Simulated Root Water Uptake Research Articles

Hydrological Coupling and Decoupling of Hydric Hemiboreal Forest Sites Inferred from Soil Water Models and Tree-Ring Chronology

The soil water regime often controls whether an ecosystem is a source of greenhouse gases such as CO2 or CH4 or is, instead, a carbon sink. The soil water regime of wetland forests is complicated by ecosystem feedback and landscape scale interactions. An in-depth understanding of these processes is needed to optimize the management of such ecosystems to balance timber production, carbon sequestration and biodiversity preservation. To investigate the soil water regime of non-riparian wetland forests, we set up a physically based Hydrus-1D soil water model for two hydric black alder Alnus glutinosa sites in a lowland hemiboreal setting informed by field observations of the soil water. Further, to gain ecohydrological insights, we explored the correlations between modeled long-term soil water parameters and local dendrochronology. We found that, at the clay soil site, the simulated root water uptake had a significant correlation (up to 0.55) with the residual tree-ring chronology. However, in the sandy soil site, the meteorological conditions—air temperature and precipitation—were better predictors for tree radial growth (correlation up to 0.42). In addition, we observed a trend towards dryer conditions during the modeling period, which might enhance the growing conditions for the considered forest stands due to a reduction in soil waterlogging.

Modeling root water uptake patterns of oil crops grown on semiarid loess

Oil crops are often used as cover crops by smallholders managing orchards on drylands, because they have an economic return and help regulate ecosystem services. Improving the ability of models in root water uptake (RWU) simulation is important to find the right crop species and predict the dynamics of these oil crops in water-limited regions. In this study, two different process-based models were used to simulate the RWU of three oil crops, i.e. soybean (Glycine max L., SYB) and two canola cultivars (turnip type rape (Brassica campestris L.), TTR, and cabbage type rape (Brassica napus L.), CTR), grown on the semiarid Loess Plateau of China, based on two-year in-situ measurements. The first model, IM-SVD, was developed by inverse modeling (IM) of the Richards equation and incorporating singular value decomposition (SVD) parameter optimization algorithms, which simulated the RWU pattern (including soil evaporation and plant transpiration). The second model was the Feddes model implemented in one-dimensional HYDRUS (HYDRUS 1-D). Both models reliably predicted the soil water content (SWC) in different layers. However, the IM-SVD model showed better modeling accuracy of SWC, and produced higher simulated RWU values than the HYDRUS 1-D model irrespective of crop. The results from both models implied that SYB had a higher daily and seasonal RWU than TTR and CTR, and its water source was derived mainly from the top 30 cm soil layer. Additionally, the simulated RWU was positively and significantly (p<0.05) correlated with fine root length density (FRLD). The RWU showed lagged correlations with SWC, which varied between crop species and growth stages, but overall were more positively correlated with the SWC on preceding days and more negatively correlated with the SWC on subsequent days. Our study provides insights into the improving RWU simulation for oil crops and similar crops in drylands.

Collaborative benchmarking of functional-structural root architecture models: Quantitative comparison of simulated root water uptake

Abstract Functional-structural root architecture models have evolved as tools for the design of improved agricultural management practices and for the selection of optimal root traits. In order to test their accuracy and reliability, we present the first benchmarking of root water uptake from soil using five well-established functional-structural root architecture models: DuMux, CPlantBox, R-SWMS, OpenSimRoot and SRI. The benchmark scenarios include basic tests for water flow in soil and roots as well as advanced tests for the coupled soil-root system. The reference solutions and the solutions of the different simulators are available through Jupyter Notebooks on a GitHub repository. All of the simulators were able to pass the basic tests and continued to perform well in the benchmarks for the coupled soil-plant system. For the advanced tests, we created an overview of the different ways of coupling the soil and the root domains as well as the different methods used to account for rhizosphere resistance to water flow. Although the methods used for coupling and modelling rhizosphere resistance were quite different, all simulators were in reasonably good agreement with the reference solution. During this benchmarking effort, individual simulators were able to learn about their strengths and challenges, while some were even able to improve their code. Some now include the benchmarks as standard tests within their codes. Additional model results may be added to the GitHub repository at any point in the future and will be automatically included in the comparison.

Construction of root tip density function and root water uptake characteristics in alpine meadows.

Accurate calculation of root water uptake (RWU) is the key to improving vegetation water use efficiency and identifying water cycle evolution patterns, and root tips play an important role in RWU. However, most of the current RWU models in the alpine meadow are calculated based on the root length density (RLD) function. In this study, a large number of roots, soil hydraulic conductivity, and physicochemical property indices were obtained by continuous field prototype observation experiments for up to 2 years. It was found that the RLD and root tip density (RTD) in alpine meadows decrease by 16.2% and 14.6%, respectively, in the wilting stage compared to the regreening stage. The RTD distribution function of the alpine meadow was constructed, and the RWU model was established accordingly. The results show that the RTD function is more accurate than the RLD function to reflect the RWU pattern. Compared with RLD, the simulated RWU model constructed by using RTD as the root index that can effectively absorb water increased by 24.64% on average, and the simulated values were more consistent with the actual situation. It can be seen that there is an underestimation of RWU calculated based on the RLD function, which leads to an underestimation of the effect of climate warming on evapotranspiration. The simulation results of the RWU model based on RTD showed that the RWU rate in the regreening stage increased by 30.24% on average compared with that in the wilting stage. Meanwhile, the top 67% of the rhizosphere was responsible for 86.76% of the total RWU on average. This study contributes to the understanding of the alpine meadow water cycle system and provides theoretical support for the implementation of alpine meadow vegetation protection and restoration projects.

Impact of Drought and Changing Water Sources on Water Use and Soil Salinity of Almond and Pistachio Orchards: 2. Modeling

California is increasingly experiencing drought conditions that restrict irrigation deliveries to perennial nut crops such as almonds and pistachios. During drought, poorer quality groundwater is often used to maintain these crops, but this use often results in secondary salinization that requires skilled management. Process-based models can help improve management guidelines under these challenging circumstances. The main objective of this work was to assess seasonal soil salinity and root water uptake as a function of irrigation water salinity and annual rain amounts. The manuscript presents a comparison of three-year experimental and numerically simulated root zone salinities in and below the root zone of almond and pistachio drip-irrigated orchards at multiple locations in the San Joaquin Valley (SJV), California, with different meteorological characteristics. The HYDRUS-1D numerical model was calibrated and validated using field measurements of soil water contents and soil solute bulk electrical conductivities at four root zone depths and measured soil hydraulic conductivities. The remaining soil hydraulic parameters were estimated inversely. Observations and simulations showed that the effects of rain on root zone salinity were higher in fields with initially low salinities than in fields with high salinities. The maximum reduction in simulated root water uptake (7%) occurred in response to initially high soil salinity conditions and saline irrigation water. The minimum reduction in simulated water uptake (2.5%) occurred in response to initially low soil salinity conditions and a wet rain year. Simulated water uptake reductions and leaching fractions varied at early and late times of the growing season, depending on irrigation water salinity. Root water uptake reduction was highly correlated with the cumulative effects of using saline waters in prior years, more than salt leaching during a particular season, even when rain was sufficient to leach salts during a wet year.

Modeling tomato root water uptake influenced by soil salinity under drip irrigation with an inverse method

The influence of soil salinity on plant root water uptake (RWU) must be considered in order to promote efficient irrigation management. However, due to challenges in parameterization, modeling RWU response to salinity is problematic. We set out to characterize the influence of soil salinity on tomato RWU under conditions of irrigation with high salinity water and to improve methods for optimizing parameters of RWU model. Experiments were conducted in a greenhouse over two seasons (2016 and 2017). Three irrigation water salinity treatments [0.4 (S1), 3.4 (S2) and 6.4 (S3) mg cm−3] were evaluated. The results showed that soil water content and salinity increased with the irrigation water salinity, especially in the top soil layers, and that RWU was reduced by the increased root zone salinity. An inverse method based on a one-dimensional equation of soil water movement was used to estimate the RWU rate distribution under drip irrigation and determined to be reliable. The parameter value of the normalized root length distribution (NRLD) function was fitted through the estimated RWU rate distributions for treatment S1 without the effect of salinity stress. The fitted NRLD function was used to optimize the parameters of the salinity stress reduction factor (β) through the measured soil moisture and salinity of treatments S2 and S3 in 2016. With the optimized parameters of β, the RWU rates of treatments S2 and S3 in 2017 were simulated. The simulated RWU rates based on the RWU model agreed well with the estimated RWU rates using the inverse method, with root mean squared error (RMSE) and normalized root mean squared error (NRMSE) less than 0.00067 cm3 cm−3 d−1 and 12.36%, and index of agreement (IA) higher than 0.94. These findings can be used to estimate the NRLD distribution and determine actual crop water requirements under saline conditions with no root distribution data required. This should contribute to efficient and sustainable irrigation scheduling.

A time-lapse imaging platform for quantification of soil crack development due to simulated root water uptake

Plants are major drivers of soil structure dynamics. Root growth creates new macropores and provides essential carbon to soil, while root water uptake may induce crack formation around roots. Cracks can facilitate root growth as they provide pathways of least resistance and improve water infiltration and soil aeration. Due to the lack of suitable quantification methods, knowledge on the effects of root water uptake on soil crack formation remains limited. In the current study, we developed a time-lapse imaging platform that allows i) simulating root water uptake through localized soil drying and ii) quantifying the development of two-dimensional crack networks. Customized soil boxes that were 50 mm wide, 55 mm high and 5 mm deep were designed. Artificial roots made of dialysis tubes were inserted into the soil boxes and polyethylene glycol solution was circulated through the tubes. This induced a gradient in osmotic potential at the contact area (150 mm2) between the soil and the dialysis tubes, resulting in controlled soil drying. Drying intensity was varied by using different polyethylene glycol concentrations. Experiments were conducted with three soils that were subjected to three drying intensities for 6.5 days. We developed a time-lapse imaging system to record soil crack formation at two-minute intervals in twelve samples simultaneously. Resulting crack networks were quantified with an automated image analysis pipeline. Across soils and drying intensities, crack network development slowed down after 24–48 h of soil drying. The extent and complexity of crack networks increased with drying intensity and crack networks were larger and more complex in the clay and clay loam soil than in the silt loam soil. Smaller and less complex crack networks were better connected than larger and more complex networks. These results demonstrate that the platform developed in this study is suitable to quantify crack network development in soil due to simulated root water uptake at high temporal resolution and high throughput. Thereby, it can provide information needed to improve our understanding on how plants modify soil structure.

Using root water uptake estimated by a hydrological model to evaluate the least limiting water range

ABSTRACT The least limiting water range (LLWR) has been extensively determined, but evaluating if LLWR can indeed indicate soil physical stress on plant growth is still a controversial issue. In this study, we used the Hydrus-1D hydrological model to simulate root water uptake (RWU) to analyze if RWU and LLWR are correlated under stress conditions. The LLWR was determined in a sandy-loam Ultisol and a clayey Oxisol. In both soils, RWU extracted by plants (leaf area index set as 3) [...]

A macroscopic soil-water transport model to simulate root water uptake in the presence of water and disease stress

Macroscopic modeling approaches based on the solution of the Richards equation with root water uptake (RWU) as a sink term can help in understanding soil-water-plant interactions within the rhizosphere. However, these models currently cannot capture the differences in RWU attributed to variations in plant health. Errors in simulating RWU from unhealthy plants are significant when disease-causing fungus inhibits water uptake rather than other usually considered plant stresses. We developed RWU reduction functions to simulate plant transpiration under combined water and disease stress conditions using linear and non-linear response models. The developed functions were implemented in the numerical model HYDRUS (2D/3D) to simulate water uptake from a root system in a radially symmetrical flow domain. Field experiments were conducted in the Vidarbha region of central India for one crop cycle on four citrus trees with varying disease intensities (healthy to severely diseased). The proposed model was rigorously tested by comparing its results with measured soil water contents and plant transpiration fluxes under various water and disease limiting conditions. Error in simulating RWU fluxes from unhealthy trees by ignoring the disease stress factor was found to be significant (15% for slightly diseased to 26% for the severely diseased tree). Parameters of the spatial root distribution and the disease stress response functions were optimized for each scenario using a genetic algorithm approach. Our results indicate that calibration targets to validate uptake reduction functions should be chosen cautiously based on the dominant stress experienced by the plant root system.

Modelling soil water dynamics and root water uptake for apple trees under water storage pit irrigation

Water storage pit irrigation is a new method suitable for apple trees. It comes with advantages such as water saving, water retention and drought resistance. A precise study of soil water movement and root water uptake is essential to analyse and show the advantages of the method. In this study, a mathematical model (WSPI-WR model) for 3D soil water movement and root water uptake under water storage pit irrigation was established based on soil water dynamics and soil moisture and root distributions. Moreover, this model also considers the soil evaporation, pit wall evaporation and water level variation in the pit. The finite element method was used to solve the model, and the law of mass conservation was used to analyse the water level variation. The model was validated by experimental data of the sap flow of apple trees and soil moisture in the orchard. Results showed that the WSPI-WR model is highly accurate in simulating the root water uptake and soil water distributions. The WSPI-WR model can be used to simulate root water uptake and soil water movement under water storage pit irrigation. The simulation showed that orchard soil water content and root water uptake rate centers on the storage pit with an ellipsoid distribution. The maximum distribution region of soil water and root water uptake rate was near the bottom of the pit. Distribution can reduce soil evaporation in the orchard and improve the soil water use efficiency in the middle-deep soil. Keywords: root water uptake, soil water dynamics, numerical simulation, water storage pit irrigation, apple tree DOI: 10.25165/j.ijabe.20191205.4005 Citation: Guo X H, Lei T, Sun X H, Ma J J, Zheng L J, Zhang S W, et al. Modelling soil water dynamics and root water uptake for apple trees under water storage pit irrigation. Int J Agric & Biol Eng, 2019; 12(5): 126–134.

Modelling soil water dynamics and root water uptake for apple trees under water storage pit irrigation

Water storage pit irrigation is a new method suitable for apple trees. It comes with advantages such as water saving, water retention and drought resistance. A precise study of soil water movement and root water uptake is essential to analyse and show the advantages of the method. In this study, a mathematical model (WSPI-WR model) for 3D soil water movement and root water uptake under water storage pit irrigation was established based on soil water dynamics and soil moisture and root distributions. Moreover, this model also considers the soil evaporation, pit wall evaporation and water level variation in the pit. The finite element method was used to solve the model, and the law of mass conservation was used to analyse the water level variation. The model was validated by experimental data of the sap flow of apple trees and soil moisture in the orchard. Results showed that the WSPI-WR model is highly accurate in simulating the root water uptake and soil water distributions. The WSPI-WR model can be used to simulate root water uptake and soil water movement under water storage pit irrigation. The simulation showed that orchard soil water content and root water uptake rate centers on the storage pit with an ellipsoid distribution. The maximum distribution region of soil water and root water uptake rate was near the bottom of the pit. Distribution can reduce soil evaporation in the orchard and improve the soil water use efficiency in the middle-deep soil. Keywords: root water uptake, soil water dynamics, numerical simulation, water storage pit irrigation, apple tree DOI: 10.25165/j.ijabe.20191205.4005 Citation: Guo X H, Lei T, Sun X H, Ma J J, Zheng L J, Zhang S W, et al. Modelling soil water dynamics and root water uptake for apple trees under water storage pit irrigation. Int J Agric & Biol Eng, 2019; 12(5): 126–134.

Root growth, water uptake, and sap flow of winter wheat in response to different soil water conditions

Abstract. How much water can be taken up by roots and how this depends on the root and water distributions in the root zone are important questions that need to be answered to describe water fluxes in the soil–plant–atmosphere system. Physically based root water uptake (RWU) models that relate RWU to transpiration, root density, and water potential distributions have been developed but used or tested far less. This study aims at evaluating the simulated RWU of winter wheat using the empirical Feddes–Jarvis (FJ) model and the physically based Couvreur (C) model for different soil water conditions and soil textures compared to sap flow measurements. Soil water content (SWC), water potential, and root development were monitored noninvasively at six soil depths in two rhizotron facilities that were constructed in two soil textures: stony vs. silty, with each of three water treatments: sheltered, rainfed, and irrigated. Soil and root parameters of the two models were derived from inverse modeling and simulated RWU was compared with sap flow measurements for validation. The different soil types and water treatments resulted in different crop biomass, root densities, and root distributions with depth. The two models simulated the lowest RWU in the sheltered plot of the stony soil where RWU was also lower than the potential RWU. In the silty soil, simulated RWU was equal to the potential uptake for all treatments. The variation of simulated RWU among the different plots agreed well with measured sap flow but the C model predicted the ratios of the transpiration fluxes in the two soil types slightly better than the FJ model. The root hydraulic parameters of the C model could be constrained by the field data but not the water stress parameters of the FJ model. This was attributed to differences in root densities between the different soils and treatments which are accounted for by the C model, whereas the FJ model only considers normalized root densities. The impact of differences in root density on RWU could be accounted for directly by the physically based RWU model but not by empirical models that use normalized root density functions.

Modeling Soil–Water–Disease Interactions of Flood‐Irrigated Mandarin Orange Trees: Role of Root Distribution Parameters

Core Ideas Soil–water–disease interactions were analyzed using field and numerical studies. ERT was able to image the soil moisture front separated by active roots. HYDRUS overpredicted RWU from diseased trees, requiring a compensation mechanism. Parameters defining the root distribution were optimized considering plant health. Variably saturated flow models that simulate soil–water–plant interactions within the rhizosphere largely ignore the effect of plant health. This makes it difficult for them to effectively simulate root water uptake (RWU) and implement alternate management practices for diseased trees. This research was aimed at understanding the hydrological and plant controls on RWU simulations dominated by the health of the tree using experimental and numerical frameworks. Two research plots, one around a healthy mature and the other around a diseased mature (Phytophthora spp. affected) orange tree (Citrus reticulate Blanco), in the Vidarbha region of India were considered for our analysis. Three‐dimensional electrical resistivity tomography (ERT) performed at the two locations revealed that the soil moisture profiles following irrigation are different between the two plots. A two‐dimensional axisymmetric form of Richards' equation was then solved using HYDRUS (2D/3D) by incorporating a root distribution function. A global sensitivity analysis was performed to identify the root distribution parameters that influence soil moisture simulations. These parameters were then optimized using a genetic algorithm for healthy and diseased conditions. We observed that the diseased orange tree had consumed less water, leaving high soil moisture in the rhizosphere, a condition favorable for the further growth of disease‐causing fungi. Hence, managing irrigation water in accordance with RWU patterns is essential for diseased trees. To understand the error propagation in RWU estimation, we simulated RWU from a diseased tree with the optimal parameters derived for a healthy case. Results show that the error in estimating RWU from the model cells progressively increases with radial distance and soil depth.

Parameterization of Root Water Uptake Models Considering Dynamic Root Distributions and Water Uptake Compensation

Core Ideas Parameters of different RWU models were optimized using data from a rhizotron facility. The same soil hydraulic properties were obtained for models considering RWU compensation. Feddes–Jarvis and Couvreur models predicted similar root‐system‐scale stress functions. The obtained RWU parameters were consistent with data reported in the literature. The models with compensation predicted similar total uptake but different local RWU. The spatiotemporal distribution of root water uptake (RWU) depends on the dynamics of the root distribution and compensatory uptake from wetter regions in the root zone. This work aimed to parameterize three RWU models with different representations of compensation: the Feddes–Jarvis model that uses an empirical function, the Feddes model without compensation, and the Couvreur model that is based on a physical description of water flow in the soil–root system. These models were implemented in HYDRUS‐1D, and soil hydraulic parameters were optimized by inverse modeling using soil water content and potential measurements and observations of root distributions of winter wheat (Triticum aestivum L.) in horizontally installed rhizotubes. Soil moisture was equally well predicted by the three models, and the soil hydraulic parameters optimized by the models with compensation were comparable. The obtained RWU parameters of the Feddes–Jarvis model were consistent with data reported in the literature, although the pressure heads h3l and h3h for lower and higher transpirations rates, respectively, could not be uniquely identified. Response surfaces of the objective function showed that the root‐related parameters of the Couvreur model could be identified using inverse modeling. Furthermore, these parameters were consistent with combined root architectural and hydraulic observations from the literature. The Feddes–Jarvis and Couvreur models simulated similar root‐system‐scale stress functions that link total RWU to the effective root zone water potential, suggesting that parameters may be transferable between the two models. Simulated RWU profiles differed due to different water redistribution by the root system, but the measurements were not sufficiently precise to observe this redistribution.

Root development and water uptake in winter wheat under different irrigation methods and scheduling for North China

A field experiment was conducted on winter wheat (Triticum aestivum L.) during 2013–2014 and 2014–2015 to study the root distribution profile and soil water dynamics under the main currently used irrigation methods in the North China Plain (NCP). The WinRHIZO system and the HYDRUS-1D model were used to identify a promising irrigation schedule. In this two-factor experiment, three irrigation methods, i.e., sprinkler irrigation (SI), surface drip irrigation (SDI) and surface flooding (SF), were scheduled to irrigate the crop as soon as the soil water content decreased to 70%, 60% and 50% of the field capacity. The results showed that both the irrigation method and irrigation schedule influenced root development, the profile root distribution pattern and the profile root water uptake (RWU). The soil surface temperature fluctuated very rapidly depending on the irrigation method and scheduling system used, whereas profile soil temperature fluctuations became more consistent with depth. The RWU was higher in the upper soil layer (0–60cm) for all irrigation methods for frequently irrigated treatments, and the maximum was observed in SDI compared to SI and SF due to the higher root length density (RLD) in the top soil under SDI. On the other hand, the RWU was higher in SF at a deep soil profile below 60cm, where it had a higher RLD compared to that of SI and SDI. SDI at 60% of FC not only improved water uptake but also resulted in better water productivity and produced the highest grain yield (9.53t/ha). The simulated RWU and soil water dynamics presented in this paper will be helpful to improve winter wheat production in the NCP and can be used as a reference for further research on water management practices.

Modeling Water Potentials and Flows in the Soil–Plant System Comparing Hydraulic Resistances and Transpiration Reduction Functions

Transpiration reduction functions are often used in hydrological modeling to estimate actual transpiration as a function of soil water status. Empirical reduction functions are most frequently used due to the higher data needs and computational requirements of mechanistic models. Empirical models, however, lack a description of physical mechanisms and their parameters require extensive calibration. We derive a process‐based reduction function predicting system potentials, resistances, and water flows. An analytical solution for a special case of Brooks and Corey soils is presented. A numerical version of the reduction function for van Genuchten soils was implemented in the Soil–Water–Atmosphere–Plant (SWAP) hydrological model, allowing predictions for layered soil profiles and root length density variations over depth. The analytical and numerical versions of the model allow an increasingly quantitative insight into the mechanism of root water uptake, such as the existence of a maximum root water uptake rate as a function of soil water status, soil hydraulic properties, root length density, and root radius, in addition to the fact that sensitivity of simulated root water uptake to the radial root conductivity and axial conductance decrease when root length density increases. The approach can be used for the estimation of threshold values for empirical reduction functions.

Evaluation of water movement and nitrate dynamics in a lysimeter planted with an orange tree

Adoption of high input irrigation management systems for South Australian horticultural crops seeks to provide greater control over timing of irrigation and fertilizer applications. The HYDRUS 2D/3D model was used to simulate water movement in the soil under an orange tree planted in a field lysimeter supplied with 68.6mm of irrigation water over 29 days. Simulated volumetric water contents statistically matched those measured using a capacitance soil water probe. Statistical measures (MAE, RMSE, tcal) indicating the correspondence between measured and simulated moisture content were within the acceptable range. The modelling efficiency (E) and the relative efficiency (RE) were in the satisfactory range, except RE at day 19. Simulated daily and cumulative drainage fluxes also matched measured values well. Cumulative drainage flux was 48.9% of applied water, indicating large water losses even under controlled water applications. High drainage losses were due to light texture of the soil and high rainfall (70mm) during the experimental period. Simulated root water uptake was 40% of applied water.The calibrated HYDRUS model was also used to evaluate several scenarios involving nitrate fertigation. The numerical analysis of NO3-N dynamics showed that 25.5% of applied fertilizer was taken up by the orange tree within 15 days of fertigation commencement. The rest of the applied NO3-N (74.5%) remained in the soil, available for uptake, but was also vulnerable to leaching later in the growing season. The seasonal simulation revealed that NO3-N leaching accounted for 50.2% of nitrogen applied as fertilizer, and plant N recovery amounted to 42.1%. The scenario analysis further revealed that timing of a nitrogen application in an irrigation event had little impact on its uptake by citrus in the lysimeter. However, slightly higher NO3-N uptake efficiency occurred when fertigation was applied late in the daily irrigation schedule, or was spread out across all irrigation pulses, rather than being applied early or in the middle. Modelling also revealed that pulsing of irrigation had little impact on nitrate leaching and plant uptake. Applying less irrigation (50% or 75% of ETC) resulted in higher nitrate uptake efficiency. This study showed that timing of water and fertilizer applications to an orange crop can be better regulated to enhance the efficiency of applied inputs under lysimeter conditions.

Lateral stress induced due root-water-uptake in unsaturated soils

A two-dimensional (2-D) lateral stress was modeled as a result of matric suction change caused by vegetative induced moisture transfer. The negative pore-water pressures are estimated through governing partial differential equations for unsaturated soils. The results of the of simulated root water uptake are used as an input for the prediction of 2-D lateral stress in a stressdeformation analysis in an uncoupled manner. The soil is allow to expand and contract free laterally, as the as water is being abstracted from the soil. A mature Lime tree located on a Boulder clay subsoil for period covering a full spring/summer drying period was used as a case study. The result shows interdependence of lateral and vertical stress generated resulting from root water-uptake.

Evaluation of soil plant system response to pulsed drip irrigation of an almond tree under sustained stress conditions

The quantification of components of the water balance is essential for designing strategies for improving water productivity of almond under different irrigation management. It is also needed for minimising the offsite movement of nutrients out of the rhizosphere. The present investigation involves the use of HYDRUS-2D simulations conducted on field data recorded for a full grown surface drip irrigated almond orchard over a season (July 20, 2009 to May 31, 2010). The model evaluated the daily fluctuations in water under full pulsed (FIp), sustained deficit pulsed (SDIp) and full continuous (FIc) irrigation. The impact of pulsing on the dynamics of the water flux was also assessed. In the SDIp treatment, water was applied to replace 65% of calculated crop evapotranspiration (ETc), compared to 100% ETC replacement in the other two treatments. The data showed that the RMSE between weekly measured and simulated moisture content in the whole domain clustered within 0.022–0.04cm3cm−3, with their magnitude equal to the standard error of the water content measurements. The variation was further reduced (0.013–0.026cm3cm−3) when considering only the 30cm soil depth, the site of maximum root density in almonds, indicating a good prediction of seasonal soil moisture distribution and plant water uptake. However, sap flow measurements underestimated water uptake by 31% as compared to the simulated root water uptake. Water uptake efficiency under SDIp (68%) was higher compared to full water application conditions under FIp and FIc (54–55%). The higher irrigation amounts (565–583mm) under 100% ETC treatments (FIp and FIc) largely contributed to non-productive water fluxes (deep drainage losses and evaporation). The leaching fraction was estimated to be 0.14 under SDIp, in spite of the fact that negligible drainage was predicted during the mid season of almond growth (November to January). The average modelled soil solution salinity (ECsw) of the profile also remained below the threshold for yield reduction during the growing season in all treatments. The seasonal water uptake by almonds under pulsed (FIp) and slow discharge continuous irrigation (FIc) remained almost on par, indicating that pulsing did not provide any added advantage, although it is a viable alternative to slow discharge continuous irrigation. The irrigation water productivity (WPI) increased substantially (37%), yield was reduced by 8% and about 35% of irrigation water was saved under sustained deficit irrigation (SDIp) compared to full irrigation (FIp). We conclude that in regions with severe water scarcity, SDIp appears to be a promising deficit irrigation strategy for almond cultivation, and irrigating almonds above the SDIp level may enhance unproductive water usage in the form of accelerated drainage, which eventually may lead to potential danger of migration of nutrients and solutes to the groundwater, thereby posing a threat to the quality of groundwater and receiving surface water bodies.

Optimizing parameters of salinity stress reduction function using the relationship between root-water-uptake and root nitrogen mass of winter wheat

Abstract Delineating the root-water-uptake (RWU) under the salinity stress condition accurately is important for making rational saline water irrigation schedules. However, RWU is very complex and still incompletely understood, especially under the salinity stress condition. In this study, the linear relationship between RWU and root nitrogen mass (RNM) of winter wheat, which was obtained under the optimal water condition (without any water or salinity stress) from the greenhouse experiments, was further verified and applied to optimize the parameters of the salinity stress reduction function ( β ), and simulate soil water dynamics under the salinity stress condition and in the field. Three saline water irrigation experiments, with winter wheat cultured in nutrient solution (Exp. 1), soil columns (Exp. 2) and field (Exp. 3) were conducted. The results of Exp. 1 showed that the daily transpiration of winter wheat decreased with increasing salinity, but was proportional to the RNM under the same salinity stress. The RWU rate distributions in Exps. 2 and 3 were estimated using an inverse method. The estimated RWU rate was also verified to be proportional to the RNM density under the optimal water condition. The verified linear relationship was then used to optimize the parameters of β , simulate the distributions of RWU rate and soil water content in the soil columns (Exp. 2) and field (Exp. 3). The simulated results matched the estimated RWU rates and the measured soil water contents well. In Exp. 2, the maximal absolute error (MAE), root mean squared error (RMSE) and absolute coefficient of residual mass (CRM) were not more than 0.0021, 0.0062 cm 3 cm −3 d −1 and 9.93% between the simulated and estimated RWU rates, and 0.007, 0.004 cm 3 cm −3 and 2.12% between the simulated and measured soil water contents, respectively. The relative errors between the calculated transpiration rates from the simulated RWU rate distributions and the measured values were less than 10%. In Exp. 3, the maximum MAE, RMSE and absolute CRM were 0.0012, 0.0007 cm 3 cm −3 d −1 and 5.81% for RWU rate, and 0.011, 0.010 cm 3 cm −3 and 3.51% for soil water content, respectively. The linear relationship between RWU and RNM of winter wheat should be reliable and rational. The optimization method should be applicable in optimizing the parameters of the salinity stress reduction function, establishing the RWU model and simulating soil water flow under the salinity stress condition in the soil–wheat system.