaturated hydraulic properties from surrogate soil data (i.e., texture and bulk density), was used to derive the saturated hydraulic conductivity, Ks, and water retention parameters for various levels of site information (i.e., only textural class; percent sand, silt, and clay (%SSC); %SSC and qb; and %SSC, qb, and Ks). Statistical distributions were derived for each soil hydraulic parameter and Monte Carlo simulations were performed to generate distributions of maximum seepage gradient. The deviation in predicted seepage gradient was calculated using assumed baseline conditions. Ranges in predicted soil hydraulic parameters and maximum seepage gradients were considerably reduced when using %SSC as compared to soil texture. Therefore, at a minimum, soil samples should be taken for particle size analysis. For qb between 1450 and 1500 kg m � 3

Estimation and analysis of soil hydraulic properties through infiltration experiments: comparison of BEST and DL fitting methods

AbstractThe BEST method (Beerkan estimation of soil transfer parameters through infiltration experiments) appears promising and easy to estimate not only saturated hydraulic conductivity but also water retention and hydraulic characteristics. However, few tests have been conducted to test the methodology. This study involved field BEST infiltration experiments for three layers (surface, 15 and 30 cm) for each of three soils with different soil textures under grassland. By comparing BEST with DL (differentiated linearization method), we found that the DL method did not produce a good estimate of the soil hydraulic properties and neither did it identify the transient flow state. The BEST method resulted in reasonable results and is therefore promising. However, with BEST we encountered some anomalies when calculating hydraulic properties in some cases with too few data points under the transient flow state. We show that the application of BEST field experiments requires a wide range of soil water content from initial to saturated states so as to include sufficient transient flow. The soil hydraulic properties determined using the BEST method showed contrasting characteristics between different soil textures with higher saturated hydraulic conductivity under coarse texture and lower values under loam textures, especially with highly compacted soils. Vertical variation in soil hydraulic properties was significant, and the surface layer had a lower saturated hydraulic conductivity partly caused by compaction (high bulk density) or by remnants of grass plants. Further research on the effects of compaction and grass plants on soil hydraulic properties is needed.

Effect of temporal variability in soil hydraulic properties on simulated water transfer under high-frequency drip irrigation

The effect of changes in the hydraulic properties of a loamy topsoil on water transfer under daily drip irrigation was studied over a cropping cycle. Soil water contents were measured continuously with neutron probes and capacitance sensors placed in access tubes (EnviroSMART) and were compared to predications made by the Hydrus-2D model. Three different sets of hydraulic parameters measured before and after irrigation started, were used. Our results demonstrated that, based on the assumptions used in this study, the accuracy of the Hydrus predictions is good. Graphical and statistical comparisons of simulated and measured soil water contents and consequently the total water storage revealed a similar trend throughout the monitoring period for the all three different sets of parameters. The soil hydraulic properties determined after irrigation started were found to be much more representative of the majority of the irrigation season, as confirmed by the accuracy of the simulation results with high values of the index of agreement and with values of RMSE similar in magnitude to the error associated with field measurements (0.020 cm 3 cm −3). The highest RMSE values (about 0.04 cm 3 cm −3) were found when the model used input soil parameters measured before irrigation started. Generally, changes in topsoil hydraulic properties over time had no significant effect on soil moisture distribution in our agro-pedo-climatic context. One possible explanation is that daily water application was conducted at the same time as maximal root water uptake. This meant the soil did not need to store total daily crop water requirements and consequently that the water redistribution phase represented a very short stage in the irrigation cycle. It is probable that irrigating in the daytime when crop evapotranspiration is highest could prevent the effects of a temporal change and other problems connected with the soil. Moreover, water will be always available for the crop. Further experiments are needed to justify the results and to study the effects of low frequency drip irrigation on soil hydraulic characterization and consequently on soil water transfer in order to improve irrigation scheduling practices.