Heterogeneous Soil Profiles Research Articles

Subsurface flow model for estimating 2D wetting pattern in drip irrigation

AbstractNumerical model using 2D Richard’s equation in cylindrical polar coordinate system was developed to assess the soil wetting patterns in a drip irrigation system. A fully implicit finite‐difference scheme was used to solve equations with suitable initial and boundary conditions. The developed model was validated with an experimental result available in the literature, which shows a high level of concordance with low values of RMSE and R2 greater than 0.93 for all discharge rates. Furthermore, the versatility and applicability of the model were demonstrated for complex subsurface conditions considering homogeneous and heterogeneous soil profiles. Water ponding on the surface was observed in clay loam soil, contrasting with deep percolation in sandy loam soil. This underscores the significance of adjusting both discharge rate and irrigation frequency in accordance with prevailing soil conditions.

The effect of soil texture, layering and water head on the infiltration rate and infiltration model accuracy

AbstractInfiltration is one of the most important physical characteristics of soil and depends on various factors. This study investigated the influence of soil texture, layering and water head on the soil water infiltration rate. It also selected the most accurate infiltration models to determine the water infiltration rate in homogeneous and heterogeneous soil profiles. Experiments were carried out in four soil containers with a length, width and height of 20 × 20 × 70 cm. Treatments consisted of two soil textures (sandy loam, SL; clay loam, CL), four soil profiles (homogeneous texture, SL and CL; and heterogeneous texture, lighter texture on the top, SL/CL, and heavier texture on the top CL/SL) and three water head sizes (4, 7 and 10 cm). Several models were used to determine the water infiltration rate under homogeneous (Kostiakov, modified Kostiakov, Philip, Horton, traditional Green–Ampt, modified Green–Ampt and HYDRUS‐1D) and heterogeneous soils (traditional Green–Ampt, modified Green–Ampt and HYDRUS‐1D). According to the results, the infiltration rate decreased over time and along the soil profile. Nevertheless, it jumped at the interface of two‐layered soils when the heavier soil was in the bottom layer (SC treatments) due to the high potential of the second layer, and then it decreased. In the reverse layering, the infiltration rate in the interface was lowest (CS treatments) because of the higher hydraulic conductivity of the second layer. Additionally, the infiltration rate increased with increasing water head, but the rate of this increase was higher by changing the water head from 7 to 10 cm. The results of infiltration models showed that the accuracy of these models was higher in clay loam texture than in sandy loam texture. The modified Green–Ampt was the most accurate model in homogeneous and layered soils, with average RMSE of 0.0204 and 0.019, respectively. The Horton model had the weakest simulation in homogeneous soils, with an average RMSE of 0.1299. Additionally, the accuracy of HYDRUS‐1D in layered soils was less than that in homogeneous soils (NS of 0.95 and 0.85, respectively), and its accuracy decreased with increasing water head in most treatments.

Soil-structure interaction effects on out-of-plane seismic response and damage of masonry buildings with shallow foundations

A significant amount of damage and casualties induced by several strong-motion earthquakes which recently stroke South-East Mediterranean area is due to the major seismic vulnerability of residential buildings. In small villages and mid-size towns, those buildings very often consist of two- to four-story, unreinforced masonry (URM) structures not designed for earthquake resistance, with direct foundations usually corresponding to an in-depth extension of load-bearing walls. For such structures, especially when founded on soft soils, site amplification and soil-foundation-structure interaction (SFSI) can significantly affect the seismic performance; conversely, such phenomena should be investigated through methods that allow a trade-off between accuracy and computational effort, hence encouraging their implementation in engineering practice. This paper provides a comprehensive updated description of the studies carried out in the last years by the authors, which are based on both linear and nonlinear, parametric, dynamic analyses of complete soil-foundation-structure (SFS) models representative of existing residential building configurations on different soils. Specifically, the parametric study investigated SFS models with different masonry types, aspect ratios, and code-conforming homogeneous and heterogeneous soil profiles. The methodology and analysis results allowed for reaching the following objectives: (i) predicting the elongation of the fundamental period and the variation of equivalent damping of the SFS system with respect to fixed-base conditions, through a simplified approach based on an equivalent simple oscillator; and (ii) estimating the probability of exceeding increasing damage levels associated with out-of-plane overturning of URM walls, through fragility functions that take into account SFS interaction. The effectiveness of these simplified tools was successfully validated against well-documented case studies, at the scales of both single instrumented buildings and urban area.

An LSTM RNN proposal for surrogate modeling the dynamic response of buried structures to earthquake plane waves in soil half-spaces

This paper presents a variant of the long short-term memory (LSTM) recurrent neural network for surrogate modeling and predicting the seismic response of buried structures to in-plane inclined shear waves in heterogeneous soil profiles. The proposed model is designed to accept scalar and time-dependent random variables as input features to facilitate Monte Carlo (MC) based uncertainty quantification tasks where both the earthquake wave field and system properties are sources of uncertainty. To this end, it employs short-duration synthetic time histories for training instead of actual earthquake records to ensure efficiency. We discuss several configurations to evaluate the performance of the proposed methodology in predicting the tunnel response at different locations. We use two application examples concerning forward uncertainty quantification and sensitivity analysis to assess further the efficacy and accuracy of the proposed surrogate modeling approach.

Influence of Soil Layers on the Infiltration Rates and Cumulative Infiltration Using Modified Green Ampt Model in the HYDROL-INF Simulation Environment

Soil profiles are generally heterogeneous and consist of various horizontal layers due to geological processes, the formation of crusts, or other artificial or man-made activities. To quantify infiltration into these heterogeneous soil profiles, the Modified Green-Ampt Model (MGAM) is a physically-based hydrologic model that can efficiently perform under both steady and unsteady rainfall events. Based on the secondary data, this study sought to determine the effect of changing soil layers (soil textures) on infiltration rates and cumulative infiltrations in in both laboratory and field settings. Different scenarios were analyzed by rearranging soil layers and evaluating their impacts on corresponding infiltration rates and cumulative infiltrations. Simulations were run with HYDROL-INF software environment using MGAM. Three scenarios were considered for a laboratory experiment with two different types of soil texture coupled with five different soil profiles. Similarly, four scenarios were considered for the field experiments with five different types of soil texture couple with eight different soil profiles. The simulated infiltration rates and cumulative infiltrations were found to vary with soil layer change scenarios. The simulated cumulative infiltrations, ponding times, infiltrating rates at ponding, and total depth of wetting front at ponding of a five-layered laboratory soil column were identical for the three scenarios. Simulated cumulative infiltrations were 33.16, 23.65, 21.29, and 42.77 cm, respectively, for scenarios (combinations) 1, 2, 3, and 4 in the eight-layered soil profile in the field scenarios. Infiltration rates among scenarios at ponding were identical (0.46 to 0.53 cm/h) with field scenario data.

Hydrological monitoring of a slope covered by stratified pyroclastic deposits and analysis of infiltration processes

AbstractIn addition to the duration and intensity of rainfall, infiltration processes are strongly affected by the hydraulic properties of the soil. In the case of heterogeneous and stratified soil profiles, the analysis of the infiltration process becomes more complex, since conflicting hydraulic properties of adjacent layers can induce locally diverted flow. Soils of volcanic origin present these characteristics, because during the various eruptive phases, layers with very different textures are deposited. In this article, data are analysed which have been recorded by a monitoring station installed on a slope made up of volcanic deposits. The station consists of a rain gauge to measure rainfall and seven tensiometers and eight TDR probes installed at different depths to measure suction and volumetric water content, respectively. The slope is made up of alternating volcanic ash (silt‐sandy‐clay paleosoils) interspersed with pumice (sandy‐gravel), due to the different eruptive phases of the volcanic complexes in the area. The analysis of the data established that the layers of coarser material (pumices), depending on the initial moisture conditions, may hinder or even favour the infiltration of water into the deeper layers. In particular, when water content is low, the pumices presented a low unsaturated conductivity which hindered infiltration. By contrast, in wetter conditions, they favoured the flow of water. Therefore, the initial moisture conditions of the soil layers must be taken into account for a correct prediction of the infiltration phenomena. Dry conditions of the pumice layers can hinder drainage into the lower layers, thus favouring the rapid accumulation of soil water during rainfall events, which could eventually lead to slope failure.

The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part.

CPTu-based approaches for cyclic liquefaction assessment of alluvial soil profiles

Over the years, methods to assess cyclic liquefaction potential based on piezocone penetration tests (CPTu) have been developed. This paper presents a comparative study between three CPTu-based methodologies, mainly in terms of the normalization procedures of overburden stresses, equivalent clean sand resistance, and magnitude scaling factor (MSF). Four CPTu profiles from a pilot site in southwest Portugal are thoroughly analysed with different methods, in terms of factor of safety against liquefaction, the Liquefaction Potential Index (LPI), and the Liquefaction Severity Number (LSN). The site presents very heterogeneous soil profiles, composed of alluvial deposits. Due to the presence of significant sand-silt–clay interbedded layers, the influence of transition zones and the use of different soil behaviour type index (Ic) cut-off values were also considered. From these analyses, a set of recommendations is presented for CPTu-based liquefaction assessment. Based on the extensive database of CPTu results in the pilot site area, a new classification relating LPI and LSN is proposed to assess liquefaction severity and damage.

Probabilistic Seismic Response and Capacity Models of Piles for Statewide Bridges in California

This study develops the probabilistic seismic response and capacity models for the broad collection of standard pile designs that are routinely incorporated into bridges in California. A review of the state bridge inventory indicates considerable variations in design details for different standard pile types used across multiple eras. For each pile type, fiber-section-based pile models attached with p-y soil springs are built to incorporate nonlinear behaviors of soil materials, a wide array of heterogeneous soil profiles, full-range damage states of piles, and realistic connection details between piles and footings (i.e., pile caps). Moreover, force-displacement responses of pile-soil systems under a large number of pushover analyses are regressed as response models consisting of five parameters [termed as response five parameter (R5P) models], which can capture all essential behaviors of laterally-loaded piles. Capacity damage states and limit state models are defined for different pile types by linking pile global responses to fiber-scale material behaviors at plastic hinge locations. Procedures are further provided to expand R5P models to pile foundation models at the regional scale, taking into account the pile group effect and capacities of pile cap backfills. R5P models for all distinct pile types are further summarized in an Excel workbook to facilitate their practical implementations. In general, this study provides a comprehensive and consistent set of response and capacity models to quantify the seismic damage potential of regional pile foundations, as well as to capture their dynamic interplays with other crucial bridge components, such as columns and abutment components. The proposed pile models are expected to significantly enhance the existing modeling capability toward improved seismic risk assessment of California’s bridge infrastructure.

In-situ determination of soil water retention curves in heterogeneous soil profiles with a novel dielectric tube sensor for measuring soil matric potential and water content

Techniques of determining soil water retention curve (SWRC) have been developed for several decades, but very few have considered the heterogeneity of SWRC at different depths caused by texture, structure and other constituents. In this study, we designed a novel dielectric tube sensor (DTS) moving in a pair of tubes installed in-situ to measure soil water content (SWC) and soil matric potential (SMP) in heterogeneous soil profiles for determining wetting-path SWRCs. A series of experiments were conducted to test the optimal size of DTS, calibrate the DTS for SWC measurement and SMP measurement, test the water proof function of the impermeable sheet and equilibrium rate of SMP tube and determine SWRCs in a layered and stony soil tank with a relatively slow drip irrigation. Commercial sensors were also employed in comparison with the DTS during the drip irrigation experiment. The results showed that when the distance of the two electrodes of the DTS was 5 mm, the measurement range, volume of sensitivity and coefficient of variation were optimal. The calibration results verified that the DTS was suitable to measure volumetric SWC (R2 > 0.9914) and SMP (R2 = 0.9992). The SMP tube exhibits good water-proof property among different layers, and the average equilibrium rate was 2.1 kPa·s−1 in loamy soil and 7.3 kPa·s−1 in pure sand. The dynamics of SWC and SMP measured by the DTS approximately agreed well with the data measured by the commercial sensors. The relatively reduced agreement between the DTS and TEROS21 for SMP measurements may attribute to their different volume of sensitivity and installed position. Based on these, the wetting-path SWRCs were determined in-situ in a soil tank containing four kinds of soil samples. Furthermore, the determined SWRCs in four soil samples verified that stones decreased soil water holding capacity.

Geophysically based analysis of breakthrough curves and ion exchange processes in soil

Abstract. Breakthrough curves (BTCs) are a valuable tool for qualitative and quantitative examination of transport patterns in porous media. Although breakthrough (BT) experiments are simple, they often require extensive sampling and multi-component chemical analysis. In this work, we examine spectral induced polarization (SIP) signals measured along a soil column during BT experiments in homogeneous and heterogeneous soil profiles. Soil profiles were equilibrated with an NaCl background solution, and then a constant flow of either CaCl2 or ZnCl2 solution was applied. The SIP signature was recorded, and complementary ion analysis was performed on the collected outflow samples. Our results confirm that changes to the pore-water composition, ion exchange processes and profile heterogeneity are detectable by SIP: the real part of the SIP-based BTCs clearly indicated the BT of the non-reactive ions as well as the retarded BT of cations. The imaginary part of the SIP-based curves changed in response to the alteration of ion mobility around the electrical double layer (EDL) and indicated the initiation and the termination of the cation exchange reaction. Finally, both the real and imaginary components of the complex conductivity changed in response to the presence of a coarser textured layer in the heterogeneous profile.

Sequential infiltration analysis of infiltration curves measured with disc infiltrometer in layered soils

The soil sorptivity, S, and saturated hydraulic conductivity, Ks, can be estimated from the inverse analysis of a cumulative infiltration curve using the quasi-exact implicit (QEI) formulation or its corresponding 4-Terms (4T) approximation. Although these models consider the soil as homogeneous media, there is no information about how heterogeneous profiles can affect the inferred soil properties. This work analyzes the influence of layered soils on Ks and S estimates using QEI and 4T models, and designs a new procedure for treating infiltration curves measured on layered soil profiles. The Sequential Infiltration Analysis (SIA) method considers a sequence of increasing time series from the cumulative infiltration data to estimate Ks and S, and its corresponding RMSE as a function of the number of samples used. A procedure to estimate the thickness of the upper uniform soil layer from the estimated wetting front advance (WFA) is also reported. The SIA method was applied on: (i) synthetic homogeneous profiles of loam soil and six layered profiles involving a 1, 2 and 3 cm thickness loam layer over silty or sandy loam soils, respectively, (ii) stratified laboratory soil columns, and (iii) 20 experimental infiltrations performed in a semiarid region of North-Eastern Spain. Similar results were found between QEI and 4T models for all cases. Erroneous estimates of Ks and S were observed when the total infiltration time series was considered for the analysis, regardless of the presence of soil layering. In opposite, estimates improved when the SIA method was applied to the layered systems. The SIA method exploits the fact that the RMSE increases when the wetting front reaches the interface between the soil layers. Such increase allows: (i) detection of the soil heterogeneity, (ii) determination of the infiltration time, to, required for the wetting front to reach the lower layer, and, (iii) accurate estimates of the upper layer Ks and S along with its thickness. Laboratory experiments on layered soils and field measurements demonstrated that the SIA method could be satisfactorily applied on different curves with contrasting shapes and magnitudes. Although soil layering encountered on most field samplings restricted the treatment of the observed infiltrations to short-medium times, the SIA method allowed robust estimates of Ks and S. These results indicate that the proposed method is a promising tool for characterizing the hydraulic properties of layered and heterogeneous soil profiles.

Inverse Estimation of Spatiotemporal Flux Boundary Conditions in Unsaturated Water Flow Modeling

AbstractAccurate estimation of boundary conditions is essential to the success of numerical schemes for modeling soil water flow processes. The flux‐type boundary condition, which is integration of many boundary fluxes and subsurface conditions, is traditionally estimated from hydrologic and meteorological information. However, such information could be rapidly variable in space and time and difficult to measure and quantify accurately. This study inversely estimates upper flux boundary conditions by calibrating interior observations of soil moisture. Unknown flux boundaries with both spatiotemporal and temporal only variabilities are studied. In the case of spatiotemporal variability, we formulate the inverse model by extending a Tikhonov regularization framework proposed in our previous study, which calibrated a macroscopic root water uptake model in one‐dimensional soil water flow. The developed method is efficient because it takes advantage of the strategy in the previous study that the inversion is transformed to a variational problem, and also because it incorporates locally one‐dimensional operator splitting technique to overcome the computation complexity when dealing with multidimensional flow processes. When flux boundaries vary with time only (special case), we have found that the inversion degenerates into a well‐posed problem and formulated the inversion explicitly without use of regularization. Numerical tests are conducted through both one‐dimensional (well‐posed) and two‐dimensional (ill‐posed) heterogeneous soil profiles to illustrate the effectiveness of the proposed inverse formulation. Particularly, the applicability of the inverse formulation in the special case is further validated in field experiments using in situ measurements in arid and semiarid northern China.

Probabilistic Settlement Analysis of Strip Footing on Spatially Random Soil

The prediction of settlement in a traditional design approach usually uses a deterministic value of modulus of elasticity (E), which is estimated as an average value by testing the soil at selected locations. However, these deterministic properties of soil may not represent the actual properties of soil and site condition. Due to numerous sources of uncertainty, the properties of soil mass are spatially varying and anisotropic in the natural field condition. In this study, a random finite element method (RFEM) is used to evaluate the reliability of settlement of the strip footing on spatially random soil. Modulus of elasticity is the only considered random parameter. For this purpose, 2000 spatially random realizations of E-field are generated using Monte Carlo Simulation. Each of these realizations of heterogeneous soil profile is passed to FEM to analyze the settlement of footing. The final settlement results measured in all these realizations are then statistically evaluated and compared. The results of analysis show that the mean and standard deviation of the footing settlement are increased with increasing spatial correlation length. The large value of isotropic correlation length led to an increase in the mean settlement value by more than 25% as compared with the deterministic settlement calculations. Also, it is concluded that the rate of increase of settlements for anisotropic correlation length is lower than the one under isotropic condition.

Regional seismic responses of shallow basins incorporating site‐city interaction analyses on high‐rise building clusters

AbstractThis study assesses the 3D amplification effects in shallow basins and quantifies the effects of site‐city interaction (SCI) on high‐rise buildings. A regional‐scale 3D spectral element simulation is conducted on the Tuen Mun‐Yuen Long basin, which contains multiple subbasins with heterogeneous and nonlinear soil profiles, while 3D city models with various building layouts are fully integrated into the basin model for our SCI study. We found a good correlation between spectral amplification factors and soil depths. Site response is significantly amplified at basin edges and centers due to surface waves generated at basin edges and the focusing effects stemming from 3D basin geometry. Transfer functions of 3D basins can be up to fourfold at fundamental frequencies as compared to 1D response, and further amplifications occur at high frequencies due to surface waves. In the SCI simulations, we observe wave trapping in the open space amid buildings resulting in energy concentration and up to twofold PGA amplifications. The wave trapping effect diminishes as the space between buildings increase beyond their range of influence (∼100 m). The SCI analyses show that destructive kinetic energy in superstructures increases 28% in one horizontal direction but decreases 22% in the other. Our study concluded that, 1D site response analysis can significantly underestimate the seismic demand in shallow basins. Site‐city interaction of high‐rise buildings increases the short‐period spectra of ground motions, leading to an increase in their story accelerations by up to 50% and to a substantial decrease in the seismic safety of short structures in their vicinity.

Effect of kinematic interaction on seismic response of offshore wind turbines on monopiles

AbstractInterest in renewable energy over the past decade has motivated a remarkable research on offshore wind energy. Due to the environmental loading conditions in the early developments of offshore wind farms, the focus of the research and engineering has been on aerodynamic and hydrodynamic subjects. However, earthquake has turned out to be a major design concern in seismic areas such as East Asia, Southern Europe, and United States. The topics include, among others, nonlinear response of foundations, soil liquefaction, and their impacts on foundation performance. An important topic in seismic soil‐structure interaction (SSI) analysis is the kinematic seismic response of foundations. This information is crucial for analyses based on the substructuring approach where both kinematic response and foundation impedances are key parameters. While considerable research has been spent on the latter topic, very little has been reported on the kinematic interaction. Large monopoles with low aspect ratios tend to rotate much more than regular piles in pile group foundations due to seismic waves. The rotation leads to additional load on the tower and the turbine. This article uses a rigorous numerical model for monopole‐soil interaction in layered soil and computes both rotational and horizontal responses at the pile head (seabed) as functions of frequency. A suite of generic homogeneous and heterogeneous soil profiles with shear moduli representative of clay (generally linear) and sand (generally parabolic) are considered in these analyses. Through representative analyses, both in frequency domain and time domain, it is illustrated how the kinematic interaction influences the earthquake loads imparted to an offshore wind turbine (OWT).

Early indicators of pedogenesis at Harrat Khaybar volcano, Saudi Arabia

The interactive effects of paleoclimates and current conditions on pedogenesis in soils developed on volcanic parent materials (VPMs) is still poorly understood, particularly for soils in dry environments. Eight representative profiles located at Harrat Khaybar, Saudi Arabia and representing different landforms and positions including alluvial plain, alluvial fans, active slopes and lava field were investigated to better understand the effect of climate and basalt lithology on pedogenesis. The morphological and physicochemical soil properties and clay mineralogy composition were evaluated to identify the primary diagnostic horizons and therefore the pedogenic pathways that created the studied soils. All investigated profiles showed the influence of slope processes, resulting in heterogeneous soil profiles. The investigated soils can be described as polygenetic due to evidence of andisolization and the presence of salic and calcic diagnostic horizons. Andisolization implies development under a sub-humid paleoclimate during initial pedogenesis, while calcic and salic diagnostic horizons, are typical of the current arid conditions. As a result, about 63% of the investigated soils were classified as Torrands/Vitric Andosols and 37% as Salids/Solonchaks based on Soil Taxonomy and World Reference Base, respectively. Values of Alo, Feo, and Sio ranged from 0.20 to 0.68%, 0.24 to 3.27%, and 0.23 to 1.03%, respectively, implying the presence of poorly-crystalline fine components (i.e., allophane and ferrihydrite). Vermiculite, kaolinite, and interstratified vermiculite-chlorite were abundant, with illite, palygorskite, and Ca-zeolites as minor crystalline minerals. A combination of paleo/present-climate influenced pedogenesis leading to andisolization and the coexistence of poorly/crystalline minerals and formation of salic and calcic diagnostic horizons. This study demonstrates that andic properties can persist in dry-hot environments, highlighting the critical role of paleo/modern climate interactions as an influential factor driving the pedogenic pathways of VPMs-derived soils.

Comparison of RZWQM2 and DNDC Models to Simulate Greenhouse Gas Emissions under Combined Inorganic/Organic Fertilization in a Subsurface-Drained Field

HighlightsRZWQM2 was compared with DNDC to predict greenhouse gas emissions.RZWQM2 was applied to simulate the greenhouse gas emissions under manure application.RZWQM2 performed better than DNDC in simulating soil water content and CO2 emissions.Abstract. N management has the potential to mitigate greenhouse gas (GHG) emissions. Process-based models are promising tools for evaluating and developing management practices that may optimize sustainability goals as well as promote crop productivity. In this study, the GHG emission component of the Root Zone Water Quality Model (RZWQM2) was tested under two different types of N management and subsequently compared with the Denitrification-Decomposition (DNDC) model using measured data from a subsurface-drained field with a corn-soybean rotation in southern Ontario, Canada. Field-measured data included N2O and CO2 fluxes, soil temperature, and soil moisture content from a four-year field experiment (2012 to 2015). The experiment was composed of two N treatments: inorganic fertilizer (IF), and inorganic fertilizer combined with solid cattle manure (SCM). Both models were calibrated using the data from IF and validated with SCM. Statistical results indicated that both models predicted well the soil temperature, but RZWQM2 performed better than DNDC in simulating soil water content (SWC) because DNDC lacked a heterogeneous soil profile, had shallow simulation depth, and lacked crop root density functions. Both RZWQM2 and DNDC predicted the cumulative N2O and CO2 emissions within 15% error under all treatments, while the timing of daily CO2 emissions was more accurately predicted by RZWQM2 (RMSE = 0.43 to 0.54) than by DNDC (RMSE = 0.60 to 0.67). Modeling results for N management effects on GHG emissions showed consistency with the field measurements, indicating higher CO2 emissions under SCM than IF, higher N2O emissions under IF in corn years, but lower N2O emissions in soybean years. Overall, RZWQM2 required more experienced and intensive calibration and validation, but it provided more accurate predictions of soil hydrology and better timing of CO2 emissions than DNDC. Keywords: CO2 emission, Corn-soybean rotation, Inorganic fertilization, Manure application, N2O emission, Process-based modeling.

Development of the DNDC model to improve soil hydrology and incorporate mechanistic tile drainage: A comparative analysis with RZWQM2

The Denitrification Decomposition model (DNDC) has known limitations for simulating soil hydrology which can strongly influence biogeochemical processes. For this study, DNDC's soil hydrological framework was enhanced by including a new sub-model for mechanistic tile drainage, improved water flux, root growth dynamics, and a deeper and heterogeneous soil profile. Comparisons were then conducted against the Root Zone Water Quality Model (RZWQM2), using measurements of soil water storage, runoff, and drainage in eastern Canada and the US Midwest. Simulation of soil water storage (DNDC 0.81 ≤ d ≤ 0.90; RZWQM2 0.76 ≤ d ≤ 0.84), daily water flow (DNDC 0.76 ≤ d ≤ 0.88; RZWQM2 0.77 ≤ d ≤ 0.90) and nitrogen loading to tile drains were improved post-development. DNDC was able to capture the observed differences in water and N losses between conventional drainage and controlled drainage management with sub-irrigation. The enhancements to DNDC's hydrological framework should enable the development of improved biogeochemical processes.

A control volume scheme using compact integrated radial basis function stencils for solving the Richards equation

A new control volume approach is developed based on compact integrated radial basis function (CIRBF) stencils for solution of the highly nonlinear Richards equation describing transient water flow in variably saturated soils. Unlike the conventional control volume method, which is regarded as second-order accurate, the proposed approach has high-order accuracy owing to the use of a compact integrated radial basis function approximation that enables improved flux predictions. The method is used to solve the Richards equation for transient flow in 1D homogeneous and heterogeneous soil profiles. Numerical results for different boundary conditions, initial conditions and soil types are shown to be in good agreement with Warrick’s semi-analytical solution and simulations using the HYDRUS-1D software package. Results obtained with the proposed method were far less dependent upon the grid spacing than the HYDRUS-1D finite element solutions.