Macroscopic scale soil moisture dynamics model for a wheat crop

Summary A numerical soil moisture dynamics model was developed for; wheat crop using either observed or generated root length densities with root sink incorporating diminishing rate of water uptake by plant roots due to decreasing soil moisture in drying cycles and loss of absorptive power of roots due to ageing. The simulated soil moisture contents were overestimated by 6.0 and 9.6% on an overall basis by the model when observed and generated root length densities were used, respectively, in comparison to observed moisture contents. The model using generated root length densities simulated less water uptake in comparison with the model which utilized observed root length densities.     

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.     

The response of sap flow in apple roots to localised irrigation

The oft-touted reason for the efficiency of drip irrigation is that roots can preferentially take up water from localised zones of water availability. Here we provide definitive evidence of this phenomenon. The heat-pulse technique was used to monitor rates of sap flow in the stem and in two large surface roots of a 14 year old apple tree (Malus domestica Borkh. cv. Braeburn). The aim was to determine the ability of an apple tree to modify its pattern of root water uptake in response to local changes in soil water content. We monitored the water status of the soil close to the instrumented roots by using time domain reflectometry (TDR) to measure the soil's volumetric water content, θ, and by using ceramic-tipped tensiometers to measure the soil's matric pressure head, h. A variation in soil water content surrounding the two roots was achieved by supplying a single localised irrigation to just one root, while the other root remained unwatered. Sap flow in the wetted root increased straight away by 50% following this drip irrigation which wetted the soil over a zone of approximately 0.6 m in diameter and 0.25 m in depth. Sap flow in the wetted root remained elevated for a period of about 10 days, that is until most of the irrigation water had been consumed. A comparative study of localised and uniform irrigation was then made. Following irrigation over the full root zone no further change in sap flow in the previously wetted root was observed when referenced to the corresponding sap flow measured in the stem of the apple tree. However sap flow in the previously dry root responded to subsequent irrigations by increasing its flow rate by almost 50%. These results show that apple roots have the capacity to transfer water from local wet areas at much higher rates than normally occurs when the entire root zone is supplied with water. They are also able to shift rapidly their pattern of uptake and begin to extract water preferentially from those regions where it is more freely available. Such an ability supports the use of drip irrigation for the efficient use of scarce water resources. We conclude that the soil-to-root pathway represents a major resistance to water uptake by apple, even at the relatively high soil water pressure heads developed during parts of this experiment, during which the tree was not even under any stress.     

Root system parameters determining water uptake of field crops

Summary The distribution of a crop rooting system can be defined by root length density (RD), root length (RL) per soil layer of depth z, sum of root length (SRL) in the soil profile (total root length) or rooting depth (z _r . The combined influence of these root system parameters on water uptake is not well understood. In the present study, field data are evaluated and an attempt is made to relate a daily maximum water uptake rate (WU_max) per unit soil volume as measured in different soil layers of the profile to relevant parameters of the root system. We hypothesize that local uptake rate is at its maximum when neither soil nor root characteristics limit water flow to, and uptake by, roots. Leaf area index and the potential evapotranspiration rate (ET _p ) are also important in determining WU_max, since these quantities influence transpiration and hence total crop water uptake rate. Field studies in Germany and in Western Australia showed that WU_max depends on RD. In general, there was a strong correlation between the maximum water uptake rate of a soil layer (LWU_max) normalized by ET _p and RL normalized by SRL. The quantity LWU_max · ET _p ^-1 was linearly related to (RL/SRL)^1/2. The data show that the single root model will not predict the influence of RD on WU_max correctly under field conditions when water-extracting neighboring roots may cause non-steady-state conditions within the time span of sequential observations. Since the rooting depth z _r was linearly related to (SRL)^1/2, the relation: LWU_max · ET _p ^-1 = f (RL^1/2/z _r ) holds. Furthermore it was found that the maximum specific uptake rate per cm root length UR_max was inversely related to RD^1/2 and to SRL^1/2 or z _r of the profile. Observed high specific uptake rates of shallow rooted crops might be explained not only by their lower RD-values but also by the additional effect of a low z _r . The relations found in this paper are helpful for realistically describing the sink term of dynamic water uptake models.Growing plants extract water from the soil to meet transpiration needs. Rates of transpiration and of water uptake are set by evaporative demand and by plant and soil factors which influence capacity to meet that demand. These factors include crop canopy size and leaf characteristics, root system characteristics and hydraulic properties of the soil and the soil-root interface. Soil and root system properties vary with depth and all factors vary in time, so that parameters related to them require constant updating over a crop season.Dynamic simulation models describe water uptake by root systems under field conditions as a function of soil depth and time. Many of these simulation approaches are based on Gardner's (1960) single root model (Feddes 1981). These simulation procedures follow the assumption that water uptake is proportional to a difference in water potential between the bulk soil and the root surface or the plant interior, to the hydraulic conductivity of the soil-plant system and to the effectiveness of competing roots in water uptake. The effectiveness factor accounts more or less empirically for the influence of various root system parameters on water uptake such as percentage of active roots absorbing water, root surface permeability, root length density determining the distance between neighbouring roots, or total root length and depth of the root system. Such models however, will not always reflect correctly the influence of root system characteristics on water uptake since these assumptions have rarely been tested under field conditions. In many instances, there is better agreement between simulated and measured total water use of plants than between predicted and observed water depletion by roots within individual layers of the soil profile (Alaerts et al. 1985).Water uptake by an expanding root system as a function of depth and time has been studied under field conditions for several crops (listed in Herkelrath et al. 1977a; Feddes 1981; Hamblin 1985). They show that the dynamics of water uptake depend on root length density and the availability of soil water. However, the combined influence of root length density, total root length and rooting depth on the water uptake pattern has not been assessed. An evaluation of root system parameters with respect to soil water extraction should aid our understanding of how roots perform under field conditions and may assist our efforts to formulate the water uptake function of roots in dynamic simulation studies more realistically.The aim of the present investigation is to develop an approach that relates measured water uptake rates to relevant parameters of the root systems. This approach will be confined to situations where water uptake in a soil layer is not restricted by unfavorable soil conditions, such as in wet soil, by insufficient aeration and, in dry soil, by reduced water flow towards roots or by increased contact resistance (Herkelrath et al. 1977b). We will define a maximum water uptake rate WU_max that is neither soil-limited nor appreciably limited by the decreasing permeability of aging roots. This WU_max will be related to relevant root system parameters as they exist when WU_max is observed. Hence, water uptake by roots in a very wet, as well as in a dry soil, has been excluded from consideration.     

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.     

滴灌紫花苜蓿根层水分稳定同位素特征分析

为了明确滴灌紫花苜蓿根层水分运移,进一步阐明滴灌节水机理,采用液态水稳定氢氧同位素技术,分析了滴灌紫花苜蓿根层水分稳定氢氧同位素分布特征。结果表明,紫花苜蓿根层水分稳定氢氧同位素在下层富集,且随土壤剖面深度的增加同位素富集量有增加的趋势。滴灌条件下,紫花苜蓿根层发育有较多细根,可迅速而高效地利用灌溉水,灌溉后紫花苜蓿对灌溉水的利用不明确偏向于某一深度土层,根层内各土层土壤水均有利用。灌溉前土壤干旱时,滴灌紫花苜蓿以30 cm上下土层土壤水作为主要水分来源的概率较高。     

Modeling water uptake by roots

Summary Most current models of the water uptake by plant roots from the soil profile solve the equation for flow of water in unsaturated soils. The boundary condition at the root-soil interface is represented, whether explicitly or implicitly by some kind of root distribution function. Such models have sufficient number of parameters so that they can be fitted to data reasonably well. Most water uptake patterns, when normalized with respect to root zone depth and plant extractable water reveal, remarkable similarities regardless of soil texture, plant species, or root distribution. This similarity is not predictable with current models. A model based upon non-linear behavior of the root membranes and described by a distributed sink moving downward through the soil profile adequately represents the uptake process. The shape of the sink function is not critical and only two parameters, a root depth parameter, and an extractable water parameter are needed.     

Analysis of soil-water uptake from a drying loess soil by an oat crop using a simulation model

Summary A simulation model of water uptake by a crop was developed to facilitate synthesis of field and laboratory observations with existing knowledge, and to analyze and predict affects of management practices, such as tillage, on water uptake from a drying soil. Radial water flow resistance in soil Rs was estimated by the single root flow model. Leaf stomata closure was represented by an observed minimal leaf water potential. Flow resistances, per unit root length Rr and in the plant Rp, were assumed to be constant and were evaluated together with an effective root length factor Frl, in the course of simulating a ten week period of observed soil water depletion by a crop of oats. Rr, Rp, and Frl were found to have similar values to those reported in the literature. Potential transpiration and evaporation and their ratio were estimated by the methods of Van Bavel (1966) and Denmead (1973). Evaporation reduction due to soil drying was estimated empirically.Cessation of soil water depletion (attainment of a permanent wilting soil water content) in the 0 – 20 cm soil layer, during the last ten-day period, was explained to be the net result of soil water extraction by the roots and backflow of water from the roots into the soil. Simulated onset of crop stress (closure of stomata) was found to be characterized by: (a) a steady decrease in average soil water potential, at a rate of about 500 cm-water per cm-soil water depletion; (b) a tenfold increase in the average soil resistance to radial flow, to about the same magnitude as average radial flow resistance in the roots; and (c) soil water diffusivities in the 0 – 50 cm layer being about 6 cm²/day. Sensitivity analyses showed that the ratio of actual to potential cumulative transpiration RCT depended primarily on potential evapotranspiration, rainfall, the unsaturated-to-saturated hydraulic conductivity exponent and plant cover. RCT was affected similarly by changes in Rr and in Rs. Under the conditions tested, zero tillage may increase RCT significantly only if it increases deep rooting beyond the 50 cm depth.Joint contribution from the Georg-August University, Göttingen, FRG, and the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, A.R.O. No. 207-E, 1980 Series     

Cotton root growth as affected by changes in soil water distribution and their impact on plant tolerance to drought

The capability of mature cotton plants (Gossypium hirsutum L.) to adjust to progressive drying of their root zone by promoting root growth to adjacent wetted zones, and the implications of this process on irrigation design were investigated. Field grown plants that developed shallow root systems in response to a drip irrigation management of daily, surface soil wettings were exposed 85 days after emergence (DAE), while in the flowering stage, to a sudden change in water distribution in the form of deep soil wetting (DSW) followed by termination of irrigation. The shallow rooted plants (SRP) failed to respond to further surface soil wetting and the progressive drying of the profile by rapid root growth to the deeper-wetted zones; consequently, the SRP suffered from water deficiency for at least two weeks, evidenced by a gradual decrease in their leaf water potential (L_w). Potted plants responded similarly. Daily irrigations of the pot surface with water amounts similar to those lost by evapotranspiration led to the development of a system in which most of the roots and available water became concentrated at the pot's upper section. A transition to irrigation from the bottom of the pot led to a reversed soil-water content gradient and failed to promote rapid root spreading to the deeper-wetted layers, in spite of the accelerated drying of the upper zone. The slow deepening of the root system was accompanied by water-stress symptoms as indicated by a considerable reduction in dry matter production. The root shoot ratio in these plants was not much greater than in non-stressed plants in which the surface wetting was continued. This indicated that preferential root growth relative to the shoot did not occur in response to the progressive drying of the shallow root zone. Rewetting of the root zone after a long period of soil water deficiency failed to promote rapid recovery of the root system in the form of root regrowth in this zone. It was concluded that the capability of mature cotton plant roots to adjust their growth to large changes in water distribution in the soil, is slow and that this should be taken into account when determining an irrigation regime in which the depth at which water is applied is changed during the growing season.Contribution from the Agricultural Research Organization, Volcani Center, Bet Dagan, Israel; No. 343-E, 1992 series     

作物根系吸水率模型的试验研究

以土壤水动力学理论为基础 ,通过取土水洗法测定了有效根量 ,考虑了目前大多用一维根系吸水率模型而忽视的一些因素 ,建立了有效根量密度分布函数。由理论分析和计算 ,得到了较为切合实际的二维根系吸水率模型。     

Water balance and pattern of soil water uptake in a peach orchard

Five-year-old peach trees were irrigated at 50% and 100% of calculated maximum evapotranspiration (MET) in order to determine the influence of water stress on the pattern of water uptake from the soil and on the actual evapotranspiration (AET) of the crop. A simplified water balance method based on the relationship between the drainage component and the soil water content averaged over the soil profile has been used to estimate AET from periodic neutron probe measurements.Maximum water uptake is from the upper 60 cm of soil when trees are well-watered. Decreased soil water content induces a shift in the soil water uptake towards deeper layers, which can be due either to upward fluxes of water or to an increased water uptake by deeper roots.AET in the 50% MET regime is reduced from July to September, compared to the 100% MET regime, partly because of stomatal closure. There is no drainage in the 50% MET treatment from June to September; it is about 1 mm day⁻¹ in the 100% MET regime until the end of August and ceases in September when the soil dries.     

黄土丘陵区红枣经济林根系分布与土壤水分关系研究

为明确半干旱黄土丘陵区不同年龄无灌溉旱作矮化修剪密植枣林的根系分布范围与其土壤水分的空间关系,利用根钻法测定枣林株间不同深度的根系分布、枣树主干就近位置的根系量,并采用土钻取土和中子仪定位测定结合了解不同年龄的枣林10 m深度的土壤水分。结果表明:随着树龄增加,1、3、5、12 a枣树根系最大深度年平均增值在减小,12 a枣林垂直根系达520 cm。枣树株间100 cm处向下的根系深度较浅,枣林的垂直根系最大和最小值之差先增加后减小,12 a枣林垂直根系之差只有180 cm。研究区枣树株间水平根系在枣林3 a时开始交汇,枣树水平根系延伸无法确定,所得到的水平方向根系实际是枣林多株树汇集的根系。枣林垂直根系对土壤水分的垂直变化作用显著,但矮化修剪密植枣林株间根系深度差异并没有造成土壤水分因此而波动。随着枣树树龄的增加根系深度和土壤水分干层均增加,0~2 m土层的土壤水分年内变化幅度也增加,而且根层范围的土壤水分随着树龄增加在降低,但是土壤干层深度稍大于测得的根系深度。     

交替隔沟灌溉下玉米根长密度分布及水分利用

为了探明交替隔沟灌溉和常规沟灌条件下玉米根长密度的分布规律及水分利用效率(WUE),研究了2种沟灌方式下玉米根长密度的空间分布和水分利用情况。结果表明,玉米根长密度在根区水平向和垂向呈指数分布。交替隔沟灌溉促进了玉米根系的水平向伸展和下扎深度,常规沟灌在垄位的大密度根系分布集中在20～60cm。交替隔沟灌溉增大了根系下扎深度,有利于根系吸收深层土壤水分,在非充分供水条件下提高了作物的水分利用效率,交替隔沟灌溉水分利用效率较常规沟灌提高5%以上。     

Roots in irrigated clay soils: Measurement techniques and responses to rootzone conditions

Summary This paper reviews research carried out at the Griffith Laboratory in Australia over the last decade on techniques for, and results of, observations of roots in irrigated clay soils. Our results emphasise the adaptability of root systems to rootzone conditions. Experiences with techniques for observing roots non-destructively in the field and both non-destructively and destructively in lysimeters are described. We concluded that the minirhizotron technique, applied in the field, was unreliable under our conditions. Horizontal root observation tubes were used in lysimeters to measure root length density (RLD) and to assess whether roots were clumped together or randomly distributed. Destructive sampling and measurement of RLD was used to establish a theoretical relationship between root intercept counts along the tubes and RLD. The application of image analysis to both destructive and non-destructive sampling in the lysimeters is outlined. The non-destructive lysimeter studies showed that roots were significantly clumped. Analysis of root intercept and root hole counts on the faces of sample cubes taken from the lysimeters showed root distribution was anisotropic over the whole soil profile for both safflower and wheat. There were many more roots and root holes present in the sampled soil cubes than was indicated by independent sampling for washed out RLD. Safflower appeared to have a faster turnover of roots than did wheat or maize. Lysimeters, equipped with horizontal root observation tubes, enabled studies to be made of many factors affecting root growth. Soils affect where and how fast roots grow, although there is also a strong species interaction. For example, soybean roots proliferated above a fresh water table in one soil but not in another; wheat had little tendency to proliferate above the water table in either soil. In wet soils, roots cease to grow once soil oxygen levels decrease below 10 mg O₂ l _soil ^-1 . This level should form the basis for soil drainage criteria. In drying soils, roots will grow successively into soil regions containing soil water: the level of adaptation being determined by soil conditions, crop growth stage and level of evaporative demand. The methods of root observation used in our studies have given quantitative assessment of root distribution. However, further research is needed to link horizontal and vertical root distribution and root adaptation more strongly to crop development and soil conditions.     

Grapefruit response to variable salinity in irrigation water and soil

Summary Results are reported from a long-term field experiment designed to determine the effect of irrigation water salinity on the yield and water uptake of mature grapefruit trees. Treatments were started in 1970 and consisted of chloride concentrations in the irrigation water of 7.1, 11.4 and 17.1 meq/1 added as NaCl+CaCl₂ at a 1 : 1 weight ratio.For the last four years of the experiment, 1973 to 1976, yield was linearly related to the mean chloride concentration in the soil saturation extract weighted according to the distribution of water uptake with depth and time (Fig. 2, Table 1). There was a 1.45% (1.68 Mg/ha) yield reduction for each 1 meq/1 increase in chloride concentration above a threshold value of 4.5 meq/1. This corresponded to a 13.5% (14.7 Mg/ha) decrease per 1 mmho/cm increase in the electrical conductivity of the soil saturation extract above a threshold value of 1.2 mmho/cm.Total water uptake was reduced as salt concentration in the soil increased (Fig. 3, Table 2). In the high salinity treatment, root concentration in, and water uptake from, the lower portion of the root zone were decreased. The maximum electrical conductivity (ECe) measured at the bottom of the root zone was 7.90 mmho/cm similar to the values of EC, obtained by linear extrapolation to zero yield and also to zero water uptake.Salt accumulation in the soil depended on the quantity and salt concentration of the irrigation water, rainfall, and on the amount of leaching. SAR and the Na⁺ concentration of the soil remained low throughout the experiment (Table 3). No leaf symptoms of either Cl^– or Na⁺ injury were observed. The results indicate an osmotic — rather than a specific ion effect — of salinity on grapefruit yield.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1977 Series No. 197-E     

Water integration by plants root under non-uniform soil salinity

Soil salinity over root zone usually demonstrates temporal and spatial variations. By changing irrigation management practices it is possible to change both the frequency of salinity fluctuations and its distribution over the root zone. The objective of this study was to experimentally investigate how plants integrate soil salinity over its rooting depth when irrigated with saline water. Consequently, detailed experiments with alfalfa were conducted in some lysimeters containing packed loamy sand soil. The target soil salinities were created by changing quantity and quality of applied saline water. Results indicated that the uptake rate preliminary reacts to soil salinity. But at given water content and salinity, the “evaporative demand” and “root activity” become more important to control the uptake pattern. The obtained results also indicate that root activity is inconstant during the stress period. By increasing salinity, the activity of that part of the root system is also increased. Thus, most water is taken from the less saline part and the uptake at other parts with higher salinities never stops. Consequently, the reduced uptake in one compartment resulting from high salinity is not only compensated from other parts with less salinities, but also from the same increment by increasing root activity.     

Groundwater uptake and sustainability of Acacia and Prosopis plantations in Southern Pakistan

Farm woodlots or plantations of salt tolerant trees may provide an economic use or reclamation treatment for salt-affected farmland within the irrigation regions of the Indus Valley, but the hydrological impact and sustainability of such plantations are unknown. Detailed measurements of plantation water use, watertable depth and soil conditions were recorded over 2 years in two small plantations with contrasting soil and groundwater salinity at Tando Jam in the Sindh province of Pakistan. The species monitored were Acacia nilotica, A. ampliceps and Prosopis pallida. Annual water use by 3- to 5-year old A. nilotica was 1248 mm on the severely saline site and 2225 mm on the mildly saline site. Water use by the other species was less than 25% of these rates, but this difference is largely explained by their lower density in terms of sapwood area per hectare. Water use by A. nilotica was considerably greater than annual rainfall, implying uptake of groundwater which was confirmed both by piezometric observations and chloride balance modelling to predict vertical water movement through the root zone. Plantation watertables fell from 1.7 m below surface in March to over 2.9 m in September, then rose again during irrigation of the surrounding farmland. Root zone salt concentrations remained high at the more saline site throughout the monitoring period, but at the less saline site there was evidence of increasing root zone salinity as salt accumulated in areas of the profile subject to root water uptake. Salt concentration in the upper profile decreased as the soil dried and water was absorbed from greater depth. Plantations using saline groundwater may be sustainable if occasional leaching and other salt-removing processes are sufficient to maintain root zone salinity at a level which does not excessively reduce tree growth.     

Effective irrigation uniformity as related to root zone depth

Summary In models used for relating the yield to irrigation uniformity it has been assumed that the spatial distribution of irrigation water, as measured at the soil surface, is indeed the water distribution at any depth throughout the root zone. In the present paper the distribution of infiltrated water within the soil bulk, as determined by an analytic solution of the two-dimensional unsaturated flow equation, did not conform to this assumption. A new alternative definition of irrigation uniformity is proposed under the assumption that water uptake by roots does not affect the flux distribution within the soil profile. In this analysis the spatial distribution of irrigation water flux at the soil surface, which is the upper boundary condition of the flow equation, is assumed to be a sine function. The solution to this problem indicates that there is a damping effect, which increases with soil depth, on the surface flux fluctuations. Furthermore, the actual irrigation uniformity at a given depth below the soil surface depends upon the initial uniformity at the surface and the distance between adjacent water sources. The closer the water sources are to each other, the shallower is the depth needed to damp the oscillations down to a certain level. This may explain why the actual uniformity of drip irrigation is high while the detailed distribution is very nonuniform and on the other hand, why the actual uniformity of sprinkler guns is low while the detailed actual distribution is close to uniform. Two uniformity coefficients are derived in this study: 1. A depth dependent coefficient which is made up of the damping factor that multiplies the flux fluctuations at the soil surface; 2. An effective uniformity coefficient, which is an average of the depth dependent coefficient over a part or the entire root zone. Different degrees of uniformity are expected when water is applied by different irrigation systems having similar uniformity coefficients at the soil surface, but dissimilar distances between the emitters. Assuming that crop yield depends to some extent on the uniformity of water depth actually available to the roots, the yields associated with such irrigation systems will probably also vary.     

涌泉根灌湿润体水氮运移特性试验研究

在陕北米脂山地微灌枣树示范基地原状土上进行了涌泉根灌肥液入渗试验,研究了湿润体特征值的变化规律及水氮运移特性.〖JP+1〗结果表明：涌泉根灌入渗能力随肥液质量浓度增大而增大,累积入渗量与入渗时间的关系符合Kostiakov模型;竖直剖面的水平和垂直方向上的湿润锋运移速度均随肥液质量浓度增大而增大,并与时间均呈良好的幂函数关系.肥液质量浓度越大,涌泉根灌相同时间内湿润体的湿润深度越深,相同位置处的土壤质量含水率越大.清水与不同肥液质量浓度的涌泉根灌土壤平均质量含水率分布规律类似,肥液质量浓度越大,相同土层深度的质量含水率越大.在一定施肥条件下,涌泉根灌肥液入渗相同深度处NO-3-N与NH+4-N质量比均随肥液质量浓度增大而增大,经过水分再分布,均于土层深度70 cm处接近本底值.     

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.