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1.
发丝滴头出流流量与工作压力成正比,与发丝微管长度成反比,与发丝微管缠绕直径成正比,并分析了各影响因素与出流量之间的关系。根据试验分析结果,得出了发丝滴头新的出流公式,经实测验证,该公式具有较高的应用价值。  相似文献   

2.
The microtube is a simple and cheap emitter that was widely used throughout the world in the early days of drip irrigation. Its length can be adjusted according to the pressure distribution along the lateral line and the discharge from the microtube can be adjusted by its length. This not only counters the pressure loss due to pipe friction but also makes it suitable for undulating and hilly conditions, where pressure in the lateral line varies considerably according to the differences in elevation. This is the major problem facing the designer, i.e., emitter flow changes as the acting pressure head changes. In this study, a novel micro-sprinkler system is proposed that uses microtube as the emitter and where the length of the microtube can be varied in response to pressure changes along the lateral to give uniformity of emitter discharges. The objective of this work is to develop and validate empirical and semi-theoretical equations for the emitter hydraulics. Laboratory testing of two microtube emitters of different diameter over a range of pressures and discharges was used in the development of the equations relating pressure and discharge, and pressure and length for these emitters. The equations proposed will be used in the design of the micro-sprinkler system, to determine the length of microtube required to give the nominal discharge for any given pressure. The semi-theoretical approach underlined the importance of accurate measurements of the microtube diameter and the uncertainty in the estimation of the friction factor for these tubes.  相似文献   

3.
To investigate the relationship between stable carbon isotope discrimination (Δ) of different organs and water use efficiency (WUE) under different water deficit levels, severe, moderate and low water deficit levels were treated at bud burst to leafing, flowering to fruit set, fruit growth and fruit maturation stages of field grown pear-jujube tree, and leaf stable carbon isotope discrimination (ΔL) at different growth stages and fruit stable carbon isotope discrimination (ΔF) at fruit maturation stage were measured. The results indicated that water deficit had significant effect on ΔL at different growth stages and ΔF at fruit maturation stage. As compared with full irrigation, the average ΔL at different growth stages and ΔF at fruit maturation stage were decreased by 1.23% and 2.67% for different water deficit levels, respectively. ΔL and ΔF among different water deficit treatments had significant difference at the same growth stage (P < 0.05). Under different water deficit conditions, significant relationships between the ΔL and WUEi (photosynthesis rate/transpiration rate, Pn/Tr), WUEn (photosynthesis rate/stomatal conductance of CO2, Pn/gs), WUEy (yield/crop water consumption, Y/ETc) and yield, or between the ΔF and WUEy and yield were found, respectively. There were significantly negative correlations of ΔL with WUEi, WUEn, WUEy and yield (P < 0.01) at the fruit maturation stage, or ΔL with WUEi and WUEn (P < 0.01) over whole growth stage, respectively. ΔF was negatively correlated with WUEy, WUEn and yield (P < 0.05), but positively correlated with ETc (P < 0.01) over the whole growth stage. Thus ΔL or ΔF can compare WUEn and WUEy, so the stable carbon isotope discrimination method can be applied to evaluate the water use efficiency of pear-jujube tree under the regulated deficit irrigation.  相似文献   

4.
Computer evaluation of sprinkler irrigation uniformity   总被引：5，自引：0，他引：5  
Summary A method for evaluating the water application rate (WAR) and uniformity coefficient (Cu) of overlapping irrigation sprinklers is given for realistic field conditions which includes wind drift of the sprinkler spray. The method requires as input — the geometry of the sprinkler arrangement, trajectories of water drops from the sprinkler nozzle as calculated by the equations of motion and the WAR distribution (discharge) profile of a single sprinkler experimentally observed under windless conditions. Wind direction with respect to the main sprinkler line is shown to have a small effect on Cu and is assumed to be parallel to the main line. Results show that the effect of wind drift of sprinkler spray on Cu can be neglected for wind velocities less than 1 ms–1 (Fig. 8). Analysis of simulated discharge profiles (Table 1) shows that the maximum value of the uniformity coefficient was obtained with triangular sprinkler discharge profiles at low values of spacing, changing to trapezoidal profiles as the spacing increases (Figs. 8 and 9). The effect of nozzle pressure on WAR was evaluated for the pressure range between 294 and 490 kPa and an optimum layout of overlapping sprinklers, designed to minimize the effect of wind drift and nozzle pressure on the uniformity of WAR distribution, is presented.Notation C D air drag coefficient of water drop - Cu uniformity coefficient - D diameter of water drop - d k reference k-th water drop - incremental scanning distance at a certain size matrix - g acceleration of gravity - h pm mean value of water application rate (mean value of WAR) - h (x, y) WAR at points P (x, y) - h o (k, n) WAR at points P o (k, n) - h p (l, m) WAR at cross points of a certain size matrix covering the unit area for calculation of Cu - i row index number (see Fig. 3) - j column index number (see Fig. 3) - K Kàrmán's constant - k index number of water drop, d k - L number of scanning points along main line - l index number of scanning point along main line - M number of scanning points along the line perpendicular to main line - m index number of scanning point along the line perpendicular to main line  相似文献   

5.
Management and control of surface irrigation, in particular furrow irrigation, is limited by spatio-temporal soil infiltration variability as well as the high cost and time associated with collecting intensive field data for estimation of the infiltration characteristics. Recent work has proposed scaling the commonly used infiltration function by using a model infiltration curve and a single advance point for every other furrow in an irrigation event. Scaling factors were calculated for a series of furrows at two sites and at four points down the length of the field (0.25 L, 0.5 L, 0.75 L and L). Differences in the value of the scaling factor with distance were found to be a function of the shape of the advance curves. It is concluded that use of points early in the advance results in a substantial loss of accuracy and should be avoided. The scaling factor was also strongly correlated with the furrow-wetted perimeter suggesting that the scaling is an appropriate way of both predicting and accommodating the effect of the hydraulic variability.  相似文献   

6.
The effect of operating pressure heads on water application uniformity in microirrigation submain units was evaluated. Research results show that water application uniformity either increases or slightly decreases as operating pressure head increases in a range when the emission exponent x ≤ 0.5 in most cases. The water application uniformity decreases as operating pressure head increases in a range when the emission exponent x > 0.5. The relationship between operating pressure head and average emitter discharge in submain units can be considered as approximately linear for operating pressure heads in a small range (usually between the allowable minimum and maximum operating pressures of the submain units). These results help to estimate the average emitter discharge rate easily in a submain unit for an increased or decreased operating pressure head when one is attempting to manage emitter discharge dynamically according to the requirements of crop root growth for different periods. Generally, a microirrigation system designed to meet the desired uniformity of water application according to the allowable minimum operating pressure head would be better when x ≤ 0.5 because water application uniformity increases as operating pressure increases if emitter discharges are being managed dynamically. However, a microirrigation system designed to meet the required water application uniformity according to the allowable maximum operating pressure head would be better when x > 0.5 because, in general, water application uniformity increases as operating pressure decreases. Received: 29 November 1999  相似文献   

7.
Design of microirrigation laterals at minimum cost   总被引：1，自引：0，他引：1  
Based on the design methods of finite elements and golden-section searches, a method was developed for designing microirrigation laterals at minimum cost. Characteristics of water application uniformity as affected by lateral diameters and lengths were analyzed. When the required average emitter discharge is known, the relationships of water application uniformity, best submain position (paired laterals), and operating pressure head as a function of the lateral diameter and length can be accurately determined using a personal computer. The lateral diameter and length can then be determined from a contour map representing water application uniformity as a function of the lateral diameter and length (computer calculation). The best submain position and operating pressure head for this lateral diameter and length is then determined by computer calculation. This method is suitable for designing microirrigation laterals on both uniformly and nonuniformly sloping fields. Received: 30 May 1997  相似文献   

8.
Emitter discharge of subsurface drip irrigation (SDI) decreases as a result of the overpressure in the soil water at the discharge orifice. In this paper, the variation in dripper discharge in SDI laterals is studied. First, the emitter coefficient of flow variation CV q was measured in laboratory experiments with drippers of 2 and 4 L/h that were laid both on the soil and beneath it. Additionally, the soil pressure coefficient of variation CV hs was measured in buried emitters. Then, the irrigation uniformity was simulated in SDI and surface irrigation laterals under the same operating conditions and uniform soils; sandy and loamy. CV q was similar for the compensating models of both the surface and subsurface emitters. However, CV q decreased for the 2-L/h non-compensating model in the loamy soil. This shows a possible self-regulation of non-compensating emitter discharge in SDI, due to the interaction between effects of emitter discharge and soil pressure. This resulted in the irrigation uniformity of SDI non-compensating emitters to be greater than surface drip irrigation. The uniformity with pressure-compensating emitters would be similar in both cases, provided the overpressures in SDI are less than or equal to the compensation range lower limit.  相似文献   

9.
The efficient use of water by modern irrigation systems is becoming increasingly important in arid and semi-arid regions with limited water resources. This study was conducted for 2 years (2005 and 2006) to establish optimal irrigation rates and plant population densities for corn (Zea mays L.) in sandy soils using drip irrigation system. The study aimed at achieving high yield and efficient irrigation water use (IWUE) simultaneously. A field experiment was conducted using a randomized complete block split plot design with three drip irrigation rates (I1: 1.00, I2: 0.80, and I3: 0.60 of the estimated evapotranspiration), and three plant population densities (D1: 48,000, D2: 71,000 and D3: 95,000 plants ha−1) as the main plot and split plot, respectively. Irrigation water applied at I1, I2 and I3 were 5955, 4762 and 3572 m3 ha−1, respectively. A 3-day irrigation interval and three-way cross 310 hybrid corn were used. Results indicated that corn yield, yield components, and IWUE increased with increasing irrigation rates and decreasing plant population densities. Significant interaction effects between irrigation rate and plant population density were detected in both seasons for yield, selected yield components, and IWUE. The highest grain yield, yield components, and IWUE were found for I1D1, I1D2, or I2D1, while the lowest were found for I3D2 or I3D3. Thus, a high irrigation rate with low or medium plant population densities or a medium irrigation rate with a low plant population density are recommended for drip-irrigated corn in sandy soil. Crop production functions with respect to irrigation rates, determined for grain yield and different yield components, enable the results from this study to be extrapolated to similar agro-climatic conditions.  相似文献   

10.
The performance of the surface renewal method to estimate latent heat fluxes (LE) over a wheat crop was evaluated by comparison against values of LE measured independently using a weighing lysimeter. High-frequency temperature readings were taken at 1.5 m above ground from 29 April to 7 June 2000 over a 0.7–0.8 m high wheat crop. Surface renewal analysis was applied for two time lags r (0.75 and 0.25 s) to estimate half-hour sensible heat flux (H) and, subsequently, LE by solving the energy balance equation, using concurrent measurements of net radiation and soil heat flux. When H was estimated using sensor measurement height (z) in the computations, indices of agreement (IA) between lysimeter and surface renewal LE were above 0.94 and relative errors varied between 8.5 and 14.9% for time lag r=0.75 s for all analyzed days but 7 June. Results were slightly poorer for time lag r=0.25 s. When z−hc or z−d (hc being the crop height and d being the zero plane displacement) were used instead of z to compute H, surface renewal LE estimates slightly improved, particularly for the z−d case. The improvement was particularly noticeable for 7 June. The use of z−hc or z−d was thus more appropriate for these measurements, with the result that it was not necessary to calibrate the weighing factor α, as required by the standard surface renewal method. Unfortunately, although of similar magnitude than those reported for other micrometeorological methods, surface renewal errors found in this paper were biased and LE was underestimated. Further research and testing of the surface renewal method is therefore required to remove biases from the estimates of LE.  相似文献   

11.
The objective of this study was to analyze the components of the water balance in an Ultisol, located in the municipality of Jaboticabal, SP, Brazil (21°20′20″S, 48°18′35″W), that was cultivated with sugarcane. The monitoring was performed during the agricultural cycle of the first ratoon between 11/16/2006 and 7/9/2007. Three treatments were established in four blocks with doses of ammonium sulfate, as follows: Treatment 1 (T1), without fertilizer; Treatment 2 (T2), 100 kg ha−1 of nitrogen (N) and 114 kg ha−1 of sulfur (S); and Treatment 3 (T3), 150 kg ha−1 of N and 172 kg ha−1 of S. Rainy precipitation (P) in the area was measured with a rain gauge. The soil water storage (H) and the soil water storage variations (ΔH) were determined by the gravimetric method, and the internal drainage (D)/capillary rise (CR) at a depth of 0.9 m was quantified by the water flux density using the Darcy–Buckingham equation. The actual evapotranspiration (ETa) was calculated as follows: ETa = P − D + CR ± ΔH. During the study period, the amount of rainfall was 1406 mm, 121 mm greater than the historic average for the region (1285 mm), with a notable peak in the month of January of 402 mm (historic average: 251 mm). The internal drainage was 300 mm under T1, 352 mm under T2, and 199 mm under T3, and this was relevant during times with elevated P, when the actual H was greater than the field capacity H. The actual evapotranspiration (T1: −897.7 mm, T2: −847.5 mm, and T3: −970.8 mm) and the water use efficiency (T1: −131.3 kg mm−1, T2: −146.6 kg mm−1, and T3: −127.5 kg mm−1) did not differ among the treatments. The dispersion of D was greater than the other components of the water balance, especially during the period of elevated P, with the errors of this process propagated in the estimation of ETa. Despite of this propagated standard deviation of ETa, it accounted less than 15% of the total ETa, showing that the method may be conveniently used in field studies with sugarcane crops.  相似文献   

12.
Pistachio can be grown in the central desert of Islamic Republic (I.R.) of Iran with adverse conditions such as shallow saline groundwater tables. The contribution of water from shallow, saline groundwater to crop water use may be important in such conditions. The objectives of this study were to determine the contributions from shallow, saline groundwater to water use of pistachio seedlings, and how this contribution was affected by groundwater depth, salinitiy, and irrigation conditions. The results indicated that an increase in groundwater depth resulted in significant increase in root depth and significant decrease in seasonal evapotranspiration (ET), transpiration, and groundwater contribution to the plant water use. Non-saline shallow (30–120 cm depth) groundwater under irrigated and non-irrigated conditions contributed 72.4–89.7% and 90.7–100.0% of plant water use, respectively. However, these contributions were 57.2–74.8% and 79.3–100.0% for irrigated and non-irrigated conditions, respectively for saline shallow (30–120 cm depth) groundwater. The effect of groundwater depths (D, cm) on groundwater contributions (q, %) was found to be influenced by the salinity levels of the groundwater (EC, dS m−1). The linear multiple regression equations were q = 97.5 − 1.24(EC) − 0.194(D) and q = 105.9 − 0.48(EC) − 0.154(D) for irrigated and non-irrigated conditions, respectively. The maximum reductions in relative plant dry weight of 80.3% and 44.8% were occurred under non-irrigated condition and saline groundwater depth of 30 cm and non-saline water depth of 60 cm, respectively. Root depth analysis indicated that vertical root growth caused the root to reach a moist layer near the groundwater. A very close to 1:1 relationship between relative reduction in top dry weight (1 − y/ym) and relative reduction in transpiration (1 − T/Tm) was obtained.  相似文献   

13.
Agricultural nonpoint-source pollution is the leading cause of water-quality degeneration of rivers and groundwater. In this context, the coast of Granada province (SE Spain) is economically an important area for the subtropical fruit cultivation. This intensively irrigated agriculture often uses excessive fertilizers, resulting to water pollution. Therefore, a 2-year experiment was conducted using drainage lysimeters to determine the potential risk of nutrient pollution in mango (Mangifera indica L. cv. Osteen) and cherimoya (Annona cherimola Mill. cv. Fino de Jete) orchards. These lysimeters were used to estimate the nutrient budgeting for each crop. NO3-N, NH4-N, PO4-P and K losses according to lysimeters were, respectively, 55.1, 12.4, 3.7, and 0.6 for mango and 61.8, 17.8, 4.9, and 0.5 kg ha−1 yr−1, for cherimoya. NO3, concentrations in the leachates ranged from 1.8 to 44.3 mg L−1, and from 23.0 to 51.0 mg L−1, for mango and cherimoya, respectively, in some cases exceeding the limits for safe drinking water. PO4 also exceeded the permitted concentrations related to eutrophication of water, ranging from 0.07 to 0.5 mg L−1 and from 0.12 to 0.68 mg L−1 from mango and cherimoya lysimeters, respectively. With respect to the nutrient balance, N, P, and K removed by cherimoya fruits was 76.4, 5.5, and 22.6 kg ha−1 yr−1, and for mango fruits 30.2, 3.3 and 27.8 kg ha−1 yr−1, respectively. Nutrient losses in the leachates were surprisingly low, considering total N, P, and K applied during the year, in mango lysimeters 3.8, 0.11, and 12.6%, and in cherimoya lysimeters 7.7, 0.23 and 16.0%, respectively, indicating a potential soil accumulation and eventual loss risk, especially during torrential rains. Crop coefficient (Kc) values of mango trees varied within ranges of 0.35–0.67, 0.55–0.89, and 0.39–0.80 at flowering, fruit set, and fruit growth, respectively. Kc values for cherimoya trees had ranges of 0.58–0.67, 0.61–0.68, and 0.43–0.62 at flowering, fruit set and fruit growth, respectively. In this study, the Kc values of mango and cherimoya were significantly correlated to julian days. Therefore, the estimated WUE in the mango and cherimoya orchards reached 21.2 and 14.0 kg ha−1 mm−1, respectively. Thus, this study highlights the urgency to establish the optimal use of fertilizers and irrigation water with respect to crop requirements, to preserve surface-water and groundwater quality, thereby achieving more sustainable agriculture in orchard terraces.  相似文献   

14.
An improved method for designing microirrigation submain units   总被引：1，自引：0，他引：1  
A method was developed for designing microirrigation submain units using the lateral flow rate equation, finite element method, and golden section search. Characteristics of water application uniformity affected by lateral parameters (length and diameter) and submain parameters (length and diameter) were analyzed using computer simulations. The design principle was then implemented based on the simulation results. When the required average emitter discharge, the required water application uniformity, one lateral parameter (length or diameter), and one submain parameter (length or diameter) are given, the optimal values of another lateral parameter, another submain parameter, best submain position (paired laterals), and operating pressure head can be accurately designed using a personal computer. A submain unit designed using this method has a minimal initial cost. This method is suitable for both submain units with uniform lateral lengths (in regular fields) and nonuniform lateral lengths (in irregular fields). Received: 28 May 1996  相似文献   

15.
A method for designing microirrigation laterals on nonuniform slopes was developed using the finite element method. Six representative nonuniform slope patterns were discussed in detail. The design principle was implemented based on the results of computer simulations. It was found that a single lateral is suitable for Pattern I while paired laterals are better for Patterns II ∼ VI in most cases. The diameter of a single lateral or paired laterals may have two solutions for a required uniformity of water application and the length may have multiple solutions. When the required average emitter discharge, required uniformity of water application, and one parameter (either length or diameter) of a single lateral or paired laterals are given, the unknown parameter, best submain position (paired laterals) and operating pressure head can be accurately designed using personal computers. The design procedures are described. Received: 2 November 1995  相似文献   

16.
The field-saturated hydraulic conductivity (Kfs) finds use in many agricultural, environmental and geotechnical investigations and designs. It is commonly derived from falling-head tests in a well, but its derivation is sometimes based on assumptions that may not always hold. The aim of this study was to develop graphs for falling head percolation tests for 340 and 60 mm diameter wells—centred in two separate cylindrical soil tanks—for use in deriving Kfs of a fine sand placed in the tanks. The graphs were obtained from results from the finite element program SEEP/W that was used to model percolation from the tank wells into sand, fine sand and silt of known Kfs; the soils were all initially field-saturated, and the tests were carried out under (i) field-saturated (by spray irrigation) and (ii) draining conditions. For the modelled field-saturated tests on the three soils, the graphs of fall of head in the well, ΔHw, against the parameter Kfs × Δt, where Δt is the time taken for the particular fall ΔHw to occur, were coincident. This indicated that for a given measured fall, ΔHw, for a particular head of water, Hw, in a well, Kfs could be calculated by dividing the parameter Kfs × Δt, by the observed time, Δt, for the fall, ΔHw, to occur. For the modelled tests on the three draining soils, there was another inverse relationship between ΔHw and the parameter Kfs × Δt for small falls in well heads after start-up. There was good agreement between the Kfs value of a fine sand measured in a cylindrical constant-head laboratory permeameter and that obtained using the graphs and the experimental results from both the field-saturated fine sand, and the same fine sand in draining mode at small falls of head in the well. This suggests that falling head tests on non-irrigated soils—initially field-saturated—could be used to determine Kfs from early time falls in well heads. Since the time parameter greatly increases with increasing well size and decreasing pressure heads in the percolation well, it is quicker to carry out falling-head percolation tests in smaller test wells and at higher heads in the well. The procedure employed for obtaining Kfs for the soils within the Kfs range examined could be extended to the field.  相似文献   

17.
A mathematical model which describes water flow under subsurface drip lines taking into account root water uptake, evaporation of soil water from the soil surface and hysteresis in the soil water characteristic curve θ(H) is presented. The model performance in simulating soil water dynamics was evaluated by comparing the predicted soil water content values with both those of Hydrus 2D model and those of an analytical solution for a buried single strip source. Soil water distribution patterns for three soils (loamy sand, silt, silty clay loam) and two discharge rates (2 and 4 l m−1 h−1) at four different times are presented. The numerical results showed that the soil wetting pattern mainly depends on soil hydraulic properties; that at a time equal to irrigation duration decreasing the discharge rate of the line sources but maintaining the applied irrigation depth, the vertical and horizontal components of the wetting front were increased; that at a time equal to the total simulation time the discharge rate has no effect on the actual transpiration and actual soil evaporation and a small effect on deep percolation. Also the numerical results showed that when the soil evaporation is neglected the soil water is more easily taken up by the plant roots.  相似文献   

18.
The cost and scarcity of water is placing increasing pressure on Australian dairy farmers to utilise water for forage production as efficiently as possible. This study aimed to identify perennial forage species with greater water-use efficiency (WUE) than the current dominant species, perennial ryegrass (Lolium perenne L.). Fifteen perennial forage species were investigated under optimum irrigation and two deficit irrigation treatments, over three years at Camden, NSW, on a brown Dermsol in a warm temperate climate. Under optimal irrigation, there was a nearly twofold difference in mean WUEt (total yield/evapotranspiration) between forages, with kikuyu (Pennisetum clandestinum Hochst. ex. chiov.) having the highest (27.3 kg ha−1 mm−1) and birdsfoot trefoil (Lotus corniculatus L.) the lowest (14.8 kg ha−1 mm−1). Kikuyu was also the most water use efficient forage under the extreme deficit irrigation treatment, although its mean WUEt declined by 15% to 23.2 kg ha−1 mm−1, while white clover (Trifolium repens L.) in the same treatment had the largest decline of 44% and the lowest WUEt of only 8.8 kg ha−1 mm−1. In order to maximise WUE for any forage, it is necessary to maximise yield, as there is a strong positive relationship between yield and WUEt.  相似文献   

19.
To balance the hydraulic and anti-clogging performances for labyrinth-channel emitters, a multi-objective optimization was performed to improve the structure of trapezoidal labyrinth channels. Two variables, pressure loss coefficient (PLC) and passage rate of particles (PRP), were proposed to evaluate the hydraulic and anti-clogging performances, respectively. The Pareto fronts of PLC and PRP were calculated using the elitist non-dominated sorting genetic algorithm version II (NSGA-II). The results show that the optimal fronts are obtained through 100 generations evolution. The associated key parameters D/W and H/D are constant, while S/H and α are different for each front. In addition, the approach to obtain a trade-off solution from these fronts was developed by arranging different weights for the two variables. This approach is very suitable for structural optimization of labyrinth channels with the same shape but different dimensions.  相似文献   

20.
Yield response factor (K y) is an important basis for implementing efficient irrigation and optimal water allocation. Because K y varies in different sites, understanding its spatial distribution plays an important role in optimization irrigation in Haihe basin. After determining the K y and ET0 of winter wheat, an exponentially increasing function was found between the two parameters. Then, spherical and exponential semivariograms were chosen as proper theoretical models for ET0 and K y, respectively, with R 2 of more than 0.970. By comparing six interpolation methods as well as two procedures, i.e. ‘calculate first, interpolate later’ (CI) and ‘interpolate first, calculate later’ (IC), IC-RK (residual kriging) was considered as an optimal method in interpolating K y. Mapping of K y for winter wheat indicated an increasing trend from the western and northern mountainous region to the eastern plain region in the basin, with the K y of 0.783–1.668 for the dry growing season, 0.760–1.460 for the average growing season and 0.749–1.293 for the wet growing season. Moreover, the K y values were more than 1.0 over the most of this basin, indicating that yield loss was more important than evapotranspiration deficit, and there were greater effect of water stress on the yield of winter wheat.  相似文献   

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