Determining leaching fraction from field measurements of soil electrical conductivity

Theory for determining leaching fraction from measurements of soil electrical conductivity is presented with experimental data to substantiate its validity.     

耕地土壤有机质空间变异性的随机模拟

有机质是土壤重要的肥力特征,研究盐渍土改良区耕地土壤有机质空间变异特征可为土壤质量提升提供科学依据。以山东省禹城市盐渍土改良区典型地块耕地土壤有机质为研究对象,在全面野外调查和室内化验分析以获得大量的土壤有机质相关信息的基础上,运用地统计学方法对有机质进行了序贯高斯模拟各次实现（SGSV）、序贯高斯模拟平均实现（SGSA）和ordinary Kriging插值（OK）,并将SGSV、SGSA、OK与实测数据进行了统计参数、变异函数、空间分布趋势等方面进行了对比分析。结果表明OK、SGSA改变了有机质数据的空间结构,具有“平滑”效应,SGSA在消除平滑影响方面优于Kriging插值;SGSV具有与实测数据相同的空间自相关结构,对预测点的模拟值具有不确定性,为揭示研究区域土壤有机质的空间结构特征提供了有力的工具,对盐渍土改良区土壤有机质空间不确定性的风险研究具有更实际的意义。     

基于EM38-MK2的滨海土壤电导率精确解译模型

以黄河三角洲为研究对象,通过土壤样品分层采集进行室内分析获得土壤电导率(1∶5)及采用EM38-MK2在采样点测量水平和垂直模式下土壤表观电导率,应用回归及多元逐步回归模拟分析的方法,研究土壤表观电导率与不同深度土层土壤电导率的相关性,并进行精度验证,以获得基于新型电磁感应仪的不同剖面深度处土壤电导率精确解译模型,实现区域尺度土壤盐分在不同剖面深度处空间分布格局快速评定.研究结果表明:多模式下测定的土壤表观电导率与不同土层土壤电导率呈现不同程度的显著性相关;联合多模式下土壤表观电导率解译土壤电导率的精度高于单一模式;表层土壤电导率协同土壤表观电导率可以提高亚表层及底层土壤电导率的预测精度.本研究可以为快速调查土壤盐渍化及盐渍土改良与利用提供思路,以节约成本和提高效率.     

土壤水盐空间异质性及尺度效应的多重分形

为了揭示研究区域林地内土壤含水率和电导率的空间分布特征及尺度效应,利用多重分形方法,对杨凌一林地内不同采样时间和不同采样面积下土壤含水率和电导率的空间异质性进行了研究。结果表明：3种采样面积下土壤含水率和电导率的空间异质性都分别随平均含水率和电导率的增大而减弱。随采样面积的增大,平均含水率和电导率较高时,土壤含水率的空间异质性趋于增强,土壤电导率的尺度效应不明显;平均含水率和电导率较低时,土壤含水率和电导率的空间分布都存在明显的斑块结构。不同采样时间和不同采样面积下土壤含水率和电导率的多重分形谱的形态有所差异,表明引起他们空间异质性的信息有所不同。多重分形分析能揭示出较多的采样林地内土壤含水率和电导率分布的局部信息。     

土壤空间变异对水氮淋失和产量影响的模拟研究

基于CERES-Maize模型,研究了土壤空间变异和水文年型对半干旱地区土壤水氮淋失和玉米产量的影响.结果表明,土壤空间变异对作物产量和土壤水氮淋失的影响程度与降雨密切相关.丰水年水氮淋失量显著高于平水年和枯水年.降雨对作物产量和农田尺度水氮淋失的空间变异有明显影响,并能在一定程度上减弱土壤空间变异对产量和农田尺度水氮淋失的影响.随着土壤空间变异程度的增大,产量降低,产量的空间变异程度增加.水分渗漏和氮淋失量随土壤空间变异的增加呈增加趋势.当土壤黏粒和粉粒含量变异系数CV≥0.2时,在水氮管理中考虑土壤空间变异有利于提高作物产量,减轻水氮淋失.     

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     

生物结皮对土壤水气传输特性的影响

为研究生物结皮对土壤气体传输、水力传导特性的影响,在陕北水蚀风蚀交错区,以4种处理土样(无结皮、去结皮、3 a生结皮和7 a生结皮)为研究对象,进行土壤导气率、饱和导水率和入渗速率的测定分析.结果表明:在土壤吸湿和脱湿过程中,相同土壤含水率所对应的土壤导气率值不同,且脱湿过程的导气率值高于吸湿过程;生物结皮导致土壤导气率降低;由于生物结皮的存在,土壤总孔隙度和毛管孔隙度增加,在增强土壤持水性的同时降低了土壤的导水性;生物结皮生长年限越久,土壤水分入渗的能力显著降低;3种入渗模型中Horton模型的决定系数R²的值大于同处理下其他模型,该模型更适合于用来模拟该区域生物结皮覆盖下的土壤入渗特征.     

Assessing temporal stability and spatial variability of soil water patterns with implications for precision water management

The temporal stability of soil water content patterns may have profound implications for precision agriculture in general and water management in particular. Spatio-temporal variability in soil water was assessed over four fields in a two-year potato (Solanum tuberosum L.) and barley (Hordeum vulgare L.) rotation to determine the potato yield implications and the potential for precision water management based on a stable spatial pattern of soil water. A hammer-driven time domain reflectometry probe was used to measure soil water content repeatedly along 10 transects. Irrigated, un-irrigated, and late irrigated treatments were employed. The temporally stable soil water pattern was mapped and compared with elevation and soil particle size classifications. A temporal stability model explained 47% of the observed variability in soil water content. An additional 20% of the variability was attributed to random measurement error. Calibrated in 2002, the model predicted water content (root mean square error of 0.05 m³ m⁻³) along transects in 2003 from a single measurement at the field edge. Field-scale trends and extended (>100 m) wet and dry segments were observed along transects. Coarser particle size class soils were generally drier. Potato yield increased linearly with water content in un-irrigated areas. Yield was comparatively high in the drier areas for the irrigated treatment but was highly variable and frequently poor in the wetter areas. For the late-irrigated treatment, a strong yield response to added water was evident in the dry areas; however, the yield response was neutral to negative in the wetter areas. Knowledge of the underlying stable soil water distribution could provide a useful basis for precision water management.     

Impacts of spatial variability of basins microtopography on irrigation performance

Land microtopography unevenness is a key variable affecting basin irrigation performances. Using stochastic modeling, a number of sets of spatially variable surface elevations were generated, and a two-dimensional basin irrigation model was used to simulate irrigation for the generated sets. Strip, narrow and wide basins, as well as graded and zero-levelled basins were analyzed. Results show that spatial variability of basin microtopography influences the infiltrated depth when the advance is completed (Z _adv) and the irrigation uniformity (DU_lq). When the degree of unevenness increases, the Z _adv value and its range of variation also increase, thus indicating that overirrigation increases with unevenness, mainly when zero leveling is adopted, inflow rates are small, and basin length is larger. Differently, DU_lq is relatively small and insensitive to unevenness in case of graded basins, but is much larger and sensitive in case of zero leveling. This indicates that when water saving is aimed, it is preferable to adopt graded basins and shorter cut-off times, while it is better to adopt zero leveling and high inflow rates when high DU_lq is pretended.     

Drainage and desalinization of heavy clay soil in Portugal

The Leziria Grande area consists mainly of poorly drained, saline clay soils of marine origin. Three experimental fields were laid out to find whether subsurface drainage can be effective in lowering the groundwater table and improving desalinization.Subsurface drainage results in a lower groundwater table than does surface drainage. With increasing spacing, the groundwater remains at a higher level for longer periods, which is expressed here by the sum of exceedances of the groundwater table above 30 cm during winter.Soil salinity, expressed as EC_1:2, and sodicity, expressed as E.S.P., decreased during the first 3 years, in which precipitation varied between 600 and 750 mm and the average drain outflow was about 250 mm. The leaching efficiency decreased with time, indicating that the removal of salt is a slow process in fine-textured soil.Application of gypsum lowered the E.S.P. The infiltration rate and the drain outflow increased. Although the total amount of salts in the drainwater was 40% higher than for the untreated plots, no lower EC_1:2 values were found. This is ascribed to spatial variability in soil salinity.     

Shallow subsurface drainage in an irrigated vertisol with a perched water table

Waterlogging and salinity are reducing the productivity of irrigated agriculture on clay soils in south east Australia. We compared five drainage treatments: (1) undrained control (Control); (2) mole drains (Mole); (3) mole drains formed beneath gypsum-enriched slots (GES) (Mole + GES); (4) shallow pipe drains installed beneath GES (Shallow Pipe); (5) deep pipe drains (Deep Pipe). The experiment was set out on a vertisol and our measurements were made during the growth of an irrigated onion crop.

Over the 3 months before the spring irrigations commenced, the perched water table on the Control was less than 400 mm below the soil surface for 27% of the time, whereas the shallow drainage treatments (Treatments 2, 3 and 4) reduced this time to less than 4%. During the irrigation season, the perched water table on the Mole + GES treatment rose above 400 mm for 3% of the time. The perched water table on the Mole treatment was above 400 mm for 14% of the time, compared with 19% of the time on the Control. The Deep Pipes were less effective in reducing the depth to the perched water table, both before and during the irrigation period.

Mole drains increased the gas-filled porosity above the drains. However, the gas-filled porosity remained below reported levels for optimum root growth. Although the drains effectively drained excess water, and lowered the water table, the hydraulic gradient was insufficient to remove all of water from the macropores. Gypsum enriched slots above the mole drains increased the gas-filled porosity in the slots but the drainable porosity in the undisturbed soil appeared to be inadequate for optimum root growth, even though some drainage occurred near the slots.

Discharge from the shallow drainage treatments averaged 58 mm for each irrigation, and was considerably more than the amount required to drain the macropores. The mole channels were in reasonably good condition at the end of the irrigation season, with at least 70% of the cross-sectional area of the channel open.

Shallow subsurface drains increased onion yield by about 38%. For each day the water table was above 400 mm, the yield declined by 0.23 tonnes per hectare. Farmer adoption of shallow subsurface drainage will depend on the long-term economic benefits (influenced by the longevity of the mole channels and yields response) and the need to develop more sustainable management practices.     

Drainage models to predict soil water regimes in drained soils: a UK perspective

Drainage is an intervention in the natural hydrology of the soil to alter the duration of adverse (waterlogged) soil conditions. The effects of drainage can be investigated by models that predict the position of the water table at a site in the presence of drainage. An inter-related series of models, which include the van Schilfgaarde non-steady state model, that have been used in the UK for the evaluation of drainage design options, are described. A simplified form of the van Schilfgaarde equation is presented, equivalent to a standard time series model, allowing both the efficient implementation of the model, and the inverse use of the model to derive performance parameters from observational data using statistical methods. A sensitivity analysis is used to investigate the relative importance of the two soil parameters, drainable porosity and soil hydraulic conductivity, on the performance of the model. This shows a far greater effect due to the variation of hydraulic conductivity.The use of a similar model to predict water tables in non-homogeneous soils has also been explored, including the investigation of a two-phase model to describe water movement in soils which are dominated by macropores. More useful, however, is the prediction of water table fluctuations in soils in which the soil hydraulic conductivity is a continuous function of soil depth, using the drainage theory of Youngs (1965). Solutions are presented for the logarithm of the hydraulic conductivity varying linearly with depth. The improvement in model performance is however gained at the expense of an additional parameter that describes the variation of hydraulic conductivity with depth. Some methods for deriving this parameter are discussed. Results from the use of this model are compared with those derived from the simple uniform conductivity model, showing superior performance.Lastly, the issue of soil lateral heterogeneity and the replicability of measurements is discussed. A detailed study of the variation of water table levels from a replicated drainage experiment indicates that in a practical situation very real limits exist on the accurate measurement of water tables, and that these present limits on our ability to verify models.     

Use of CERES-Maize to study effect of spatial precipitation variability on yield

The objective of this study was to determine the usefulness of on-farm precipitation measurement, through determining spatial and temporal precipitation variability and its effect on corn yield. CERES-Maize (DSSAT version 3.5) was used with three precipitation data sources, for an Indiana farm—an on-farm National Weather Service (NWS) station, the nearest non-urban NWS station with electronic reporting (27 km from the farm), and a weighted mean of the three nearest such stations (27–35 km away)—to simulate 31 years of crop yield on 1-ha grid cells. Described as a percentage of the mean, spatial precipitation variability among the three data sources by corn phenological phase was 21–104%, while temporal (year-to-year) variability was 20–49%. The difference in simulated yield based on spatial precipitation variability was 15.8%, while year-to-year yield variability was 21.5%. The apparent yield difference based on spatial precipitation variability was of the same order as year-to-year variability, which suggests having on-farm precipitation data may be necessary for accurate yield modeling.     

Assessment of spatial variability in nitrate leaching to reduce nitrogen fertilizers impact on water quality

Nitrogen leaching has caused a growing societal concern over N fertilizer impact on water quality. One way to decrease nitrogen loss through leaching is to adjust fertilizer inputs to site-specific conditions. This study was conducted to investigate spatial variability of NO₃ leaching parameters on a 5 ha commercial wheat field (Typic Ustifluent) located 25 km north of Tokat, Turkey, for the purpose of dividing the field into small cells in which application rates can be kept constant. NO₃ leaching parameters were calculated using the monthly analysis version of computer program NLEAP (nitrate leach and economic analysis package) on a regular grid spacing of 25 m, and semi-variogram for each parameter was calculated using the computer program GEAOES. The values for parameter NL (nitrate leached) were between 24.64 (low) and 77.28 kg ha⁻¹ (medium), for NAL (nitrate available for leaching) 42.46 (low) and 274.40 kg ha⁻¹ (high), and for MRI (movement risk index) 0.28 (low) and 0.35 (medium). Values for parameter ALRP (annual leaching risk potential) varied from high (index=4) to moderate (index=3). A moderately significant correlation (r=0.54, P<0.01) was found between measured and model-estimated values for the parameter NAL, indicating that the NLEAP model adequately simulated the NO₃ leaching in the study area. Values for range were 360 m for NAL, 350 m for NL and 180 m for MRI, and nugget effect was 0.72 for MRI, 0.45 for NAL and 0.25 for NL, and mean correlation distances (MCD) were 145 m for NAL and 61 m for NL. Although, the spatial patterns for the parameters NAL and NL were similar, the upper cell limit for parameter NAL was higher than two times that of parameter NL, suggesting that calculation of input for continuous control of nitrogen application rate in a variable rate nitrogen fertilizer application program be based on the spatial pattern of NL but not on that of NAL.     

塔里木河下游地下水位空间变异性研究

塔里木河下游干旱缺水,地下水资源是生态健康的基本保障.本文采用线性回归法对塔里木河下游地下水位进行回归分析,并采用地统计学方法对当地地下水位的空间变异规律进行分析.结果表明:塔里木河下游各月地下水位与其测点空间坐标呈线性关系,具有很强的空间相关性;对于局部变化剧烈的地下水位,采用克里金平滑效应修正法可明显减小克里金插值所带来的平滑效应,但整体插值精度略低于普通克里金与泛克里金法.     

Spatial variability of soil sampling for salinity studies in Southwest Iran

Summary This study addresses the technology for soil sampling of large agricultural fields which are inherently variable in both space and time. Three several hundred ha fields in southwest Iran initially sampled on an arbitrarily selected grid of 80 m to ascertain soil salinity levels were analyzed using both geostatistical and classical statistical methods. The results from two fields showed that the variance structure of the salinity observations were spatially dependent, and hence, geostatistical techniques allowed best linear unbiased estimates of salinity values interpolated between spatially observed sampling locations to yield contour lines of isosalinity. In the third field, salinity observations were found to be spatially independent, and hence, were analyzed by classical methods to yield the number of soil samples necessary to observe the fiducial limits within which it is expected that the true mean salinity exists for given levels of probability.     

暗管控制排水对棉田排水的影响

对几次较大规模降雨产生排水后排水量、排水氮素含量、地下水位等进行观测,结果发现暗管排水使得地表、地下排水量被重新分配,控制水位排水使地表排水量所占比例提高、而总排水量比常规排水减少36.4%～82.7%、地下排水峰值量较常规排水减少7.2%～85.4%。地表、地下排水硝氮质量浓度较低,一般低于4 mg/L;地表排水总氮质量浓度在2.3～11.5 mg/L之间,地下排水总氮质量浓度在0.6～9.1 mg/L之间。要减少氮素流失总量,减少排水量是关键。     

The effect of microorganisms,salinity and turbidity on hydraulic conductivity of irrigation channel soil

The introduction of polysaccharide producing benthic algae and bacteria could provide a low cost technique for seepage control in irrigation channels. The ability of algae and bacteria to produce polysaccharides proved to be successful in reducing the hydraulic conductivity of irrigation channel soil. Hydraulic conductivity was reduced to less than 22% of its original value within a month of inoculating soil columns with algae. Chlorophyll and polysaccharide concentrations in irrigation channel soil were measured in order to assess the growth of algae and extent of polysaccharide production, and their correlation with hydraulic conductivity of channel soil. Increases in polysaccharide occurred in the top layer (0–5 mm) of the soil column. The reduction of hydraulic conductivity was highly correlated with the amount of polysaccharides produced (r ² = 0.92). Hydraulic conductivity decreased with increasing algal and bacterial numbers. The first few millimetres of the soil core where microbial activity was concentrated, seemed effective in controlling seepage. Incorporation of extra nitrate and phosphate into algal medium did not increase the production of polysaccharides by algae in channel soil. The effect of salinity and turbidity of irrigation channel water on channel seepage was studied by measuring the effects on hydraulic conductivity of channel soils. When the electrical conductivity (EC) of the water increased above a threshold value, the hydraulic conductivity increased because of the flocculating effects on clay particles in channel soils. A relationship between sodium adsorption ratio (SAR) and EC of the channel water was established which indicated 15% increase in channel seepage due to increases in salinity. Increasing the turbidity of irrigation water (by increasing the concentration of dispersed clay) resulted in lowering the hydraulic conductivity of the channel soil due to the sealing of soil pores by dispersed clay particles. When the turbidity of the water was 10 g clay l^–1, the hydraulic conductivity was reduced by 100%. An increase in clay concentration above 1 g l^–1 resulted in significant reduction in hydraulic conductivity. Soil bowl experiments indicated that clay sealing with a coating of hydrophobic polymer on the surface could also effectively prevent seepage of saline water.     

Effect of temporal variability in soil hydraulic properties on simulated water transfer under high-frequency drip irrigation

The effect of changes in the hydraulic properties of a loamy topsoil on water transfer under daily drip irrigation was studied over a cropping cycle. Soil water contents were measured continuously with neutron probes and capacitance sensors placed in access tubes (EnviroSMART) and were compared to predications made by the Hydrus-2D model. Three different sets of hydraulic parameters measured before and after irrigation started, were used.Our results demonstrated that, based on the assumptions used in this study, the accuracy of the Hydrus predictions is good. Graphical and statistical comparisons of simulated and measured soil water contents and consequently the total water storage revealed a similar trend throughout the monitoring period for the all three different sets of parameters. The soil hydraulic properties determined after irrigation started were found to be much more representative of the majority of the irrigation season, as confirmed by the accuracy of the simulation results with high values of the index of agreement and with values of RMSE similar in magnitude to the error associated with field measurements (0.020 cm³ cm⁻³). The highest RMSE values (about 0.04 cm³ cm⁻³) were found when the model used input soil parameters measured before irrigation started.Generally, changes in topsoil hydraulic properties over time had no significant effect on soil moisture distribution in our agro-pedo-climatic context. One possible explanation is that daily water application was conducted at the same time as maximal root water uptake. This meant the soil did not need to store total daily crop water requirements and consequently that the water redistribution phase represented a very short stage in the irrigation cycle. It is probable that irrigating in the daytime when crop evapotranspiration is highest could prevent the effects of a temporal change and other problems connected with the soil. Moreover, water will be always available for the crop. Further experiments are needed to justify the results and to study the effects of low frequency drip irrigation on soil hydraulic characterization and consequently on soil water transfer in order to improve irrigation scheduling practices.     

Modeling the soil water balance based on time-dependent hydraulic conductivity under different tillage practices

A simulation model with time-dependent hydraulic conductivity parameters was used to predict the effects of three different tillage practices: conventional tillage (CT), no-tillage (NT) and subsoiling tillage (ST) on the components of the soil water balance during the summer maize growing season. The predictive capability of the model was improved, particularly for the subsoiling tillage case. The simulation results show that temporal changes in soil hydraulic conductivity induced by different tillage practices can affect percolation, water storage, transpiration and evaporation. Differences in the simulated components of the water balance were found to be small between CT and NT practices, but larger in the ST case. Compared with the conventional and no-tillage methods, subsoiling promotes infiltration and deep percolation, thereby favoring a possible recharge of the groundwater. Actual evaporation is always lower in the subsoiled plots, whatever the hydrological year. Transpiration is similar for the three treatments, suggesting no significant differences in water availability, except in wet years where it is higher in subsoiled soils.