Estimation of saturated hydraulic conductivity from double‐ring infiltrometer measurements

This research aims to determine soil vertical saturated hydraulic conductivity (K_s) in situ from the measured steady infiltration rate (I), initial soil properties and double‐ring infiltrometer (DRI) test data. Characterizing the effects of these variables on the measured steady infiltration rate will enable more accurate prediction of K_s. We measured the effects of the ring diameter, head of ponding, ring depth, initial effective saturation and soil macroscopic capillary length on measured steady infiltration rates. We did this by simulating 864 DRI tests with the finite element program HYDRUS‐2D and by conducting 39 full‐scale in situ DRI tests, 30 Mini‐Disk infiltrometer experiments and four Guelph Permeameter tests. The M5′ model trees and genetic programming (GP) methods were applied to the data to establish formulae to predict the K_s of sandy to sandy‐clay soils. The nine field DRI tests were used to verify the computer models. We determined the accuracy of the methods with 30% of the simulated DRI data to compare I/K_S values of the finite element models with estimates from the suggested formulae. We also used the suggested formulae to predict the K_s values of 30 field DRI experiments and compared them with values measured by Guelph Permeameter tests. Compared with the GP method, the M5′ model was better at predicting K_S, with a correlation coefficient of 0.862 and root mean square error (RMSE) of 0.282 cm s^?1. In addition, the latter method estimated K_svalues of the field experiments more accurately, with an RMSE of 0.00346 cm s^?1.     

利用圆盘入渗仪推求含碎石土壤饱和水力传导度(英)

在模拟土柱中，利用圆盘入渗仪对碎石对土壤饱和水力传导度的影响进行了分析。结果表明：含碎石土壤饱和水力传导度可以通过对不同负压下土壤稳定入渗速率进行非线性回归获得。含碎石土壤饱和水力传导度与去除碎石后的土壤饱和水力传导度及碎石形状指数密切相关。试验中含碎石土壤的饱和水力传导度随碎石含量的增加而呈指数降低趋势。     

Time dependence of hydraulic parameters estimation from transient analysis of mini disc infiltrometer measurements

The transient analysis of mini disc infiltrometer (MDI) measurements is an established method for characterising near-surface hydraulic characteristics of soils. The reliability of hydraulic characteristics obtained from transient analysis depends on the (1) adequacy of model, (2) adequacy of data, (3) measurement time and (4) measurement footprint. The measurement time dependence recommendations are reported only for a few soil textures, initially wet samples and tension infiltrometer (TI) with a higher measurement footprint than the MDI. This study investigated the adequacy of infiltration data (using cumulative linearization [CL] and differentiated linearization [DL]) and measurement time influence on the hydraulic parameters determined from the transient analysis of MDI measurements for six soil textures. The objective of the study is to identify suitable MDI measurement durations for different soil textures for the initially dry state, considering both adequacy of data and time fractionation (measurement time influence). The data adequacy time obtained from the DL (T_DL) was found to be 0.8 times less than the value obtained from CL (T_CL). The marginal difference in T_DL and T_CL had a significant influence on the determination of infiltration equation coefficient C₁ and negligible influence on coefficient C₂. The time fractionation procedure adopted for identifying adequate MDI measurement time (T_m) was found to be comparable based on sorptivity (S₀) and hydraulic conductivity (K₀). The average T_m was also comparable with T_DL and T_CL with a strong positive correlation. The C₂ values obtained based on T_m, T_DL, and T_CL were in better agreement than the corresponding C₁ values. The adequate MDI measurement times identified by considering T_m, T_DL, and T_CL were texture dependent, ranging from 45 min for silt to 120 min for silt loam and silty clay loam. For loamy sand, it was 50 min; for sand, it was 70 min, followed by 60 min for loam.     

基于Guelph法的土壤饱和导水率测定方法对比

为探寻该区域Ks的最优测定方法,应用Guelph入渗仪测量了川中低山丘陵区的林地和坡耕地土壤的饱和导水率,对比分析单水头法(LA法、USH法)和双水头法(TH法、BH法)所测得饱和导水率(Ks)的差异,同时比较了田间原位Guelph法与室内降水头法测定结果的差异。对于同一土层深度,不同方法估算所得的Ks有较大的差异:1)双水头TH法测得的值最大(坡耕地土层>20～40cm除外),林地0～20cm,>20～40cm土层的Ks值分别为0.134,0.266mm/min,坡耕地0～20cm土层的Ks值为0.860mm/min。单水头USH法5cm水头所得值最小,林地0～20和>20～40cm土层的Ks值分别为0.015和0.022mm/min,坡耕地对应土层则分别为0.040和0.022mm/min;2)单水头USH法10cm水头(USH2)测得的Ks大于5cm水头(USH1)所得值,采用前者所测得林地0～20和>20～40cm土层、坡耕地0～20和>20～40cm土层的Ks值分别为0.031,0.056,0.211,0.031mm/min;3)田间原位BH法和USH2法(压力水头为10cm)测定的Ks均大于室内降水头法所测得的值,这可能与室内环刀法在采样中对土壤大孔隙通道的切断与破坏、所测定土壤样品的体积较小有关。综合上述结果,并考虑到单水头法操作简便,故而该研究推荐在川中低山丘陵紫色土地区使用单水头USH2法,压力水头为10cm。     

Determining soil saturated hydraulic conductivity and sorptivity from single ring infiltration tests

The difference between the cumulative infiltration occurring during three‐dimensional axisymmetric and one‐dimensional vertical flow is a linear function of time. The slope of this line is a function of the source radius, initial and final volumetric soil water contents and the soil sorptivity. This allows the determination of the sorptivity and saturated conductivity of the soil from data of axisymmetric flow in a single ring of small diameter under negligible head of water. The method is based on the optimization of the sorptivity and saturated conductivity on the one‐dimensional vertical cumulative infiltration inferred from axisymmetric flow data. To examine the reliability of the method to determine these parameters, numerical three‐ and one‐dimensional data are generated on soils with known hydrologic properties from the literature. The linearity versus time of the difference of the two types of flow is verified. Several physically based expressions for the vertical cumulative infiltration as a function of time are considered. The optimized values of the sorptivity and saturated conductivity are compared to the their real known values. Despite the large errors on the optimized parameters, namely the saturated conductivity, the error on the vertical predicted cumulative infiltration is limited to 10%. This makes possible the application of this method on a large scale for hydrological modelling purposes.     

用自动盘式吸渗仪测定土壤导水率

盘式吸渗仪已成为测定田间土壤水力参数的重要工具之一,自动监测装置的应用将进一步提高试验的精确度并加快试验进程。该文介绍了一种试验装置,通过连接在储水管上下两端的传感器实时压差监测,以此确定入渗水量的变化过程,并通过一系列试验,对试验装置的可靠性进行了验证,将试验装置应用到实际测定土壤水力参数中。试验结果表明：自动监测装置通过测定储水管的压差,并通过相应关系代换为储水管水位变化来采集试验数据的方法是可靠的。通过与人工读数方法测定的结果对比表明,其精度可以达到试验要求;对于本试验土壤,用自动监测装置采集的数据来确定吸渗率的时间可以控制在30 s～4 min之间。     

双环入渗仪的缓冲指标对测定土壤饱和导水率的影响

双环入渗仪在测定田间土壤饱和导水率时被广泛采用。本文采用不同直径的双环入渗仪(内环直径分别为20 cm、40 cm、80 cm和120 cm)和不同的内外环直径比,即不同的缓冲指标(0.2、0.33、0.5和0.71),进行了16组定水头积水入渗试验,研究了双环入渗仪缓冲指标对土壤饱和导水率测定的影响。结果表明:内环直径较小的入渗仪,其累积入渗过程曲线的分布较分散;随着入渗环直径逐渐增大,分散范围逐渐缩小。另外,随着缓冲指标的逐渐增大,测定的土壤饱和导水率并没有明显的增大或减小趋势,但内环直径20 cm的入渗仪测定的土壤饱和导水率的波动最大,而80 cm和120 cm内径的双环入渗仪测定的土壤饱和导水率最稳定,并且始终非常接近。因此,相对于双环入渗仪内环大小或土壤非均质性的影响,双环入渗仪的缓冲指标对于土壤饱和导水率测定的影响要小。     

Estimating detailed soil water profile records from point measurements

Temporal and spatial patterns of soil water content affect many soil processes including evaporation, infiltration, ground water recharge, erosion and vegetation distribution. This paper describes the analysis of a soil moisture dataset comprising a combination of continuous time series of measurements at a few depths and locations, and occasional roving measurements at a large number of depths and locations. The objectives of the paper are: (i) to develop a technique for combining continuous measurements of soil water contents at a limited number of depths within a soil profile with occasional measurements at a large number of depths, to enable accurate estimation of the soil moisture vertical pattern and the integrated profile water content; and (ii) to estimate time series of soil moisture content at locations where there are just occasional soil water measurements available and some continuous records from nearby locations. The vertical interpolation technique presented here can strongly reduce errors in the estimation of profile soil water and its changes with time. On the other hand, the temporal interpolation technique is tested for different sampling strategies in space and time, and the errors generated in each case are compared.     

The use of horizontal piezometers for in situ measurements of hydraulic conductivity below the water table

To interpret experimental results from installations of horizontal piezometers for the purpose of obtaining in situ estimates of hydraulic conductivity of saturated soils below the water table, shape factors for a range of cylindrical cavities at the ends of piezometers were obtained using an electric analogue. Those for the particular piezometers used in a field experiment in a clay soil were obtained in the same way and found to be smaller because of the limited number of perforations in the tip forming the cavity. Hydraulic conductivity values obtained with the piezometers were similar to those obtained in laboratory measurements on undisturbed cores of the same soil.     

Ammonium decreases muskmelon root system hydraulic conductivity

Abstract

The flow of water through plant roots is controlled by two driving forces, the transpiration rate (?P) and osmotic potential difference between the soil solution and inside the root (?π), and the root system hydraulic conductivity [L_root (L?m^‐2)]. Plant water status is affected by the source of nitrogen (N) supplied to the plant. This study was undertaken to isolate the effect of ammonium (NH₄)‐N on L_root from other factors affecting water transport through plants. The effect of NH₄‐N on muskmelon (Cucumis melo L.) L_root was determined by estimating conductance at high water flux rates where osmotic effects are negligible. Ammonium decreased L_root by about 50%. At a given transpiration rate, the NH₄‐N‐induced decrease in L_root decreased leaf water potential [ψ_leaf (MPa)] which, in turn, may alter the behavior of the leaves as observed in other studies.     

Estimating leaf area distribution in savanna trees from terrestrial LiDAR measurements

Vegetation structure parameters are key elements in the study of ecosystem functioning and global scale ecosystemic interactions. The detailed retrieval of many of these parameters by direct measurements is impractical due to the quantity of plant material in trees. Terrestrial LiDAR Scanners (TLSs) have been shown to hold great potential as an indirect means of estimating plant structure parameters with a high level of detail, while some studies identified a number of challenges inherent to this approach. In this study we investigate the use of a voxel-based approach to retrieve leaf area distribution of individual trees. The approach is based on the contact frequency method applied to co-registered TLS returns from two or more scanning positions. The contact frequency was computed for voxels being 10, 30, and 50 cm in size and subsequently corrected for the influence of occlusion effects, leaf inclination, the presence of non-photosynthetic material, and the laser beam size. The leaf area of voxels for which occlusion effects were too pronounced was estimated using modeled values based on the availability of light. We compared the TLS derived leaf area estimates against direct measurements, obtained by the harvesting of leaves, in a broad-leaved savanna of central Mali. The measured leaf area values of the sampled trees ranged from 30 to 530 m², and crown LAI values between 0.8 and 7.2. The leaf area estimates lay on average 14% from the reference measurements (general bias). Our method provides vertical as well as radial distributions of leaf area in individual trees, and lends itself to the estimation of savanna vegetation structural parameters with a high level of detail.     

Estimating the saturated hydraulic conductivity in a spatially variable soil with different permeameters: a stochastic Kozeny–Carman relation

The spatial variability of the saturated hydraulic conductivity (K_s) of a greenhouse banana plantation volcanic soil was investigated with three different permeameters: (a) the Philip-Dunne field permeameter, an easy to implement and low cost device; (b) the Guelph field permeameter; (c) the constant head laboratory permeameter. K_s was measured on a 14×5 array of 2.5 m×5 m rectangles at 0.15 m depth using the above three methods. K_s differences obtained with the different permeameters are explained in terms of flow dimensionality and elementary volume explored by the three methods. A sinusoidal spatial variation of K_s was coincident with the underlying alignment of banana plants on the field. This was explained in terms of soil disturbances, such as soil compaction, originated by management practices and tillage. Soil salinity showed some coincidence in space with the hydraulic conductivity, because of the irrigation system distribution, but a causal relationship between the two is however difficult to support. To discard the possibility of an artefact, the original 70 point mesh was doubled by intercalation of a second 14×5 grid, such that the laboratory K_s was finally determined on a 140 points 2.5 m×2.5 m square grid. Far from diluting such anisotropy this was further strengthened after inclusion of the new 70 points. The porosity (φ) determined on the same laboratory cores shows a similar sigmoid trend, thus pointing towards a plausible explanation for such variability. A power-law relationship was found between saturated hydraulic conductivity and porosity, K_sφⁿ (r²=0.38), as stated by the Kozeny–Carman relation. A statistical reformulation of the Kozeny–Carman relation is proposed that both improved its predictability potential and allows comparisons between different representative volumes, or K_s data sets with different origin. Although the two-field methods: Guelph and Philip-Dunne, also follow a similar alignment trend, this is not so evident, suggesting that additional factors affect K_s measured in the field. Finally, geostatistical techniques such as cross correlograms estimation are used to further investigate this spatial dependence.     

Comparison of unconfined and confined unsaturated hydraulic conductivity

The field tension infiltrometer (TI) and the laboratory unit hydraulic gradient (UHG) methods are widely applied to determine the near-saturated soil hydraulic conductivity, K. Comparison between the two methods is relevant given that they differ in the explored soil volume (undetached or detached) and in the flow process (unconfined or confined). The objective of this investigation was to compare unconfined and confined measurements of unsaturated hydraulic conductivity. Twenty TI experiments were conducted in a relatively coarse-textured soil having an appreciable hysteretic behavior by using two different dry-to-wet-to-dry (DTWTD) sequences of pressure head, h₀, values that differed by the highest h₀ value imposed within the sequence (i.e. h₀ = − 150, − 75, − 30, + 5, − 30, − 75, − 150 in site A or h₀ = − 150, − 75, − 30, − 10, − 30, − 75, − 150 in site B). The same pressure head sequences were applied on twenty undisturbed soil cores, collected at the exact location of the TI measurements, to perform the laboratory UHG measurements. Regardless of the type of experiment (i.e. unconfined or confined) and the applied pressure head sequence (i.e. site A or B), higher K₀ values were obtained with a drying sequence of h₀ values (K_0,d) than with a wetting one (K_0,w)and the discrepancies between K_0,w and K_0,d decreased as the imposed h₀ value increased, as it was expected due to hysteresis. A tendency of the UHG method to overestimate the K₀ values was detected (ratios of mean K_0,1D to mean K_0,3D values ranging from 0.93 to 4.35), but the statistical significance of the observed differences varied with the considered sequence of pressure head values. It was concluded that both the TI and the UHG methods were effective in detecting hysteresis effects on K₀, but the laboratory method resulted in K₀ values that were higher and more variable probably as a consequence of a more substantial effect of macropore flow on the measured flow rates.     

土壤饱和导水率的田间测定

本文简述了圆盘渗透仪(disc permeameter) 在田间条件下测定土壤饱和导水率的原理及方法.该方法在测定时田间土壤饱和导水率附加了一个负压Ψo,因而可以控制土壤入渗孔隙的孔径大小、排除土壤裂缝和蚯蚓孔洞对测定的影响,具有操作简便,测定精度高等优点.     

Estimating the number of true null hypotheses from a histogram of p values

In an earlier article, an intuitively appealing method for estimating the number of true null hypotheses in a multiple test situation was proposed. That article presented an iterative algorithm that relies on a histogram of observed p values to obtain the estimator. We characterize the limit of that iterative algorithm and show that the estimator can be computed directly without iteration. We compare the performance of the histogram-based estimator with other procedures for estimating the number of true null hypotheses from a collection of observed p values and find that the histogram-based estimator performs well in settings similar to those encountered in microarray data analysis. We demonstrate the approach using p values from a large microarray experiment aimed at uncovering molecular mechanisms of barley resistance to a fungal pathogen.     

Scaling the saturated hydraulic conductivity of an alfisol

Alfisols exhibit a high degree of spatial variability in their physical properties. As a result, it is difficult to use information on physical parameters measured at one location to model larger-scale hydrologic processes. In this study, the saturated hydraulic conductivity, K_S , of an Alfisol was determined on 109 undisturbed monoliths using the falling-head permeameter method. The model developed by Arya & Paris (1981) was used to calculate the pore volume from sand and clay fractions. Scaling factors were calculated from the measured K_s , sand pore-volume, clay pore-volume, clay content and effective porosity, using the similar media concept. Prediction of K_s of gravelly Alfisol using clay pore-volume is confounded by high gravel content which, when discounted, improves the prediction remarkably. The scaled mean saturated hydraulic conductivity K* for all horizons of the Alfisol was approximately l.0x 10⁻⁵ms⁻¹.     

植被群落演替对土壤饱和导水率的影响

土壤饱和导水率是表征土壤入渗能力的重要参数,对不同土地利用类型反应敏感。为了揭示植被演替对土壤剖面上饱和导水率的影响规律,采用恒定水头法测定了天童林区155 a植物群落演替序列60 cm深土壤剖面上的饱和导水率。结果表明,不同演替阶段饱和导水率均随土壤深度增加迅速降低,在0～20 cm土层内,各演替阶段饱和导水率均存在极显著差异,0～60 cm土层内饱和导水率的平均值从裸地、石栎+檵木灌丛、马尾松林、木荷+马尾松林、木荷林到栲树林升高极为显著,植物群落演替到灌丛阶段,平均饱和导水率已与裸地存在显著差异,演     

Evaluation method dependency of measured saturated hydraulic conductivity

Saturated hydraulic conductivity (Ks) is one of the most important hydraulic properties affecting water flow in soils. Spatial and seasonal variability as well as scale dependency are key factors which make it more difficult to accurately measure the saturated hydraulic conductivity. The uncertainty of the Ks values due to different evaluation methods was investigated using raw measured data obtained on two different Hungarian soils with three different in situ measuring devices (double ring, tension disc and mini disc infiltrometers), as well as with two laboratory methods. Since the very same raw infiltration data could result in significantly different Ks values, we have introduced the evaluation method dependency of the measured Ks values. Our investigations found that the effect of the applied evaluation method for assessing raw measured data can be just as significant as the effect of other factors, such as the scale effect, as well as the spatial and temporal variability.