不同采样密度的土壤水分特征参数预测

利用不同取样精度的土壤,将土壤质地(砂土、淤泥、粘土含量)和容重作为输入值,探讨了使用基于土壤转换函数的BP神经网络模型来预测0~20 cm表层土壤水分特征曲线参数,用甘肃省称钩河流域小流域的土样进行预测并进行了误差分析。结果表明,使用线性回归能够减小预测误差与实测值差距;使用BP神经网络来预测饱和体积含水量,其准确性比使用BP神经网络预测剩余体积含水量和田间持水量要高。为了进一步提高预测精度,还应尽可能地包括土壤结构、有机质含量等信息。     

Simulation of root water uptake: III. Non-uniform transient combined salinity and water stress

Six different reduction functions for combined water and salinity stress are used in the macroscopic root water extraction term. The reduction functions are classified as linear additive, non-linear multiplicative, and that which is neither additive nor multiplicative. All these reduction functions are incorporated in the numerical simulation model HYSWASOR. The relation between the experimental relative transpiration and the joint soil water osmotic and pressure heads appears to be linear (with an exception for the salinity near the threshold value). As the mean soil solution salinity increases, the trend becomes more linear. The simulations indicated that for most treatments the newly proposed reduction term provides the closest agreement with the experimental transpiration. Soil water content, and particularly soil solution salinity simulated with this equation agree reasonably with the experimental data: in spite of the observed differences, the trend of the simulated data is good. A reason for the disagreement between the simulated and experimental water contents can be attributed to the influence of roots and the soil solution concentration on the soil hydraulic conductivity. The input soil hydraulic parameters were obtained from soil samples without roots and salinity and assumed constant during the simulations.     

基于联合多重分形的土壤水分特征曲线土壤传递函数

利用联合多重分形方法识别在不同尺度上对土壤水分特征曲线空间变异性都具有显著影响的因素,基于得出的结论建立考虑尺度效应的小尺度和大尺度van Genuchten模型参数的土壤传递函数,探讨了将小尺度上得出的联合多重分形结论进行尺度扩展,应用到大尺度上的可行性。结果表明:小尺度和大尺度上基于0～20 cm和20～40 cm土层van Genuchten模型参数土壤传递函数预测的土壤含水率均方根误差分别为0.038 6、0.047 3和0.027 0、0.030 4,可将小尺度上进行联合多重分形分析得出的结论进行尺度扩展,应用到大尺度上,且建立的土壤传递函数具有较强的理论基础和较高的预测精度。     

黄土高原南北样带不同土层土壤容重变异分析与模拟

为探明黄土高原南北样带土壤容重空间分布特征,为土壤水文过程模拟与预测提供水力参数,采用经典统计学方法,分析了样带不同土层深度(0~10 cm、10~20 cm、20~40 cm)土壤容重的空间变异特征,并用多元逐步回归、传递函数方程和一阶自回归状态空间模型方法分别对土壤容重的空间分布进行了模拟。结果表明:样带0~20 cm深度土壤容重的变异为中等程度变异,20~40 cm为弱变异。状态空间方程转换系数表明,不同土层深度土壤容重的影响因素不同,0~10 cm主要为有机碳含量、黏粒和砂粒体积分数,10~20 cm为有机碳含量、黏粒和砂粒体积分数和降水量,20~40 cm为黏粒和砂粒体积分数、降水量和土地利用。状态空间模型的模拟效果均优于经典统计的多元逐步回归方程和传递函数方程,基于黏粒和砂粒体积分数、降水量和土地利用的状态空间模型可以解释样带20~40 cm容重92.3%的变异。一阶自回归状态空间模型可用于田间条件下土壤容重分布特征的预测。     

Developing a reliable strategy to infer the effective soil hydraulic properties from field evaporation experiments for agro-hydrological models

The Richards equation has been widely used for simulating soil water movement. However, the take-up of agro-hydrological models using the basic theory of soil water flow for optimizing irrigation, fertilizer and pesticide practices is still low. This is partly due to the difficulties in obtaining accurate values for soil hydraulic properties at a field scale. Here, we use an inverse technique to deduce the effective soil hydraulic properties, based on measuring the changes in the distribution of soil water with depth in a fallow field over a long period, subject to natural rainfall and evaporation using a robust micro Genetic Algorithm. A new optimized function was constructed from the soil water contents at different depths, and the soil water at field capacity. The deduced soil water retention curve was approximately parallel but higher than that derived from published pedo-tranfer functions for a given soil pressure head. The water contents calculated from the deduced soil hydraulic properties were in good agreement with the measured values. The reliability of the deduced soil hydraulic properties was tested in reproducing data measured from an independent experiment on the same soil cropped with leek. The calculation of root water uptake took account for both soil water potential and root density distribution. Results show that the predictions of soil water contents at various depths agree fairly well with the measurements, indicating that the inverse analysis is an effective and reliable approach to estimate soil hydraulic properties, and thus permits the simulation of soil water dynamics in both cropped and fallow soils in the field accurately.     

Simulating soil water flow and nitrogen dynamics in a sunflower field irrigated with reclaimed wastewater

Soil water flow and nitrogen dynamics were simulated in sunflower field during and after the growing period, in Northern Greece. Soil water and nitrogen dynamics were evaluated using a one-dimensional simulation model based on the Galerkin finite element method. We examined the effects of irrigation with reclaimed wastewater and nitrogen fertilizer applications on plant growth, water and nitrogen distribution in the soil profile, water and nitrogen balance components and nitrogen leaching to groundwater. The model simulated the temporal variation of soil water content with reasonable accuracy. However, an over estimation of the measured data was observed during the simulation period. Relatively good agreement was found between the simulated and measured NH₄-N and NO₃-N concentrations over time and depth, whereas fluctuations at greater depths were relatively small. Most of the cumulative nitrate-N leaching (44.7 kg N ha⁻¹) occurred during the winter.     

In situ water transmission characteristics of a tropical soil under rice-based cropping systems

In soils under rice-based cropping systems in Asia water movement and distribution in the root zone of rice and dryland crops are important for efficient water management. Saturated hydraulic conductivities in the wetland soil profile were evaluated from measurements of hydraulic gradients and percolation rates in the field. The subsoil layer (15–60 cm) restricted percolation rate to a greater degree than the puddled top soil.Unsaturated hydraulic conductivities and soil water diffusivities in the soil profile under dryland conditions were obtained from simultaneous measurements of soil water content using the neutron moderation technique and the soil matric potential by tensiometers over time and soil depth. Soil matric potential versus hydraulic conductivity and soil water content versus soil water diffusivity relations of various soil depths were established. At equivalent soil matric potentials, the hydraulic conductivity of surface soil was greater than that of the subsoil layers. Soil water diffusivity at different depths responded similarly. The study describes a simple in situ technique to measure percolation rates in wetland rice fields and evaluation of water transmission properties of field soil profiles.     

叶尔羌河灌区棉花灌溉试验

棉花试验设计了五个灌水下限处理,每个处理四个重复,试验提供了各种处理的１ｍ 土层储水量动态过程,进行了田间土壤水量平衡,分析了棉花生长期耗水强度与灌水量关系,棉花产量与灌水量关系,棉花产量及棉花水分生长率与全生育期总耗水量关系     

Simulation of soil water in space and time using an agro-hydrological model and remote sensing techniques

Complete knowledge of all components of the water balance is essential to optimize water use in irrigated agriculture. However, most water balance components are very difficult to measure in terms of the required time interval and due to the complexity of the processes. An unsaturated zone model is a useful tool for predicting the effects of agricultural management on crop water use and can be used to optimize agricultural practices in view of minimizing the agricultural water use. For the irrigated areas in Minqin County of northwest China, the physically based one-dimensional agro-hydrological model SWAP (Soil, Water, Atmosphere and Plant) for water movement and crop growth was applied to reveal all the components of the water balance at multiple sites. This model has a varying level of abstraction referring to simulated processes in time and space. A combination of field, meteorological and aerial data was used as input to the model. Inverse modeling of evapotranspiration (ET) fluxes was followed to calibrate the soil hydraulic functions by using the parameter estimation package PEST. Surface Energy Balance System (SEBS) was used to estimate actual ET fluxes from NOAA AVHRR satellite images. Simulations were carried out for 15 different sites in Minqin County by using wheat (Triticum aestivum L.) as a test crop, but only three sites were selected for model calibration and evaluation. The period of simulation for the whole wheat growing season was from 1 April 2004 to 30 July 2004 and detailed analyses were performed for all sites. SWAP simulated soil water dynamics well and the distributed SWAP model is a useful tool to analyze all water balance components.     

Time-variable soil hydraulic properties in near-surface soil water simulations for different tillage methods

Simulating near-surface soil water dynamics is challenging since this soil compartment is temporally highly dynamic as response to climate and crop growth. For accurate simulations the soil hydraulic properties have to be properly known. Although there is evidence that these properties are subject to temporal changes, they are set constant over time in most simulations studies. The objective of this study was to improve near-surface soil water simulations by accounting for time-variable hydraulic properties. Repeated tension infiltrometer measurements over two consecutive seasons were used to inversely estimate the hydraulic properties of a silt loam soil under different tillage - conventional (CT), reduced (RT), and no-tillage (NT). Simulated water dynamics with constant and time-variable hydraulic parameters were compared to observed data in terms of the soil water content and water storage in the near-surface soil profile (0-30 cm). The measurements indicate a considerable temporal variability in the saturated hydraulic conductivity, the field-saturated water content and the parameter α of the van Genuchten/Mualem model. Temporal variability was largest for CT and RT, whereas under NT, replicates of measured water contents and hydraulic properties showed a considerable large spatial variability. Simulations with time-constant hydraulic parameters led to underestimations of soil water dynamics in winter and early spring and overestimations during late spring and summer. The use of time-variable hydraulic parameters significantly improved simulation performance for all treatments, resulting in average relative errors below 13%. Since simulation results agreed with observed water dynamics in two seasons, the applicability of inversely estimated hydraulic properties for soil water simulations is demonstrated. Thus, simulations that address applied questions in agricultural water management may be improved by using time-variable hydraulic parameters. The simulated water balance indicated that RT and NT result in better water storage than CT and therefore may increase water efficiency under water-limited climatic conditions.     

Simulation of nitrogen leaching from a fertigated crop rotation in a Mediterranean climate using the EU-Rotate_N and Hydrus-2D models

Two different modeling approaches were used to simulate the N leached during an intensively fertigated crop rotation: a recently developed crop-based simulation model (EU-Rotate_N) and a widely recognized solute transport model (Hydrus-2D). Model performance was evaluated using data from an experiment where four N fertigation levels were applied to a bell pepper-cauliflower-Swiss chard rotation in a sandy loam soil. All the input data were obtained from measurements, transfer functions or were included in the model databases. Model runs were without specific site calibration. The use of soil input parameters based on the same pedotransfer functions in both models resulted in a very similar simulation of soil water content in spite of the different nature of the approaches. Good correlations were found between the simulated water draining below 60 cm and that calculated by water balance. Accuracy of the predicted nitrate nitrogen (NO₃-N) contents in the 0-90 cm soil profile was acceptable with both models, with values of the mean absolute error (MAE) below the average standard deviation of the observations. The uptake of nitrate was better simulated with EU-Rotate_N where specific crop N demand algorithms are involved. In the simulations with Hydrus-2D the evapotranspiration demand was a limiting factor for N uptake, resulting in an increasing underestimation of uptake with decreasing N fertilizer rates. Simulated N leaching below a depth of 60 cm was higher with Hydrus-2D due to a higher nitrate concentration in percolated water. Comparison of the observed and predicted yield response to N applications with EU-Rotate_N demonstrated that the best fertigation strategy could be identified and the risk of nitrate leaching quantified with this model. The results showed that for a successful solving of the problem studied, Hydrus-2D probably would need a more complex calibration, and that the EU-Rotate_N model can provide acceptable predictions by adjusting basic parameters for the growing conditions. Further research with other crops and soil types will allow up-scaling the quantification of N leaching from a field level to regional and national levels, identifying best management strategies in relation to N use from an environmental and economic perspective.     

华北冬小麦-夏玉米农田水分动态模拟研究

冬小麦-夏玉米连作是华北地区主要的粮食作物种植模式。根据华北季节性冻土区的特点,将全年划分为作物生长期与越冬期,分别建立了作物生长条件下农田水分运移模型、冻融条件下土壤水热运移模型。前一模型主要包括参照腾发量计算、腾发量分配、作物根系吸水、土壤表面蒸发、土壤水分特征参数和土壤水分运动等子模型;后一模型主要包括冻土水热耦舍迁移、地气水热交换等子模型。应用以上模型对冬小麦-夏玉米连作条件下的土壤水分过程进行模拟,根据北京永乐店试验资料对模型进行检验。模拟了不同降水水平年、不同灌溉处理下的农田灌溉制度及土壤水分过程,分析了降水、灌溉对农田蒸散、土壤水利用、深层渗漏等的影响。     

A scaling based estimation of soil unsaturated hydraulic properties at a field scale

This paper deals with the prediction of the soil water retention h(S) and the soil unsaturated hydraulic conductivity K(S) functions of a clay-loam soil at a field scale (1 ha) where the variable S represents water saturation. The Van Genuchten model and the corresponding Mualem-Van Genuchten model were used to predict h(S) and K(S) functions respectively. The field data (tensiometric and neutron probe measurements) used in this study were provided by the soil water balance (four neutron sites, 0.35 to 1.55 m soil layer) of a soybean crop over a 78 days growing season. The advantages of the scaling approach for describing the field variability of the h(S) function were confirmed. The scaling approach accounted for 73% of the field variability of the soil matrix potential. A simple procedure was proposed in order to predict the K(S) function using scaling theory. This was done by simultaneously applying a ``zero flux method' and ``deep flux method' to compute the soil water balance and fit the saturated hydraulic conductivitiy (K _sat), the only unknown parameter in K(S). Received: 15 November 1995     

Modelling soil-water-plant relationships in the Cerrado soils of Brazil: The case of maize (Zea mays L.)

A soil-water balance simulation model developed for the Cerrado soils of central Brazil is presented. The model calculates daily soil water evaporation, plant transpiration and soil-water balance for fourteen soil layers of 15 mm each. The model includes a subroutine to calculate capillary water movement. Computer simulations of daily soil water levels at five soil depths (15, 30, 45, 60 and 90 cm) for a field of maize are compared with actual field measurements over an 80-day period. Results indicated that the developed model can, in general, estimate the soil-water balance of the various depths within ± 10% of actual measurements.     

SWAP, CropSyst and MACRO comparison in two contrasting soils cropped with maize in Northern Italy

The quantification of the water balance terms within soil-crop-climate systems is required to derive proper management for plant growth and irrigation. A large number of available models use the well known Richards’ equation for the simulation of water redistribution at field scale. Despite their common basis of the representation of water flow in the unsaturated zone, apparently similar hydrological models give different answers if applied in the same pedological, climatic and agronomic scenarios.The objective of the present study was evaluating and comparing the performance of three well known models (SWAP, MACRO and CropSyst) based on the solution of the Richards’ equation: in a structured fine soil (Calciustepts located in Cerese, Mantova, Italy) and in a structured fine loamy over sandy soil (Hapludalf located in Caviaga, Lodi, Italy), both cropped with maize. The models were compared on the basis of their reliability to predict soil water content, measured by TDR, at 10 depths over 2 years.We compared the three models on the basis of difference-based indexes (CRM and RMSE) and correlation statistics (r and EF): at three depths (0-0.15, −0.4 and −1.0 m), in terms of soil water content profile following a drainage process on bare soil and on soil water content over the whole soil profiles.Although water retention and hydraulic conductivity curves were properly measured in laboratory on undisturbed soil samples, all three models required calibration and validation to obtain good quality simulations. The performances of the three models were quite similar: the average of all (models, sites and depths) root mean square error (RMSE) was 0.032 cm³ cm⁻³ (±0.007).Generally, SWAP had the best performance especially in simulating surface infiltration and drying processes, followed by CropSyst and then MACRO.The better performance of SWAP respect the other two models seemed rely on the hydraulic properties parameterization (van Genuchten-Mualem vs. Campbell equation), and to the different techniques used for the numerical solutions of Richards’ equation close to the bottom and upper boundaries. Moreover, despite its rather good performance, CropSyst, due to its internal numerical constraints in the parameterization of the retention and conductivity functions, needed a very strong calibration then loosing part of its “physical basis” towards an increasing of its empiricism.     

Hydraulic properties and water balance of a clay soil cropped with cotton

A field study was carried out in the Cukurova Region, Southern Turkey to investigate the magnitude of the components of water balance, and the water uptake by cotton roots in relation to hydraulic properties of a clay soil. A plot cropped with cotton and with bare soil only were equipped with tensiometers, gypsum blocks, and access tubes for neutron probe to monitor soil water potential and water content.The hydraulic conductivity values, evaporation and drainage rates, and water withdrawal of roots were determined from field data with numerical calculations based on water flow equations.Results showed that the evaporation from bare soil was generally high during the three month period May to July varying between 4.5 and 1.0 mm/day. However, when soil water potential at 10 cm depth had decreased to -0.065 and -0.070 MPa in the drying phase, the evaporation from the soil decreased to 0.4 mm/day. The drainage rates were influenced by rainfall.The highest values of capillary flux toward the surface layer, and drainage rate from the cropped soil, were 2.0 and 1.8 mm/day respectively. Rates of water uptake by roots from the soil profile, not including the 0–10 cm layer, were high when compared with drainage and upward fluxes, changing between 7.7 and 1.4 mm/day during the experimental period. A good agreement between root length densities and water uptake was found; up to 80% of all roots were in the top 50 cm of the soil and 78% of the total water uptake was extracted from the same layer. Evapotranspiration was found to decline as a cubic function of the available water content of the top 120 cm of the soil profile.     

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO₃-N) and ammonium (NH₄-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO₃-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO₃-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (K_s), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO₃-N and NH₄-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.     

Field determination of soil hydraulic properties for simulation of the optimum watertable regime

A field experiment to evaluate accurate cost and time efficient methodologies for determining soil hydraulic properties was done at the NIAB Research Station at Faisalabad, Pakistan. The experiment was performed on a freely draining loamy soil. This soil type is representative of 75% of the topsoil in a tile drainage area known as the Fourth Drainage Project. Redistribution of water was monitored at five locations, for seven depths, following a steady state infiltration for prolonged time. The data were analyzed with Darcian flow analysis, three simplified methods, and two parameter optimization programs to calculate unsaturated hydraulic conductivity. The Darcian flow analysis was used as a reference against which the simplified methods were compared. Two simplified methods produced satisfactory results with less effort. The drawback is that the simplified methods alone do not provide enough information for use in simulation models. The advantage of the two optimization programs — SFIT and RETC — is that they are based on a continuous function which describes complete h() and K() curves. This is a requirement for computer simulation of salt and water movement in the unsaturated soil. The results of the optimizations were evaluated by their correspondence to field measurements and to laboratory measurements and by their ability to simulate soil water flow. Both programs fit the observed field data well, but only the SFIT optimized parameters were suitable for soil water flow simulations.Abbreviations IWASRI International Waterlogging and Salinity Research Institute - NRAP Netherlands Research Assistance Project - NIAB Nuclear Institute for Agriculture and Biology - SSP Soil Survey of Pakistan Revised paper from Field determination of soil hydraulic properties presented in ICID, CIID IDW5, Lahore, Pakistan (1–55 — 1–64).     

Crop rotations in Argentina: Analysis of water balance and yield using crop models

Cropping schemes have developed in east-central Argentina for rainfed soybean (Glycine max Merr.) production that invariably employ no-tillage management. Often these schemes include growing soybean in a sequence of crops including wheat (Triticum aestivum L.) and maize (Zea mays L.). The full impact of various rotation schemes on soil water balance through a sequence of seasons has not been explored, although the value of these rotations has been studied experimentally. The objective of this work was to investigate through simulations, potential differences in temporal soil water status among rotations over five years. In this study, mechanistic models of soybean (Soy), maize (Maz), and wheat (Wht) were linked over a five-years period at Marcos Juárez, Argentina to simulate soil water status, crop growth, and yield of four no-till rotations (Soy/Soy, Soy/Wht, Soy/Maz, and Soy/Maz/Wht). Published data on sowing dates and initial soil water contents in the first year from a no-till rotation experiment were used as inputs to the model. After the first year, soil water status output from the model was used to initiate the next crop simulation in the sequence. The results of these simulations indicated a positive impact on soil water balance resulting from crop residue on the soil surface under no-till management. Continuous soybean and the two-year soybean/maize rotation did not efficiently use the available water from rainfall. Residue from maize was simulated to be especially effective in suppressing soil evaporation. Thus, the Soy/Maz simulation results indicated that this rotation resulted in enhanced soil water retention, increased deep water percolation, and increased soybean yields compared with continuous soybean crops. The simulated results matched well with experimental observations. The three-crop rotation of Soy/Maz/Wht did not increase simulated soybean yields, but the additional water retained as a result of decreased soil evaporation resulting from the maize residue allowed the addition of a wheat crop in this two-year rotation. Simulated soybean yields were poorly correlated with both the amount of soil water at sowing and the rainfall during the cropping period. These results highlight the importance of temporal distribution of rainfall on final yield. These models proved a valuable tool for assessing the consequences of various rotation schemes now being employed in Argentina on temporal soil water status, and ultimately crop yield.     

The feasibility of using variable rate water application under a central pivot irrigation system

The purpose of this paper was to assess the feasibility and significance of applying spatially variable irrigation under a central pivot system at the Federal German Agricultural Research Center, Braunschweig, Germany. The assessment was based on soil moisture holding capacity, soil depth variation and root development. Soil texture analysis was carried out by sampling on a 60 meter grid. The German Agro-Meteorological Model was applied to simulate the water balance in the crop-soil-atmosphere system for the growing season 2003/4. The research findings are presented in terms of six scenarios: 20, 30, 40 mm water application depths per irrigation under both variable rate application and uniform application. The comparison revealed that the loss of water was higher for the uniform application scenarios than that for the variable rate application (VRA) scenarios for the applications of 20 and 30 mm. The VRA scenario of 20 mm water application was found out to be the best option for water conservation.