Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)an...Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)and soil moisture(SM)conditions on a land surface energy and water simulation in the permafrost region in the Tibetan Plateau(TP)using the Community Land Model version 5.0(CLM5.0).The results indicate that the default initial schemes for ST and SM in CLM5.0 were simplistic,and inaccurately represented the soil characteristics of permafrost in the TP which led to underestimating ST during the freezing period while overestimating ST and underestimating SLW during the thawing period at the XDT site.Applying the long-term spin-up method to obtain initial soil conditions has only led to limited improvement in simulating soil hydrothermal and surface energy fluxes.The modified initial soil schemes proposed in this study comprehensively incorporate the characteristics of permafrost,which coexists with soil liquid water(SLW),and soil ice(SI)when the ST is below freezing temperature,effectively enhancing the accuracy of the simulated soil hydrothermal and surface energy fluxes.Consequently,the modified initial soil schemes greatly improved upon the results achieved through the long-term spin-up method.Three modified initial soil schemes experiments resulted in a 64%,88%,and 77%reduction in the average mean bias error(MBE)of ST,and a 13%,21%,and 19%reduction in the average root-mean-square error(RMSE)of SLW compared to the default simulation results.Also,the average MBE of net radiation was reduced by 7%,22%,and 21%.展开更多
A frozen soil parameterization coupling of thermal and hydrological processes is used to investigate how frozen soil processes affect water and energy balances in seasonal frozen soil. Simulation results of soil liqui...A frozen soil parameterization coupling of thermal and hydrological processes is used to investigate how frozen soil processes affect water and energy balances in seasonal frozen soil. Simulation results of soil liquid water content and temperature using soil model with and without the inclusion of freezing and thawing processes are evaluated against observations at the Rosemount field station. By comparing the simulated water and heat fluxes of the two cases, the role of phase change processes in the water and energy balances is analyzed. Soil freezing induces upward water flow towards the freezing front and increases soil water content in the upper soil layer. In particular, soil ice obviously prevents and delays the infiltration during rain at Rosemount. In addition, soil freezingthawing processes alter the partitioning of surface energy fluxes and lead the soil to release more sensible heat into the atmosphere during freezing periods.展开更多
The California Simulation of Evapotranspiration of Applied Water (CaI-SIMETAW) model is a new tool developed by the California Department of Water Resources and the University of California, Davis to perform daily s...The California Simulation of Evapotranspiration of Applied Water (CaI-SIMETAW) model is a new tool developed by the California Department of Water Resources and the University of California, Davis to perform daily soil water balance and determine crop evapotranspiration (ETo), evapotranspiration of applied water (ETaw), and applied water (AW) for use in California water resources planning. ETaw is a seasonal estimate of the water needed to irrigate a crop assuming 100% irrigation efficiency. The model accounts for soils, crop coefficients, rooting depths, seepage, etc. that influence crop water balance. It provides spatial soil and climate information and it uses historical crop and land-use category information to provide seasonal water balance estimates by combinations of detailed analysis unit and county (DAU/County) over Califomia. The result is a large data base of ETc and ETaw that will be used to update information in the new California Water Plan (CWP). The application uses the daily climate data, i.e., maximum (Tx) and minimum (Tn) temperature and precipitation (Pcp), which were derived from monthly USDA-NRCS PRISM data (PRISM Group 2011) and daily US National Climate Data Center (NCDC) climate station data to cover California on a 4 kmx4 km change grid spacing. The application uses daily weather data to determine reference evapotranspiration (ETo), using the Hargreaves-Samani (HS) equation (Hargreaves and Samani 1982, 1985). Because the HS equation is based on temperature only, ETo from the HS equation were compared with CIMIS ETo at the same locations using available CIMIS data to determine correction factors to estimate CIMIS ETo from the HS ETo to account for spatial climate differences. CaI-SIMETAW also employs near real-time reference evapotranspiration (ETo) information from Spatial CIMIS, which is a model that combines weather station data and remote sensing to provide a grid of ETo information. A second database containing the available soil water holding capacity and soil depth information for all of California was also developed from the USDA-NRCS SSURGO database. The Cal-SIMETAW program also has the ability to generate daily weather data from monthly mean values for use in studying climate change scenarios and their possible impacts on water demand in the state. The key objective of this project is to improve the accuracy of water use estimates for the California Water Plan (CWP), which provides a comprehensive report on water supply, demand, and management in California. In this paper, we will discuss the model and how it determines ETaw for use in water resources planning.展开更多
The land-atmosphere energy and turbulence exchange is key to understanding land surface processes on the Tibetan Plateau(TP). Using observed data for Aug. 4 to Dec. 3, 2012 from the Bujiao observation point(BJ) of the...The land-atmosphere energy and turbulence exchange is key to understanding land surface processes on the Tibetan Plateau(TP). Using observed data for Aug. 4 to Dec. 3, 2012 from the Bujiao observation point(BJ) of the Nagqu Plateau Climate and Environment Station(NPCE-BJ), different characteristics of the energy flux during the Asian summer monsoon(ASM) season and post-monsoon period were analyzed. This study outlines the impact of the ASM on energy fluxes in the central TP. It also demonstrates that the surface energy closure rate during the ASM season is higher than that of the post-monsoon period. Footprint modeling shows the distribution of data quality assessments(QA) and quality controls(QC) surrounding the observation point. The measured turbulent flux data at the NPCE-BJ site were highly representative of the target land-use type. The target surface contributed more to the fluxes under unstable conditions than under stable conditions. The main wind directions(180° and 210°) with the highest data density showed flux contributions reaching 100%, even under stable conditions. The lowest flux contributions were found in sectors with low data density, e.g., 90.4% in the 360° sector under stable conditions during the ASM season. Lastly, a surface energy water balance(SEWAB) model was used to gap-fill any absent or corrected turbulence data. The potential simulation error was also explored in this study. The Nash-Sutcliffe model efficiency coefficients(NSEs) of the observed fluxes with the SEWAB model runs were 0.78 for sensible heat flux and 0.63 for latent heat flux during the ASM season, but unrealistic values of-0.9 for latent heat flux during the post-monsoon period.展开更多
Understanding the energy balance on the Tibetan Plateau is important for better prediction of global climate change. To characterize the energy balance on the Plateau, we examined the radiation balance and the respons...Understanding the energy balance on the Tibetan Plateau is important for better prediction of global climate change. To characterize the energy balance on the Plateau, we examined the radiation balance and the response of albedo to environmental factors above an alpine meadow and an alpine wetland surfaces in the eastern Tibetan Plateau, using 2014 data. Although our two sites belong to the same climatic background, and are close geographically, the annual incident solar radiation at the alpine meadow site(6,447 MJ/(m2·a)) was about 1.1 times that at the alpine wetland site(6,012 MJ/(m2·a)),due to differences in the cloudiness between our two sites. The alpine meadow and the alpine wetland emitted about 38%and 42%, respectively, of annual incident solar radiation back into atmosphere in the form of net longwave radiation; and they reflected about 22% and 18%, respectively, of the annual incident solar radiation back into atmosphere in the form of shortwave radiation. The annual net radiation was 2,648 and 2,544 MJ/(m2·a) for the alpine meadow site and the alpine wetland site, respectively, accounting for only about 40% of the annual incident solar radiation, significantly lower than the global mean. At 30-min scales, surface albedo exponentially decreases with the increase of the solar elevation angle; and it linearly decreases with the increase of soil-water content for our two sites. But those relationships are significantly influenced by cloudiness and are site-specific.展开更多
Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and...Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil heat flux (G) observed by a fluxplate at the depth of 5-em below the soil surface was conducted and its effect on the SEB was studied. Secondly, the surface soil heat flux (Gs) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined. The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary linear regression (OLR) of turbulent fluxes (H+LE) against available energy (Rn G) increased from 0.835 to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the SEB. When Gs, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE) against (Rn-Gs) reached 0.979, thereby the imelosure ratio of SEB was reduced to only 2.1%.展开更多
Actual evapotranspiration(ETa)is a key component of water balance.This study aimed to investigate the spatial variability and time stability of ETa along a hillslope and to analyze the key factors that control the spa...Actual evapotranspiration(ETa)is a key component of water balance.This study aimed to investigate the spatial variability and time stability of ETa along a hillslope and to analyze the key factors that control the spatiotemporal variability of ETa.The potential evaporation,surface runoff and 0–480 cm soil water profile were measured along a 243 m long transect on a hillslope of the Loess Plateau during the normal(2015)and wet(2016)water years.ETa was calculated using water balance equation.Results indicated that increasing precipitation during the wet water year did not alter the spatial pattern of ETa along the hillslope;time stability analysis showed that a location with high time stability of ETa could be used to estimate the mean ETa of the hillslope.Time stability of ETa was positively correlated with elevation(P<0.05),indicating that,on a hillslope in a semi-arid area,elevation was the primary factor influencing the time stability of ETa.展开更多
In the mountain area of inland Heihe River Basin at Hexi Corridor of northwest China during the vegetation growing season from May to September, the Simultaneous Heat and Water (SHAW) model of Soil-Vegetation-Atmosphe...In the mountain area of inland Heihe River Basin at Hexi Corridor of northwest China during the vegetation growing season from May to September, the Simultaneous Heat and Water (SHAW) model of Soil-Vegetation-Atmosphere Transfer (SVAT) system is applied to simulating and studying energy and water balance of the soil-residue-plant canopy layers in the Picea crassifolia forest and the grassland by the forest at the shaded slope and the grassland at the sun-facing slope. The simulation of energy balance indicates that net radiation of the grass- land at the sun-facing slope is more than that of the Picea crassifolia forest and the grassland by the forest at the shaded slope. The energy outgoing components are the first latent heat and next sensible heat from the grassland both at the shaded slope and the sun-facing slope, but those at the former are less. The energy outgoing components are the first sensible heat and next latent heat from the Picea crassifolia forest. The composition and distribution of energy in the soil-residue-plant canopy layers in the Picea crassifolia forest and the grassland by the forest at the shaded slope make the soil layer receive less energy, which therefore, especially the forest possesses the energy conditions for soil water conservation. The simulation of water balance indicates that the water loss of the grassland at the sun-facing slope is mainly caused by soil evaporation, while evapotranspiration of the Picea crassifolia forest and the grassland by the forest at the shaded slope is less than that of the grassland at the sun-facing slope. Half of the evapotranspiration of the Picea crassifolia forest and the grassland by the forest at the shaded slope is consumed by transpiration. After precipitation, the soil water storage is increased much more for the Picea crassifolia forest and also more for the grassland by the forest at the shaded slope. Therefore the shaded slope vegetation, especially the forest is favorable for soil water storage.展开更多
Soil moisture has a significant influence on water, energy, and carbon biogeochemical cycles. A numerical method for solving Richards' equation is usually used for simulating soil moisture. Selection of a lower bo...Soil moisture has a significant influence on water, energy, and carbon biogeochemical cycles. A numerical method for solving Richards' equation is usually used for simulating soil moisture. Selection of a lower boundary condition for Richards' equation will further affect the simulation results for soil moisture, water cycle, energy balance, and carbon biogeochemical processes. In this study,the soil water movement dynamic sub-model of a hydrologically based land surface model, the variable infiltration capacity(VIC)model, was modified using the finite difference method(FDM) to solve a mixed form of Richards' equation. In addition, the VIC model was coupled with a terrestrial biogeochemical model, the Carnegie Ames Stanford Approach model of carbon, nitrogen, and phosphorus(CASACNP model). The no-flux boundary(NB) and free-drainage boundary(FB) were selected to investigate their impacts on simulations of the water, energy, and soil carbon cycles based on the coupling model. The NB and FB had different influences on the water, energy, and soil carbon simulations. The water and energy simulations were more sensitive, while the soil carbon simulation was less sensitive to FB than to NB. Free-drainage boundary could result in lower soil moisture, evaporation, runoff,and heterotrophic respiration and higher surface soil temperature, sensible heat flux, and soil carbon content. The impact of the lower boundary condition on simulation would be greater with an increase in soil permeability. In the silt loam soil case, evaporation, runoff,and soil respiration of FB were nearly 16%, 13%, and 1% smaller, respectively, compared to those of NB.展开更多
Water repellency(WR) is a phenomenon known from many soils around the world and can occur in arid as well as in humid climates;few studies,however,have examined the effect of soil WR on the soil-plant-atmosphere energ...Water repellency(WR) is a phenomenon known from many soils around the world and can occur in arid as well as in humid climates;few studies,however,have examined the effect of soil WR on the soil-plant-atmosphere energy balance.The aim of our study was to estimate the effects of soil WR on the calculated soil-atmosphere energy balance,using a solely model-based approach.We made out evapotranspiration to have the largest influence on the energy balance;therefore the effect of WR on actual evapotranspiration was assessed.To achieve this we used climate data and measured soil hydraulic properties of a potentially water-repellent sandy soil from a site near Berlin,Germany.A numerical 1D soil water balance model in which WR was incorporated in a straightforward way was applied,using the effective cross section concept.Simulations were carried out with vegetated soil and bare soil.The simulation results showed a reduction in evapotranspiration of 30-300 mm year^(-1)(9%-76%) at different degrees of WR compared to completely wettable soil,depending on the severity degree of soil WR.The energy that is not being transported away by water vapor(i.e.,due to reduced evapotranspiration) had to be transformed into other parts of the energy balance and thus would influence the local climate.展开更多
陆面过程模式是气候模式和天气模式的核心组成部分之一.在土壤—植被—大气耦合模式(Soil-PlantAtmosphere Model,SPAM)的基础上,发展了新一代北京大学陆面过程模式PKULM(Peking University Land Model).本文首先介绍了PKULM的辐射传输...陆面过程模式是气候模式和天气模式的核心组成部分之一.在土壤—植被—大气耦合模式(Soil-PlantAtmosphere Model,SPAM)的基础上,发展了新一代北京大学陆面过程模式PKULM(Peking University Land Model).本文首先介绍了PKULM的辐射传输、湍流输送、光合作用、土壤水热输送等过程的参数化方案;采用隐式迭代计算框架,发展并应用了一个快速的线性方程组求解算法,提高了模式计算稳定性;提出并使用了二分搜索算法计算气孔阻抗,避免了CLM(Community Land Model)等使用的迭代方法在干旱区不稳定的情况,提高了模式的适用性;采用水势为基础的土壤水分扩散方程,使模式能够模拟土壤饱和区的水分输送过程,为进一步与水文过程模式耦合奠定了基础;还发展了一个地表积水与径流过程的机理模型,提高了模式对地表水分平衡过程的模拟能力;最后,使用"中国西北干旱区陆—气相互作用观测试验"平凉站的资料对模式进行了检验并与NOAH(National Center for Environmental Prediction,Oregon State University,Air Force,and Hydrology Lab model)陆面过程模式的模拟结果进行了比较,结果表明PKULM能够较好地模拟西北半干旱区农田下垫面地气交换过程.展开更多
Observations collected in the Badan Jaran desert hinterland and edge during 19-23 August 2009 and in the Jinta Oasis during 12-16 June 2005 are used to assess three methods for calculating the heat storage of the5-20-...Observations collected in the Badan Jaran desert hinterland and edge during 19-23 August 2009 and in the Jinta Oasis during 12-16 June 2005 are used to assess three methods for calculating the heat storage of the5-20-cm soil layer.The methods evaluated include the harmonic method,the conduction-convection method,and the temperature integral method.Soil heat storage calculated using the harmonic method provides the closest match with measured values.The conduction-convection method underestimates nighttime soil heat storage.The temperature integral method best captures fluctuations in soil heat storage on sub-diurnal timescales,but overestimates the amplitude and peak values of the diurnal cycle.The relative performance of each method varies with the underlying land surface.The land surface energy balance is evaluated using observations of soil heat flux at 5-cm depth and estimates of ground heat flux adjusted to account for soil heat storage.The energy balance closure rate increases and energy balance is improved when the ground heat flux is adjusted to account for soil heat storage.The results achieved using the harmonic and temperature integral methods are superior to those achieved using the conduction-convection method.展开更多
蒸散是水量平衡和能量平衡的重要组成部分,也是农田生态系统水分消耗的主要途径。为探究黄土塬区农田蒸散的日动态变化规律,运用涡度相关法、土壤水分及常规微气象观测系统等,于2013年作物生长季(4—10月)对试验区农田作物(冬小麦、...蒸散是水量平衡和能量平衡的重要组成部分,也是农田生态系统水分消耗的主要途径。为探究黄土塬区农田蒸散的日动态变化规律,运用涡度相关法、土壤水分及常规微气象观测系统等,于2013年作物生长季(4—10月)对试验区农田作物(冬小麦、春玉米)蒸散特征及影响因素进行分析。结果表明,降水对蒸散的影响较为显著,降水过后的日蒸散量较降水前会有所增加;农田0~100 cm土壤含水量变异系数较大,土壤水分变化剧烈,作物根系的集中分布范围在0~80 cm之间,因此0~100 cm土壤水分主要参与蒸散过程;晴天蒸散的累积量大于阴天,晴天和阴天的日均蒸散量分别为4.5、3.8 mm d-1,相差0.7 mm d-1。阴天蒸散开始的时间较晴天晚,阴天条件下的蒸散更易受到气象因子的扰动;不同天气条件下净辐射均为蒸散的主要影响因子,蒸散速率与净辐射变化趋势一致,但在时间上滞后于净辐射;在不同的土壤水分环境条件下,蒸散的过程和强度差异较大,水分胁迫条件下,全天蒸散量水平较低,"蒸散高地"的持续时间较长;而水分相对充足时,全天蒸散水平较高,"蒸散高地"持续时间较短,维持较高的蒸散速率的时间较长。展开更多
基金the National Natural Science Foundation of China(Grant No.U20A2081)West Light Foundation of the Chinese Academy of Sciences(Grant No.xbzg-zdsys-202102)the Second Tibetan Plateau Scientific Expedition and Research(STEP)Project(Grant No.2019QZKK0105).
文摘Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)and soil moisture(SM)conditions on a land surface energy and water simulation in the permafrost region in the Tibetan Plateau(TP)using the Community Land Model version 5.0(CLM5.0).The results indicate that the default initial schemes for ST and SM in CLM5.0 were simplistic,and inaccurately represented the soil characteristics of permafrost in the TP which led to underestimating ST during the freezing period while overestimating ST and underestimating SLW during the thawing period at the XDT site.Applying the long-term spin-up method to obtain initial soil conditions has only led to limited improvement in simulating soil hydrothermal and surface energy fluxes.The modified initial soil schemes proposed in this study comprehensively incorporate the characteristics of permafrost,which coexists with soil liquid water(SLW),and soil ice(SI)when the ST is below freezing temperature,effectively enhancing the accuracy of the simulated soil hydrothermal and surface energy fluxes.Consequently,the modified initial soil schemes greatly improved upon the results achieved through the long-term spin-up method.Three modified initial soil schemes experiments resulted in a 64%,88%,and 77%reduction in the average mean bias error(MBE)of ST,and a 13%,21%,and 19%reduction in the average root-mean-square error(RMSE)of SLW compared to the default simulation results.Also,the average MBE of net radiation was reduced by 7%,22%,and 21%.
基金supported by the National Basic Research Program of China under Grant No 2006CB400504National Natural Science Foundation of China under Grant Nos 40605027 and 40775050
文摘A frozen soil parameterization coupling of thermal and hydrological processes is used to investigate how frozen soil processes affect water and energy balances in seasonal frozen soil. Simulation results of soil liquid water content and temperature using soil model with and without the inclusion of freezing and thawing processes are evaluated against observations at the Rosemount field station. By comparing the simulated water and heat fluxes of the two cases, the role of phase change processes in the water and energy balances is analyzed. Soil freezing induces upward water flow towards the freezing front and increases soil water content in the upper soil layer. In particular, soil ice obviously prevents and delays the infiltration during rain at Rosemount. In addition, soil freezingthawing processes alter the partitioning of surface energy fluxes and lead the soil to release more sensible heat into the atmosphere during freezing periods.
基金supported and funded by the California Department of Water Resources(DWR)
文摘The California Simulation of Evapotranspiration of Applied Water (CaI-SIMETAW) model is a new tool developed by the California Department of Water Resources and the University of California, Davis to perform daily soil water balance and determine crop evapotranspiration (ETo), evapotranspiration of applied water (ETaw), and applied water (AW) for use in California water resources planning. ETaw is a seasonal estimate of the water needed to irrigate a crop assuming 100% irrigation efficiency. The model accounts for soils, crop coefficients, rooting depths, seepage, etc. that influence crop water balance. It provides spatial soil and climate information and it uses historical crop and land-use category information to provide seasonal water balance estimates by combinations of detailed analysis unit and county (DAU/County) over Califomia. The result is a large data base of ETc and ETaw that will be used to update information in the new California Water Plan (CWP). The application uses the daily climate data, i.e., maximum (Tx) and minimum (Tn) temperature and precipitation (Pcp), which were derived from monthly USDA-NRCS PRISM data (PRISM Group 2011) and daily US National Climate Data Center (NCDC) climate station data to cover California on a 4 kmx4 km change grid spacing. The application uses daily weather data to determine reference evapotranspiration (ETo), using the Hargreaves-Samani (HS) equation (Hargreaves and Samani 1982, 1985). Because the HS equation is based on temperature only, ETo from the HS equation were compared with CIMIS ETo at the same locations using available CIMIS data to determine correction factors to estimate CIMIS ETo from the HS ETo to account for spatial climate differences. CaI-SIMETAW also employs near real-time reference evapotranspiration (ETo) information from Spatial CIMIS, which is a model that combines weather station data and remote sensing to provide a grid of ETo information. A second database containing the available soil water holding capacity and soil depth information for all of California was also developed from the USDA-NRCS SSURGO database. The Cal-SIMETAW program also has the ability to generate daily weather data from monthly mean values for use in studying climate change scenarios and their possible impacts on water demand in the state. The key objective of this project is to improve the accuracy of water use estimates for the California Water Plan (CWP), which provides a comprehensive report on water supply, demand, and management in California. In this paper, we will discuss the model and how it determines ETaw for use in water resources planning.
基金supported by the National Natural Science Foundation of China (Grant Nos. 91337212, 41175008)Cold and Arid Regions Environmental and Engineering Research Institute Youth Science Technology Service Network initiative (STS)+1 种基金the China Exchange Project (Grant No. 13CDP007)the National Natural Science Foundation of China (Grant Nos. 40825015 and 40675012)
文摘The land-atmosphere energy and turbulence exchange is key to understanding land surface processes on the Tibetan Plateau(TP). Using observed data for Aug. 4 to Dec. 3, 2012 from the Bujiao observation point(BJ) of the Nagqu Plateau Climate and Environment Station(NPCE-BJ), different characteristics of the energy flux during the Asian summer monsoon(ASM) season and post-monsoon period were analyzed. This study outlines the impact of the ASM on energy fluxes in the central TP. It also demonstrates that the surface energy closure rate during the ASM season is higher than that of the post-monsoon period. Footprint modeling shows the distribution of data quality assessments(QA) and quality controls(QC) surrounding the observation point. The measured turbulent flux data at the NPCE-BJ site were highly representative of the target land-use type. The target surface contributed more to the fluxes under unstable conditions than under stable conditions. The main wind directions(180° and 210°) with the highest data density showed flux contributions reaching 100%, even under stable conditions. The lowest flux contributions were found in sectors with low data density, e.g., 90.4% in the 360° sector under stable conditions during the ASM season. Lastly, a surface energy water balance(SEWAB) model was used to gap-fill any absent or corrected turbulence data. The potential simulation error was also explored in this study. The Nash-Sutcliffe model efficiency coefficients(NSEs) of the observed fluxes with the SEWAB model runs were 0.78 for sensible heat flux and 0.63 for latent heat flux during the ASM season, but unrealistic values of-0.9 for latent heat flux during the post-monsoon period.
基金supported by the National Natural Science Foundation of China (Grant Nos. 91537106, 41405016, 41275016, 91537104, and 41605011)
文摘Understanding the energy balance on the Tibetan Plateau is important for better prediction of global climate change. To characterize the energy balance on the Plateau, we examined the radiation balance and the response of albedo to environmental factors above an alpine meadow and an alpine wetland surfaces in the eastern Tibetan Plateau, using 2014 data. Although our two sites belong to the same climatic background, and are close geographically, the annual incident solar radiation at the alpine meadow site(6,447 MJ/(m2·a)) was about 1.1 times that at the alpine wetland site(6,012 MJ/(m2·a)),due to differences in the cloudiness between our two sites. The alpine meadow and the alpine wetland emitted about 38%and 42%, respectively, of annual incident solar radiation back into atmosphere in the form of net longwave radiation; and they reflected about 22% and 18%, respectively, of the annual incident solar radiation back into atmosphere in the form of shortwave radiation. The annual net radiation was 2,648 and 2,544 MJ/(m2·a) for the alpine meadow site and the alpine wetland site, respectively, accounting for only about 40% of the annual incident solar radiation, significantly lower than the global mean. At 30-min scales, surface albedo exponentially decreases with the increase of the solar elevation angle; and it linearly decreases with the increase of soil-water content for our two sites. But those relationships are significantly influenced by cloudiness and are site-specific.
基金Supported by the National Basic Research Program of China(2012CB955304)National Natural Science Foundation of China(40830957and40175008)China Postdoctoral Scientific Research Fund(20110490854)
文摘Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil heat flux (G) observed by a fluxplate at the depth of 5-em below the soil surface was conducted and its effect on the SEB was studied. Secondly, the surface soil heat flux (Gs) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined. The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary linear regression (OLR) of turbulent fluxes (H+LE) against available energy (Rn G) increased from 0.835 to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the SEB. When Gs, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE) against (Rn-Gs) reached 0.979, thereby the imelosure ratio of SEB was reduced to only 2.1%.
基金the Programme of Introducing Talents of Discipline to Universities of China(D18013)the independent subject of State Key Laboratory of Plateau Ecology and Agriculture,Qinghai University,China(2019-ZZ-05)+3 种基金the Project of Qinghai Science&Technology Department(2019-ZJ-Y01)the Natural Science Foundation of Qinghai Province,China(2020-ZJ-967Q)the Thousand High Innovative Talents Program of Qinghai Province(2019)the Program for Changjiang Scholars and Innovative Research Team in University,Ministry of Education,China(IRT_17R62).
文摘Actual evapotranspiration(ETa)is a key component of water balance.This study aimed to investigate the spatial variability and time stability of ETa along a hillslope and to analyze the key factors that control the spatiotemporal variability of ETa.The potential evaporation,surface runoff and 0–480 cm soil water profile were measured along a 243 m long transect on a hillslope of the Loess Plateau during the normal(2015)and wet(2016)water years.ETa was calculated using water balance equation.Results indicated that increasing precipitation during the wet water year did not alter the spatial pattern of ETa along the hillslope;time stability analysis showed that a location with high time stability of ETa could be used to estimate the mean ETa of the hillslope.Time stability of ETa was positively correlated with elevation(P<0.05),indicating that,on a hillslope in a semi-arid area,elevation was the primary factor influencing the time stability of ETa.
基金supported by the Chinese Academy of Sciences(Grant Nos.KZCX1-10-03-01 and KZCX3-SW-329)the National Natural Science Foun dati on ofChina(Grant No.49731030)AIACC-AS25,UNEP.
文摘In the mountain area of inland Heihe River Basin at Hexi Corridor of northwest China during the vegetation growing season from May to September, the Simultaneous Heat and Water (SHAW) model of Soil-Vegetation-Atmosphere Transfer (SVAT) system is applied to simulating and studying energy and water balance of the soil-residue-plant canopy layers in the Picea crassifolia forest and the grassland by the forest at the shaded slope and the grassland at the sun-facing slope. The simulation of energy balance indicates that net radiation of the grass- land at the sun-facing slope is more than that of the Picea crassifolia forest and the grassland by the forest at the shaded slope. The energy outgoing components are the first latent heat and next sensible heat from the grassland both at the shaded slope and the sun-facing slope, but those at the former are less. The energy outgoing components are the first sensible heat and next latent heat from the Picea crassifolia forest. The composition and distribution of energy in the soil-residue-plant canopy layers in the Picea crassifolia forest and the grassland by the forest at the shaded slope make the soil layer receive less energy, which therefore, especially the forest possesses the energy conditions for soil water conservation. The simulation of water balance indicates that the water loss of the grassland at the sun-facing slope is mainly caused by soil evaporation, while evapotranspiration of the Picea crassifolia forest and the grassland by the forest at the shaded slope is less than that of the grassland at the sun-facing slope. Half of the evapotranspiration of the Picea crassifolia forest and the grassland by the forest at the shaded slope is consumed by transpiration. After precipitation, the soil water storage is increased much more for the Picea crassifolia forest and also more for the grassland by the forest at the shaded slope. Therefore the shaded slope vegetation, especially the forest is favorable for soil water storage.
基金supported by the National Science Foundation for Distinguished Young Scholars of China (No. 51309245)supported by the US Department of Energy and National Aeronautics and Space Administration
文摘Soil moisture has a significant influence on water, energy, and carbon biogeochemical cycles. A numerical method for solving Richards' equation is usually used for simulating soil moisture. Selection of a lower boundary condition for Richards' equation will further affect the simulation results for soil moisture, water cycle, energy balance, and carbon biogeochemical processes. In this study,the soil water movement dynamic sub-model of a hydrologically based land surface model, the variable infiltration capacity(VIC)model, was modified using the finite difference method(FDM) to solve a mixed form of Richards' equation. In addition, the VIC model was coupled with a terrestrial biogeochemical model, the Carnegie Ames Stanford Approach model of carbon, nitrogen, and phosphorus(CASACNP model). The no-flux boundary(NB) and free-drainage boundary(FB) were selected to investigate their impacts on simulations of the water, energy, and soil carbon cycles based on the coupling model. The NB and FB had different influences on the water, energy, and soil carbon simulations. The water and energy simulations were more sensitive, while the soil carbon simulation was less sensitive to FB than to NB. Free-drainage boundary could result in lower soil moisture, evaporation, runoff,and heterotrophic respiration and higher surface soil temperature, sensible heat flux, and soil carbon content. The impact of the lower boundary condition on simulation would be greater with an increase in soil permeability. In the silt loam soil case, evaporation, runoff,and soil respiration of FB were nearly 16%, 13%, and 1% smaller, respectively, compared to those of NB.
基金Supported by the German Research Foundation(DFG)(No.WE1125/29-1)
文摘Water repellency(WR) is a phenomenon known from many soils around the world and can occur in arid as well as in humid climates;few studies,however,have examined the effect of soil WR on the soil-plant-atmosphere energy balance.The aim of our study was to estimate the effects of soil WR on the calculated soil-atmosphere energy balance,using a solely model-based approach.We made out evapotranspiration to have the largest influence on the energy balance;therefore the effect of WR on actual evapotranspiration was assessed.To achieve this we used climate data and measured soil hydraulic properties of a potentially water-repellent sandy soil from a site near Berlin,Germany.A numerical 1D soil water balance model in which WR was incorporated in a straightforward way was applied,using the effective cross section concept.Simulations were carried out with vegetated soil and bare soil.The simulation results showed a reduction in evapotranspiration of 30-300 mm year^(-1)(9%-76%) at different degrees of WR compared to completely wettable soil,depending on the severity degree of soil WR.The energy that is not being transported away by water vapor(i.e.,due to reduced evapotranspiration) had to be transformed into other parts of the energy balance and thus would influence the local climate.
文摘陆面过程模式是气候模式和天气模式的核心组成部分之一.在土壤—植被—大气耦合模式(Soil-PlantAtmosphere Model,SPAM)的基础上,发展了新一代北京大学陆面过程模式PKULM(Peking University Land Model).本文首先介绍了PKULM的辐射传输、湍流输送、光合作用、土壤水热输送等过程的参数化方案;采用隐式迭代计算框架,发展并应用了一个快速的线性方程组求解算法,提高了模式计算稳定性;提出并使用了二分搜索算法计算气孔阻抗,避免了CLM(Community Land Model)等使用的迭代方法在干旱区不稳定的情况,提高了模式的适用性;采用水势为基础的土壤水分扩散方程,使模式能够模拟土壤饱和区的水分输送过程,为进一步与水文过程模式耦合奠定了基础;还发展了一个地表积水与径流过程的机理模型,提高了模式对地表水分平衡过程的模拟能力;最后,使用"中国西北干旱区陆—气相互作用观测试验"平凉站的资料对模式进行了检验并与NOAH(National Center for Environmental Prediction,Oregon State University,Air Force,and Hydrology Lab model)陆面过程模式的模拟结果进行了比较,结果表明PKULM能够较好地模拟西北半干旱区农田下垫面地气交换过程.
基金Supported by the National Science and Technology Support Program of China(2012BAH29B03)National(Key) Basic Research and Development(973)Program of China(2009CB421402)
文摘Observations collected in the Badan Jaran desert hinterland and edge during 19-23 August 2009 and in the Jinta Oasis during 12-16 June 2005 are used to assess three methods for calculating the heat storage of the5-20-cm soil layer.The methods evaluated include the harmonic method,the conduction-convection method,and the temperature integral method.Soil heat storage calculated using the harmonic method provides the closest match with measured values.The conduction-convection method underestimates nighttime soil heat storage.The temperature integral method best captures fluctuations in soil heat storage on sub-diurnal timescales,but overestimates the amplitude and peak values of the diurnal cycle.The relative performance of each method varies with the underlying land surface.The land surface energy balance is evaluated using observations of soil heat flux at 5-cm depth and estimates of ground heat flux adjusted to account for soil heat storage.The energy balance closure rate increases and energy balance is improved when the ground heat flux is adjusted to account for soil heat storage.The results achieved using the harmonic and temperature integral methods are superior to those achieved using the conduction-convection method.
文摘蒸散是水量平衡和能量平衡的重要组成部分,也是农田生态系统水分消耗的主要途径。为探究黄土塬区农田蒸散的日动态变化规律,运用涡度相关法、土壤水分及常规微气象观测系统等,于2013年作物生长季(4—10月)对试验区农田作物(冬小麦、春玉米)蒸散特征及影响因素进行分析。结果表明,降水对蒸散的影响较为显著,降水过后的日蒸散量较降水前会有所增加;农田0~100 cm土壤含水量变异系数较大,土壤水分变化剧烈,作物根系的集中分布范围在0~80 cm之间,因此0~100 cm土壤水分主要参与蒸散过程;晴天蒸散的累积量大于阴天,晴天和阴天的日均蒸散量分别为4.5、3.8 mm d-1,相差0.7 mm d-1。阴天蒸散开始的时间较晴天晚,阴天条件下的蒸散更易受到气象因子的扰动;不同天气条件下净辐射均为蒸散的主要影响因子,蒸散速率与净辐射变化趋势一致,但在时间上滞后于净辐射;在不同的土壤水分环境条件下,蒸散的过程和强度差异较大,水分胁迫条件下,全天蒸散量水平较低,"蒸散高地"的持续时间较长;而水分相对充足时,全天蒸散水平较高,"蒸散高地"持续时间较短,维持较高的蒸散速率的时间较长。