人工模拟降雨的能量相似及其实现

 能量相似是降雨模拟的基本原则。通过对雨滴粒径、雨滴击溅速度、降雨均匀度等能量相似要素的分析和探讨,提出实现降雨模拟能量相似的重要手段,即在降雨高度、雨滴发生器等条件不变的情况下,通过调节雨强来实现降雨能量的相似;通过加大供水压力等手段可提高降雨动能,所以在降雨模拟实践中,降雨高度不一定需要达到使所有粒径的雨滴均达到终速所需的高度。     

Vergleich deutscher und schweizer Regensimulatoren - Einfluß der Regeneigenschaften auf die Bodenoberflächenrauhigkeit

Comparison of German and Swiss Rainfall Simulators - Influence of Rainfall Characteristics on Soil Surface Roughness Six different field rainfall simulators were compared. Soil surface roughness was tested by a simple field method before and after rainfall simulation. Preparation of the test site with a rotary cultivator inadvertently entailed spatial variability of roughness and aggregate size. Thus, the plots with finer seedbed showed higher runoff and soil loss. The relative decrease of roughness could best be correlated with the total kinetic energy as well as with the R factor of the different simulators (r = 0.87 resp. 0.91). Thus, the degree of soil surface leveling was shown to be due to the rainfall characteristics of the different simulator types. However, there was no relation between change of soil surface roughness and runoff. The relative classification of the kinetic energy of the rainfall simulators explained the leveling of the soil surface better than the classifications previously used in many studies (r = 0.91 as against r = 0.73).     

Ein Vergleich deutscher und schweizerischer Regensimulatoren nach Regenstruktur und kinetischer Energie

Comparison of German and Swiss Rainfall Simulators - Rain Structure and Kinetic Energy The drop size distributions of the investigated rain are significantly different. The rainfall simulators Swanson, Bonn, Basel and Trier culminate in the 2 - 3 mm drop diameter class (～40%) and thus simulate best the drop size distribution of natural heavy rainfall. The structure and the kinetic energy of the rains of eight rainfall simulators have been investigated. The data are compared to those registered during a thunder-shower at the research station of Mertesdorf near Trier/FRG. The results are summarized as follows:

• The impact velocity of artificial raindrops depends on the water pressure within the rainfall simulator, the initial velocity of the raindrops and the fall height. A correction factor to match the kinetic energy of natural rainfall, which is based on the percentage of deviation from the terminal velocity, can be applied only to some extend.
• With respect to various fall velocities of the artificial raindrops all tested rainfall simulators except one yield a rainfall energy between 19 and 27 J/m² per mm of rainfall.
• The actual amount of precipitation measured at the end of the rainfall experiments sometimes differs significantly from the aimed value of 60 mm/h. Due to this variation, the total rainfall energy during the experiments accumulated to 9 - 15 MJ/ha.
• This high variability depends on the used rainfall simulator. But it also demonstrates the importance of precipitation measurements on the plot during simulations in order to exactly calculate the rainfall energy.

     

Using meshes to change the characteristics of simulated rainfall produced by spray nozzles

Rainfall simulators have been used for many years contributing to the understanding of soil and water conservation processes. Nevertheless, rainfall simulators’ design and operation might be rather demanding for achieving specific rainfall intensity distributions and drop characteristics and are still open for improvement. This study explores the potential of combining spray nozzle simulators with meshes to change rainfall characteristics, namely drop properties (drop diameters and fall speeds). A rainfall simulator laboratory set-up was prepared that enabled the incorporation of different wire meshes beneath the spray nozzles. The tests conducted in this exploratory work included different types of spray nozzles, mesh materials (plastic and steel), square apertures and wire thicknesses, and positions of the meshes in relation to the nozzles. Rainfall intensity and drop size distribution and fall speed were analysed. Results showed that the meshes combined with nozzles increased the mean rainfall intensity on the 1 m² control plot below the nozzle and altered the rain drops’ properties, by increasing the mass-weighted mean drop diameter, for example.     

Comparison between rainfall simulator erosion and observed reservoir sedimentation in an erosion-sensitive semiarid catchment

Estimating catchment scale soil loss based on rainfall simulators is often hampered by the difficulty to scale up simulator results. Our objective was to develop and test a method for estimating catchment scale soil loss based on observed rainfall using a variable intensity rainfall simulator in an erosion-sensitive catchment in semiarid Tunisia. A 7-year period, 1992–1999, with observed sedimentation amounts in a downstream reservoir was chosen to test a methodology. The methodology was based on (1) energy adjustment for the used simulator due to the difference in kinetic energy of simulated and natural rainfall at equal intensities and (2) upscaling of simulated erosion in which rill erosion was estimated by adjusting the difference between slope lengths for the plots versus the catchment after onset of runoff. The comparison between calculated soil loss from rainfall simulator experiments and observed sedimentation in the downstream reservoir displayed good overall results. Calculated soil loss was found to be about 96%, 36%, and 80% for different observed subperiods, respectively. The observed low value for the second period was probably due an exceptionally intense rainfall event during this period, which appears to have led to gully erosion, soil slide, and riverbank collapse. Therefore, during this event, siltation in the reservoir may essentially be due to unaccounted erosion processes such as gully erosion. Overall, however, it appears that plot-scale variable intensity rainfall simulators can rather successfully estimate catchment scale soil losses.     

A mathematical model for evaluating the effect of wind on downward-spraying rainfall simulators

Rainfall simulators are used to study a variety of different processes (e.g., water erosion, infiltration, overland flow, irrigation, movement of agrochemicals, etc.). Wind affects field experiments that make use of rainfall simulators. Water-drop trajectories and velocities are altered, affecting water application and kinetic energy distributions.In this study, a three-dimensional numerical model was developed from the movement of individual drops after their release from the nozzle of a downward-spraying rainfall simulator. Drag forces, wind and gravity affect the original momentum of a single drop. Water application and kinetic energy were estimated from the coupling of a hydrodynamic model for drop movement, a drop generator representing a single full-cone spray nozzle, and an appropriate interception algorithm at the soil surface.The mathematical model should facilitate the selection of single full-cone spray nozzles and the size and configuration of the spray area for rainfall simulation in order to achieve high application uniformity values on the plot area. It can contribute to the adequate choice of nozzles as well as operating conditions necessary for laboratory and field purposes. Laboratory and field experiments were conducted to evaluate the adequacy of the proposed methodology.     

Preliminary study on mechanics-based rainfall kinetic energy

A raindrop impact power observation system was employed to observe the real-time raindrop impact power during a rainfall event and to analyze the corresponding rainfall characteristics. The experiments were conducted at different simulated rainfall intensities. As rainfall intensity increased, the observed impact power increased linearly indicating the power observation system would be satisfactory for characterizing rainfall erosivity. Momentum is the product of mass and velocity (Momentum=MV), which is related to the observed impact power value. Since there is no significant difference between momentum and impact power, observed impact power can represent momentum for different rainfall intensities. The relationship between momentum and the observed impact power provides a convenient way to calculate rainfall kinetic energy. The value of rainfall kinetic energy based on the observed impact power was higher than the classic rainfall kinetic energy. The rainfall impact power based kinetic energy and the classic rainfall kinetic energy showed linear correlation, which indicates that the raindrop impact power observation system can characterize rainfall kinetic energy. The article establishes a preliminary way to calculate rainfall kinetic energy by using the real-time observed momentum, providing a foundation for replacing the traditional methods for estimating kinetic energy of rainstorms.     

Comparison of German and Swiss Rainfall Simulators - Experimental Setup

Rainfall simulators, that are commonly used in erosion research should produce comparable runoff and soil loss. At present five types of field rainfall simulators are used in Germany and Switzerland. The experimental setup on a Hapludalf soil site for testing these simulators and one laboratory rainfall simulator are described. Fifty rainfall simulations were carried out during two days on a homogeneous field. Hence, small deviations in slope steepness (< 2%) resulted in 40% deviations in the S factor of the USLE. This strong influence of slope steepness necessitates a high accuracy in the measurement of this parameter. Soil moisture during the two days was very high. This reduced slaking due to air inclusions and should have helped to detect differences in soil detachment due to the different rain structures and test procedures.     

泾河南小河沟流域自然降雨特性

采用德国HSC-OTT Persivel32激光雨滴谱探测仪监测南小河沟流域自然降雨的雨滴谱,分析不同雨型自然降雨雨滴特征及其分布规律,并分析了降雨特性各要素间的相互关系。结果表明:(1)该区自然降雨雨滴直径集中在0.3~1.5mm,与黄土高原利用传统的滤纸色斑法测定结果基本一致,但略小于东北和闽南地区。当雨强I10mm/h时,雨滴中数直径随雨强的增大而增大;当I≥10mm/h时,雨滴中数直径骤然下降,这在一定程度上和雨强增大后大雨滴破裂有关。该地区雨谱随雨强的变化可用Best分布拟合。(2)普通型降雨雨滴数量随直径的分布曲线较平缓,其动能与雨强呈线性相关;而短阵型降雨的雨滴数量随直径的分布曲线较陡峭,其动能与雨强呈幂函数关系。两种雨型的降雨动能均随雨强的增大而增大,但短阵型降雨动能增大的更快,因此短阵型降雨的侵蚀力更大。     

Methodologies for interrill soil erosion studies

Due to wide range of experimental techniques reported in the literature for determining interrill erodibility and soil loss values, meaningful comparisons between experiments often cannot be made. Furthermore, inaccurate concepts are developed because erosion processes are dependent upon methodologies. The purpose of this paper is to discuss problems related to both laboratory and field rainfall simulator experiments. Rainfall simulators cannot duplicate a wide range of rainfall intensities and, at the same time, have similar energies as natural rainstorms, unless several different nozzles are used. Rainfall intensity in most simulators is created by varying the frequency of spray oscillation. This intermittent spray characteristic of most simulators, and the constant drop size characteristic of other simulators, greatly affects results. Erosion pan design for laboratory studies and preparation of soil samples placed in the pans also can influence erosion results. We conclude that standardization of rainfall simulator design and test procedures will allow better comparison of erosion results to be made among researchers.     

Mechanisms and processes of crust formation on artificial aggregates

Crust formation process initiates from aggregate disinategration occurred through the combined effects of inherent soil properties and external actions. Rainfall plays a major role in determining the moisture conditions of the soil surface and in triggering aggregate disintegration mainly through slaking and the impact off raindrops. To analyze the relations between rainfall characteristics and crust formation, three artificially granulated aggregates were exposed to natural rainfall events. The rainfall characteristics were expressed in terms of the distribution pattern of rainfall intensity, I _10min; mean weighted drop diameters, D _n; raindrop concentration, N _Dn,; rainfall energy, Et;and kinetic energy, KE; as well as conventionally used parameters, such as total amount and mean intensity of rainfall. The degree of crusting was evaluated in terms of the water permeability and morphological observation just after the rainfall treatment. Based on a comparison of the rainfall characteristics and degree of crusting, we confirmed the following relations. Rainfall characteristics at the onset of the event considerably influence the degree of crusting, i.e. initial rainfall with high I _10min; and large drop size promotes aggregate disintegration by slaking due to rapid wetting, whereas initial gentle rainfall prevents slaking by slow wetting and subsequent crusting. When initial rainfall is not saaficient for slaking but Beads to a weakening sf the aggregates, subsequent disintegration and crusting are promoted by impact together with the residual influence of slaking. Impact is a dominant factor for aggregate disintegration under wet conditions after initial rainfall which is so gentle that slaking never occurs. Responses of the soils under different natural rainfall conditions were almost consistent with the results under simulated rainfall reported in the previous papers (Tanaka et $1.1997: Soil Sci. Plant Nutr., 43, 99-107, 109-115). Crusting was not observed in the Fujino 1–3 soil gender any rainfall events. In the Wokuei 1–3 soil, characterized by a susceptibility to slaking and stability to impact, crusting was only formed under the rainfall with high I _{10 min} at the onset. For the Shimazu 1 soil, impact played a dominant role in crusting under any rainfall events, and a larger degree of crusting was found when the effects of impact and slaking were combined. The characteristics of rainfall monitored in this study may be within a normal range as natural rainfall in terms of the amount and the mean intensity, suggesting that crusting likely to occur if the aggregate stability of soils is Bower than or similar to that sf the Shimaza 1 and the Hokuei 1–3 soils.     

模拟降雨下坡面草带分布对产流产沙过程的影响

为深入探讨坡面植被分布对产流产沙过程的影响,定量分析植被分布与坡面产流产沙的关系,通过人工降雨试验,在15°坡面条件下,对不同植被覆盖度（40%,60%）、不同降雨强度（30,60,90 mm/h）和不同植被分布位置（相对距离为0,0.2,0.4,0.6,0.8,1.0）条件下的坡面产流产沙进行观测,分析不同植被分布条件下的水土保持作用,提出了不同覆盖度下控制水土流失的植被优化配置。结果表明：（1）在一定坡度和降雨强度条件下,不同植被分布条件下坡面产流率和产沙率均表现为先迅速增加后趋于稳定的变化趋势;（2）坡面平均产流率和产沙率随着相对距离的增加表现出先减小后增大的趋势,植被相对距离为0.2的坡面的平均产流率在不同实验条件下均为最小;（3）通过随机森林算法发现,降雨强度和植被覆盖度对产流具有重要影响,降雨强度和植被相对距离对产沙具有重要影响;（4）当植被覆盖度为40%时,以减少径流和泥沙为主要目标的最优化植被相对位置分别为0~0.36,0~0.31;当植被覆盖度为60%时,以减少径流和泥沙为主要目标的最优化植被相对位置分别为0~0.43,0~0.22。表明坡面植被分布对产流产沙有重要的影响,在相同植被覆盖度条件下,草带的相对距离越小,对减少径流和泥沙的作用较好。研究成果可为生态恢复过程中植被的优化配置提供理论依据和数据支撑。     

便携式人工模拟降雨装置的设计与率定

用人工模拟降雨研究径流,可以大大缩短试验周期,加速雨水入渗规律和土壤侵蚀规律的研究进程。目前的人工模拟降雨装置普遍存在体积庞大、操作复杂、试验成本高等问题,在现实应用中难以推广。该文针对以上问题设计了一种由喷淋系统和供水系统构成的便携式人工模拟降雨装置。该装置采用单喷头、下喷式模拟降雨结构,通过控制输水管道供水压力和改变喷头型号,实现了不同雨强、不同历时的人工模拟降雨过程。通过降雨特性试验表明,该装置可实现降雨强度为0～140mm/h的模拟降雨,降雨均匀性能保持在80%以上,雨滴直径主要分布在0.1～5.5mm之间,能够满足不同直径的雨滴获得2～2.9m/s的终点速度。试验结果与天然降雨有较高的相似性,可有效实现模拟降雨。     

用于土壤侵蚀试验的降雨模拟器研究进展

述评悬线式、针头式、管网式、喷头式4种降雨模拟器的特性及其应用现状与前景。悬线式和针头式降雨模拟器具有降雨强度下限低、均匀性好等优点,但在模拟降雨时,控制面积小,雨滴粒径偏大而且单一,所以应用渐少。管网式与喷头式降雨模拟器所产生的雨滴粒径与能量分布与天然降雨更为接近,广泛应用于土壤侵蚀试验研究中。目前,智能化、自动化、大型化的降雨模拟大厅不断涌现,同时根据雨滴发生基本原理搭建的手控、简易的降雨模拟器仍有其强大的生命力。     

Increasing the Rainfall Kinetic Energy of Spray Nozzles by using Meshes

Rainfall simulators are an important tool in studying soil erosion, which is a key process contributing to land degradation. The kinetic energy of simulated rain is central to these studies and it is used as an indicator of the raindrops' ability to detach particles from the soil surface. The main purpose of this experimental work was to explore the usefulness of incorporating meshes underneath pressurised nozzles' rain simulators that intercept the drops sprayed out by the nozzles and change the simulated rain characteristics, namely by increasing the rainfall kinetic energy. The laboratory experiments included testing four types of spray nozzles (discharge from 2·3 to 11·9 L min⁻¹), combined with a high‐density polyethylene mesh (square aperture of 20 mm). The effect of the mesh was studied for three vertical distances between the nozzle and the mesh (0·20, 0·40 and 0·60 m). A laser disdrometer was used to measure the diameter and fall speed of the simulated raindrops. For the mesh‐free simulations, the nozzles produced drops having on average a mean equivalent diameter of around 0·6 mm and a mean fall speed of about 1·5 m s⁻¹. The mesh increased the formation of bigger drops (>2·5 mm) and, consequently, increased the rainfall kinetic energy of the simulated rain; the magnitude of this increase varied with the spray produced by the nozzles. Results show that meshes can be useful for increasing the kinetic energy of the rainfall simulated by nozzles within soil erosion studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparison of German and Swiss Rainfall Simulators - Accuracy of Measurement and Effect of Rainfall Sequence on Runoff and Soil Loss Rates

An important characteristic of the suitability of a specific rainfall simulator is the accuracy that can be obtained in measuring runoff rate, sediment concentration and soil loss rate. The accuracy of the different simulators currently in use in Germany and Switzerland was determined. The simulators differed greatly in the accuracy of sediment concentrations, runoff rates and soil loss rates they produced. Major sources of error were small plot sizes and wind drift. However, most of the simulators were suitable for erosion research (Average confidence interval: 30%). The time measurement of the runoff rates limits the accuracy of the best measuring method actually available. The smallest increment that can be measured during a run is one second. With high runoff rates, rounding to the next full second can cause the runoff rate to alternate by 10%. In erosion experiments, a sequence of rains is often applied on a plot to examine different soil conditions. Several sequences are in use. To facilitate comparison of the sediment concentration, runoff rates, and soil loss rates obtained with different sequences, it was concluded that the total rain duration should at least be 90 minutes and that the first run should last at least one hour. Therefore, a sequence is recommended that has a 1 h first run, a 0.25 h break and a 0.5 h second run. Sequences with a second and third run on the following day are disadvantageous in most cases.     

鄂东南花岗岩红壤不同土层侵蚀过程

为了探讨不同降雨动能条件下花岗岩红壤各层次(淋溶层、淀积层、母质层)在坡面侵蚀过程中的产流产沙特征,研究采用室内人工模拟降雨试验方法,对其水动力学参数变化特征及其与土壤侵蚀速率之间的定量关系进行了研究。结果表明:(1)降雨动能对各层次土壤坡面的产沙产流特征影响显著。随着降雨动能增加,各层次土壤初始产流时间不断递减,且径流含沙量和总产沙量均不断增大;(2)各层次土壤之间的侵蚀规律差异显著。初始产流时间、径流含沙量及总产沙量均表现出显著差异; 通过回归分析,发现土壤侵蚀速率与降雨动能之间有良好的线性关系;(3)水流功率和单位水流功率与土壤侵蚀速率均呈显著的相关性,且能够用幂函数准确地描述它们之间的关系,引入土壤黏粒含量可使得模型更加可靠,在实际应用中有更广的应用范围和现实价值。综上,降雨动能和土壤质地对花岗岩红壤坡面侵蚀的形成和发展过程具有显著影响。     

西北地区降雨集中度时空演变及其影响因素

在全球气候变化背景下,深入研究中国西北地区降雨集中度时空演变规律具有重要的现实意义。该研究基于1960—2017年逐月栅格降雨数据,利用Mann-Kendall趋势检验方法、Mann-Kendall突变点检验方法、Morlet小波方法和冷热点分析方法分析了西北地区降雨集中度的时空变化特征,并通过交叉小波变换探讨大气环流因子变化与降雨集中度的关系,同时讨论地貌分布对降雨集中度的影响。结果表明:1)在1960—2017年,西北地区及其3个子区域的降雨集中度平均值呈现减少趋势,且存在显著突变点(P0.05),降雨量年内分配不均,存在明显的季节性变化,部分区域降雨量年内异常集中且降雨集中度未来将会持续减少;2)整个研究区及其3个子区域在1960—2017年平均降雨集中度变化均存在一个40 a左右的主周期和一个24 a左右的次周期,各个子区域降雨集中度的变化与研究区整体的周期变化保持一致性;3)在1960—2017年,降雨集中度空间分布存在冷热点,冷点区域降雨集中度呈现显著减少趋势(P0.05)。热点区域降雨集中度呈现不显著减少趋势,年际变化幅度高于冷点区域;4)北大西洋涛动和太平洋十年涛动指数等大气环流因子变化对降雨集中度的变化具有较强的影响,不同的大气环流因子对降雨集中度的影响存在差异。研究成果将有助于进一步深化对西北地区降雨年内分配变化的认识,为西北地区生态环境保护和水资源规划制定提供一定的科学依据。     

极端降雨对森林集水区之冲击与因应对策

近年来由于全球暖化,极端气候日渐加剧,暴雨所带来的土砂灾害,将破坏森林集水区的地景结构与功能,如何减低或因应极端气候所带来的森林集水区冲击,已是未来集水区经营必须面临的重要课题。该研究以高屏溪集水区为研究范围,以17 a自动测站雨量资料,进行极端降雨趋势分析,并以2001年与2009年桃芝与莫拉克台风所带来的不同降雨量,以卫星影像探讨集水区内不同土地使用分区崩塌比及其变化,以及相对应之降雨量资料,探讨极端降雨与崩塌发生与土地使用分区之关系。研究结果显现2005年以后,高屏溪集水区发生极端气候之频、降雨量与降雨强度有升高趋势。两场不同台风之最大24 h延时降雨量,莫拉克台风为桃芝台风的1.92倍。桃芝台风所造成之崩塌比,自然保护区由灾前的0.64%提升至灾后的0.86%,国土保安区由灾前的0.95%提升至灾后的1.15%,该场台风降雨以地形较为陡峻之自然保护区及国土保安区新增崩塌地比例相对较高,其冲击较为严重,而莫拉克台风则因属超大豪雨（单日累积降雨量达350 mm以上）,其降雨范围小且集中,所造成之新增的崩塌范围高出桃芝台风的11.81倍,对于地形相对较为平坦之经济林区,亦造成大量的新增崩塌地,显现超大豪雨级以上的极端降雨,造成林地脆弱,使诱发崩塌之地形条件门槛降低,完整的植生覆盖已无法抵挡极端降雨的冲击,未来林地使用分区经营必须要有新的思维。     

The Wageningen Rainfall Simulator: Set‐up and Calibration of an Indoor Nozzle‐Type Rainfall Simulator for Soil Erosion Studies

The set‐up and characterisation of an indoor nozzle‐type rainfall simulator (RS) at Wageningen University, the Netherlands, are presented. It is equipped with four Lechler nozzles (two nr. 460·788 and two nr. 461·008). The tilting irrigation plot is 6 m long and 2·5 m wide. An electrical pump supplies the constant flow during the experiments. The spatial distribution of the rainfall was measured with 60 rain gauges equally distributed on the experimental plot. Thies^® Laser Precipitation Monitor was used to measure the size and falling velocity of the raindrops. Four different flow rates were applied (Q1–4). From the collected data, spatial rainfall intensity and spatial kinetic energy distribution maps were created; Christiansen uniformity coefficient was calculated for each flow rate. The results of the experiments revealed that the rainfall parameters (spatial rainfall intensity, kinetic energy, raindrop size distribution and fall velocity) in the RS are not homogeneous (Christiansen uniformity ranges from 68·5% to 83·2%). Accordingly, the whole plot can only be irrigated irregularly applying a wide range of intensities and rainfall energies. The RS offers a good opportunity to study great variety of process intensities such as splash erosion, runoff generation, soil aggregate stability, organic matter migration and scaled landscape development. Copyright © 2015 John Wiley & Sons, Ltd.