Root Water Uptake Model Research Articles

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.

Effect of Salinity on Moisture Flow and Root Water Uptake in Sandy Loam Soil

Abstract The present study analyzes the effect of salinity on crop growth and root zone soil moisture dynamics. A newly developed root water uptake (RWU) model was used for simulation of soil moist...

Application of a root water uptake model and numerical simulation to walnut trees in arid areas of northwest China

Water shortages restrict the sustainable development of agriculture in arid areas of northwest China. Quantitative studies of root water uptake (RWU) and soil water dynamics can guide the optimal regulation of irrigation. We studied walnut trees under drip irrigation in Hongqopo Farm Experimental Base in Aksu Prefecture, Xinjiang. Our aim was to parameterize the root distribution of walnut trees, establish a flexible and practical macroscopic two-dimensional RWU model, and numerically simulate soil moisture. Multiple regression analysis was applied to establish the root length density (RLD) distribution function and RWU model. The finite difference method was applied to the RWU rate inversion solution, and the alternating direction implicit difference (ADI) method was applied to numerically simulate soil moisture. The RLD distribution function accurately described the location of maximum root density and zero root density using four spatial distribution parameters (R2 = 0.86). The RWU model comprised three parameters: transpiration intensity, RLD distribution, and soil moisture distribution. There was a close agreement between simulated and measured RWU rates. The RWU was concentrated at depths of 10–60 cm. The mean absolute error of soil moisture numerical simulation for 16 days was 0.027 cm3·cm-3, which indicated that the proposed RLD distribution function, RWU model, and numerical simulation method were reliable. These approaches have the advantages of simple parameter measurement and flexible consideration of the spatial variability of the RWU rate and are applicable to other fruit trees. They provide a theoretical basis for water-saving irrigation and scientific management of orchards in arid areas.

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.

Comparison of root water uptake models in simulating CO<sub>2</sub> and H<sub>2</sub>O fluxes and growth of wheat

Abstract. Stomatal regulation and whole plant hydraulic signaling affect water fluxes and stress in plants. Land surface models and crop models use a coupled photosynthesis–stomatal conductance modeling approach. Those models estimate the effect of soil water stress on stomatal conductance directly from soil water content or soil hydraulic potential without explicit representation of hydraulic signals between the soil and stomata. In order to explicitly represent stomatal regulation by soil water status as a function of the hydraulic signal and its relation to the whole plant hydraulic conductance, we coupled the crop model LINTULCC2 and the root growth model SLIMROOT with Couvreur's root water uptake model (RWU) and the HILLFLOW soil water balance model. Since plant hydraulic conductance depends on the plant development, this model coupling represents a two-way coupling between growth and plant hydraulics. To evaluate the advantage of considering plant hydraulic conductance and hydraulic signaling, we compared the performance of this newly coupled model with another commonly used approach that relates root water uptake and plant stress directly to the root zone water hydraulic potential (HILLFLOW with Feddes' RWU model). Simulations were compared with gas flux measurements and crop growth data from a wheat crop grown under three water supply regimes (sheltered, rainfed, and irrigated) and two soil types (stony and silty) in western Germany in 2016. The two models showed a relatively similar performance in the simulation of dry matter, leaf area index (LAI), root growth, RWU, gross assimilation rate, and soil water content. The Feddes model predicts more stress and less growth in the silty soil than in the stony soil, which is opposite to the observed growth. The Couvreur model better represents the difference in growth between the two soils and the different treatments. The newly coupled model (HILLFLOW–Couvreur's RWU–SLIMROOT–LINTULCC2) was also able to simulate the dynamics and magnitude of whole plant hydraulic conductance over the growing season. This demonstrates the importance of two-way feedbacks between growth and root water uptake for predicting the crop response to different soil water conditions in different soils. Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, particularly in heterogeneous fields. The newly coupled soil–plant model marks a promising approach but requires further testing for other scenarios regarding crops, soil, and climate.

In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

Abstract. The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies. Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.

Evaluation of Non-Linear Root Water Uptake Model Under Different Agro-Climates

Evaluation of Non-Linear Root Water Uptake Model Under Different Agro-Climates

Long-term analysis of the shrinkage and swelling of clayey soils in a climate change context by numerical modelling and field monitoring

The shrinkage swelling phenomenon of clayey soils is affected by Soil-Vegetation-Atmosphere (SVA) interactions and can cause severe structural damage to lightly loaded constructions such as residential buildings. A soil-atmosphere interaction method was primarily presented along with a coupled hydro-thermal soil model. This approach was established to determine primarily, the natural time variable boundary conditions at the considered soil surface and secondly to determine the spatial–temporal changes of the soil suction, water content and temperature. Thereafter, the influence of the water uptake by vegetation was incorporated in the source term of the unsaturated water flow theory, using an existing root water uptake model. Subsequently, the variations of the soil suction were related to the volume change using a linear model. The associated volumetric indices in the void ratio-log suction plan, along with the complementary parameters of the linear model were correlated with basic geotechnical parameters. Comparisons between the simulation results and observation from a monitored site showed an acceptable performance of the predictions. The approach was then extended to study the influence of future climate projections (2050) on soil displacements using three Representative Concentration Pathway (RCP) climate change scenarios which revealed different possible behavior in both short and long-term.

Parameterization of the water stress reduction function based on soil–plant water relations

Rational parameterization of the soil water stress reduction function in root water uptake model is crucial for accurate description of root water uptake and simulation of soil water dynamics in a soil–plant system. In this study, we propose three improvements to a popular transpiration-based approach to parameterize the water stress reduction function in a widely used macroscopic root water uptake model. The improvements are based on the interdependent relationships between soil and plant water status and consideration of effects of (1) relative distribution of soil water to roots on transpiration; (2) differences in growth levels of plants exposed to different levels of water stresses on potential transpiration; and (3) hysteresis of water stress on parameter optimization through identifying and discarding the data involved in the recovery periods when the discrepancy between soil and plant water availability is significant. Lysimetric experiments with winter wheat planted alternatively in greenhouse soil columns and in a field were conducted to test the proposed improvements. Through minimizing the residuals between the measured and estimated actual transpiration, the optimized parameterization was used to set up the root water uptake model. Thereupon, actual transpiration and relative transpiration were estimated and soil water content distributions were simulated. The estimated actual (RMSE ≤ 0.09 cm day−1) and relative (RMSE = 0.06) transpiration agreed well with the measurements. The simulated soil water content distributions also matched the measured values well for both experiments (RMSE ≤ 0.023 cm3 cm−3). Omitting any of the three proposed improvements reduced the estimation accuracy of relative transpiration, as the individual contribution ratio for each improvement was between 21.2 and 51.2%. The improvements should be reasonable in providing rational parameter estimation for the water stress reduction function, from which root water uptake models can be established to accurately evaluate plant transpiration and simulate soil water flow in a soil–plant system. The parameterization strategy for the water stress reduction function of root water uptake not only benefits accurate evaluation of plant transpiration under drought conditions but also contributes to further study and description regarding the apparent hysteresis of root water uptake after re-watering.

Introducing water stress hysteresis to the Feddes empirical macroscopic root water uptake model

During wetting-drying cycles, divergence is often found between the immediately improved soil water conditions after re-watering and the recovery of plant water status from stress, which ensues only gradually. Such an apparent hysteresis effect of water stress (HEWS) is usually neglected in simulating root-water-uptake (RWU) by empirical models. To consider HEWS in the empirical macroscopic RWU model of Feddes, a water stress recovery coefficient (δ) was introduced based on two lysimetric experiments under greenhouse and field conditions for winter wheat. The integrated effects of historical water stress events were investigated by assuming that the normalized influence weight of each past stress event declines with the increase of time interval before simulation as an exponential function of attenuation rate. Although δ could be described by an exponential function of an integrative index representing the general historical stress extent (R2 = 0.65, P < 0.001), with an attenuation rate smaller than 0.13, it is challenging to establish such a function practically. An attenuation rate close to zero means HEWS is mainly dominated by the water stress on the previous day, validated by a significant relationship between the relative transpiration or stomatal conductance on the day after irrigation and the water stress extent on the day before irrigation. Therefore, a simplification, substituting the integrative index in the exponential function with the stress extent on the previous day, was proposed for estimating δ. Compared to the traditional RWU model, the revised model considering HEWS was more successful in simulating relative transpiration and soil water dynamics. Root mean square error of relative transpiration was reduced by 65.9 % and of soil water by 30 % in the greenhouse experiment and by 7.4 % and 12.5 %, respectively, in the field experiment.

Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: when rooting depth matters in labeling studies

Abstract. Isotopic labeling techniques have the potential to minimize the uncertainty of plant root water uptake (RWU) profiles estimated using multisource (statistical) modeling by artificially enhancing the soil water isotopic gradient. On the other end of the modeling continuum, physical models can account for hydrodynamic constraints to RWU if simultaneous soil and plant water status data are available. In this study, a population of tall fescue (Festuca arundinacea cv. Soni) was grown in amacro-rhizotron and monitored for a 34 h long period following the oxygen stable isotopic (18O) labeling of deep soil water. Aboveground variables included tiller and leaf water oxygen isotopic compositions (δtiller and δleaf, respectively) as well as leaf water potential (ψleaf), relative humidity, and transpiration rate. Belowground profiles of root length density (RLD), soil water content, and isotopic composition were also sampled. While there were strong correlations between hydraulic variables as well as between isotopic variables, the experimental results underlined the partial disconnect between the temporal dynamics of hydraulic and isotopic variables. In order to dissect the problem, we reproduced both types of observations with a one-dimensional physical model of water flow in the soil–plant domain for 60 different realistic RLD profiles. While simulated ψleaf followed clear temporal variations with small differences across plants, as if they were “onboard the same roller coaster”, simulated δtiller values within the plant population were rather heterogeneous (“swarm-like”) with relatively little temporal variation and a strong sensitivity to rooting depth. Thus, the physical model explained the discrepancy between isotopic and hydraulic observations: the variability captured by δtiller reflected the spatial heterogeneity in the rooting depth in the soil region influenced by the labeling and may not correlate with the temporal dynamics of ψleaf. In other words, ψleaf varied in time with transpiration rate, while δtiller varied across plants with rooting depth. For comparison purposes, a Bayesian statistical model was also used to simulate RWU. While it predicted relatively similar cumulative RWU profiles, the physical model could differentiate the spatial from the temporal dynamics of the isotopic composition. An important difference between the two types of RWU models was the ability of the physical model to simulate the occurrence of hydraulic lift in order to explain concomitant increases in the soil water content and the isotopic composition observed overnight above the soil labeling region.

Modelling root water uptake under deficit irrigation and rewetting in Northwest China

AbstractThe spatial and temporal distribution of root water uptake (RWU) under deficit irrigation are critical factors for crop growth. The SWAP (soil–water–atmosphere–plant) model was applied to analyze the pattern of RWU for winter wheat (Triticum aestivum L.) under three irrigation levels: no water deficit (100% evapotranspiration [ET]), moderate water deficit (80% ET) and severe water deficit (60% ET). The 2–yr experiments indicated that SWAP was highly accurate (mean relative error [MRE] &lt;21.7%, root mean square error [RMSE] &lt;0.07 cm3 cm−3) in simulating the soil water content (SWC). Root water uptake was significantly (P &lt; 0.01) different in the 0‐ to 60‐cm soil layer. The 0‐ to 60‐cm soil layer was the main source of RWU, and the average value accounted for 89.4% of the total root zone. Water stress had the greatest adverse effect on heading to grain filling, reducing RWU by 0.0026 cm3 cm−3 d−1. The critical SWC was 67.9% of the field capacity, when the RWU dropped to 95% of the control treatment. After rewetting, compensation and hysteresis effects on RWU were observed. The ranking of RWU recovery ability after rewetting was: emergence to jointing &gt; jointing to heading &gt; grain filling to maturity &gt; heading to grain filling. Recovery time of RWU was 2 to 11 d and gradually increased with growth stage. The simplified RWU model established using path analysis and regression performed well (R2 = 0.836; P &lt; 0.01) for RWU. This provided a more convenient way to accurately estimate RWU with fewer variables.

Modeling soil-crack–water–atmospheric interactions: a novel root water uptake approach to simulate the evaporation through cracked soil and experimental validation

Few studies numerically investigated evaporation through cracked soil using macro pore size distribution approach (conventional approach). In previous studies, the experimental results used for the simulation were usually obtained by evaporation from a very small mould. Moreover, those results are prone to size and boundary effects and can influence simulated soil moisture distribution around crack. In addition, difference between volumetric water contents (VWCs) computed using conventional approach and measured VWCs is relatively high. The main objective of the present study is to propose a new approach for simulating evaporation through cracked soil. The new approach draws analogy from root water uptake model, which includes a sink term. For validation, a large-scale experimental setup was designed and developed to obtain the experimental results by minimizing boundary effects. The experimental setup was instrumented to measure suction and VWC. Commercial finite element package “HYDRUS” was used to numerically solve (simulate) the Richards equation coupled with sink term. This study revealed that, root water uptake approach is appropriate for simulation of large reduction in VWCs under evaporation. Maximum difference between computed VWCs using conventional method and measured VWCs was observed to be 21%. Whereas, maximum difference between VWCs computed using novel root water uptake approach and measured VWCs was observed to be 16%. The proposed approach was able to predict soil moisture distributions more reasonably as it allows loss of water through cracks on soil surface.

Model of crop growth, water flow, and solute transport in layered soil

Agro-eco-hydrological models are essential for managing scarce water resources and ensuring crop productivity. Here, a one-dimensional agro-eco-hydrological model, LAWSTAC, capable of simulating water and solute transport in layered soil coupled with crop growth, is presented and validated. LAWSTAC considers eight hydraulic conductivity discretization methods to address the nonlinearity of Richards equation for soil water flow. LAWSTAC includes two root water uptake models and a flexible root distribution model for reliable transpiration simulation. Simulation of infiltration in layered soil demonstrated that the methods of arithmetic mean, geometric mean and triadic mean performed well among the eight discretization methods. The model was further verified by comparison with results from two widely used models, HYDRUS-1D and SWAP, based on the measured data in a spring wheat field for 2007 and 2008 in the Northwest China. The models produced similar results for flow in layered soil, although SWAP showed some instability in the salinity dynamics. LAWSTAC models crop growth with a more efficient parameterization than SWAP. The root mean square error for soil moisture, soil salinity concentration and LAI simulated by LAWSTAC was less than 0.06 cm3 cm−3, 3.56 g L-1, and 0.43, respectively. In conclusion, LAWSTAC is suitable for simulating soil water and salinity dynamics, crop growth and their interactions.

Incorporating a root water uptake model based on the hydraulic architecture approach in terrestrial systems simulations

A detailed representation of plant hydraulic traits and stomatal closure in land surface models (LSMs) is a prerequisite for improved predictions of ecosystem drought response. This work presents the integration of a macroscopic root water uptake (RWU) model based on the hydraulic architecture approach in the LSM of the Terrestrial Systems Modeling Platform. The novel RWU approach is based on three parameters derived from first principles that describe the root system equivalent conductance, the compensatory RWU conductance, and the leaf water potential at stomatal closure, which defines the water stress condition for the plants. The developed RWU model intrinsically accounts for changes in the root density as well as for the simulation of the hydraulic lift process. The standard and the new RWU approach are compared by performing point-scale simulations for cropland over a sheltered minirhizotron facility in Selhausen, Germany, and validated against transpiration fluxes estimated from sap flow and soil water content measurements at different depths. Numerical sensitivity experiments are carried out using different soil textures and root distributions in order to evaluate the interplay between soil hydrodynamics and plant characteristics, and the impact of assuming time-constant plant physiological properties. Results show a good agreement between simulated and observed transpiration fluxes for both RWU models, with a more distinct response under water stress conditions and with uncertainty in the soil parameterization prevailing to the differences due to changes in the model formulation. The hydraulic RWU model exhibits also a lower sensitivity to the root distributions when simulating the onset of the water stress period. Finally, an analysis of variability across the soil and root scenarios indicates that differences in soil water content are mainly influenced by the root distribution, while the transpiration flux in both RWU models is additionally determined by the soil characteristics.

Measurements and simulation of leaf xylem water potential and root water uptake in heterogeneous soil water contents

The relationship between leaf water potential, transpiration rate and soil water potential is complex, particularly when the soil water potential in the root zone is not uniform, which is the rule rather than the exception in soils. Our objectives were: 1) to measure the effect of heterogeneous soil water potentials on the relation between leaf water potential and transpiration rate and 2) to test whether root water uptake models could predict this relation. To this end, we combined the root pressure chamber technique, which allows measuring the suction in the leaves of transpiring plants, with two models of root water uptake, a simple one where soil and roots are presented as resistances in series and a more detailed 3D root architecture model. The experiments were carried out with lupines grown in sandy soil, for which the root architecture and root hydraulic properties had been previously estimated. The soil was partitioned in two layers separated by a coarse sand layer that allowed the roots to grow through but limited the water redistribution between the layers. Three scenarios (wet-wet, dry-wet, dry-dry) were tested. The results showed that the relation between transpiration and leaf water potential was linear in all scenarios. As the upper soil layer severely dried, the conductance of the soil-plant system decreased by ca. 1.65 times compared to the conductance of the plant-soil system in a uniform wet soil. As both layers dried, the conductivity was 8.26 times lower compared to the uniform-wet case. The combination of the experiment and modelling showed that a simple model is capable to reproduce the relation between transpiration, leaf water potential and soil water potential (despite an offset in the leaf water potential). Both simplified and the 3D root architecture models were capable of reproducing the measured changes in hydraulic conductance of the plant-soil system due to the soil drying. However, both models overestimated the measured leaf water potential by 0.1 MPa, probably because of a gradient in osmotic potential between the xylem and the soil. The simulations predicted the occurrence of hydraulic lift, even at day time conditions, although the hydraulic lift was relatively more important at low transpiration rates. The simulation suggested that a root architecture model is needed to estimate the variations of water uptake along the individual roots and this might be crucial to properly model hydraulic lift.

Continuum multiscale model of root water and nutrient uptake from soil with explicit consideration of the 3D root architecture and the rhizosphere gradients

Although modelling of water and nutrient uptake by root systems has advanced considerably in recent years, steep local gradients of nutrient concentration near the root-soil interface in the rhizosphere are still a central challenge for accurate simulation of water and nutrient uptake at the root system scale. Conventionally, mesh refinement is used to resolve these gradients. However, it results in excessive computational costs. The object of the study is to present a multiscale approach which resolves the steep gradient of nutrient concentrations at rhizosphere scale and simulates nutrient and water fluxes within the entire root zone at macroscale scale in a computationally efficient way. We developed a 3D water and nutrient transport model of the root-soil system with explicit consideration of the 3D root architecture. To capture the nutrient gradients at root surfaces, 1D axisymmetric soil models at rhizosphere scale were constructed and coupled to the coarse 3D root-system-scale simulations using a mass conservative approach. The multiscale model was investigated under different scenarios for water and potassium (K+) uptake of a single root, multiple roots, and whole 3D architecture of a Zea mays L. root system in conditions of dynamic soil water and different soil buffer capacity of K+. The steep gradients of K+ concentrations were efficiently resolved in the multiscale simulations thanks to the 1D model at the rhizosphere scale. In comparison with the refinement method, the multiscale model achieved a significant accuracy of K+ uptake prediction with a relative error below 5%. Meanwhile, the simulation at macroscale with coarse mesh could overestimate the K+ uptake in one order of magnitude. Moreover, the computational cost of multiscale simulations was decreased considerably by using coarse soil mesh. The newly developed model can describe the effect of the drying and nutrient transport in the root zone on nutrient uptake. It also allows to simulate processes in larger and complex root systems because of the considerable reduction in computational cost.

Whole root system water conductance responds to both axial and radial traits and network topology over natural range of trait variation

Current theory and supporting research suggests that radial transport is the most limiting factor to root water uptake, raising the question whether only absorbing root length and radial conductivity matter to water uptake. Here, we extended the porous pipe analytical model of root water uptake to entire root networks in 3D and analysed the relative importance of axial and radial characteristics to total uptake over parameter ranges reported in the literature. We found that network conductance can be more sensitive to axial than radial conductance of absorbing roots. When axial transport limits uptake, more dichotomous topology, especially towards the base of the network, increases water uptake efficiency, while the effect of root length is reduced. Whole root system conductance was sensitive to radial transport and length in model lupin (Lupinus angustifolius L.), but to axial transport and topology in wheat (Triticum aestivum L.), suggesting the root habit niche space of monocots may be constrained by their loss of secondary growth. A deep tap root calibrated to oak (Quercus fusiformis J. Buchholz) hydraulic parameters required 15 times more xylem volume to transport comparable amounts of water once recalibrated to parameters from juniper (Juniperus ashei Small 1901), showing that anatomical constraints on axial conductance can lead to significant trade-offs in woody roots as well. Root system water uptake responds to axial transport and can be limited by it in a biologically meaningful way.

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.

Establishing and validating a root water uptake model under the effects of superabsorbent polymers

AbstractIn‐depth understanding of the patterns of root water uptake (RWU) is the key to improving the water‐use efficiency of plants. Superabsorbent polymers (SAPs) can repeatedly absorb and then release water to plants when needed, which substantially increases the water‐use efficiency of plants in arid and semiarid farming. However, it also creates a challenge in modeling RWU due to unstable soil hydraulic parameters and the unclear response of root growth to SAP application. A controlled soil column experiment was conducted to investigate the response of two important root indexes, that is, root length density (RLD) and root nitrogen mass density (RND), to SAP application, as well as their relationships with RWU. The results showed that the use of the SAP caused a steady increase in RND, and it also substantially increased the RLD in the early stage, but the rate of increase gradually slowed over time. The effect of unstable soil hydraulic parameters on RWU was analyzed in a numerical experiment, which indicated that the time‐dependent effect of SAP on soil hydraulic parameters should be considered to avoid errors in calculating RWU. A specific method for calculating the RWU source/sink term in the Richards equation was proposed and ultimately used as the theoretical value of RWU to verify the RWU models based on RLD and RND to ensure the best one was selected. The results showed that RND was better able to be used for development of the RWU model to simulate the patterns of RWU under SAP application.