Soil Water Flow Research Articles

Estimating soil water suction from texture, bulk density and electrical resistivity

Information on soil water suction (h) is essential to study water flow and solute transport in soils, and to understand engineering behaviors of unsaturated soils. Due to limited availability of field methods that can accurately measure h, numerous studies have been performed to estimate h from more readily available soil properties. In this study, a new relationship between h and soil electrical resistivity (ρ), developed from the Gardner water retention model and Archie’s second law, was used to estimate h values lower than the air entry value and a formation factor (expressed on log-scale) less than 1. The two model parameters (A and B), which are functions of soil texture and bulk density, were obtained by fitting the model to ρ and h data measured on soil columns of eight textures and various bulk densities. Laboratory and field evaluations with independent h and ρ data showed that the model estimated h values agreed well with the measured values, with root mean square errors less than 0.85 kPa. The model provides a new opportunity to evaluate in situ h dynamics and study coupled transport of water and solutes in the field.

Quantifying the GCM-related uncertainty for climate change impact assessment of rainfed rice production in Cambodia by a combined hydrologic - rice growth model

The effects of climate change on agriculture are a major concern for global food security. In this study, the impacts of climate change on rainfed rice production in the granary of Cambodia were examined on a basin scale by developing and applying a combined model consisting of a crop model and a basin-scale distributed hydrological model. The response of rice production to soil-water availability was simulated for past (1981–2000) and future (2041–2060, 2081–2100) periods. From 34 general circulation models (GCMs) that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5), 5 GCMs were selected by evaluating monthly rainfall in the past. Although annual rainfall was projected to increase by all five selected GCMs, notable decreases in rainfed rice production were projected with 3 GCMs, while small changes were projected with the other 2 GCMs. The main factor restricting future rice production was soil water availability, brought by the projected change in the seasonal distribution of rainfall and the projected more severe dry spells in the early monsoon season. The results suggest the importance of the selection and bias correction of GCMs to force rice crop models and of the simulation of soil water flow on a basin scale for the assessment of rain-fed rice production. In particular, improvements in projections of rainfall amounts over shorter periods rather than annual or seasonal periods, which fit within the time scales of rice plant growth, were suggested to be important.

Simulation of Soil Water and Heat Flow under Plastic Mulching and Different Ridge Patterns

The ridge–furrow mulching system with plastic film (RFMS) has been widely used in semi-arid areas in order to improve soil water and heat conditions, crop yields and water use efficiency. It is of practical significance to study the effect of mulching and ridge types on soil water and heat in order to optimize mulching measures and improve the effectiveness of the ridge and furrow system. To clarify the combined effect of soil water and heat beneath the system and the influence of ridge morphology on it, field experiments were conducted with three treatments, including conventional planting in bare land (CK), a ridge–furrow (wide ridge with 70 cm width and 10 cm height, narrow ridge with 40 cm width and 15 cm height) mulching system with complete plastic film (RFWN) and a ridge–furrow (equal ridge with 55 cm width and 15 cm height) mulching system with complete plastic film (RFE). An insufficient irrigation system was adopted and the two-dimensional numerical software HYDRUS-2D was used to simulate the soil water and heat flow under the experimental conditions. The model was calibrated and verified according to test data for the period of 2018 to 2019, which showed good agreement between the simulated and measured values. The simulation results revealed that the ground temperatures of RFWN and RFE were much higher than that of CK, and the average value of 0–25 cm during the growth period could increase by 2.29–4.61%. Compared with CK, RFWN and RFE reduced soil evaporation (84.71–93.73%) and field evapotranspiration (12.02–21.75%), while they increased root water uptake (25.87–40.98%) and T/ET (48.85–80.15%). Plastic film mulching and ridge morphologies affected the infiltration range and the direction of soil water movement, increased soil moisture when there was no rainfall or irrigation and reduced soil water and heat fluctuations, which was more conducive to crop growth, especially under the RFWN system. The simulation method proposed in this paper is an effective technique for calculating the soil water and heat dynamics under different ridge and furrow sections under the condition of film mulching, and it can be used for the optimal management of soil water and heat in this area.

Estimation of soil hydraulic and solute transport properties from field infiltration experiments

To model water flow and solute transport in soils, hydrodynamic and hydrodispersive parameters are required as input data in the mathematical models. This work aims to estimate the soil hydraulic and solute transport properties using a ponded axisymmetric infiltration experiment using a single-ring infiltrometer along with a conservative tracer (Cl-) in the field. Single ring infiltration experiments were accomplished on an Oxisol in Areia in the state of Paraíba, Brazil (6o 58' S, 35o 41' W, and 645 m), in a 50 x 50 m grid (16 points). The unsaturated hydraulic conductivity (K) and the sorptivity (S) were estimated for short or long time analysis of cumulative three-dimensional infiltration. The single tracer technique was used to calculate mobile water fraction (Ф) by measuring the solute concentration underneath the ring infiltrometer at the end of the infiltration. Two solute transfer numerical models based on the mobile-immobile water concept were used. The mobile water fraction (Ф), the dispersion coefficient (D), and the mass transfer coefficient (a) between mobile and immobile were estimated from both the measured infiltration depth and the Cl- concentration profile underneath the infiltrometer. The classical convection-dispersion (CD) and the mobile-immobile (MIM) models showed a good agreement between calculated and experimental values. However, the lowest standard errors to the fitted parameters were obtained by the CD model.

Simulating upward soil-water flow from buried pipes with variable hydraulic characteristics

Differences in the boundary condition near buried corrugated pipes used for combined drainage and irrigation show that the hydraulic resistance due to perforations are not identical under both drainage and subsurface irrigation modes. No studies to date have sufficiently examined how perforation characteristics such as size and configuration affect the exit head loss of buried corrugated pipes used for subsurface irrigation. This research proposes a new dimensionless parameter called the exit resistance (αx) that accounts for variable perforations and can be used to compute the exit head loss due to capillary rise under subsurface irrigation. Datasets obtained from 189 numerical simulations were used to investigate how the size and configuration of rectangular slots affect αx. The results show that the number of perforation lines (N) and longitudinal spacing of slots (ay) on the pipe wall have the largest impact on αx. The difference in αx can be as large as 50% when N varies between 4 and 10, while slots placed in every corrugation valley (ay = 1.645 cm) provide less than 60% of the exit resistance compared to those spaced in every third valley (ay = 4.935 cm). Exit resistance was also found to vary with the pressure head on the subsurface irrigation pipes. The impact of heterogeneity from layered soils on αx of buried perforated pipes was also investigated. The results indicated that αx is not influenced by soil heterogeneity. In general, the findings from this study implies that it is better to perforate buried corrugated pipes with slots in every corrugation valley to reduce exit head losses and maximize upward soil-water fluxes in subsurface irrigation systems.

Vertical distribution of soil water repellency in ortsteinic soils in relation to land use

Land use, soil organic carbon content and bulk density are important factors influencing the occurrence of soil water repellency (SWR) and water flow through the soil profile. This study elucidated the relations between the repellency index (RI) and contents of soil organic carbon (SOC), organic-complexed aluminium (Alp) and iron (Fep), pH, specific surface area (SSA) and bulk density in the ortstein horizon and the overlying and underlying horizons in two podzolic soils under grassland and forest. The soil under grassland was originally used as a cropland. The RI was quantified from the ethanol/water sorptivity ratio. The results showed that the RI in the ortstein horizon (20.6–27.3) in the air-dried state in both soils was many times greater than in the overlying (7.2–19.0) and underlying (6.1–11.5) horizons, whereas in moist conditions (from soil water content at sampling and air-dried state), all horizons were not repellent. The RI in the air-dried soil in the overlying horizon was higher under grassland than forest, whereas the inverse was true in both the ortstein and underlying horizons. SOC, organic-complexed Alp and Fep contents, and SSA were the highest in the ortstein horizons. The pH value increased from 5.3 in the overlying horizon to 6.0 in the underlying horizon in the grassland soil. The corresponding values in the forest soil were 4.2 and 5.0. The RI values in the dry condition were significantly correlated with Alp (R2 = 0.976), Fep (R2 = 0.798), SSA (R2 = 0.724), and SOC (R2 = 0.638). The study provides new information about the vertical distribution of soil water repellency in ortsteinic soils depending on the soil water status and the type of land use. The potential mechanisms of the distribution of SWR are discussed.

Assessing the nitrate vulnerability of shallow aquifers under Mediterranean climate conditions

The EU Nitrates Directive calls for urgent integration of process-oriented indicators of nitrate fate with map overlay approaches for assessing nitrate vulnerable zones (NVZs). In the region of Campania (southern Italy), groundwater contamination represents a serious concern because of the presence of intensive agricultural practices and livestock farming. A protocol was proposed to assess the probability distribution of the following three indicators of nitrate transport across the vadose zone: i) annual cumulative nitrate flux entering the shallow aquifer, ii) annual cumulative root nitrate uptake, and iii) nitrate transit time across the vadose zone. This method involves numerical simulations of soil water flow and solute transport using Hydrus-1D for a representative 10-m-thick soil profile beneath an irrigated maize plot located in a study area within the Sele plain, Campania. Two scenarios are built by running a set of one hundred (20-yr-long) simulations at a daily time resolution: i) 195 kg N ha-1 of nitrate fertilizer (urea) is applied annually to estimate the probability distributions of annual cumulative nitrate leaching and root nitrate uptake (Scenario 1), ii) 195 kg N ha-1 of nitrate fertilizer (urea) is applied once to estimate the nitrate transit time distribution across the soil profile (Scenario 2). In each Scenario, the simulations consider two agricultural practices with either one or three annual nitrate fertilizer applications subject to randomly generated climate forcing using a Monte Carlo approach. Variations in soil and rainfall properties are described by the Miller-Miller geometric similitude and the Poisson parameterization, respectively.In Scenario 1, roots absorb on average 80.3 kg N ha-1 every year (corresponding to 40% of applied nitrate), and the median annual cumulative nitrate flux across the soil profile bottom is 74.9 kg N ha-1 (representing about 38% of applied nitrate) when urea is applied in a single treatment. In contrast, when fertilizer is applied in three treatments, 112.4 kg N ha-1 (corresponding to almost 60% of applied nitrate) of nitrate is removed by root water uptake, and the median annual cumulative nitrate leaching is 52.4 kg N ha-1 (corresponding to about 27% of applied nitrate). In Scenario 2, mean transit time values are 2741 days, 2707 days, and 2650 days when urea is applied on April 1st, June 1st, and August 1st, respectively. Our model simulations provide useful indicators of nitrate transport and can be integrated with map overlay procedures for delineating nitrate vulnerable zones.

Validation of the ISAREG model for the irrigation management of the melon crop in the state of Ceará

The research aimed to validate the ISAREG model, introducing it to water management studies in irrigation in the State of Ceará, comparing results of experimental research, with results of simulations, carried out with the use of software, analyzing the following variables: crop evapotranspiration, variation of soil water storage and water flow in the soil. A bibliographic survey was carried out to obtain soil, climate and crop information required by the model to perform the soil water balance. Aiming at the validation of ISAREG, the model was fed with the following data: reference evapotranspiration, precipitation, phenological phases of the crop, effective depth of the root system, water availability factor in the soil, crop coefficient and soil information. Subsequently, the irrigation management option "dates and irrigation depths" was selected, and ISAREG performed the simulation of the soil water balance. The ISAREG demonstrated a detailed soil water balance, being validated in this study, because when its results were compared to the experimental ones, there was similarity in the trends of the variables analyzed, despite the reduced correlation verified regarding the variation of water storage in the soil.

From hydraulic root architecture models to macroscopic representations of root hydraulics in soil water flow and land surface models

Abstract. Root water uptake is an important process in the terrestrial water cycle. How this process depends on soil water content, root distributions, and root properties is a soil–root hydraulic problem. We compare different approaches to implement root hydraulics in macroscopic soil water flow and land surface models. By upscaling a three-dimensional hydraulic root architecture model, we derived an exact macroscopic root hydraulic model. The macroscopic model uses the following three characteristics: the root system conductance, Krs, the standard uptake fraction, SUF, which represents the uptake from a soil profile with a uniform hydraulic head, and a compensatory matrix that describes the redistribution of water uptake in a non-uniform hydraulic head profile. The two characteristics, Krs and SUF, are sufficient to describe the total uptake as a function of the collar and soil water potential, and water uptake redistribution does not depend on the total uptake or collar water potential. We compared the exact model with two hydraulic root models that make a priori simplifications of the hydraulic root architecture, i.e., the parallel and big root model. The parallel root model uses only two characteristics, Krs and SUF, which can be calculated directly following a bottom-up approach from the 3D hydraulic root architecture. The big root model uses more parameters than the parallel root model, but these parameters cannot be obtained straightforwardly with a bottom-up approach. The big root model was parameterized using a top-down approach, i.e., directly from root segment hydraulic properties, assuming a priori a single big root architecture. This simplification of the hydraulic root architecture led to less accurate descriptions of root water uptake than by the parallel root model. To compute root water uptake in macroscopic soil water flow and land surface models, we recommend the use of the parallel root model with Krs and SUF computed in a bottom-up approach from a known 3D root hydraulic architecture.

River Buffer Effectiveness in Controlling Surface Runoff Based on Saturated Soil Hydraulic Conductivity

In tropical Indonesia, rainforests are managed by an intensive forest management system (IFMS). The IFMS has promoted selective logging for timber harvesting and intensive line planting to enrich the standing stock. The implementation of the IFMS has reduced the forest canopy cover, disturbed the surface soil, changed the soil hydraulic properties, and increased direct runoff and soil erosion. Investigation of the IFMS impact on soil hydraulic properties and the generation of surface runoff using a saturated hydraulic conductivity model is needed. Soil hydraulic properties were investigated on 11 plots, including one virgin forest plot and 10 plots at different operational periods of the IFMS. A two-dimensional saturated soil water flow simulation was applied to generate surface runoff from different periods of the IFMS. The main parameters of canopy cover, net rainfall, and saturated hydraulic conductivity were used in the simulations. A simulation scenario of a surface runoff hydrograph in different forest operations was used to analyze the river buffer effectiveness. The results showed that fundamental IFMS activities associated with mechanized selective logging and intensive line planting have reduced the soil hydraulic conductivity within the near-surface profile. The recovery time for near-surface Ks on non-skidder tracks was between 10 and 15 years, whereas on the skidder tracks it was more than 20 years. Forest disturbances have altered the typical surface hydrological pathways, thereby creating the conditions for more surface runoff on disturbed surfaces than on undisturbed surfaces. Maintaining the buffer area is an effective means to reduce the peak discharge and surface runoff in the stream channel.

Soil water dynamics based on a contrastive experiment between vegetated and non-vegetated sites in a semiarid region in Northwest China

Vegetation plays an active role in soil water dynamics and water balance in groundwater-soil-plant-atmosphere continuum systems. Therefore, we characterized soil water transport at depths of 0–4.0 m induced by root water uptake (RWU) at a groundwater-dependent Salix site during the whole growth stage and compared the results with those from a non-vegetated soil site, in the semiarid Ordos basin of China. The results showed that: during the whole experimental period, evapotranspiration at the vegetated site was more than twice evaporation at the bare site, indicating that the transpiration exceeded the evaporation and was the main output of soil water. For the bare site, the increase of soil water storage in the lysimeter was 97.9 mm in total, accounting for 27.5% of the precipitation, with 16.8% of rainfall as retention in the shallow vadose zone and 10.7% of rainfall as seepage below 150 cm depth. Conversely, in the vegetated site, rainfall-driven soil water was mainly consumed by vegetation, and the decrease of soil water storage was 254.4 mm in total, with no seepage being observed. Although the root system as the preferred channel improved the soil infiltration capacity, it did not mean that the deep soil water recharge increased. The infiltrated rainfwater could not offset the water consumption of the root system. An exponential normal composite RWU model obtained by the reverse method showed that the water deficit soil layers caused by RWU changed the soil water flow field and blocked soil water interchange. Consequently, deep soil water and groundwater could not be replenished by precipitation. Our results emphasize ascertaining the hydrodynamic processes of vegetation in semiarid regions and help to strike a balance between vegetation restoration and groundwater management.

A gradient-enhanced sequential nonparametric data assimilation framework for soil moisture flow

Soil water content (SWC) is a vital variable in the hydrological cycle, while simulation of it often relies on resolving the soil water flow equation. To cope with the unavailability or poor quality of physical models, various nonparametric data assimilation (DA) schemes have been established. However, there tends to be two significant common challenges in such methods: (1) the difficulty in capturing locally changing behaviors of time series, especially the peaks and troughs, and (2) poor statistical interpolation capability in time and space. These two challenges are no doubt attributed to the complete renunciation of physical constraints. Unlike previous physics-informed approaches that incorporated physical governing equations and engineering control into the loss functions, this study attempts to introduce additional physical constraints from data gradient into the model-free DA framework. As a follow-up study of Wang et al. (2021), a gradient-enhanced version of nonparametric DA schemes (i.e., GE-EnKFGP) is proposed. The temporal (daily) and spatial (vertical) gradients of the SWC are merged into the construction of the unsaturated flow dynamical models based on the Gaussian process (GP), while the Kalman update formulation is used to reconcile real-time observations. With the aid of a series of real-world cases, the performance of the GE-EnKFGP was compared with the original EnKFGP and its gradient-based version (GB-EnKFGP), where the temporal gradients of the SWC were used as the proxy for the SWC as the GP output. The results showed that the enhancement of the gradient information in the GE-EnKFGP led to a better estimation than the initial EnKFGP due to its more accurate identification of multiple local extrema. This should be attributed to the mass conservation constraint hidden within the temporal gradients and the implicit constraint of the driving force (or upper boundary) from the spatial gradients. Spatial gradients of the SWC outperformed temporal ones under various application scenarios. The GB-EnKFGP and GE-EnKFGP exhibited superior performances in retrieving surface SWC than that in the deeper layer. Hence, an enhancement scheme using only the temporal gradients of the surface layer was recommended. In the context of spatial extrapolation, the assistance of spatial gradients yielded an improved estimate of the deeper SWC quite robustly through GP training and assimilation of easy-to-access surface data. However, the implementation of the GB-EnKFGP and temporal gradient-enhanced EnKFGP [i.e., GE-EnKFGP (t)] run the risk of triggering a performance collapse due to the delayed response of SWC profiles to rainfall events.

Observation and simulation of water movement and runoff in a coarse texture water repellent soil

Water repellent or non-wetting soils show poor water infiltration, which leads to an increased runoff in both natural and agricultural setting. Some forest areas of northern Iran are characterized by extreme water repellency, which makes their soil hydrological response highly complex. A field experiment was carried out to study the effect of water repellency on water movement and runoff in coarse-textured soil covered in Pinus teada in northwestern Iran. An adapted version of the popular agrohydrological model SWAP (Soil, Water, Atmosphere and Plant) has been used to evaluate the modelling of soil water flow and runoff generation. Water repellency was accounted for by multiplying the soil water content and the unsaturated hydraulic conductivity of soil with the F factor. This factor is equal to the volumetric fraction of soil occupied by preferential flow paths. Twenty-seven rainfall experiments were carried out under artificial rainfall using soils with various wettability levels and various rainfall intensities (14 mm/h, 27 mm/h and 34 mm/h). The result show that during the summer and early autumn (May to October) the soil became extreme repellent and runoff was formed when the soils were dry (θ<2%) and the rain intensity was more than 27 mm/h. Also, we investigated the role of the repellent layer on evaporation losses. The results of laboratory and simulation data confirm that water repellency reduces evaporation from the soil surface. Comparing modelling results with field data shows that an assumption of uniform flow in water repellent soil leads to an overestimation of soil water content and an underestimation of runoff. After incorporating water repellency in SWAP with the mobile–immobile concept, the agreement between measured and model simulation data considerably improved.

Parameterization of two‐dimensional approaches in HYDRUS‐2D: Part 2. Solute transport at the field and column scale

AbstractIn this study, a tracer field experiment was conducted to study the temporal dynamics of bromide movement in a loamy tile‐drained agricultural landscape. Moreover, tritium leaching experiments were performed on undisturbed soil columns from the same field. The HYDRUS‐2D software package was used to model water and solute transport. Three water flow models developed in Part 1—a single‐porosity approach with modified soil hydraulic functions (SP), a dual‐porosity approach (DP), and a dual‐permeability approach (DUP)—were used as a foundation for building solute transport models, of which the initial parameterization was based on a suggested hydrogeological tool from previous studies. The selected solute transport models were an SP, an SP with immobile water, a DP, a DUP, and a DUP with immobile water. The model predictions were compared against measurements of bromide concentrations in soil‐water. The DP captured the degree of the initial peaks and predicted well the shape of the observed concentrations. However, DP presented low capability to match the timing of bromide concentration peaks. In contrast, the DUP with immobile water illustrated better predictive ability by introducing an additional pore region into the matrix domain. Validation of field‐scale solute transport models was implemented using the data from the following leaching experiments. The already suggested parameterization concept was further evaluated for its capability to provide a sufficient initial representation of the internal pathways. The output from such hydrogeological studies shows that macropores and their interconnection with tile drains are key to understanding the water flow and solute transport processes in loamy structured soils.

Effects of water flow on performance of soil microbial fuel cells: Electricity generation, benzo[a]pyrene removal, microbial community and molecular ecological networks

Soil microbial fuel cells with water flow (W-SMFCs) as a driven force of substrate transport were constructed. Electricity generation, benzo[a]pyrene (BaP) removal, microbial communities and microbial molecular ecological networks were compared between W-SMFCs and their control reactors (without water flow, C-SMFCs) in 240 days of operation. The W-SMFCs started up faster than C-SMFCs (37 days vs. 50 days) and output higher startup voltage (148.45 mV vs. 111.90 mV). The water flow caused higher removal efficiency of BaP at sites >1 cm from the anode (S > 1 cm) than at sites <1 cm from the anode (S < 1 cm) in W-SMFCs, whereas in C-SMFCs, the removal efficiency of BaP at S< 1 cm was higher than that at S> 1 cm. The removal efficiency of BaP at S> 1 cm in W-SMFCs was up to 1.7 times higher than that at S> 1 cm in C-SMFCs on the 91st day. After 240 days of operation, the biodegradation efficiency of absolute BaP amount was 45.95% in W-SMFCs, being 20% higher than that in C-SMFCs (38.17%). Moreover, the water flow caused highly tight interaction among the microbial species, which could be beneficial to BaP biodegradation. Conclusively, the water flow in soil was very beneficial for startup and biodegradation of BaP in SMFCs.

Promise and pitfalls of modeling grassland soil moisture in a free-air CO2 enrichment experiment (BioCON) using the SHAW model

Free-air carbon dioxide (CO2) enrichment (FACE) experiments provide an opportunity to test models of heat and water flow under novel, controlled situations and eventually allow use of these models for hypothesis evaluation. This study assesses whether the United States Department of Agriculture SHAW (Simultaneous Heat and Water) numerical model of vertical one-dimensional soil water flow across the soil-plant-atmosphere continuum is able to adequately represent and explain the effects of increasing atmospheric CO2 on soil moisture dynamics in temperate grasslands. Observations in a FACE experiment, the BioCON (Biodiversity, CO2, and Nitrogen) experiment, in Minnesota, USA, were compared with results of vertical soil moisture distribution. Three scenarios represented by different plots were assessed: bare, vegetated with ambient CO2, and similarly vegetated with high CO2. From the simulations, the bare plot soil was generally the wettest, followed by a drier high-CO2 vegetated plot, and the ambient CO2 plot was the driest. The SHAW simulations adequately reproduced the expected behavior and showed that vegetation and atmospheric CO2 concentration significantly affected soil moisture dynamics. The differences in modeled soil moisture amongst the plots were largely due to transpiration, which was low with high CO2. However, the modeled soil moisture only modestly reproduced the observations. Thus, while SHAW is able to replicate and help broadly explain soil moisture dynamics in a FACE experiment, its application for point- and time-specific simulations of soil moisture needs further scrutiny. The typical design of a FACE experiment makes the experimental observations challenging to model with a one-dimensional distributed model. In addition, FACE instrumentation and monitoring will need improvement in order to be a useful platform for robust model testing. Only after this can we recommend that models such as SHAW are adequate for process interpretation of datasets from FACE experiments or for hypothesis testing.

The potential impact of biochar: Soil hydraulics and responses of maize under soil drying cycles

Biochar (BC) has been shown to positively impact soil hydraulic properties. However, its effect on water flow in unsaturated soil and root access to water have been poorly understood. This study was conducted to investigate the potential impact of BC application on soil water accessibility and plant responses under soil drying cycles.A grapevine wood-based BC was mixed with a sandy loam soil at the rates of 0, 20, and 40 g kg−1 and preincubated in the PVC pots for two months. Then, maize plants were grown in preincubated pots and regularly irrigated during their vegetative phase (64 days) to maintain soil moisture at optimum condition (0.30 cm3 cm−3). Thereafter, plants were subjected to repeated soil drying (via plant transpiration) and wetting cycles (via irrigation from the top) with soil water content varying between 0.18 and 0.30 cm3 cm−3, respectively. Some selected plant growth parameters and plant physiological responses to soil drying cycles were determined. In parallel, soil water retention and hydraulic conductivity curves of different treatments were measured by a combination of the sandbox, pressure plate apparatus, and Hyprop.The results showed that plant dry shoot biomass was slightly increased by the BC application (9% on average). In contrast, the leaf area and root volume did not show any significant difference. As soil dried during a soil drying cycle, the transpiration rate decreased among all treatments. However, the drop in transpiration rate was smaller in the BC-amended soils than in the control. At soil water content of 018 cm3 cm−3, plants grown in BC-amended soils had on average 83% and 164% higher transpiration and photosynthesis rate than plants grown in control soil, respectively. These comparisons suggest that BC improved water accessibility to plants during soil drying, resulting in 76 and 58% increase in grain yield and grain water productivity. The application of BC had little effect on soil water retention in the wet range and had no impacts in the drier range, while the unsaturated hydraulic conductivity (Kh) and calculated matric flux potential (φ) were positively impacted in a wide range of soil water content.The findings of this study revealed that BC mixed with sandy loam soil could prevent a big drop in Kh as soil dries, thereby improving root access to water and transpiration rate under drought stress.

Mechanistic modeling of pesticide uptake with a 3D plant architecture model

Meaningful assessment of pesticide fate in soils and plants is based on fate models that represent all relevant processes. With mechanistic models, these processes can be simulated based on soil, substance, and plant properties. We present a mechanistic model that simulates pesticide uptake from soil and investigate how it is influenced, depending on the governing uptake process, by root and substance properties and by distributions of the substance and water in the soil profile. A new root solute uptake model based on a lumped version of the Trapp model (Trapp, 2000) was implemented in a coupled version of R-SWMS-ParTrace models for 3-D water flow and solute transport in soil and root systems. Solute uptake was modeled as two individual processes: advection with the transpiration stream and diffusion through the root membrane. We set up the model for a FOCUS scenario used in the European Union (EU) for pesticide registration. Considering a single vertical root and advective uptake only, the root hydraulic properties could be defined so that water and substance uptake and substance fate in soil showed a good agreement with the results of the 1D PEARL model, one of the reference models used in the EU for pesticide registration. Simulations with a complex root system and using root hydraulic parameters reported in the literature predicted larger water uptake from the upper root zone, leading to larger pesticide uptake when pesticides are concentrated in the upper root zone. Dilution of root water concentrations at the top root zone with water with low pesticide concentration taken up from the bottom of the root zone leads to larger uptake of solute when uptake was simulated as a diffusive process. This illustrates the importance of modeling uptake mechanistically and considering root and solute physical and chemical properties, especially when root-zone pesticide concentrations are non-uniform.

Evapotranspiration estimation by inverse soil water flow modelling

Evapotranspiration estimation by inverse soil water flow modelling

A method for solving heat transfer with phase change in ice or soil that allows for large time steps while guaranteeing energy conservation

Abstract. The accurate simulation of heat transfer with phase change is a central problem in cryosphere studies. This is because the non-linear behaviour of enthalpy as function of temperature can prevent thermal models of snow, ice, and frozen soil from converging to the correct solution. Existing numerical techniques rely on increased temporal resolution in trying to keep corresponding errors within acceptable bounds. Here, we propose an algorithm, originally applied to solve water flow in soils, as a method to solve these integration issues with guaranteed convergence and conservation of energy for any time step size. We review common modelling approaches, focusing on the fixed-grid method and on frozen soil. Based on this, we develop a conservative formulation of the governing equation and outline problems of alternative formulations in discretized form. Then, we apply the nested Newton–Casulli–Zanolli (NCZ) algorithm to a one-dimensional finite-volume discretization of the energy–enthalpy formulation. Model performance is demonstrated against the Neumann and Lunardini analytical solutions and by comparing results from numerical experiments with integration time steps of 1 h, 1 d, and 10 d. Using our formulation and the NCZ algorithm, the convergence of the solver is guaranteed for any time step size. With this approach, the integration time step can be chosen to match the timescale of the processes investigated.