Multistep Outflow Experiments Research Articles

A framework for estimating soil water characteristic curve and hydraulic conductivity function of permeable reactive media

The unsaturated behavior of permeable reactive barriers (PRB) is a critical component in predicting the removal efficiency through the adsorption of contaminants. This study investigates the framework to estimate the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF) for iron oxide-coated sand (IOCS) and zeolite, which are common materials used in PRBs. A multistep outflow (MSO) experiment was performed and the results of the MSO experiment were used to optimize associated parameters in Kosugi's SWCC and HCF. In addition, three scenarios of optimization analysis were investigated to evaluate the best-fitting model for estimating SWCC and HCF. The low root mean square error (RMSE) of fitted parameters indicates the Kosugi model well described the observed suction profiles in MSO experiments. In addition, the lowest RMSE and coefficient of variation suggested the inclusion of the additional parameter β provided the best estimation of the three materials (clean sand, IOCS, and zeolite). The physically reasonable estimation of SWCC and HCF of the three materials from the optimized parameters suggests the proposed framework is a reasonable model for the unsaturated behavior of PRBs.

Inverse modeling towards parameter estimation of the nonlinear soil hydraulic functions using developed multistep outflow procedure

Modeling and monitoring vadose zone processes have a key role to effectively identify and solve important emerging issues in hydrology and related disciplines. However, major gaps and challenges remain concerning the further conceptualization of the zone complexities and parameterization of relevant models. Consequently, the main focus of this investigation was to establish a modern developed version of the multistep outflow experiments for the vadose zone characterization and modeling. Besides, soil hydraulic properties (K-θ-h) were determined by the Mualem/Van Genuchten model based on the inverse solution of the Richards equation for the four loamy textured soils. The adjustable parameters were estimated with the least-square approach through the implementation of the generalized reduced gradient (GRG) algorithm. Then, the inverse manner was evaluated by comparing optimized hydraulic functions with those measured independently using an equilibrium condition. The results revealed the optimized attributes complied well with those determined using equilibrium analysis and steady-state situations. The reliable estimation of the parameters to inversely optimize the K-θ-h functions was possible only by measuring the cumulative outflow as a function of time. The simultaneous optimization of the unsaturated hydraulic functions carried out from a single transient experiment. This protocol appears well suited for vadose zone simulation. Furthermore, along with the tensiometer elimination and no independent measurements of Ks and θs, the developed version is generally more cost-effective, faster and more convenient than the conventional method. Finally, we conclude the combination of proposed procedure with GRG algorithm can provide a reasonable procedure to inversely estimate the hydraulic parameters.

Error correction of water evaporation loss on the unsaturated hydraulic conductivity

Water evaporation loss occurs during multistep outflow experiments from the soil surface and the outflow vessel. The main focus of this paper is to introduce an improved pressure plate instrument that can correct for the error of water evaporation loss on the unsaturated hydraulic conductivity. Experiments using silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were conducted with an improved instrument. The results showed that the measured value of the water content will be overestimated and the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss.

Multi-step outflow and evaporation experiments – Gaining large undisturbed samples and comparison of the two methods

The aim of the study was the comparison of the soil hydraulic properties determined based on two different experimental methods of four large and macroporous soil samples. An undisturbed cylindrical soil column (h = 60 cm and d = 30 cm) was obtained at a natural slope and quartered horizontally in the laboratory. The four sub-samples were saturated and subsequently drained using the extended multi-step outflow method (XMSO). Retention curves and unsaturated hydraulic conductivity curves were derived via inverse modeling based on the measured drainage and corresponding pore water pressure. After resaturation the evaporation method (EVA) was used on the same four samples. The samples were exposed to evaporation and the effective water contents and the average pore water pressures were recorded over time to determine soil hydraulic functions.While both methods yielded similar results in the medium moisture range, the advantage of the XMSO-method is its higher information content with respect to the hydraulic conductivity close to water saturation whereas the advantage of the EVA-method is its fast execution. Moreover, the EVA-method is easier to handle and has significantly lower computational requirements than the XMSO-method.Furthermore, since the four soil samples represent a soil profile in the field, a depth dependent characterizing of the soil profile was possible and showed that the soil properties near saturation are greatly dominated by the ratio of macropores in the sample and that a decrease in the macropore ratio with soil depth was clearly reflected in the hydraulic functions.

Effect of hysteresis on hydraulic properties of soils under multiple drying and wetting cycles

The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood. Many studies on the soil water retention curve (SWRC) and the relative coefficient of permeability under drying and wetting cycles have been performed. However, most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying–wetting cycles with limited soil experiments. This might be because the measurement of both the SWRC and the coefficient of permeability of unsaturated soil is complex and time consuming. In this study, the hydraulic properties of silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were studied for the same soil specimen under five cycles of drying and wetting in multistep outflow experiments. Significant hysteresis of SWRCs was observed under the first drying and wetting cycle for both soils. However, the hysteresis effect decreased as the number of drying and wetting cycles increased. Furthermore, the coefficient of permeability was estimated using the van Genuchten–Mualem (VGM) model. Results also show that hysteresis plays an important role in the coefficient of permeability for both soils subjected to multiple drying and wetting cycles.

A Method for Directly Measuring the Hydraulic Conductivity of Unsaturated Soil

Abstract Numerous methods, such as the Gardner outflow analysis method and the van Genuchten-Mualem prediction model, have been developed to estimate the hydraulic conductivity of unsaturated soil in multistep outflow experiments. In the Gardner outflow analysis method, the hydraulic conductivity is estimated based on the diffusivity calculated from the line fitted to the water outflow. In contrast, in the van Genuchten-Mualem prediction model, it is predicted via the curve-fitting parameters based on the measured soil water retention curve (SWRC). However, a major drawback of these two approaches is that variation of hydraulic conductivity during each step of the outflow process cannot be represented. In this paper, an improved pressure plate instrument with an evaporation compensation system is developed to measure the transient water content curve (TWCC) of the soil in multi-step outflow experiments. The SWRCs of silicon micro-powder (SMP) and Guangxi Guiping clay (GGC) are derived from this transient curve. Therefore, a direct measurement method is presented to describe the change of hydraulic conductivity during each step. Comparison with the Gardner outflow analysis method and the van Genuchten-Mualem prediction model indicates that our approach shows better agreement with test data for these two types of soils.

Unsaturated hydraulic behaviour of a permeable pavement: Laboratory investigation and numerical analysis by using the HYDRUS-2D model

An adequate hydrological description of water flow in permeable pavement systems relies heavily on the knowledge of the unsaturated hydraulic properties of the construction materials. Although several modeling tools and many laboratory methods already exist in the literature to determine the hydraulic properties of soils, the importance of an accurate materials hydraulic description of the permeable pavement system, is increasingly recognized in the fields of urban hydrology. Thus, the aim of this study is to propose techniques/procedures on how to interpret water flow through the construction system using the HYDRUS model. The overall analysis includes experimental and mathematical procedures for model calibration and validation to assess the suitability of the HYDRUS-2D model to interpret the hydraulic behaviour of a lab-scale permeable pavement system. The system consists of three porous materials: a wear layer of porous concrete blocks, a bedding layers of fine gravel, and a sub-base layer of coarse gravel. The water regime in this system, i.e. outflow at the bottom and water contents in the middle of the bedding layer, was monitored during ten irrigation events of various durations and intensities. The hydraulic properties of porous concrete blocks and fine gravel described by the van Genuchten functions were measured using the clay tank and the multistep outflow experiments, respectively. Coarse gravel properties were set at literature values. In addition, some of the parameters (Ks of the concrete blocks layer, and α, n and Ks of the bedding layer) were optimized with the HYDRUS-2D model from water fluxes and soil water contents measured during irrigation events. The measured and modeled hydrographs were compared using the Nash-Sutcliffe efficiency (NSE) index (varied between 0.95 and 0.99) while the coefficient of determination R2 was used to assess the measured water content versus the modelled water content in the bedding layer (R2=0.81÷0.87). The parameters were validated using the remaining sets of measurements resulting in NSE values greater than 0.90 (0.91÷0.99) and R2 between 0.63 and 0.91. Results have confirmed the applicability of HYDRUS-2D to describe correctly the hydraulic behaviour of the lab-scale system.

The impact of the permanent grass cover or conventional tillage on hydraulic properties of Haplic Cambisol developed on paragneiss substrate

AbstractThis study is focused on the comparison of soil structure and soil hydraulic properties of a Haplic Cambisol on paragneiss under two different land managements. Soil samples were taken from all diagnostic horizons (A, Bw and C) of the soil profile under the permanent grass cover (grassland) and under the conventional tillage (arable land). Basic soil properties were measured. Aggregate stability was assessed using the WSA index. Soil composition was evaluated using micromorphological images. Tension disk infiltrometers with two diameters of 2.22 and 10.25 cm (and applied pressure head of −2 cm) and Guelph permeameter were used to measure unsaturated and saturated hydraulic conductivities, respectively. Soil hydraulic properties were measured in the laboratory using the multistep outflow experiment, which was performed on the undisturbed 100 cm

Determination of Hydraulic Properties of Unsaturated Soils Based on Nonequilibrium Multistep Outflow Experiments

AbstractIn this paper, a new procedure for rapidly determining the hydraulic properties of unsaturated soils is developed. The proposed procedure is used to characterize the evolution of the instant degree of saturation in an unsaturated soil during a multistep outflow experiment. By introducing the concept of capillary relaxation, a theoretical model is developed and adopted to describe the dynamic hydraulic properties of unsaturated soils under constant-volume conditions. A series of nonequilibrium multistep outflow experiments are performed on various types of unsaturated soils. It is shown that the evolution of the instant degree of saturation or the outflow mass generally follows an exponential law. Both experimental results and theoretical simulations illustrate that flow instability and water blockage can occur in the soil during multistep outflow experiments. The theoretical predictions are compared to the experimental results, showing that the proposed procedure can be effectively used to determi...

A comparison of two Bayesian approaches for uncertainty quantification

Statistical calibration of model parameters conditioned on observations is performed in a Bayesian framework by evaluating the joint posterior probability density function (pdf) of the parameters. The posterior pdf is very often inferred by sampling the parameters with Markov Chain Monte Carlo (MCMC) algorithms. Recently, an alternative technique to calculate the so-called Maximal Conditional Posterior Distribution (MCPD) appeared. This technique infers the individual probability distribution of a given parameter under the condition that the other parameters of the model are optimal. Whereas the MCMC approach samples probable draws of the parameters, the MCPD samples the most probable draws when one of the parameters is set at various prescribed values. In this study, the results of a user-friendly MCMC sampler called DREAM(ZS) and those of the MCPD sampler are compared. The differences between the two approaches are highlighted before running a comparison inferring two analytical distributions with collinearity and multimodality. Then, the performances of both samplers are compared on an artificial multistep outflow experiment from which the soil hydraulic parameters are inferred. The results show that parameter and predictive uncertainties can be accurately assessed with both the MCMC and MCPD approaches.

Modeling dynamic non-equilibrium water flow observations under various boundary conditions

Summary Experimental evidence of variably-saturated flow in soils often shows a non-equilibrium between water content and water potential. Such phenomena, summarized by the term “dynamic effects” often occur in laboratory experiments specifically designed for the identification of soil hydraulic properties which are a mandatory input for water flow simulations at the field scale using the Richards equation. Depending on boundary conditions, dynamic non-equilibrium leads either to a relaxation of pressure head for constant water content or vice versa. Diamantopoulos et al. (2012) presented a model which describes dynamic effects in the case of multistep outflow experiments by assuming that the total water content is portioned into two fractions: one fraction which is in instantaneous equilibrium with the pressure head, and a second fraction for which the equilibration of the water content with the pressure is described by a first-order kinetics which leads to a temporal shift in water content at constant pressure head. In this work we applied this non-equilibrium model to four different observations of dynamic non-equilibrium effects presented in the literature and demonstrate that it successfully describes (1) non-equilibrium in water absorption experiments which results in a non-uniqueness of the diffusivity vs water content function, (2) the flow-rate dependence of soil hydraulic properties, (3) dynamic effects in multistep outflow and (4) dynamic effects in multistep flux experiments. We conclude the article by a discussion of possible macroscopic explanations, the consideration of dynamic effects for modeling water flow at the field scale, the uniqueness of the estimated non-equilibrium parameters among different experiments, and further theoretical considerations concerning dynamic effects.

Application of Reactive Transport Modeling to Growth and Transport of Microorganisms in the Capillary Fringe

In the process of modeling and simulation of bacterial growth, the calibration of a numerical model to the specific experimental setup is often necessary. This study addressed the question of whether it is possible to predict the growth of microorganisms based only on independent measurements without additional calibration. To this end, flow‐through experiments with Escherichia coli (HB101 K12 pGLO) in a Hele–Shaw cell filled with quartz sand were simulated with a newly developed numerical code for reactive multiphase multicomponent flow based on an operator splitting approach. The hydraulic parameters of the sand were determined by multistep outflow experiments and the parameters of the microbial growth model from batch experiments. There was very good agreement between predicted and measured cell concentrations nondestructively determined from the fluorescence intensities of a green fluorescent protein produced by the bacterial strain. The correct description of the kinetics of gas–water phase exchange in the unsaturated zone and of cell attachment to the solid phase proved to be crucial. The operator splitting approach significantly reduced numerical dispersion in a convection‐dominated test case. It allowed a flexible adaptation of the solution strategy to the specific problem.

Impact of various surface covers on water and thermal regime of Technosol

Summary Different soil covers influence water and thermal regimes in soils within urban areas. Knowledge of these regimes is needed, particularly when assessing effectiveness of energy gathering from soils using horizontal ground heat exchangers. The goal of this study was to calibrate the model HYDRUS-1D for simulating coupled water and thermal regime in Technosol type soils with grass cover, and to use this model for predicting water and thermal regimes under different materials covering the soil surface. For this purpose soil water contents were measured at depths of 10, 20, 30, 40, 60 and 100 cm at 4 locations and temperatures were measured at depths of 20, 40, 80, 120, 150 and 180 cm at three locations (all covered by grass) from June 2011 to December 2012. In addition sensors for simultaneous measuring soil water contents and temperatures were installed under different soil covers (grass, bark chips, sand, basalt gravel and concrete paving) at a depth of 7. The parameters of soil hydraulic properties were obtained on the 100-cm3 undisturbed soil samples using the multi-step outflow experiment and numerical inversion of the measured transient flow data using HYDRUS-1D. HYDRUS-1D was then used to simulated the water regime within the soil profile under the grass cover using climatic data from June 2011 to December 2012 and some of the soil hydraulic parameters were additionally numerically optimized using soil water contents measured at all depths. Water flow and heat transport were then simulated using these parameters, measured thermal properties and temperatures measured close to the surface applied as a top boundary condition. Simulated temperatures at all depths successfully approximated the measured data. Next, water and thermal regimes under another 4 different surface covers were simulated. Soil hydraulic properties of different materials were partly measured and partly optimized when simulating soil water regime from June 2011 to December 2012 using the soil water contents measured at the depth of 7 cm (sand, bark chips, and concrete paving) or set (basalt gravel). The greatest soil water content variability was obtained (measured at the depth of 7 cm and simulated at different depths) for grass and gradually decreased for sand, basalt gravel, bark chips and concrete paving. However, in the case of concrete paving the measured soil water contents at the depth of 7 cm could not be successfully approximated using HYDRUS-1D due to the fact that measured soil water contents reflected 3D character of water flow below the paving. The largest temperature oscillations during each day at the depth of 7 cm were measured for concrete paving (due to this large daily data oscillation measured values could not be influence by surrounding area) and decreased as follows: basalt gravel, sand, grass and bark chips. As result also temperature oscillations simulated at different depths using measured temperatures as top boundary conditions followed the same trends. The highest temperatures at different depths during the warm periods were simulated for concrete paving and decreased as daily temperature oscillations. The highest temperatures during the cold period were simulated for basalt gravel and decreased for grass, bark chips, sand and concrete paving. Thus the basalt gravel scenario appeared to be the most efficient for increasing heating systems effectiveness.

Multiobjective parameter estimation of hydraulic properties for a sandy soil in Oman

The estimation of soil hydraulic parameters for the description of the highly non-linear hydraulic conductivity and water retention curve is of crucial importance for numerical modeling of water and solute transport in the vadose zone. Soil cores have been taken from different depths of a sandy soil of an agricultural field site located in the Batinah region of the Sultanate of Oman and subjected to a comprehensive laboratory analysis, comprising simplified multistep outflow experiments without the use of microtensiometers, additional retention measurements with the pressure plate apparatus and particle size analysis. The measured retention curve and the dynamic cumulated outflow time series—characterizing the hydraulic conductivity curve—are used to estimate the soil hydraulic parameters of the Mualem-van Genuchten (MvG) soil model. Multiobjective parameter optimization runs were performed by coupling the HYDRUS-1D model with the AMALGAM algorithm to allow an enhanced diagnostics of the soil and model behavior by analyzing the trade-offs between two objectives. Furthermore, two different formulations of the MvG model were investigated regarding their ability to reproduce the measurements: (1) P1—with fixed shape parameter \(\tau\) and (2) P2—considering the shape parameter \(\tau\) as an additional fitting parameter. The estimated parameters for the investigated soil profile exhibit a considerable degree of heterogeneity with saturated hydraulic conductivities varying by three orders of magnitude within the soil profile. The multiobjective analysis showed that considerable trade-offs exist between the two objectives for some of the soil cores, indicating that the fit to the cumulated outfow data can be increased on the cost of the fit to the water retention. However, satisfying fits to both objectives simultaneously were achieved for both formulations of the MvG model.

A comparison between simulation and experiment for hysteretic phenomena during two-phase immiscible displacement

[1] The paper compares a theory for immiscible displacement based on distinguishing percolating and nonpercolating fluid parts with experimental observations from multistep outflow experiments. The theory was published in 2006 in Physica A, volume 371, pages 209–225; the experiments were published in 1991 in Water Resources Research, volume 27, pages 2113. The present paper focuses on hysteretic phenomena resulting from repeated cycling between drainage and imbibition processes in multistep pressure experiments. Taking into account, the hydraulic differences between percolating and nonpercolating fluid parts provides a physical basis to predict quantitatively the hysteretic phenomena observed in the experiment. While standard hysteretic extensions of the traditional theory are nonlocal in time the theory used in this paper is local in time. Instead of storing the pressure and saturation history, it requires only the current state of the system to reach the same quantitative agreement.

Complex Electrical Measurements on an Undisturbed Soil Core: Evidence for Improved Estimation of Saturation Degree from Imaginary Conductivity

Recent studies have shown that induced polarization (IP) coupled with electrical resistivity surveys can be used for in situ lithological and hydrologic discrimination of the subsurface; yet the driving factors behind the effects of water content dynamics on IP are relatively understudied. We sought to improve the understanding of the relationship of IP on variations in water saturation degree for an undisturbed agricultural soil. Our experiment consisted of collecting IP measurements concurrently with hydraulic data during multistep outflow experiments. We determined the hydraulic properties of the undisturbed soil samples and correlated saturation degree with IP data. Due to an increase in pore fluid conductivity in the column pore water with decreasing saturation degree, we found that imaginary conductivity (σ″) may offer distinct advantages for determining water content over real conductivity (σ′) measurements. Although σ″ exhibits a weaker dependence on saturation degree compared to σ′, the relative insensitivity of σ″ to pore fluid conductivity results in a simpler dependence on saturation change in the presence of varying salinities. As changes in pore fluid conductivity are likely to occur in the field simultaneously with water content variations, we suggest that although IP has mostly been used to discriminate lithology, time lapse IP measurements may additionally provide a robust indicator of changes in saturation degree.

Using inverse analysis to estimate hydraulic properties of unsaturated sand from one-dimensional outflow experiments

A one-dimensional (1-D) vertical draining test for Jumunjin sand was carried out in one-step and multi-step outflow conditions. An inverse analysis method was conducted to determine the hydraulic properties of unsaturated soil. A non-linear optimization method combining a finite element code and inversion analysis was used to minimize the objective function, defined by the difference between observed and predicted data. Unsaturated hydraulic parameters in van Genuchten model were estimated using soil suction measurements in 10cm intervals and an outflow rate at the bottom of a sand column. The predicted hydraulic properties and the experimental results were in close agreement when the measurements were compared from the one-step and multi-step outflow experiments. The results also showed that the multi-step outflow experiment was more appropriate in determining the unsaturated hydraulic properties than the one-step outflow experiment. The comparison between predicted and measured results concluded that the inverse analysis based on the 1-D outflow experiment was reliable and useful to determine the hydraulic properties of unsaturated soils.

Conductivity and Hydraulic Nonequilibrium across Drainage and Infiltration Fronts

Recently, a new approach was introduced to directly measure unsaturated hydraulic conductivity in flux-controlled experiments—the multistep flux experiment. Thereby an overshoot in matric potential h m across drainage and infiltration fronts was observed. We extended this experimental approach to simultaneously measure the volumetric water content θ within the sample and applied the method to a sand and a clay loam soil. The detailed trajectories within the h m –θ space were obtained during a number of decreasing and increasing steps in infiltration rate. This clearly demonstrates the type and magnitude of hydraulic nonequilibrium under transient conditions where water content and matric potential deviate from a well-defined static relation. We also compared the directly measured hydraulic conductivities with those obtained from classical multistep outflow experiments and found that nonequilibrium dynamics might lead to an underestimation of hydraulic conductivity when obtained from an inverse solution of Richards’ equation. We provide a qualitative explanation of nonequilibrium that depends on the structure of the material and the type and magnitude of external forcing. The new experimental setup is considered to be a valuable tool to actually quantify nonequilibrium effects. This will make it possible to represent this relevant phenomenon in future modeling concepts.

Estimating effective model parameters for heterogeneous unsaturated flow using error models for bias correction

Estimates of effective parameters for unsaturated flow models are typically based on observations taken on length scales smaller than the modeling scale. This complicates parameter estimation for heterogeneous soil structures. In this paper we attempt to account for soil structure not present in the flow model by using so‐called external error models, which correct for bias in the likelihood function of a parameter estimation algorithm. The performance of external error models are investigated using data from three virtual reality experiments and one real world experiment. All experiments are multistep outflow and inflow experiments in columns packed with two sand types with different structures. First, effective parameters for equivalent homogeneous models for the different columns were estimated using soil moisture measurements taken at a few locations. This resulted in parameters that had a low predictive power for the averaged states of the soil moisture if the measurements did not adequately capture a representative elementary volume of the heterogeneous soil column. Second, parameter estimation was performed using error models that attempted to correct for bias introduced by soil structure not taken into account in the first estimation. Three different error models that required different amounts of prior knowledge about the heterogeneous structure were considered. The results showed that the introduction of an error model can help to obtain effective parameters with more predictive power with respect to the average soil water content in the system. This was especially true when the dynamic behavior of the flow process was analyzed.

Application of the swap model to predict impact of climate change on soil water balance in a Sicilian vineyard

In Sicily, the increasing scarcity of quality water is leading to irrigation with saline water in soils having a considerable susceptibility to salinization and degradation. In this paper, water flow and solute transport in a Sicilian irrigated soil was simulated by using the SWAP model, and the simulated results compared to measured values of soil moisture and salinity. The investigation was carried out in a vineyard located in Sicily (Mazara del Vallo, Trapani Province) within the framework of the Project: Evolution of cropping systems as affected by climate change (CLIMESCO). The soil hydraulic parameters were obtained by inverse method based on multi-step outflow experiments. The results obtained using field measurements from two soil profiles showed that SWAP provided reliable predictions of water content, θ, and of electrical conductivity of saturated extract, ECsat. The effect of climate change on soil water balance was predicted by application of SWAP under the soil-crop and irrigation management conditions currently practiced in the vineyard. Simulations were carried out with climatic data generated by the statistical downscaling process based on the global CGM HADCM3 for two IPCC scenarios (A2 and B1) and for the 2070-2100 time interval. The two mentioned IPCC scenarios (A2 and B1) were then compared to a third reference scenario (BASELINE) obtained by using the climatic data recorded from 1951 to 2004. The results indicated that the only significant effect of climate change was to shorten and to advance the crop cycle length, which decreased of 16 and 27 days respectively in B1 and A2 compared to values obtained for the BASELINE scenario. Further application of SWAP carried out simulating an advance in the irrigation starting time according to the advance in the crop cycle indicated that this could be considered a kind of adaptation strategy to be applied in the future in the vineyard to counterbalance the impact of climate change. This means that in the future farmers might be likely to anticipate the irrigation season in order to avoid any significant reduction in the crop productivity.