Überprüfung eines Schätzverfahrens zur Ermittlung von Kennwerten der Wasserbindung anhand der Labordatenbank des Niedersächsischen Bodeninformationssystems

Testing a method for estimating water retention parameters using the laboratory database of the Lower Saxony Soil Information System The validity of the method used for estimating field capacity (pF > 1.8), plant available water (pF 1.8 – 4.2), air capacity (pF < 1.8), and total pore volume from soil texture, packing density (bulk density + 0.009 % clay) and humus content described by the Arbeitsgruppe Bodenkunde (1982) was checked on the basis of 1693 pF curves of the laboratory database of the Lower Saxony Soil Information System (NIBIS). The positive and negative corrections for humus content applied in this method to the above parameters are clearly too small. Use of tables for estimating the pore volume of humus-free soils leads to overestimation. It will only be possible to work out an alternative method applicable to all classes of soils when the database has been extended.     

两种非饱和导水参数推求方法在紫色土上的应用

土壤非饱和导水参数的难于获取,限制了非饱和水流数值模拟技术的实际应用。本研究选择颇具代表性的土壤水分特征曲线推求法和简单入渗法两种方法,分别推求不同质地紫色土导水参数,并进行了分析比较。结果显示,土壤水分特征曲线推求法和简单入渗法,推求的非饱和导水率与计算值均具有较好的一致性。鉴于土壤水分特征曲线的易测优势和简单入渗法实验简便省时特点,采用这两种方法进行紫色土非饱和导水参数的推求或预报是可行的。     

两种非饱和导水参数推求方法在紫色土上的应用

土壤非饱和导水参数的难于获取,限制了非饱和水流数值模拟技术的实际应用。本研究选择颇具代表性的土壤水分特征曲线推求法和简单入渗法两种方法,分别推求不同质地紫色土导水参数,并进行了分析比较。结果显示,土壤水分特征曲线推求法和简单入渗法,推求的非饱和导水率与计算值均具有较好的一致性。鉴于土壤水分特征曲线的易测优势和简单入渗法实验简便省时特点,采用这两种方法进行紫色土非饱和导水参数的推求或预报是可行的。     

Evaluation of porous medium hydraulic properties using experimental methods and RETC code

Two experimental procedures were used to determine both hydraulic properties, soil water retention θ(h) curve and unsaturated hydraulic conductivity K(θ), of a sand sample. Knowledge of hydraulic properties is essential, since they generally control soil water dynamics. A steady-state laboratory method was used for the simultaneous determination of θ(h) and K(θ). A one-step outflow method was used for the determination of diffusivity D(θ) and subsequently K(θ) from soil water retention data which were measured independently on the same sample and using the same apparatus. The comparison of K(θ) measured values from the above-mentioned methods showed very good agreement of the results. Also, the comparison between the experimental K(θ) and θ(h) functions and the predictions obtained using retention curve (RETC) code by simultaneous fit of experimental soil water retention and hydraulic conductivity data from outflow data, assuming the Mualem-van Genuchten model, showed very good agreement. It is noted that the main disadvantage of the one-step outflow method is the weakness to predict K(θ) values near saturation. This disadvantage could be overcome using RETC code with the above procedures, since the K(θ) values between the predictive approach and the steady-state method were similar.     

保护性耕作对黄土旱塬箭筈豌豆地土壤呼吸的影响

[目的]揭示保护性耕作对土壤呼吸的影响,为旱区保护性农业的发展提供理论依据。[方法]采用多通道土壤碳通量系统监测传统耕作(T)、传统耕作+秸秆覆盖(TS)、免耕(NT)和免耕+秸秆覆盖(NTS)下箭筈豌豆(Vicia sativa)地的土壤呼吸速率。[结果]各措施下花期呼吸速率比收获期高10.45%~45.09%,NTS处理下土壤呼吸速率最低别比TS,NT和T处理显著减少39.17%,21.37%和30.25%(p0.01)。耕作处理(T,TS)下日均土壤呼吸速率高于免耕处理(NT,NTS)(p0.05)。晴天土壤呼吸变化呈单峰曲线,最大值出现在14:00。耕作下土壤呼吸速率与气温显著线性相关,免耕下(NT,NTS)与气温呈指数关系(p0.01)。不同耕作措施间气温敏感性Q10值大小依次为:T(1.97)NT(1.62)TS(1.58)NTS(1.52)。[结论]免耕加秸秆还田处理对减少温室气体排放有一定的贡献。     

黄土区两种植物篱不同部位土壤持水特征对比

[目的]研究植物篱不同部位土壤持水性能的分异特征,指导植物篱系统的设计与经营管理。[方法]在测定柠条、山杏两种典型植物篱土壤水分物理参数的基础上,利用van Genuchten模型比较分析植物篱系统带内、带间的土壤水分特征曲线及其参数。[结果](1)两种植物篱在土壤总孔隙度、水稳性团聚体含量、渗透系数等方面均高于耕地(对照),其中柠条植物篱带内土壤总孔隙度(66.36%)、水稳性微团聚体含量(35.50%)和有机质含量(44.1g/kg)比带间分别高7.34%,1.95%和25.64%。山杏植物篱的这3项指标带内比带间分别高11.93%,29.33%和21.48%。(2)柠条、山杏植物篱带内土壤脱湿过程中凋萎系数分别是耕地的7倍和3.25倍,田间持水量分别是耕地的1.46和1.25倍,柠条植物篱土壤保水优势更明显。(3)在相同的土壤水吸力下,土壤吸湿与脱湿过程中带内土壤的水分含量均明显高于带间,带内土壤的持水性更高,这与带内较高的黏粒含量相一致;柠条带内土壤水分含量高于山杏带内,二者均高于耕地。[结论]植物篱的二相结构能明显促进带内黏粒、微团聚体和养分的富集(有机质),进而提高带内土壤的持水性能和饱和渗透性能。     

Estimating the wetting branch of the soil water retention curve from grain-size fractions

The development of a theoretical method for estimating the wetting branch of the soil water retention curve (SWRC) is required for knowledge of the soil infiltration process. The aim of this study was to derive the theoretical functions to represent the wetting branch of the SWRC based on the Jensen method, and then compare the Jensen method and Kool & Parker (KP87) model for estimating the wetting branch of the SWRC. Fifteen soil samples with varying basic properties (e.g., grain-size distribution and bulk density (BD)) were selected from the Unsaturated Soil Hydraulic Database (UNSODA) to test these two methods. Results showed that the Jensen method (root mean squared error (RMSE) = 0.057 cm³ cm⁻³) produced a substantially better performance in predicting the wetting branch of the SWRC than the KP87 (RMSE = 0.089 cm³ cm⁻³) for the 15 samples. The range of the scaled mean bias error (SMBE) between the Jensen method-predicted and measured soil water contents at all pressure heads was −0.529 to 0.402. A positive linear relationship between the SMBE and silt content was observed for the Jensen method. The findings of this study have practical significance for simulating the soil infiltration in the unsaturated zone.     

The spatial variability of soil nitrates in arable and pasture landscapes: implications for the development of geographical information system models of nitrate leaching

Abstract. In response to the European Community Nitrate Directive (91/676) a catchment scale Geographical Information System (GIS) model of nitrate leaching has been developed to map nitrate vulnerability and predict average weekly fluxes of nitrate from agricultural land units to surface water. This paper presents a pilot study which investigated the spatial variability of soil nitrates in order to: (1) define an appropriate pixel size for modelling N leaching; (2) quantify the within-unit variability of soil nitrate concentrations for pasture and arable fields; and (3) assist in the design of an efficient sampling strategy for estimating mean nitrate concentrations. Soil samples, taken from two 800 m transects in early September 1994, were analysed for water soluble nitrate. The arable soils had a mean nitrate-nitrogen concentration of 0.693 μg/g (S.E. 0.054 μg/g) and the pasture soils had a higher mean nitrate-nitrogen concentration of 0.86 μg/g (S.E. 0.085 μg/g). Spatial variability was investigated using variograms. The pasture data had a weak spatial relationship, whereas the arable data exhibited a strong spatial relationship which fitted a spherical variogram model (r² 0.87), with a range of 40 m. A pixel size of 40 m is suggested for nitrate modelling within the GIS based on the arable variogram and an improved sampling strategy for model validation is suggested, involving bulking sub-samples over a 40 m grid for estimating mean nitrate concentrations in combined land use and soil units.     

基于土壤质地数据估算土壤保水性能Van Genuchten参数

The van Genuchten (vG) function is often used to describe the soil water retention curve (SWRC) of unsaturated soils and fractured rock. The objective of this study was to develop a method to determine the vG model parameter m from the fractal dimension. We compared two approaches previously proposed by van Genuchten and Lenhard et al. for estimating m from the pore size distribution index of the Brooks and Corey (BC) model. In both approaches we used a relationship between the pore size distribution index of the BC model and the fractal dimension of the SWRC. A dataset containing 75 samples from the UNSODA unsaturated soil hydraulic database was used to evaluate the two approaches. The statistical parameters showed that the approach by Lenhard et al. provided better estimates of the parameter m. Another dataset containing 72 samples from the literature was used to validate Lenhard’s approach in which the SWRC fractal dimension was estimated from the clay content. The estimated SWRC of the second dataset was compared with those obtained with the Rosetta model using sand, silt, and clay contents. Root mean square error values of the proposed fractal approach and Rosetta were 0.081 and 0.136, respectively, indicating that the proposed fractal approach performed better than the Rosetta model.     

Point pedotransfer functions for prediction of water retention of selected soil series in a semi-arid region of western Iran

This study was conducted to derive point pedotransfer functions (PPTFs) for soil water retention (SWR) in western Iran. Topsoil and subsoil of 63 soil series, which were representative of different regions of Hamadan province, were sampled. Soil water retention was determined by the sand box and pressure plate at matric suctions (h _m) of 0, 1, 2, 5, 10, 25, 50, 100, 200, 500, 1000 and 1500 kPa. PPTFs were derived through multiple linear regressions for the topsoils and subsoils. These used particle size distribution, bulk density, organic matter, calcium carbonate and gravel contents as easily-available inputs. To increase the accuracy of the PPTFs, saturated water content was also included as an input variable in a group of PPTFs but they are not better as assessed using the Akaike Information Criterion. All of the PPTFs were statistically significant (p < 0.001) and could be used to predict the SWR. The absolute effect of bulk density on the SWR diminished as h _m increased. Bulk density decreased the SWR for low h _m and increased it for high h _m. In the wet range, organic matter increased the SWR. Clay and silt increased SWR whereas gravel decreased it. The effect of calcium carbonate on SWR was negligible.     

Water repellency of organic growing media related to hysteretic water retention properties

With respect to soils, most growing media can exhibit hysteresis during drying/wetting cycles, which greatly affects their hydraulic properties. In the case of organic substrates, hydrophobicity during desiccation could be considered as one of the main factors leading to hysteretic behaviour. The purpose of this study was to estimate the influence of changes in wettability on the water retention properties, θ(ψ), of peat and pine bark during a drying/wetting cycle. Major differences in the hydraulic behaviour of the two organic materials studied were observed. For peat, hysteresis was found in the water retention curve (21%) and also in the contact angle/water potential relationship, (α(ψ), 20%), whereas in pine bark, this phenomenon was less pronounced in the water retention curve (10%) and even more limited in the α(ψ) curve (> 5%). Water retention hysteresis was successfully modelled using a modified van Genuchten‐Durner approach (VG_α model), which took into account the local hydrophobicity of each poral domain of the porous media, regardless of the extent of hysteresis. Incorporating the parameters of the VG_α water retention model into a α(ψ) equation to characterize overall or average changes in the hydrophobicity of the material during desiccation resulted in values very similar to those of the contact angles calculated with the capillary rise method. These results indicate that water retention properties of these organic substrates are strongly influenced by hydrophobicity.     

A world dataset of derived soil properties by FAO–UNESCO soil unit for global modelling

A standardized dataset of derived soil properties for the 106 soil units considered on FAO-UNESCO's 1:5 million scale Soil Map of the World is presented. It was derived from a statistical analysis of the 4353 soil profiles held in the WISE (World Inventory of Soil Emission) database, which was developed at the International Soil Reference and Information Centre (ISRIC) for the geographic quantification of soil factors that control processes of global change. Median values are presented by soil unit for selected soil properties including: pH(H₂O); organic carbon content; cation exchange capacity; sum of exchangeable Ca²⁺, Mg²⁺ and K⁺; exchangeable sodium percentage; bulk density; total porosity; available water capacity; soil drainage class; and gravel content class. Medians for these variables are presented both for the topsoil (0–30 cm) and subsoil (30–100 cm), where applicable. The data set can help to refine ratings for soil quality in global environmental models pending the availability of comprehensive georeferenced databases on soil and terrain resources such as SOTER, the World Soil and Terrain Database. In a Geographical Information System (GIS) it can be linked to the units shown on the digital Soil Map of the World through the legend code.     

Evaluation of similar-media scaling and a one-parameter model for estimating the soil water characteristic

Two simple methods for estimating the soil water characteristic curve from saturated soil water content, or even just the soil bulk density, plus one other measured point on the curve (such as the water content at a matric potential of –33 kPa) were evaluated. A similar-media scaling approach and the one-parameter model of Gregson et al. were compared for eight diverse soils from within the USA. The scaling method also requires knowledge of one complete curve to serve as a reference. The one-parameter model was extended to include the residual water content, and the soils were divided into three textural groups for the analysis. Visual comparison of the calculated vs. measured soil water content for both methods showed less scatter in the relationship to the 1:1 line, concomitant with smaller calculated error terms, for the one-parameter model. These results demonstrate that the one-parameter model is better than similar-media scaling in estimating soil water content.     

Comparison of different measuring and calculating methods to quantify the hydraulic conductivity of unsaturated soil

The double-plate method and the evaporation method provide comparable results on the hydraulic conductivity of soil (K(h_s)). Adaptation of the water retention curve according to the method of Vachaud and Vauclin proved to be very reliable. The course of the K(h_s) function is well shown by all indirect identification methods included in the comparison. In some cases, the absolute deviations of the measured from the calculated values are, however, considerable and primarily determined by the initial value of the hydraulic conductivity. In quantifying the capillary water rise, best approximation to the measured values was reached in 11 of 13 cases with the van Genuchten method.     

漓江上游猫儿山3种典型植被不同层次土壤的含水量

[目的]揭示漓江上游森林植被对降水产流的调节作用,客观评估漓江上游水资源潜力,为流域水资源管理和森林经营提供科学依据。[方法]以漓江上游猫儿山林区的毛竹林(Phyllostachys pubescens)、荷木林(Schima superba)和杉木林(Cunninghamia lanceolata)3种典型森林植被为研究对象,比较不同层次土壤含水量及其影响因素。[结果](1)由于荷木林冠层结构复杂,与毛竹林和杉木林相比,荷木林蓄水能力更强;毛竹林、荷木林和杉木林3种植被垂直结构明显,导致土壤含水量垂直变化趋势不同;(2)荷木林土壤含水量都在高值变化,而杉木林土壤含水量都在低值变化;各层次土壤含水量与年降雨分配关系密切,随着雨旱两季变化,毛竹林、荷木林和杉木林不同层次土壤水分随降雨变化趋势基本一致;(3)毛竹林土壤孔隙度大于木荷林和杉木林,但毛竹林的浅根性和速生性对表层土壤水含量影响更大。[结论]大气降水是土壤含水量变化的主要因素,土壤含水量是降雨与植被垂直结构及蒸腾作用共同作用的结果。     

封育年限对干旱河谷灌丛土壤理化性质的影响

[目的]探讨不同封育年限对干旱河谷灌丛土壤理化性质的影响,为该地区的天然植被恢复与管理提供理论依据。[方法]采用野外调查和实验室分析相结合的方法开展研究。[结果](1)封育后灌木层的盖度、密度、平均冠幅、高度明显升高,而草本层则相反。(2)随着封育年限的增加,土壤容重呈V形变化,封育13a达到最小值,而群落植被特征、土壤孔隙度、土壤持水量、土壤养分,呈倒V形变化,封育13a达到最大值。0—40cm,封育13a的土壤平均速效N,P,K较未封育草地分别增加26.42%,107.35%,33.92%;全量N,P,K含量分别增加了168.42%,47.62%和31.12%。(3)封育后,土壤理化性质有所改善,具有明显的表聚现象,土壤固碳能力有所提高,土壤有机碳密度表现为:F_(13)F_(22)F_(30)F_5CK;土壤有机碳储量表现为:F_5F_(13)F_(22)F_(30)CK,说明在狼牙刺群落封育恢复过程中,土壤环境有较为明显的改善。(4)采取封育措施后,外界干扰减少,狼牙刺灌丛覆盖度增加,从而增加地上生物量,使生产力得到提升,生物量与封育年限存在线型关系。[结论]采取封育措施后,干旱河谷地带的狼牙刺群落植被特征和土壤理化性质及有机碳密度和储量均优于未封育,植被自然恢复状态良好。     

Alternative methods in the development of pedotransfer functions for soil hydraulic characteristics

Soil hydraulic properties are needed in the modeling of water flow and solute movement in the vadose zone. Pedotransfer functions (PTFs) have received the attention of many researchers for indirect determination of hydraulic properties from basic soil properties as an alternative to direct measurement. The objective of this study was to compare the performance of cascade forward network (CFN), multiple-linear regression (MLR), and seemingly unrelated regression (SUR) methods using prediction capabilities of point and parametric PTFs developed by these methods. The point PTFs estimated field capacity (FC), permanent wilting point (PWP), available water capacity (AWC), and saturated hydraulic conductivity (Ks) and the parametric PTFs estimated the van Genuchten retention parameters. A total of 180 soil samples was extracted from the UNSODA database and divided into two groups as 135 for the development and 45 for the validation of the PTFs. The model performances were evaluated with three statistical tools: the maximum error (ME), the model efficiency (EF), and the D index (D) using the observed and predicted values of a given parameter. Despite the fact that the differences among the three methods in prediction accuracies of the point and parametric PTFs were not statistically significant (p > 0.05) except θ_r and α (p < 0.05) based on the ANOVA test, overall MLR and SUR were somewhat better than CFN in prediction of the point PTFs, whereas CFN performed better than the other two methods in prediction of the parametric PTFs. The F.F values of FC and θ_r for CFN, MLR, and SUR methods were 0.705. 0.805, 0.795 and 0.356, −0.290, −0.290, respectively, which refer to the best and worst predictions. Properties (Ks, θ_r, α) having some difficulty in prediction were better predicted by CFN and SUR methods, where these methods predict all hydraulic properties from basic soil properties simultaneously rather than individually as in MLR. This suggests that multivariate analysis using such functional relationships between hydraulic properties and basic soil properties can be utilized in developing more accurate point and parametric PTFs with less time and effort.     

从连续的粒径大小分布数据估算土壤水分特征曲线

Soil moisture characteristic curve (SMC) is a fundamental soil property and its direct measurement is tedious and time consuming. Therefore, various indirect methods have been developed to predict SMC from particle-size distribution (PSD). However, the majority of these methods often yield intermittent SMC data because they involve estimating individual SMC points. The objectives of this study were 1) to develop a procedure to predict continuous SMC from a limited number of experimental PSD data points and 2) to evaluate model predictions through comparisons with measured values. In this study, an approach that allowed predicting SMC from the knowledge of PSD, parameterized by means of the closed-form van Genuchten model (VG), was used. Through using Mohammadi and Vanclooster (MV) model, the parameters obtained from fitting of VG to PSD data were applied to predict SMC curves. Since the residual water content (θ r ) could not be obtained through fitting of VG-MV integrated model to PSD data, we also examined and compared four different methods estimating θ r . Results showed that the proposed equation (MV-VG integrated model) provided an excellent fit to all the PSD data and the model could adequately predict SMC as measured in forty-two soils sampled from different regions of Iran. For all soils, the method in which θ r was obtained through parameter optimization procedure provided the best overall predictions of SMC. The two methods estimating θ r with Campbell and Shiozawa (CS) model resulted in less accuracy than the optimization procedure. Furthermore, the proposed model underestimated the moisture content in the dry range of SMC when the value of θ r was assumed to equal zero. θ r could be attributed to the incomplete desorption of water coated on soil particles and the accurate estimation of θ r was critical in prediction of SMC, especially for fine-textured soils at high suction heads. It could be concluded that the advantages of our approach were the continuity, robustness, and independency of model performance on soil type, allowing to improve predictions of SMC from PSD at the field and watershed scales.