Root Water Uptake Model Research Articles

Comparison of Spring Maize Root Water Uptake Models Under Water and Salinity Stress Validated with Field Experiment Data

AbstractCrop root water uptake has strong correlations with transpiration and yield. An accurate prediction of crop root water uptake can benefit understanding of the law of transpiration and soil water movement. Under water and salinity stress, maize root water uptake will decrease because of soil water pressure and osmotic head. Usually such reduction of water uptake can be expressed by models such as the additive model and the multiplicative model. Based on a field irrigation experiment, we calculated spring maize root water uptake in this study by setting different combinations of water amount and saline concentration based on the water balance method. One additive model and four multiplicative models of root water uptake were calibrated and verified in order to simulate the impact of water and salt stress on root water uptake. The result shows that multiplicative models are better than the additive model in simulating the experimental data from different treatments. Therefore, the multiplicative model is recommended as the optimal model of root water uptake of spring maize under water and salinity stress in the arid region of north‐west China. Copyright © 2015 John Wiley & Sons, Ltd.

Compensatory uptake functions in empirical macroscopic root water uptake models – Experimental and numerical analysis

Macroscopic empirical root water uptake (RWU) models are often used in hydrological studies to predict water dynamics through the soil–plant–atmosphere continuum. RWU in macroscopic models is highly dependent on root density distribution (RDD). Therefore, compensatory uptake mechanisms are being increasingly considered to remedy this weakness. A common formulation of compensatory functions is to relate compensatory uptake rate to the plant water-stress status. This paper examines the efficiency of such compensatory functions to reduce the sensitivity of simulated actual transpiration (Ta), drainage (Draina) and RWU patterns to RDD. The possibility to replace the compensatory RWU functions by an adequate description of RDD is also discussed. The study was based on experimental and numerical analysis of two-dimensional soil-water dynamics of 11 maize plots, irrigated using sprinkler (Asp), subsurface drip (SDI) systems, or rainfed (RF). Soil-water dynamics were simulated using a physically-based soil-water flow model coupled to a macroscopic empirical compensatory RWU model. For each plot, simulation scenarios involved crossing 6 RDD profiles with 6 compensatory levels. RDD was found to be the main factor in the determination of RWU patterns, Ta and Draina rates, with and without the compensatory mechanism. The use of a water-tracking RDD, i.e., higher uptake intensity in expected wetter soil regions, was found a surrogate for compensatory RWU functions in surface-watering simulations (Asp and RF). However, in SDI simulations, a water-tracking RDD should be combined to a high level of compensatory uptake to satisfactorily reproduce real RWU patterns. Our results further suggest that the compensatory RWU process is independent of the plant stress status and should be seen as a response to heterogeneous soil-water distribution. Our results contribute to the identification of optimum parameterization of empirical RWU models as a function of watering methods.

Assessment of improved soil hydraulic parameters for soil water content simulation and irrigation scheduling

The efficient irrigation systems utilizing numerical models based on Richard’s flow equation require key input parameters, soil hydraulic parameters (SHPs). The present study proposes a method to determine the SHPs from the soil water contents at field capacity and wilting point, which are determined for irrigation scheduling. It also compares them with the SHPs estimated by various pedotransfer functions (PTFs) to simulate respective soil water retention curves (SWRCs). High efficiencies of 70–80 % were obtained in simulating the SWRCs by the proposed method as compared to PTFs. In order to further assess the applicability of the SHPs as determined, the experiments were conducted under real field conditions for wheat crop in Roorkee, India, and SWRCs were experimentally determined. These different sets of SHPs, along with experimentally determined saturated permeability, were then used as input parameters in root water uptake model and the results of observed and simulated soil water contents were compared under three different irrigation treatments. It was found that the experimentally obtained SHPs and those obtained by the proposed method were able to simulate the soil water contents with efficiencies of 70–80 % at all the depths for all the three irrigation treatments, while the PTFs performed poorly.

Using measured soil water contents to estimate evapotranspiration and root water uptake profiles – a comparative study

Abstract. Understanding the role of plants in soil water relations, and thus ecosystem functioning, requires information about root water uptake. We evaluated four different complex water balance methods to estimate sink term patterns and evapotranspiration directly from soil moisture measurements. We tested four methods. The first two take the difference between two measurement intervals as evapotranspiration, thus neglecting vertical flow. The third uses regression on the soil water content time series and differences between day and night to account for vertical flow. The fourth accounts for vertical flow using a numerical model and iteratively solves for the sink term. None of these methods requires any a priori information of root distribution parameters or evapotranspiration, which is an advantage compared to common root water uptake models. To test the methods, a synthetic experiment with numerical simulations for a grassland ecosystem was conducted. Additionally, the time series were perturbed to simulate common sensor errors, like those due to measurement precision and inaccurate sensor calibration. We tested each method for a range of measurement frequencies and applied performance criteria to evaluate the suitability of each method. In general, we show that methods accounting for vertical flow predict evapotranspiration and the sink term distribution more accurately than the simpler approaches. Under consideration of possible measurement uncertainties, the method based on regression and differentiating between day and night cycles leads to the best and most robust estimation of sink term patterns. It is thus an alternative to more complex inverse numerical methods. This study demonstrates that highly resolved (temporally and spatially) soil water content measurements may be used to estimate the sink term profiles when the appropriate approach is used.

Comparison of drought tolerances in a root water uptake model for two co-occurring grass species in Mongolia

Comparison of drought tolerances in a root water uptake model for two co-occurring grass species in Mongolia

Evaluation of root water uptake in the ISBA-A-gs land surface model using agricultural yield statistics over France

Abstract. The simulation of root water uptake in land surface models is affected by large uncertainties. The difficulty in mapping soil depth and in describing the capacity of plants to develop a rooting system is a major obstacle to the simulation of the terrestrial water cycle and to the representation of the impacts of drought. In this study, long time series of agricultural statistics are used to evaluate and constrain root water uptake models. The inter-annual variability of cereal grain yield and permanent grassland dry matter yield is simulated over France by the Interactions between Soil, Biosphere and Atmosphere, CO2-reactive (ISBA-A-gs) generic land surface model (LSM). The two soil profile schemes available in the model are used to simulate the above-ground biomass (Bag) of cereals and grasslands: a two-layer force–restore (FR-2L) bulk reservoir model and a multi-layer diffusion (DIF) model. The DIF model is implemented with or without deep soil layers below the root zone. The evaluation of the various root water uptake models is achieved by using the French agricultural statistics of Agreste over the 1994–2010 period at 45 cropland and 48 grassland départements, for a range of rooting depths. The number of départements where the simulated annual maximum Bag presents a significant correlation with the yield observations is used as a metric to benchmark the root water uptake models. Significant correlations (p value < 0.01) are found for up to 29 and 77% of the départements for cereals and grasslands, respectively. A rather neutral impact of the most refined versions of the model is found with respect to the simplified soil hydrology scheme. This shows that efforts should be made in future studies to reduce other sources of uncertainty, e.g. by using a more detailed soil and root density profile description together with satellite vegetation products. It is found that modelling additional subroot-zone base flow soil layers does not improve (and may even degrade) the representation of the inter-annual variability of the vegetation above-ground biomass. These results are particularly robust for grasslands, as calibrated simulations are able to represent the extreme 2003 and 2007 years corresponding to unfavourable and favourable fodder production, respectively.

Evaluation of root water uptake models – a review

Estimation of root water uptake and water movement in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere through plants. Present paper is aimed to illustrate and discuss how mathematical modeling can be used for understanding of plants and, in particular, plant–soil interactions. Soil water movement due to root water uptake is a key process for plant growth and transport of water in the soil plant system. There are different root water uptake models which can be used for estimation of moisture uptake by plants. The accuracy of moisture uptake prediction by plants depends on selection of proper mathematical models. Root uptake models are available in different forms i.e. linear, nonlinear, and exponential. In this paper, a detailed review of the most commonly used models which are suitable for the estimation of moisture uptake by plants in different agro-climatic regions is presented. Further, an effort has been made to compare the performance of different widely used models under controlled condition and future research needs are outlined.

Effect of parameter choice in root water uptake models – the arrangement of root hydraulic properties within the root architecture affects dynamics and efficiency of root water uptake

Abstract. Detailed three-dimensional models of root water uptake have become increasingly popular for investigating the process of root water uptake. However, they suffer from a lack of information on important parameters, particularly on the spatial distribution of root axial and radial conductivities, which vary greatly along a root system. In this paper we explore how the arrangement of those root hydraulic properties and branching within the root system affects modelled uptake dynamics, xylem water potential and the efficiency of root water uptake. We first apply a simple model to illustrate the mechanisms at the scale of single roots. By using two efficiency indices based on (i) the collar xylem potential ("effort") and (ii) the integral amount of unstressed root water uptake ("water yield"), we show that an optimal root length emerges, depending on the ratio between roots axial and radial conductivity. Young roots with high capacity for radial uptake are only efficient when they are short. Branching, in combination with mature transport roots, enables soil exploration and substantially increases active young root length at low collar potentials. Second, we investigate how this shapes uptake dynamics at the plant scale using a comprehensive three-dimensional root water uptake model. Plant-scale dynamics, such as the average uptake depth of entire root systems, were only minimally influenced by the hydraulic parameterization. However, other factors such as hydraulic redistribution, collar potential, internal redistribution patterns and instantaneous uptake depth depended strongly on the arrangement on the arrangement of root hydraulic properties. Root systems were most efficient when assembled of different root types, allowing for separation of root function in uptake (numerous short apical young roots) and transport (longer mature roots). Modelling results became similar when this heterogeneity was accounted for to some degree (i.e. if the root systems contained between 40 and 80% of young uptake roots). The average collar potential was cut to half and unstressed transpiration increased by up to 25% in composed root systems, compared to homogenous ones. Also, the least efficient root system (homogenous young root system) was characterized by excessive bleeding (hydraulic lift), which seemed to be an artifact of the parameterization. We conclude that heterogeneity of root hydraulic properties is a critical component for efficient root systems that needs to be accounted for in complex three-dimensional root water uptake models.

Application of the water table fluctuation method for estimating evapotranspiration at two phreatophyte-dominated sites under hyper-arid environments

Shallow groundwater is primarily discharged via evapotranspiration (ETg) in arid and semi-arid riparian systems; however, the quantification of ETg remains a challenge in regional water resource assessments of such systems. In this study, the diagnostic indicators of groundwater evapotranspiration processes and the principles of applying the water table fluctuation (WTF) method to estimate ETg based on seasonal groundwater level changes were presented. These techniques were then used to investigate groundwater evapotranspiration processes at two sites dominated by phreatophytes (Tamarix ramosissima and Populus euphratica) within hyper-arid desert environments in northwestern China for the period 2010-2012. The results indicate that steady declines in the water table, which are commonly attributed to groundwater evapotranspiration, occurred at both sites during the growing season. Based on the proposed WTF method, the estimated ETg was 0.63-0.73 mm/d at the Tamarix ramosissima site and 1.89-2.33 mm/d at the Populus euphratica site during the summer months (June-August). Numerical simulations using a one-dimensional root water uptake model indicate that the seasonal variations in ETg at both sites were primarily dependent on the potential evaporation rates. Comparisons with previous studies on plant transpiration at similar sites in this area show that these results are reasonable. It is apparent that the WTF method can provide a simple and relatively inexpensive method of estimating ETg on a large scale in arid/semi-arid regions. However, there are significant uncertainties associated with time-dependent lateral flow rates, which creates a challenge when applying this method. In addition, the selection of calculation periods that show steady declines in the groundwater level can be somewhat subjective. To enhance the performance of the WTF method based on seasonal water table declines, further research on the estimation of lateral flow rates should be performed using an effective network of groundwater monitoring. (C) 2014 Elsevier B.V. All rights reserved.

Root water compensation sustains transpiration rates in an Australian woodland

We apply a model of root-water uptake to a woodland in Australia to examine the regulation of transpiration by root water compensation (i.e., the ability of roots to regulate root water uptake from different parts of the soil profile depending on local moisture availability). We model soil water movement using the Richards equation and water flow in the xylem with Darcy’s equation. These two equations are coupled by a term that governs the exchange of water between soil and root xylem as a function of the difference in water potential between the two. The model is able to reproduce measured diurnal patterns of sap flux and results in leaf water potentials that are consistent with field observations. The model shows that root water compensation is a key process to allow for sustained rates of transpiration across several months. Scenarios with different root depths showed the importance of having a root system deeper than about 2m to achieve the measured transpiration rates without reducing the leaf water potential to levels inconsistent with field measurements. The model suggests that the presence of more than 5% of the root system below 0.6m allows trees to maintain sustained transpiration rates keeping leaf water potential levels within the range observed in the field. According to the model, a large contribution to transpiration in dry periods was provided by the roots below 0.3m, even though the percentage of roots at these depths was less than 40% in all scenarios.

Evaluation of soil water dynamics and crop yield under furrow irrigation with a two-dimensional flow and crop growth coupled model

Aiming at investigating an appropriate furrow irrigation management strategy for high melon yields and water productivity (WP), a new coupled model was developed based on the CHAIN_2D and the crop growth model of EPIC. In the coupled model, the root water uptake model of Vrugt was coupled with the root depth growth model in order to consider the interaction between root water uptake and crop growth. The coupled model was calibrated and validated with the observed values obtained from melon field experiment conducted in 2008 and 2009 in Gansu province, Northwest China. Simulation of total water use, leaf area index, melon yield and soil water dynamics fitted well with the field observations. The calibrated model was then used to predict the yield and water productivity (WP) of melon under different furrow irrigation scenarios. The relative yield and WP for different irrigation depth were considered as the criteria for investigating the appropriate irrigation management practices. Results showed that the relative yield and WP increased and decreased, respectively, as the relative irrigation increased through a quadratic function. The appropriate irrigation amounts for melon in the study area were 209mm and 218mm in 2008 and 2009, respectively.

Modelling of unsaturated ground behaviour influenced by vegetation transpiration

Vegetation contributes to weak soil stabilisation through reinforcement of the soil, dissipation of excess pore pressure and increasing the shear strength by induced matric suction. This paper describes the way vegetation influences soil matric suction, shrinkage and ground settlement in the vadose zone through transpiration. A mathematical model for the rate of root water uptake, including the root growth rate considering ground conditions, type of vegetation and climatic parameters, has been developed. A finite element approach is employed to solve the transient coupled flow-deformation equations. The finite element mesh is built using partially saturated soil elements capable of representing the salient aspects of unsaturated permeability and the soil water characteristic curve. The model formulation is based on the effective stress theory of unsaturated soils. Based on this proposed model, the distribution of the ground matric suction profile adjacent to the tree is numerically analysed. Current field measurements of soil matric suction and moisture content collected from Miram site located in Victoria State, Australia by the authors are compared with the numerical predictions. The results indicate that the proposed root water uptake model incorporated in the numerical analysis can be used for prediction of ground properties influenced by tree roots.

Modelling hourly evapotranspiration and soil water content at the grass-covered boundary-layer field site Falkenberg, Germany

The study analyses a 2-year period of hourly rates of real evapotranspiration (ETr) derived from eddy covariance measurements and soil water contents at depths from 8 to 90 cm, monitored by time domain reflectometry probes at the grass-covered boundary-layer field site Falkenberg of the Lindenberg Meteorological Observatory – Richard-Aßmann-Observatory, operated by the German Meteorological Service (DWD). The ETr rates and soil water contents were compared with the results of a modelling approach consisting of the Penman-Monteith equation and the soil water balance model Hydrus-1D using a noncompensatory and a compensatory root-water uptake model. After optimization of soil hydraulic parameters by inverse modelling, using measured soil water contents as the objective function, simulated and measured model outputs showed good agreement for soil water contents above 90 cm depth and for ETr rates simulated by our modelling approaches using noncompensatory root-water uptake. The application of a compensatory root-water uptake model led to a decrease in the simulation quality for the total investigation period.Editor Z.W. KundzewiczCitation Wegehenkel, M. and Beyrich, F., 2014. Modelling of hourly evapotranspiration and soil water content at the grass-covered boundary-layer field site Falkenberg, Germany. Hydrological Sciences Journal, 59 (2), 376–394.

Compensation in Root Water Uptake Models Combined with Three‐Dimensional Root Length Density Distribution

Because root water uptake is a considerable component in the soil water balance, a lot of attention has been paid to defining and applying root water uptake models. These models can be grouped into empirical vs. mechanistic root uptake models. Intermodel comparisons are valuable in understanding the different concepts used. Such a comparison is sometimes difficult because the level of information required by the models is different, for example, information on the root length density distribution. Here a three‐dimensional root length density distribution function is introduced that makes it possible to compare two empirical uptake models with a more mechanistic uptake model. Adding a compensation component to the more empirical model results in prediction of root water uptake distributions in the root zone similar to those predicted by the more complex model.

Mucilage exudation facilitates root water uptake in dry soils.

As plant roots take up water and the soil dries, water depletion is expected to occur in the rhizosphere. However, recent experiments showed that the rhizosphere was wetter than the bulk soil during root water uptake. We hypothesise that the increased water content in the rhizosphere was caused by mucilage exuded by roots. It is probably that the higher water content in the rhizosphere results in higher hydraulic conductivity of the root-soil interface. In this case, mucilage exudation would favour the uptake of water in dry soils. To test this hypothesis, we covered a suction cup, referred to as an artificial root, with mucilage. We placed it in soil with a water content of 0.03cm3cm-3, and used the root pressure probe technique to measure the hydraulic conductivity of the root-soil continuum. The results were compared with measurements with roots not covered with mucilage. The root pressure relaxation curves were fitted with a model of root water uptake including rhizosphere dynamics. The results demonstrated that when mucilage is added to the root surface, it keeps the soil near the roots wet and hydraulically well conductive, facilitating the water flow from dry soils towards the root surface. Mucilage exudation seems to be an optimal plant trait that favours the capture of water when water is scarce.

A Physically Based Microscopic Model of Root-Water Uptake

The present study involves development of a microscopic numerical model for simulating radial and vertical moisture flow towards plant root, root-water uptake and finally flow through the plant to the atmosphere. The model essentially comprises a numerical solution of the Richard’s equation in axis-symmetric polar co-ordinate system by finite difference method. The simulation of the subsequent water movement (from root to canopy and from canopy to atmosphere) is accomplished indirectly through a boundary condition at the root-soil interface. This boundary condition is derived by invoking the meteorological conditions prevailing over the plant leaf surface, and a plant conductivity parameter (plant transmissivity) is defined herein. The model provides distribution of soil moisture content, and the root-water uptake at discrete times. The plant transmissivity has been estimated for wheat crop. KeywordsRoot-Water Uptake; Microscopic Model; Richard’s Equation; Transpiration

Root Water Uptake: From Three‐Dimensional Biophysical Processes to Macroscopic Modeling Approaches

The process description of plant transpiration and soil water uptake in macroscopic root water uptake models is often based on simplifying assumptions that no longer reflect, or even contradict, the current status of knowledge in plant biology. The sink term in the Richards equation for root water uptake generally comprises four terms: (i) a root resistance function, (ii) a soil resistance function, (iii) a stress function, and (iv) a compensation function. Here we propose to use a detailed three‐dimensional model, which integrates current knowledge of soil and root water flow equations, to deduct a one‐dimensional effective behavior at the plant scale and to propose improvements for the four functions used in the macroscopic sink term. We show that (i) root hydraulic resistance may be well defined by the root length density but only for homogeneous lateral conductances and no limiting xylem conductance—in other cases a new function depending on the root hydraulic architecture should be used; (ii) soil resistance cannot be neglected, in particular in the rhizosphere where specific processes may occur that alter the soil hydraulic properties and therefore affect uptake; (iii) stress and compensation are two different processes, which should not be linked explicitly; (iv) there is a need for a clear definition of compensatory root water uptake independent of water stress; (v) stress functions should be defined as a maximal actual transpiration in function of an integrated root–soil interface water head rather than in terms of local bulk water heads; and (vi) nonlinearity in the stress function is expected to arise if root hydraulic resistances depend on soil matric head or when it is defined as a function of the bulk soil water head.

Dynamics of root water uptake and water use efficiency under alternate partial root-zone irrigation

The distribution of root length density of maize in ridge culture under alternate partial root-zone furrow irrigation (APFI) and conventional furrow irrigation (CFI) was investigated, and two-dimensional models of root length density and root uptake were developed to investigate the water use efficiencies under different furrow irrigation modes. Results indicated that APFI increased root depth and horizontal extended distance of maize, as well as water use efficiency (WUE). Values of the mean absolute error (MAE), the root mean square error (RMSE), and the index of agreement (di) between measured and simulated values of root length density of maize under APFI ranged from 0.26 to 0.68, 0.03 to 1.51, and 0.64 to 0.85 cm m−3, respectively; and the coefficient of determination (R2) was greater than 0.80. Values of MAE, RMSE, and di between measured and simulated values of root length density of maize under CFI ranged from 0.01 to 0.26, 0.01 to 0.60, and 0.61 to 0.96 cm m−3, respectively; and R2 was higher than 0.86. Dynamics of root uptake at different sites were simulated using the model of root water uptake. Root growth of maize under furrow irrigation was modeled using the 2D model of root length density with reasonable accuracy. Compared with CFI, water consumption rate of the maize in APFI was lower in key water requirement period, root uptake rate was higher in the end period, and WUE in APFI was higher by 5% than that in CFI.

Simulation of stand transpiration based on a xylem water flow model for individual trees

Quantifying the water exchange between a forest stand and the atmosphere is of major interest for the prediction of future growth conditions and the planning of silvicultural treatments. In the present study, we address (i) the uncertainties of sap flow estimations at the tree level and (ii) the performance of the simulation of stand transpiration. Terrestrial laser scan images (TLS) of a mature beech stand (Fagus sylvatica L.) in Southwestern Germany serve as input data for a representation of the aboveground tree architecture of the study stand. In the single-tree xylem water flow model (XWF) used here, 98 beech trees are represented by 3D graphs of connected cylinders with explicit orientation and size. Beech-specific hydraulic parameters and physical properties of individual trees determine the physiological response of the tree model to environmental conditions.The XWF simulations are performed without further calibration to sap flow measurements. The simulations reliably match up with sap flow estimates derived from sap flow density measurements. The density measurements strongly depend on individual sapwood area estimates and the characterization of radial sap flow density gradients with xylem depth. Although the observed pure beech stand is even-aged, we observe a high variability in sap flow rates among the individual trees. Simulations of the individual sap flow rates show a corresponding variability due to the distribution of the crown projection area in the canopy and the different proportions of sapwood area.Stand transpiration is obtained by taking the sum of 98 single-tree simulations and the corresponding sap flow estimations, which are then compared with the stand-level root water uptake model (RWU model) simulation. Using the RWU model results in a 35% higher simulation of seasonal stand transpiration relative to the XWF model. These findings demonstrate the importance of individual tree dimensions and stand heterogeneity assessments in estimating stand water use. As a consequence of species-specific model parameterization and precise TLS-based stand characterization, the XWF model is applicable to various sites and tree species and is a promising tool for predicting the possible water supply limitations of pure and mixed forest stands.

Effect of hydraulic conductivity on soil moisture uptake under saline conditions for wheat crop

Salinity in soil can decrease plant available water and cause plant stress. The pattern of root water uptake for wheat was studied for saline as well as non-saline condition using non-linear root water uptake model. Experiments have been conducted using a salinity level of 4 dS/m and freshwater condition. The effect of salinity on soil moisture has been studied by varying the crop coefficient as well as hydraulic conductivity. The correction factor to the crop coefficient approach is found to be unsuccessful. However, the correction factor to the hydraulic conductivity for a non-saline condition improves the simulation of soil moisture uptake in case of saline soils. The exponential form of the equation is established for the hydraulic conductivity to soil moisture relationship under salinity level and freshwater sample. The present work also substantiates that the non-linearity parameter of root water uptake model (O-R model) is successful in simulation of soil moisture depletion in the crop root zone and does not vary more than 10% in case of saline soils.