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.     

Determining water consumption in olive orchards using the water balance approach

Efficient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature ‘Manzanilla’ olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip’s theory. The results were validated for a 3 l h⁻¹ dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coefficients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997–1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip’s theory showed a good agreement with the values measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap flow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree.     

北京地区等高草篱防治坡耕地水土流失效果

在北京地区采用自然降雨和模拟降雨相结合的方法,研究了狼尾草和野古草两种等高草篱在不同坡度（5%、10%、15%、20%）下对裸露坡耕地水土流失的影响。结果一致表明,狼尾草和野古草2种草篱均可显著降低坡耕地水土流失,且狼尾草草篱的效果显著好于野古草草篱。在自然降雨下（平均雨强13.3 mm/h）,狼尾草草篱可减少72.7%的地表径流和86.3%的土壤流失,野古草草篱可减少53.8%的地表径流和64.1%的土壤流失;在模拟降雨下（平均雨强49.5 mm/h）,狼尾草草篱可减少80.7%的地表径流和94.5%的土壤流失,野古草草篱可减少59.5%的地表径流和71.5%的土壤流失。另外,2种等高草篱防治水土流失的效果与坡度成显著负相关关系,随坡度增加2种草篱的水土保持功能逐渐减弱,但至20%坡度时,其对径流和土壤流失的降低作用仍维持在40%和50%以上。回归分析结果显示,草篱因素已经超过坡度和雨强,成为控制坡耕地水土流失的首要因素。     

Relationship between root uptake-weighted mean soil water salinity and total leaf water potentials of alfalfa

Summary Alfalfa was grown in five laboratory soil columns and irrigated at a fixed average amount per day. One column received tapwater at 6-day intervals; the others saline water (h _o=–12 m) at intervals of 4, 6, 8, and 12 days. The alfalfa was harvested at 24-day intervals. The resulting widely varying distributions of soil water content, pressure potential and osmotic potential were measured in detail. From these data variously weighted mean soil water potentials were calculated and correlated with measured total leaf water potentials. This indicated that in the moist, saline soil columns the alfalfa plants tended to maximize the root uptake-weighted mean total soil water potential and, since the pressure potentials were generally high compared with the osmotic potentials, also the uptake-weighted mean osmotic soil water potential (minimize the uptake-weighted mean salinity). For the drier nonsaline soil column the leaf water potentials were much lower than expected from the soil water retention function. This was attributed to dominant resistance for water flow through the soil and across the soil-root interface.     

灌区灌溉用水时空优化配置方法

将传统的灌溉水量在作物间的优化分配模型和建立的渠系工作制度多目标优化模型与地理信息系统相集成,提出了基于空间决策支持系统的灌区灌溉用水优化配置的新方法.综合考虑了灌区内作物、土壤、气象站点、渠系布置的空间差异、年季间气象以及作物不同生育阶段对应参数的时间差异.与传统优化方法相比,该方法可根据管理者对优化精度的要求,灵活选择优化尺度,同时,简化了求解时空优化配水问题的繁琐程度,结果表现形式更加丰富.在此基础上建立的空间决策支持系统界面友好,运行效率高,可移植性和通用性强.经实例验证,优化后的配水方案与原配水方案相比较,灌溉总用水量减少296％,产量增加243％,水分生产率提高05 kg／m3,灌溉净效益增加168％.优化后配水方案具有将有限的水资源向经济价值较高作物转移的趋势.该方法为灌区灌溉用水优化配置提供了新思路.     

Estimating plant root water uptake using a neural network approach

Water uptake by plant roots is an important process in the hydrological cycle, not only for plant growth but also for the role it plays in shaping microbial community and bringing in physical and biochemical changes to soils. The ability of roots to extract water is determined by combined soil and plant characteristics, and how to model it has been of interest for many years. Most macroscopic models for water uptake operate at soil profile scale under the assumption that the uptake rate depends on root density and soil moisture. Whilst proved appropriate, these models need spatio-temporal root density distributions, which is tedious to measure in situ and prone to uncertainty because of the complexity of root architecture hidden in the opaque soils. As a result, developing alternative methods that do not explicitly need the root density to estimate the root water uptake is practically useful but has not yet been addressed. This paper presents and tests such an approach. The method is based on a neural network model, estimating the water uptake using different types of data that are easy to measure in the field. Sunflower grown in a sandy loam subjected to water stress and salinity was taken as a demonstrating example. The inputs to the neural network model included soil moisture, electrical conductivity of the soil solution, height and diameter of plant shoot, potential evapotranspiration, atmospheric humidity and air temperature. The outputs were the root water uptake rate at different depths in the soil profile. To train and test the model, the root water uptake rate was directly measured based on mass balance and Darcy's law assessed from the measured soil moisture content and soil water matric potential in profiles from the soil surface to a depth of 100 cm. The ‘measured’ root water uptake agreed well with that predicted by the neural network model. The successful performance of the model provides an alternative and more practical way to estimate the root water uptake at field scale.     

Estimating field-measured, plant extractable water from soil properties: beyond statistical models

For effective irrigation management we need to know the water storage capacity of the soil reservoir. Though plant extractable water is best measured in the field, sometimes it is useful to estimate it. Laboratory-derived retention curves do not necessarily reflect field conditions. Statistical models to estimate plant extractable water from other soil properties are restricted by assumptions that are difficult to check, and they can look very complicated. We propose to test a physical-based model that exploits the similarity between the particle size distribution curve and the soil water retention curve. A large data set of soil properties from the USA was used. Detailed particle size fraction data enabled the construction of simulated soil water retention curves for 388 samples. The physical-based model was compared against a statistical model that was derived from a subset of the data base. The statistical model fit the data better than the physical-based model. On the other hand, the statistical model overpredicted the soil water limits of those soils that were not used in the derivation of the statistical model. The strength of the physical model is that it represents a cause and effect relationship between particle size distribution and soil water retention. Also, it is conceptually simple and requires few inputs. The physical model may be improved by considering soil structure and type of clay.     

水肥耦合对南疆沙区滴灌红枣光合特性及叶绿素相对含量的影响

为了探索南疆沙区水肥耦合对红枣光合特性及叶绿素相对含量的影响,以大田“8a成龄骏枣树”为供试材料,在滴灌条件下进行水肥二因素三水平完全处理小区试验.结果表明:红枣叶片净光合速率(P_n)与气孔导度(G_s)日变化呈“双峰型”,蒸腾速率(T_r)日变化呈 “单峰型”,细胞间隙CO₂浓度(C_i)和叶片水分利用效率(WUE)日变化呈 “单谷型”;W2F1处理的P_n,T_r,G_s第一次峰值的最大值比常规漫灌(CK)分别提高34.8%, 10.74%,7.14%; W2F3处理C_i和WUE谷值比常规漫灌分别提高29.04%和8.33%,叶绿素相对含量和氮含量均在W3F3处理获得最大值,比常规漫灌分别提高6.00%,9.49%;W3F3处理与W2F1处理差异不具有统计学意义(P>0.05);红枣叶片P_n与T_r,G_s之间密切相关,P_n与叶绿素相对含量及氮含量有一定相关关系,但不密切,叶绿素相对含量与氮含量之间密切相关,与WUE之间相关性较低.根据试验结果,初步认定灌水量820 mm、施肥量450 kg/hm²(W2F1处理)为南疆沙区节水节肥的最佳水肥供应模式.     

Measurement and simulation of evaporation from soil in olive orchards

Evaporation from the soil (E _s) beneath an olive orchard was characterised in a semi-arid Mediterranean climate (Córdoba, Spain). First, the microlysimeter method was modified to measure accurately E _s beneath tree orchards. The variability in irradiance reaching the soil beneath the orchard caused spatial variations in E _s during both evaporation stages. In the first days of the drying cycle, E _s was higher for high irradiance locations but the opposite occurred the subsequent days, although daily differences in E _s between locations progressively declined. For the energy-limiting stage, linear relationships between E _s values and incident photosynthetically active radiation were found for different times throughout the season. The slopes of the relationships were similar, but their intercepts differed substantially, showing the importance of a variable aerodynamic component in determining E _s. A simple functional model was formulated to estimate E _s at daily time steps. During the energy-limiting stage, E _s is calculated as the sum of the equilibrium evaporation at the soil surface and an aerodynamic term, derived from the Penman equation. For the falling rate stage, Ritchie's (1972) approach is adopted for the E _s calculations. The model was successfully tested in an orchard of 6×6 m spacing, typical of intensive olive orchards, under a wide range of evaporative demand conditions. Trees covered around 36% of the soil surface. The model predicted an average seasonal E _s of 286 mm, which represents around one third of the estimated olive evapotranspiration and about 50% of the average seasonal rainfall of the area. Received: 3 August 1998     

Comparative monitoring of temporal and spatial changes in tree water status using the non-invasive leaf patch clamp pressure probe and the pressure bomb

Real-time monitoring of plant water status under field conditions remains difficult to quantify. Here we give evidence that the magnetic-based leaf patch clamp pressure (LPCP) probe is a non-invasive and online-measuring method that can elucidate short- and long-term temporal and spatial dynamics of leaf water status of trees with high precision in real time. Measurements were controlled remotely by telemetry and data transfer to the Internet. Concomitant measurements using the pressure chamber technique (frequently applied for leaf water status monitoring) showed that both techniques yield in principle the same results despite of the high sampling variability of the pressure chamber data. There was a very good correlation between the output pressure signals of the LPCP probe and the balancing pressure values (on average r² = 0.90 ± 0.05; n = 8), i.e. the external pressure at which water appears at the cut end of a leaf under pressure chamber conditions. Simultaneously performed direct measurements of leaf cell turgor pressure using the well-established cell turgor pressure probe technique evidenced that both techniques measure relative changes in leaf turgor pressure. The output pressure signals of the LPCP probe and the balancing pressure values were inversely correlated to turgor pressure. Consistent with this, the balancing pressure values and the cell turgor pressure values could be fitted quite well by the same firm theoretical backing derived recently for the LPCP probe (Zimmermann et al., 2008). This finding suggests that use of the LPCP probe technique in agricultural water management can be built up on the knowledge accumulated on spot leaf or stem water potential measurements.     

基于水分胁迫系数的枣树园土壤含水率估算

为了实现黄土高原地区滴灌条件下枣树园土壤含水率的小范围快捷监测,根据FAO-56水分胁迫系数的定义和相关计算公式,得到了基于土壤水分胁迫系数的黄土高原地区滴灌条件下枣树根层土壤平均含水率估算公式．2009年4-9月将该公式应用于西北农林科技大学陕西米脂孟岔试验站的枣树试验园,配置了3种不同的土壤含水率控制下限,对枣树2个重要生育期的土壤含水率进行了估算,模拟了水分动态变化过程,并对估算值和实测值进行对比和误差分析．结果表明:采用基于FAO-56水分胁迫系数的计算公式对土壤含水率的动态模拟达到了较高的精度,估算值与实测值之间误差较小:其中开花坐果期各处理(灌水下限为60％,50％,40％的田间持水率)的估算值与实测值之间的相关系数分别为0828 0,0907 3,0935 1;标准误差分别为0055,0093,0068．果实膨大期各处理的相关系数分别为0777 2,0766 7,0905 5;标准误差分别为0057,0092,0079．估算值与实测值之间的相关系数随土壤含水率的增大而减小,随土壤水分胁迫程度的增大而增大,即土壤含水率较高时对公式精度有一定的影响．该方法较适用于黄土高原半干旱地区,对农业用水管理具有一定参考价值．     

An improved water use efficiency of cereals under temporal and spatial deficit irrigation in north China

Water shortage is the major bottleneck that limits sustainable development of agriculture in north China. Crop physiological water-saving irrigation methods such as temporal (regulated deficit irrigation) and spatial (partial root zone irrigation) deficit irrigation have been tested with much improved crop water use efficiency (WUE) without significant yield reduction. Field experiments were conducted to investigate the effect of (1) spatial deficit irrigation on spring maize in arid Inland River Basin of northwest China during 1997–2000; (2) temporal deficit irrigation on winter wheat in semi-arid Haihe River Basin during 2003–2007 and (3) temporal deficit irrigation on winter wheat and summer maize in Yellow River Basin during 2006–2007. Results showed that alternate furrow irrigation (AFI) maintained similar photosynthetic rate (P_n) but reduced transpiration rate (T_r), and thus increased leaf WUE of maize. It also showed that the improved WUE might only be gained for AFI under less water amount per irrigation. The feasible irrigation cycle is 7d in the extremely arid condition in Inner River Basin of northwest China and less water amount with more irrigation frequency is better for both grain yield and WUE in semi-arid Haihe River Basin of north China. Field experiment in Yellow River Basin of north China also suggests that mild water deficit at early seedling stage is beneficial for grain yield and WUE of summer maize, and the deficit timing and severity should be modulated according to the drought tolerance of different crop varieties. The economical evapotranspiration for winter wheat in Haihe River Basin, summer maize in Yellow River Basin of north China and spring maize in Inland River Basin of northwest China are 420.0 mm, 432.5 mm and 450.0 mm respectively. Our study in the three regions in recent decade also showed that AFI should be a useful water-saving irrigation method for wide-spaced cereals in arid region, but mild water deficit in earlier stage might be a practical irrigation strategy for close-planting cereals. Application of such temporal and spatial deficit irrigation in field-grown crops has greater potential in saving water, maintaining economic yield and improving WUE.     

秸秆覆盖对不同初始含水率土壤产沙过程的影响

秸秆覆盖是防治水土流失最有效的措施之一。该文通过模拟降雨（降雨强度为60 mm/h,降雨历时1 h）研究坡耕地在不同土壤初始含水率状态下秸秆覆盖变化对产沙过程的影响。试验地位于中国科学院红壤生态实验站,土壤类型为耕作铝质湿润淋溶土,试验小区为12 m×3 m,坡度为9%。处理分为5个水平的覆盖度（0、15%、30%、60%和90%）和2种土壤初始含水率（干态和湿态）。结果表明,在0、15%、30%、60%和90%覆盖度下,干态土壤平均产沙速率依次为24.5、15.8、10.4、11.2和1.0 g/(m2·h),同一覆盖度下产沙速率在模拟降雨时段内略微增大。湿态土壤条件下平均产沙速率依次为115.6、70.0、49.6、34.8和31.9 g/(m2·h),同一覆盖度下产沙速率在模拟降雨时段内下降明显。0、15%、30%、60%和90%覆盖度下平衡时产沙速率依次为52.5、30.5、22.8、19.8和15.4 g/(m2·h)。另外,5个水平的覆盖度中,30%的花生秸秆覆盖降低不同前期含水率下土壤产沙速率50%以上。因此,不同前期含水率情况下土壤产沙速率对秸秆覆盖度变化的响应非常明显,30%秸秆覆盖具有较为经济的水土保持效果。     

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。     

A method for spatial prediction of daily soil water status for precise irrigation scheduling

Available water holding capacity (AWC) and field capacity (FC) maps have been produced using regression models of high resolution apparent electrical conductivity (EC_a) data against AWC (adj. R² = 0.76) and FC (adj. R² = 0.77). A daily time step has been added to field capacity maps to spatially predict soil water status on any day using data obtained from a wireless soil moisture sensing network which transmitted hourly logged data from embedded time domain transmission (TDT) sensors in EC_a-defined management zones. In addition, regular time domain reflectometry (TDR) monitoring of 50 positions in the study area was used to assess spatial variability within each zone and overall temporal stability of soil moisture patterns. Spatial variability of soil moisture within each zone at any one time was significant (coefficient of variation [% CV] of volumetric soil moisture content (θ) = 3-16%), while temporal stability of this pattern was moderate to strong (bivariate correlation, R = 0.52-0.95), suggesting an intrinsic soil and topographic control. Therefore, predictive ability of this method for spatial characterisation of soil water status, at this site, was limited by the ability of the sensor network to account for the spatial variability of the soil moisture pattern within each zone. Significant variability of soil moisture within each EC_a-defined zone is thought to be due to the variable nature of the young alluvial soils at this site, as well as micro-topographic effects on water movement, such as low-lying ponding areas. In summary, this paper develops a method for predicting daily soil water status in EC_a-defined zones; digital information available for uploading to a software-controlled automated variable rate irrigation system with the aim of improved water use efficiency. Accuracy of prediction is determined by the extent to which spatial variability is predicted within as well as between EC_a-defined zones.     

Quantitative evaluation of soil salinity and its spatial distribution using electromagnetic induction method

In the Lower Yellow River Delta, soil salinity is a problem due to the presence of a shallow, saline water table and marine sediments. Spatial information on soil salinity at the field level is increasingly needed, particularly for better soil management and crop allocation in this area. In this paper, a mobile electromagnetic induction (EMI) system including EM38 and EM31 is employed to perform field electromagnetic (EM) survey, and fast determination and quantitative evaluation of the spatial pattern of soil salinity is discussed using the field EM survey data. Optimal operation modes of EM38 and EM31 are determined to establish multiple linear regression models for estimating salinity from apparent soil electrical conductivity (EC_a). Spatial trend and semivariogram are illustrated and spatial distribution of field salinity status is further visualized and quantitatified. The results suggest that EC_a (EM38 and EM31) data is highly correlated with salinity, and that the interpretation precision of soil salinity at various layers can be improved using EM_38h and EM_31h (where h represents the horizontal mode of EM measurement). Both EM_38h and EM_31h exhibit significant geographic trend. Nested spherical models fit the semivariance of EM_38h and EM_31h better than single spherical models. Spatial autocorrelation of EM_31h is stronger than that of EM_38h, and short-range variation is the chief constitute of spatial heterogeneity for both EM_38h and EM_31h. Quantitative classification shows that soil salinity exhibits the trend of accumulation in the root zone. In 0-1.0 m solum, heavy salinized and saline soils are the predominant soil types, accounting for 54% and 41% of total survey area, respectively. The area of light and moderate salinized soils is comparatively small, which accounts for only 0.4% and 4.6%, respectively.     

Evaluation of pedotransfer functions in predicting the soil water contents at field capacity and wilting point

Thirteen pedotransfer functions (PTFs), namely Rosetta PTF, Brakensiek, Rawls, British Soil Survey Topsoil, British Soil Survey Subsoil, Mayr-Jarvis, Campbell, EPIC, Manrique, Baumer, Rawls–Brakensiek, Vereecken, and Hutson were evaluated for accuracy in predicting the soil moisture contents at field capacity (FC) and wilting point (WP), of fine-textured soils of the Zagros mountain region of Iran. PTFs were developed using the laboratory measurements made on soil moisture at FC and WP, particle-size distribution, bulk density, and organic matter content. PTFs were evaluated on the basis of mean-squared deviation (MSD) between the observed and predicted values. Results agreed with the concept that the PTFs developed on soils of similar properties to the ones under study generally perform better than the others. In the case of the Zagros mountain soils, the “British Soil Survey” and “Brakensiek” PTFs were found to be the best methods. Since the soils under study had a wide range of organic matter contents (0.2–5.5%), the better performance of these PTFs may also be explained by the fact that they happen to be the only ones that require organic matter content as input. Rosetta, a software package that involves an artificial neural network approach, was of intermediate value in estimating soil moistures of the soils in question. This was attributed to the fact that the texture and the bulk density of the Zagros soils were not in the range of those used to develop Rosetta.     

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.     

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.     

涌泉根灌在黄土坡地的水分运移规律试验

为了了解涌泉根灌这项微灌技术灌水后的水分运移情况,在野外黄土坡地利用剖面法对涌泉根灌在不同孔径、孔深条件下土壤水分运移规律进行了研究.结果表明,在不同孔径、孔深处理下,湿润体水平扩散半径、向上入渗距离和向下入渗深度有不同的影响,且均与时间有显著的幂函数关系;涌泉根灌停止后24 h内的土壤湿润体水平及竖直方向扩散相对变化超过了47%,湿润体平均含水量相对降低了30%;24 h后的扩散较小,平均含水量下降较小.涌泉根灌停止后24 h时的湿润体特征值可作为涌泉根灌系统设计的依据;推荐涌泉根灌适宜的孔洞深度为30~40 cm,孔径为φ6 cm.研究结果可为涌泉根灌技术的实际应用提供理论参考.