The effect of uncertainty in aerodynamic resistance on evaporation estimates from the combination model

Evaporation estimates from a soybean crop calculated from the combination model are insensitive to aerodynamic resistance. The insensitivity arises from a strong link between evaporation and the vapour pressure deficit of the air and bulk stomatal resistance. The sensitivity of aerodynamic resistance to errors in surface roughness and zero-plane displacement is considered. The resistance is found to be more sensitive to errors in surface roughness than to errors in zero-plane displacement. However, large errors in these have little effect on calculated evaporation. Both surface roughness and zero-plane displacement are empirically related to crop height and leaf area index. Errors incurred by ignoring bluff-body effects and atmospheric stability are small in estimating both resistance and evaporation. Evaporation can be calculated adequately from empirical estimates of surface roughness and zero-plane displacement and single-level measurements of windspeed.     

Utility of Radiometric–aerodynamic Temperature Relations for Heat Flux Estimation

In many land-surface models using bulk transfer (one-source) approaches, the application of radiometric surface temperature observations in energy flux computations has given mixed results. This is due in part to the non-unique relationship between the so-called aerodynamic temperature, which relates to the efficiency of heat exchange between the land surface and overlying atmosphere, and a surface temperature measurement from a thermal-infrared radiometer, which largely corresponds to a weighted soil and canopy temperature as a function of radiometer viewing angle. A number of studies over the past several years using multi-source canopy models and/or experimental data have developed simplified methods to accommodate radiometric–aerodynamic temperature differences in one-source approaches. A recent investigation related the variability in the radiometric–aerodynamic relation to solar radiation using experimental data from a variety of landscapes, while another used a multi-source canopy model combined with measurements over a wide range in vegetation density to derive a relationship based on leaf area index. In this study, simulations by a detailed multi-source soil–plant–environment model, Cupid, which considers both radiative and turbulent exchanges across the soil–canopy–air interface, are used to explore the radiometric–aerodynamic temperature relations for a semi-arid shrubland ecosystem under a range of leaf area/canopy cover, soil moisture and meteorological conditions. The simulated radiometric-aerodynamic temperatures indicate that, while solar radiation and leaf area both strongly affect the magnitude of this temperature difference, the relationships are non-unique, having significant variability depending on local conditions. These simulations also show that soil–canopy temperature differences are highly correlated with variations in the radiometric–aerodynamic temperature differences, with the slope being primarily a function of leaf area. This result suggests that two-source schemes with reliable estimates of component soil and canopy temperatures and associated resistances may be better able to accommodate variability in the radiometric–aerodynamic relation for a wider range in vegetated canopy cover conditions than is possible with one-source schemes. However, comparisons of sensible heat flux estimates with Cupid using a simplified two-source model and a one-source model accommodating variability in the radiometric-aerodynamic relation based on vegetation density gave similar scatter. On the other hand, with experimental data from the shrubland site, the two-source model generally outperformed the one-source scheme. Clearly, vegetation density/leaf area has a major effect on the radiometric–aerodynamic temperature relation and must be considered in either one-source or two-source formulations. Hence these adjusted one-source models require similar inputs as in two-source approaches, but provide as output only bulk heat fluxes; this is not as useful for monitoring vegetation conditions.     

黑河地区土壤及小麦体内水分动态观测分析

研究了甘肃黑河地区麦田土壤水分的变化动态。对春小麦同部位叶片及不同土壤分析的叶水势、气孔导度和蒸腾强度的日变化规律进行了观测和分析。对春小麦叶鞘、叶片和茎水势的变化民政部进行了比较。分析了土壤水分对叶水势、气孔导度和蒸腾强度的影响。建立了相对应的函数关系。     

UNCERTAINTY IN THE SPECIFICATION OF SURFACE CHARACTERISTICS: A STUDY OF PREDICTION ERRORS IN THE BOUNDARY LAYER

The effects of uncertainty in the specification of surface characteristics on simulated atmospheric boundary layer (ABL) processes and structure were investigated using a one-dimensional soil-vegetation-boundary layer model. Observational data from the First International Satellite Land Surface Climatology Project Field Experiment were selected to quantify prediction errors in simulated boundary-layer parameters. Several numerical 12-hour simulations were performed to simulate the convective boundary-layer structure, starting at 0700 LT 6 June 1987.In the control simulation, measured surface parameters and atmospheric data were used to simulate observed boundary-layer processes. In the remaining simulations, five surface parameters – soil texture, initial soil moisture, minimum stomatal resistance, leaf area index, and vegetation cover – were varied systematically to study how uncertainty in the specification of these surface parameters affects simulated boundary-layer processes.The simulated uncertainty in the specification of these five surface parameters resulted in a wide range of errors in the prediction of turbulent fluxes, mean thermodynamic structure, and the depth of the ABL. Under certain conditions uncertainty in the specifications of soil texture and minimum stomatal resistance had the greatest influence on the boundary-layer structure. A lesser but still moderately strong effect on the simulated ABL resulted from (1) a small decrease (4%) in the observed initial soil moisture (although a large increase [40%] had only a marginal effect), and (2) a large reduction (66%) in the observed vegetation cover. High uncertainty in the specification of leaf area index had only a marginal impact on the simulated ABL. It was also found that the variations in these five surface parameters had a negligible effect on the simulated horizontal wind fields. On the other hand, these variations had a significant effect on the vertical distribution of turbulent heat fluxes, and on the predicted maximum boundary-layer depth, which varied from about 1400–2300 m across the 11 simulations. Thus, uncertainties in the specification of surface parameters can significantly affect the simulated boundary-layer structure in terms of meteorological and air quality model predictions.     

A comparison of two methods of estimating transpiration rates from a Sitka spruce plantation

Estimates of hourly transpiration from a 16–17 yr old Sitka spruce forest were calculated from the Penman-Monteith combination equation and compared with estimates from an eddy correlation/energy balance method.Canopy conductances were estimated from stomatal conductances measured using null balance diffusion porometers and took account of canopy variations of stomatal conductance and needle area index.Vertical heat fluxes were measured by the eddy correlation method; transpiration fluxes were then estimated from an energy balance of the forest.There was not a 1:1 relationship between the estimates of transpiration from the two methods. The major sources of error were concluded to be (i) difficulties of estimating the variation in stomatal conductance and leaf area through the canopy, (ii) errors in the value of total leaf area index, and (iii) errors in stomatal conductance measurements.The eddy correlation method was suggested as the more useful for future studies of the variation of forest transpiration in time or space, because the Penman-Monteith equation requires extensive biological measurements.     

土壤-植被-大气系统水分能量传输模拟和验证

本文建立了包括地下水的土壤-植被-大气系统水分能量传输综合模型,并对模型中冠层动量湍流交换反梯度传输现象的描述进行了改进。在此基础上,依据冠层内动量和热量的交换以及辐射传输过程,同时求解地表、冠层能量平衡方程,进而模拟饱和-非饱和土壤的水热传输。用浅地下水地区冬小麦田间试验资料对模型进行验证,结果表明,系统能量平衡各分量和土壤含水量的模拟与观测结果相当一致。模型敏感性分析发现,叶面积指数对总蒸散量的影响随叶面积指数的增加而逐渐减弱;叶片最小气孔阻力对总蒸散量的影响,在该阻力较小时更显着;地下水位对蒸散量的影响在它小于1.5m时不显着,而在1.5～1.75m之间时,蒸散减小较快,主要由于土壤蒸发减小显着,冠层蒸腾稍有增加。     

土壤水分对冬小麦影响机制研究

文中通过试验系统地研究了冬小麦叶片气孔形态与土壤湿度的关系,结果表明:土壤干旱使气孔密度增加,上表皮的密度大于下表皮;气孔开张度随土壤湿度下降而变小;气孔导度与土壤湿度呈指数相关,随土壤含水量的下降呈指数减少。随土壤湿度的改变小麦的生理过程也发生改变,蒸腾速率随土壤湿度下降呈指数减小。并研究了土壤干旱对叶绿素超微结构的影响及与脯氨酸的关系。     

Factors affecting the canopy resistance of a Douglas-fir forest

The physiological nature of canopy resistance was studied by comparing the stomatal and canopy resistance of a 10-m high Douglas-fir forest. Stomatal resistance of the needles was measured using porometry, while the canopy resistance was calculated using energy balance/Bowen ratio measurements of evapotranspiration. A typical steady increase in the forest canopy resistance during daytime hours, even at high soil water potentials, was observed. A similar trend in the stomatal resistance indicated that increasing canopy resistance during the daytime was caused by gradually closing stomata. During a dry period when soil water potentials declined from 0 to –10.5 bars, the mean daytime value of canopy resistance increased in proportion to the mean daytime value of the stomatal resistance. Values of canopy resistance calculated from stomatal resistance and leaf area index measurements agreed well with those calculated from energy balance measurements. The dependences of stomatal resistance on light, vapour pressure deficit, twig and soil water potentials art summarized.     

半干旱区陆面模式参数对水分循环的敏感性研究

植被覆盖对陆气之间物质和能量交换过程具有极其重要的影响,但植被覆盖对于交换过程的影响因子很多,关系复杂.作者研究了各种植被因子对陆气之间水分循环的作用和相对重要性.首先通过单点NO-AH模式对吉林通榆农田下垫面2004年土壤和边界层各物理量进行模拟,并与观测结果比较和评价,肯定了单点NOAH模式模拟能力.使用这一模式进行敏感试验,将与植被有关的参数分别在其取值范围取较大与较小值,比较水分循环各物理量如土壤湿度、土壤蒸发、植被蒸腾等的变化情况.试验表明在各参数中植被气孔阻抗、根系深度、土壤湿度初值和反照率对水分循环的影响较大,而叶面积指数、粗糙度和冠层阻抗则影响较小.     

土壤-植被-大气系统水分散失机理的数值模拟

以Eeardorff(1978)提出的陆面参数化方案和Noilhan等人(1989)土壤水分参数化方案为基础,对陆面物理过程参数化方案进行了改进,在模式中较详细地考虑了植被和地面的各种物理参量如地面和叶面的反射率和发射率,净叶面面积指数,植被的物理阻抗等,并与大气边界层模式耦合。应用该模式模拟了沙漠及绿洲地区不同植被覆盖率情况下的蒸散量、土壤含水量和表面温度的日变化和连续变化特征;对不同植被覆盖率的热量平衡特征进行了比较。结果表明该模式较好地反映了地表蒸散3阶段的变化趋势特征,揭示出下垫面热量平衡分量间的相互转换过程。该模式可以用于中尺度的气象和区域气候模式,模拟和预测不同植被覆盖情况下近地层的热量输送和水分散失情况。     

用Priestley-Taylor模式计算棉田实际蒸散量的研究

在农田蒸散试验资料的基础上,综合考虑影响棉田实际蒸散的气象条件,棉花生物学特性和土壤水分等因素,利用Priestley-Taylor模式、棉花叶面积指数和相对有效土壤湿度建立了棉田实际蒸散量的计算模式。该模式仅需常规气象和农业气象资料,具有较高的精度,便于在干旱区推广使用。     

A generalized combination equation derived from a multi-layer micrometeorological model

The discrete multi-layer model originally devised by Waggoner and Reifsnyder (1968) is used as a theoretical basis to describe the vegetation-atmosphere interaction. Mathematical development of the basic equations yields Ohm's law-type formulae for sensible and latent heat fluxes from which it is possible to derive a combination equation very close in form to Penman-Monteith's equation. A bulk aerodynamic resistance and a bulk stomatal resistance can be defined and expressed in terms of the elementary resistances of the multi-layer model. This new combination equation offers an alternative to the attempts undertaken by Shuttleworth (1976) to unify multi-layer and single-layer approaches.     

基于吉林省观测土壤水分的WOFOST模型模拟研究

利用吉林省白城站试验数据进行模型参数调整,通过独立的观测资料对生育期、叶面积指数、地上部分各器官生物量进行模拟验证与评价。以白城站和榆树站代表吉林省西部玉米种植区和中部黄金玉米带参数,利用农业气象观测站发育期资料、气象资料和经过质量控制后的逐日土壤水分自动站观测数据进行模拟。为了提高WOFOST模型模拟精度,将由模型通过降水量计算的土壤体积含水量替换为实测土壤水分计算的体积含水量,采用替换后的土壤体积含水量参与模型下一步运算,以此来模拟2001—2016年春玉米穗生物量变化状况,构建玉米土壤体积含水量改善率(PD)指标,来表征降水驱动和土壤水分驱动对作物模型模拟结果的影响。结果表明:(1)模型对白城站春玉米生育期、叶面积、地上部分总生物量和叶生物量较准确,而穗生物量模拟效果一般。(2)从代表站白城来看,穗生物量模拟值与降水量存在明显正相关,降水偏少的年份土壤模拟效果明显优于降水驱动。(3)从区域来看,以盐碱土为主的地区或降水量偏少的年型下土壤水分驱动效果优于降水驱动;在以黑土为主的区域或降水偏多的年型下,两者模拟效果基本接近。(4)总体来说,利用观测土壤水分替换降水量参与模型能够显著提高模型模拟精度。     

Satellite Monitoring of the Surface Water and Energy Budget in the Central Tibetan Plateau

The water and energy cycle in the Tibetan Plateau is an important component of Monsoon Asia and the global energy and water cycle. Using data at a CEOP （Coordinated Enhanced Observing Period）-Tibet site, this study presents a first-order evaluation on the skill of weather forecasting from GCMs and satellites in producing precipitation and radiation estimates. The satellite data, together with the satellite leaf area index, are then integrated into a land data assimilation system （LDAS-UT） to estimate the soil moisture and surface energy budget on the Plateau. The system directly assimilates the satellite microwave brightness temperature, which is strongly affected by soil moisture but not by cloud layers, into a simple biosphere model. A major feature of this system is a dual-pass assimilation technique, which can auto-calibrate model parameters in one pass and estimate the soil moisture and energy budget in the other pass. The system outputs, including soil moisture, surface temperature, surface energy partition, and the Bowen ratio, are compared with observations, land surface models, the Global Land Data Assimilation System, and four general circulation models. The results show that this satellite data-based system has a high potential for a reliable estimation of the regional surface energy budget on the Plateau.     

Prediction of Changes in Soil Moisture Associated with Climatic Changes and Their Implications for Vegetation Changes: Waves Model Simulation on Taihang Mountain, China

The WAVES model was used to simulate the effect of global warming on soil moisture on the semi-arid Taihang Mountain in China. Parameters of the WAVES model were first adjusted according to soil moisture data from a field global warming experiment. Then, the reliability of WAVES in predicting soil moisture changes induced by climatic change was confirmed by comparing the simulated and observed soil moisture values under different climatic conditions and plant growth rates of another field treatment. Next, 10 climate change scenarios incorporating increases in temperature and changes in precipitation were designed. When a simulation was conducted using the leaf area index (LAI) growth pattern from a field experiment under the present climatic conditions, the results suggested that the combination of temperature increase and precipitation decrease would greatly decrease soil water content throughout the entire simulation period. On the other hand, only when precipitation increased by 20% and temperatureincreased by 2 °C, the effect of precipitation increase on soil moisture was obviously positive. Although soil moisture conditions in the T2P1 (temperature increase by 2 °C and precipitation increase by 10%) and T4P2 (temperature increase by 4 °C and precipitation increase by 20%) scenarios were slightly better during the rainy season and notmuch changed before the rainy season, the positive effect of 10%precipitation increase on soil moisture was totally offset by moisture decrease caused bya 4 °C temperature increase in the T4P1 scenario. At the same time, the trends of soil-moisture change were highly coincident with predicted changes in productivity. Finally, the predicted LAI values from other studies were combined with the climatic change scenarios and used in the simulation. The results showed that changes in LAI alleviated, at least to some extent, the effects of temperature and precipitation changes on soil moisture.     

Daytime variations of ozone eddy fluxes to maize

The vertical fluxes of ozone, momentum and heat in the atmospheric surface layer have been measured by eddy correlation above both mature and senescent maize canopies. Aerodynamic formulae are applied to find that the bulk canopy surface resistancer _c to ozone uptake and destruction varies between 4.0 and 0.5 s cm⁻¹ during the daytime. Apparently, surface properties tend to control the removal of ozone at the surface of the earth. For a lush canopy, the stomatal diffusion resistance is the most important property, while changes in surface temperature have little effect. Destruction at the soil and exterior plant surfaces appears to account for 20–50% of the total loss if leaf mesophyll resistances are assumed to be very small. Free water at leaf surfaces may at times inhibit ozone removal by both senescent and healthy plants.     

Modelling of Seasonal Evapotranspiration from an Agricultural Field Using the Canadian Land Surface Scheme (CLASS) with a Pedotransfer Rule and Multicriteria Optimization

The performance of the Canadian Land Surface Scheme (CLASS 3.5) was assessed using turbulent fluxes derived from data recorded at two micrometeorological stations located in a potato field in Quebec, Canada. The minimum stomatal resistance, the maximum leaf area index, and the initial water content of the third soil layer were optimized using the Non-Dominated Sorting Genetic Algorithm-II and the mean square error of the latent heat flux. With respect to benchmark solutions, the optimization improved the sensible and latent heat fluxes by 31 and 23%, respectively. The use of a pedotransfer rule in adjustment of the water content of mineral soils having small percentages of organic matter provided better estimates of the evapotranspiration during the growing stage. However, like the original version of the model (without the pedotransfer rule), it underestimated evapotranspiration throughout the maturity stage. It is noteworthy that the original version produced a good estimate of cumulative evapotranspiration over the entire season as a result of over- and underestimates at the beginning and maturity stage of the growing season, respectively.     

唐古拉地区活动层水热状况及地气系统能水平衡分析

活动层水热状况与地-气系统间能水交换直接影响着寒区生态环境、水文过程以及多年冻土的稳定性。利用唐古拉站2007年实测资料和SHAW模型,对研究点活动层土壤剖面温湿度进行了模拟。土壤温度方面,模型的纳什效率系数NSE≥0.93;水分方面,纳什效率系数的平均值为0.69,说明SHAW模型可用于多年冻土区活动层内水热动态变化的模拟研究。基于模型的输出结果,对唐古拉站活动层土壤冻融过程中的水分动态、地表能量收支的变化特征进行了分析讨论。结果表明:(1)活动层冻融过程中,土壤水分的冻结和融化响应时间随土壤深度的增加而逐渐滞后,水分迁移通量随土壤深度的增加逐渐减小;(2)地表能量平衡收支在季风活动引起的降水与活动层的冻融循环共同影响下,表现出明显的季节性变化特征。同时,通过改变SHAW模型植被输入参数中的叶面积指数,分析了植被覆盖变化对多年冻土区土壤蒸散发的影响。结果表明:植被蒸腾量、土壤蒸发量与总的蒸散发量与植被的叶面积指数呈正相关关系,而浅层土壤含水率(20 cm)则表现为负相关,当叶面积指数在-100%(裸土)~100%变化时,总蒸散发量的变化幅度为-5%~13%。     

Aerodynamic and canopy resistance of short-rotation forest in relation to leaf area index and climate

The aerodynamic and canopy resistances of a willow short-rotation stand were estimated during the course of a growing season on the basis of micrometeorological measurements. The normalized roughness length (z ₀/h) decreased from about 0.10 at a leaf area index of one to 0.05 at a leaf area index of seven. This implies that the aerodynamic resistance at peak leaf area index is more than twice the value at zero leaf area index, all other variables unchanged. The canopy resistance depended strongly on air water concentration deficit and on leaf area index. The Lohammar equation showed good agreement between estimated and measured canopy resistances over the whole course of leaf development. The stand was well-coupled to the atmosphere only for very small values of leaf area indices, less than one, and it was practically de-coupled for leaf area indices above two. From the point of view of factors controlling evaporation, this type of stand acts as a traditional forest at the beginning and end of the season and as an agricultural crop in the middle of the season.