RETC Software Research Articles

Comment on “Modified expression for unsaturated hydraulic conductivity according to Mualem–van Genuchten to allow proper computations at low pressure heads” by M. Heinen

AbstractThis comment concerns evaluation of the Mualem–van Genuchten (MvG) unsaturated soil hydraulic functions at very low (negative) pressure heads as described recently in an important study by Heinen. He showed that below some critical pressure head, the unsaturated hydraulic conductivity should be approximated by a power function, even when evaluated using double precision computations. We show that a more precise approximation is possible when the approximation is formulated in terms of effective fluid saturation (Se) rather than the pressure head, h. More general constraints are also provided when the approximated hydraulic conductivity equation should be used. The alternative formulation and constraints were implemented earlier in the SOHYP and RETC software packages.

Long-term analysis of soil water regime and nitrate dynamics at agricultural experimental site: Field-scale monitoring and numerical modeling using HYDRUS-1D

Intensive agricultural practices increase agrochemical pollution, particularly nitrogen (N) based fertilizers, which present an environmental risk. This study aims to evaluate long-term (2009–2020) data on soil water regime and nitrate dynamics at an agricultural experimental site on fine-textured soils and to better understand the implications of N management in relation to groundwater pollution. The field site is located in the Biđ field (eastern Croatia), in the proximity of the Sava river. Zero-tension lysimeters were installed at six selected locations. Lysimeters were used to monitor the water regime, i.e., outflows in which nitrate concentration was measured, while additional soil-water samples were collected via 4 and 15-meter-deep monitoring wells. Soil hydraulic parameters were estimated by combining the laboratory measurements, and estimation in RETC software. Water regime and nitrate leaching in lysimeters were simulated using HYDRUS-1D for each year to allow crop rotation and to evaluate their effects individually. The HYDRUS-1D model successfully reproduced lysimeter outflows and nitrate dynamics, which was confirmed with high R2 values (water: 93% above 0.7, and nitrate: 73% above 0.7) indicating the good performance of the model simulating nitrification chain reactions. Principal component analysis (PCA) was performed to identify the relationships among all soil properties and environmental characteristics. The results showed the complex interaction of soil hydraulic properties, precipitation patterns, plant uptake, and N application. All locations have a decreasing trend of nitrate leaching over the investigation period. Most of the lysimeter outflows and elevated nitrate concentrations were connected to the wet period of the year when the soil was saturated, and evapotranspiration was low. The results of this study show that it is important to optimize N fertilizer applications for each particular environmental condition to reduce nitrate loss. The study indicates the importance of long-term field studies, key for agro-hydrological modeling and the improvement of agricultural practices.

Hydraulic conductivity and diffusivity of an Oxisol cultivated with sugarcane fertigated with nitrogen and potassium

This study had the objective to evaluate the effect of irrigation and fertigation (NK) in the hydraulic conductivity and diffusivity of an Oxisol cultivated with sugarcane. The experimental design comprised randomized blocks in a 5 × 2 factorial scheme, with four replications. Treatments consisted of five levels of water replacement (100, 75, 50, 25 and 0%), with and without fertirrigation (NK). The planting of sugarcane, cultivar RB85-5453, was performed in a double row (W-shaped), 8 m long, with 1.80 m spacing between the double rows, the distance between the crops in the double row was 0.40 m, with a total area of 52,8 m2 in each paddock. For treatments with water, replacement (WR) a drip tube was placed in the ground at a depth of 0.20 m among the furrows of the double row. The drip tube (DRIPNET PC 16150) comprised a thin wall, 1.0 bar pressure, nominal discharge 1.0 L h-1, and 0.50 m spacing between drippers. Nitrogen was applied by fertirrigation at a dose of 100 Kg ha-1, at 30-day intervals, with 10 applications throughout the development of the sugarcane culture. Potassium fertilization was done partially, in 30% of the furrows, and the remaining part was treated with the irrigation water. Nitrogen and potassium were spread only in the treatment with 0% water replacement. Was evaluated hydraulic conductivity and diffusivity versus logarithmic pressure head, at a depth of 10 cm, using RETC software. The hydraulic diffusivity for water replacement of 25 and 50% with fertigation was 160.3 and 14.9 cm2 days-1 for the lower values of the logarithm of the pressure head.

Small-scale spatial variability of hydro-physical properties of natural and degraded peat soils

The spatial variability of soil properties plays an important role in water and carbon cycles in peatlands. The objectives of this study were to analyze the spatial variation of hydro-physical properties of peat soils and to establish pedotransfer functions (PTFs) to estimate the hydraulic properties of peat using readily available soil properties. We selected three study sites, each representing a different state of peat degradation (natural, degraded and extremely degraded). At each site, 72 undisturbed soil cores were collected from 5 m by 5 m grid cells in an area of 35 m by 40 m. The saturated hydraulic conductivity (Ks), soil water retention curves, total porosity, macroporosity (pore diameter >30 μm), bulk density and soil organic matter content (SOM) were determined for all sampling locations. The Van Genuchten (VG) model parameters (θs, α, and n) were optimized using the RETC software package. A strong positive correlation between macroporosity and Ks was observed irrespective of the degradation stage of the peat. However, the relationships between macroporosity and Ks differed between the natural and the drained peatlands. Adding macroporosity to the PTFs substantially improves the prediction of Ks as well as VG parameters. Results show that the soil physical and hydraulic properties (e.g. Ks and VG model parameters) exhibit different levels of spatial heterogeneity depending on the peat degradation stage. The geostatistical analysis suggests that the spatial dependence of soil hydro-physical properties varies depending on the considered property as well as land management (e.g. drainage). Bulk density and SOM are spatially dependent, whereas Ks and macroporosity are spatially independent if the peat is severely degraded. In conclusion, the peat degradation stage plays an important role and should be generally considered in the spatial analysis of peatlands. The obtained semivariograms may serve as a basis for 2D and 3D hydrological modelling as well as peatland restoration studies.

Effect of straw addition on soil infiltration characteristics and model-fitting analysis

This study explored the influence of straw addition on the soil water characteristic curves and the soil water holding capacity. Using two different soil bulk densities, soil infiltration was investigated using differently treated straw, i.e., comminuted ammoniated, and blended with inorganic soil conditioner. The experimental results showed that the addition of long straw significantly obstructed soil infiltration. Compared with the control experiment and the comminuted straw treatment, both the addition of ammoniated comminuted straw and the inorganic soil conditioner (Fe (OH)3) effectively improved the soil stable infiltration rate and the cumulative infiltration. After 30 days of culturing, the ammoniated straw treatment affected a more remarkable improvement of the soil structure by reducing the soil bulk density and increasing the soil porosity than did the inorganic soil conditioner treatment. Using a straw and inorganic soil conditioner in combination did not result in a positive interaction for soil improvement. The relative ranking of each treatment on soil infiltration performance was the same for both soil bulk densities. For the soil water characteristic curve of each treatment, both the Philip infiltration model and the van Genuchten (VG) fitting model using RETC software had excellent fitting accuracy. The excellent soil water holding capacity arising from the use of ammoniated comminuted straw at low suction pressures is of significance for soil water preservation in the dry season of the humid region of southern China.

Brooks–Corey Modeling by One-Dimensional Vertical Infiltration Method

The laboratory methods used for the soil water retention curve (SWRC) construction and parameter estimation is time-consuming. A vertical infiltration method was proposed to estimate parameters α and n and to further construct the SWRC. In the present study, the relationships describing the cumulative infiltration and infiltration rate with the depth of the wetting front were established, and simplified expressions for estimating α and n parameters were proposed. The one-dimensional vertical infiltration experiments of four soils were conducted to verify if the proposed method would accurately estimate α and n. The fitted values of α and n, obtained from the RETC software, were consistent with the calculated values obtained from the infiltration method. The comparison between the measured SWRCs obtained from the centrifuge method and the calculated SWRCs that were based on the infiltration method displayed small values of root mean square error (RMSE), mean absolute percentage error (MAPE), and mean absolute error. SWMS_2D-based simulations of cumulative infiltration, based on the calculated α and n, remained consistent with the measured values due to small RMSE and MAPE values. The experiments verified the proposed one-dimensional vertical infiltration method, which has applications in field hydraulic parameter estimation.

Hydraulic properties of drained and cultivated fen soils part I - Horizon-based evaluation of van Genuchten parameters considering the state of moorsh-forming process

Soil hydraulic properties of drained and cultivated fen soils vary considerably due to structural and physical alteration in the long-term. Although hydrological modelling is a key method to determine soil water conditions for a large number of aims, for instance the utilization of fen soils, general estimators of water retention that reflect the progressive nature of fen soil alteration are rare. Based on 405 horizons from 30 fen soil profiles at 12 different peatland sites in Germany soil hydraulic properties were estimated. Water retention and bulk density measurements were conducted fivefold per each horizon mapped after the German Soil Taxonomy. Considering the varying phases of moorsh-forming process, bulk densities and water retention data were clustered according to horizon category by applying the k-means method. This approach reduced the range of water contents measured at considered pressure heads (pF) and recommended their subdivision into three groups of graduated bulk density (high, moderate, low) per each horizon category. This subdivision resulted in 18 separated data sets for parameterisation of the selected van Genuchten model using the RETC software. Estimated water content at saturation ranged between 0.68 and 0.90cm3cm−3 and 0.10 and 0.35cm3cm−3 at a pF-value of 4.2. Shape parameter α varied stronger than n, especially for the strongly altered fen soil horizons. The continuing alteration of the moorshified, earthified, aggregated, and shrunken fen soil horizons classified by increasing bulk density levels was reflected by the estimated van Genuchten parameters. For horizons inventoried as unchanged peat substrates, less variability of bulk densities in combination with deviating van Genuchten parameters indicated the influence of additional factors on soil hydraulic properties.

Deforestation effects on soil quality and water retention curve parameters in eastern Ardabil, Iran

The land use change from natural to managed ecosystems causes serious soil degradation. The main objective of this research was to assess deforestation effects on soil physical quality attributes and soil water retention curve (SWRC) parameters in the Fandoghlou region of Ardabil province, Iran. Totally 36 surface and subsurface soil samples were taken and soil water contents measured at 13 suctions. Alfa (α) and n parameters in van Genuchten (1980) model were estimated by fitting SWRC data by using RETC software. The slope of SWRC at inflection point (SP) was calculated by Dexter (2004) equation. The results indicated that with changing land use from forest (F) to range land (R) and cultivated land (C), and also with increasing soil depth from 0–25 to 75–100 cm in each land use, organic carbon, micropores, saturated and available water contents decreased and macropores and bulk density increased significantly (P < 0.05). The position of SWRC shape in F was higher than R and C lands at all soil depths. Changing F to R and C lands and also increasing soil depth in each land use significantly (P < 0.05) increased α and decreased n and SP. The average values of SP were obtained 0.093, 0.051 and 0.031 for F, R and C, respectively. As a result, deforestation reduced soil physical quality by affecting SWRC parameters.

Estimating S-index, unsaturated hydraulic conductivity and diffusivity through RETC in Indian Agricultural Research Institute farm

The hydraulic properties of vadoze zone like hydraulic conductivity, diffusivity and soil water retention characteristics are required to study soil water, nutrient and pollutant dynamics and their management. Surface layer of ten master profiles of Indian Agricultural Research Institute (IARI) farm, New Delhi were studied for different physicochemical and hydraulic properties. All ten master profiles were grouped under the order Inceptisol, subgroup Typic Haplustept and hyperthermic soil temperature regime. The soil water retention data were measured for all the sample layers and fitted to van Genuchten model using RETC software. The R2 value of fitted curve varied between 0.98 to 0.99 with residual sum of squares from 0.001 to 0.003. The θr value varied between 0.0332 to 0.0760 cm3 cm−3 and the saturated water content between 0.4665 to 0.3613 cm3 cm−3. In the present study, α ranged between 0.0018 to 0.0164 cm−1. The n value varied between 1.381 to 2.437. Combined for all profiles the prediction equation developed for K(θ) is K(θ) = 1.17×106×θ11.93 and D(θ) is D(θ) = 2.07×107× θ7.71 with R2 value of 0.77 both for K(θ) and D(θ). The Sindex varied between 0.068 to 0.128. The S-index was significantly positively correlated with θr (r = 0.73*), θs (r = 0.64*), n (r = 0.97**) and significantly negatively correlated with α (r = -0.80**). The S-index value for all the 10 master profiles were more than 0.050 indicating very good physical quality for all the 10 master profiles of IARI experimental farm.

Risk assessment of gas oil and kerosene contamination on some properties of silty clay soil.

Soil and ground water resource pollution by petroleum compounds and chemical solvents has multiple negative environmental impacts. The aim of this research was to investigate the impacts of kerosene and gas oil pollutants on some physical and chemical properties, breakthrough curve (BTC), and water retention curve (SWRC) of silty clay soil during a 3-month period. Therefore, some water-saturated soils were artificially contaminated in the pulse condition inside some glassy cylinders by applying half and one pore volume of these pollutants, and then parametric investigations of the SWRC were performed using RETC software for Van Genukhten and Brooks-Corey equations in the various suctions and the soil properties were determined before and after pollution during 3 months. The results showed that gas oil and kerosene had a slight effect on soil pH and caused the cumulative enhancement in the soil respiration, increase in the bulk density and organic matter, and reduction in the soil porosity and electrical and saturated hydraulic conductivity. Furthermore, gas oil retention was significantly more than kerosene (almost 40%) in the soil. The survey of SWRC indicated that the contaminated soil samples had a little higher amount of moisture retention (just under 15% in most cases) compared to the unpolluted ones during this 3-month period. The parametric analysis of SWRC demonstrated an increase in the saturated water content, Θ s, from nearly 49% in the control sample to just under 53% in the polluted ones. Contaminants not only decreased the residual water content, Θ r, but also reduced the SWRC gradient, n, and amount of α parameter. The evaluation of both equations revealed more accurate prediction of SWRC's parameters by Van Genukhten compared to those of Brooks and Corey.

Evaluation of petroleum impacts on some properties of loamy sand soil with the main focus on hydraulic properties

In recent years, petroleum products and by-products have been widely used chemicals in soil habitats, which change the soil properties and quality of groundwater due to solubility, volatility and biodegradability of these pollutants. The aim of this research was to investigate the impacts of kerosene and gas oil pollutants on the breakthrough curve (BTC), soil water retention curve (SWRC) and some physical and chemical properties of the laboratory-contaminated soils. For this purpose, some water-saturated soils were artificially contaminated in the pulse condition inside some glass cylinders by applying half and one pore volume of these pollutants. Furthermore, parametric investigations of the soil retention curves were performed using RETC software by Van Genukhten, and Brooks–Corey equations in the various suctions and the soil properties were determined before and after pollution during a 3-month period. The BTC revealed that kerosene flowed out of the soil with a sharper gradient and greater velocity than gas oil, especially on introduction of one volume of kerosene (triple times more than the 0.5 volume). The RETC results regarding the Van Genukhten model demonstrated slightly lower water-holding capacity in the contaminated samples than the unpolluted one. Contamination not only diminished the saturation moisture (Θs), but also reduced the residual moisture (Θr) and the SWRC gradient (n), while Brooks–Corey equation represented only one set of data for all conditions. It can be concluded that all apparent physical and chemical properties of soil have been changed by spill of petroleum hydrocarbon on the soil surface. Evaluation of both equations demonstrated more accurate prediction of the SWRC’s parameter by Van Genukhten compared to Brooks–Corey. The results of this research can be generalizable for the assessment of petroleum pollution impacts on the quality and changes of soil properties in terms of pulse contamination for approximately the same condition on a wider scale.

Study on soil shrinkage characteristics during soil water characteristic curve measurement

Clay loam soil from Yunnan(KM),sandy loam soil from Liaoning(SY) and Xinjiang(KL),and loam soil from Shandong(LY) were selected for analyzing the characteristics of soil shrinkage duringsoil water characteristic curve measurement in order to explore the suitability of fitting models for soil watercharacteristic curve,and the changes of soil shrinkage. In this paper,soil water characteristic curves ofthe four soil types were measured by centrifuge method and were fitted by RETC software. The resultsshowed that as for KM,SY,KL and LY soils,firstly,soil moisture content gradually decreased with soilsuction increasing from 0 to 7000 cm. And the rate of soil water reduction decreased,and then increased,and then decreased again. Secondly,all fitting models presented in RETC software were of high accuracyfor the simulation of soil water characteristic curves,and the appropriate fitting models for KM,SY,KLand LY soils were VG-M(m,n),LND-M,VG-M(1-1/n,n) and VG-B(m,n),respectively. And VGmodel could be considered as the optimal model for simulation of soil water characteristic curves of thefour soils. Thirdly,a logarithmic function was found between degree of effective soil shrinkage and soil suc-tion during soil water characteristic curve measurement. The soil shrinkage was ranked in the order of SY KM LY KL,and the bulk density of the four soils increased by 0.19 ~ 0.46 g/cm3. Fourthly,soil shrink-age process could be roughly divided into pseudo-saturated stage,structural stage and super-normal stage.And as for KM,SY,KL and LY soils,the three stages could be described by linear function,but theranges of soil suction corresponding to three soil shrinkage stages were different.

Determination of pore size distribution and hydraulic properties using nuclear magnetic resonance relaxometry: A comparative study of laboratory methods

In this study, we evaluate the feasibility of using nuclear magnetic resonance (NMR) relaxometry measurements to characterize pore size distribution and hydraulic properties in four porous samples with different texture and composition. We compare NMR with two classical techniques based on water retention and mercury intrusion measurements. Both T2 and T1 NMR relaxation measurements at 6.47 MHz were carried out for three saturated model samples (medium sand, fine sand, and a homogenous sand/kaolin clay mixture) and one saturated natural silt loam soil. Cumulative pore size distribution functions and mean pore diameters were calculated assuming average surface relaxivity parameters and a cylindrical capillary model of the pores. The mean pore diameters derived from T2 and T1 distributions as well as the cumulative pore size distribution functions agree satisfactorily with those derived from mercury intrusion and retention curves. The observed deviations are due to limitations of each method, sample preparation, and sample composition. To evaluate the influence of the variations observed in the hydraulic properties of the samples, the pore size distribution functions were scaled back to water retention functions, and the van Genuchten hydraulic parameters were estimated by inversion using the RETC software. The comparison shows that both T2 and T1 NMR relaxation measurements can be used to estimate pore size distribution and mean pore diameter, as well as the retention function and corresponding hydraulic properties.