Linking Surface Temperature Based Approaches for Estimating Soil Heat Flux with Error Propagation

Soil heat flux is an inseparable component of the surface energy balance. Accurate estimation of regional soil heat flux is valuable to studies of meteorology and hydrology. Conventional measurement of using soil heat flux plates at the site scale is impractical to estimate large-scale flux. Other approaches generally require soil temperature to?be measured in at least two soil layers, which is also difficult to implement at the regional scale. In the last decade, single-layer based approaches were developed to fulfill the regional requirement. This study used a simple but more general approach for estimating soil heat flux solely with surface temperature. The generalized approach can be conditionally linked to two existing single-layer based approaches but has fewer restrictions or assumptions. Error analysis revealed that measurement error in surface temperature would have limited effects on soil heat flux estimated from the new approach. Model simulations showed that soil heat flux estimated from the approach agreed with those simulated from the heat transfer equation. Furthermore, case examinations at two sites with contrasting climate regimes demonstrated that the generalized approach had better performance than the existing single-layer approaches. It achieved the highest correlation of determination and the lowest mean, standard deviation, and root mean squared error of the differences between the estimates and the field measures at either site. The generalized approach can estimate soil heat flux at a depth but it requires only surface temperature data as input, which is an advantage to remote sensing applications.

Effects of the Soil Heat Flux Estimates on Surface Energy Balance Closure over a Semi-Arid Grassland

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%.

An improved algorithm to estimate the surface soil heat flux over a heterogeneous surface: A case study in the Heihe River Basin

Surface soil heat flux(G0) is an indispensable component of the surface energy balance and plays an important role in the estimation of surface evapotranspiration(ET). This study calculated G0 in the Heihe River Basin based on the thermal diffusion equation, using the observed soil temperature and moisture profiles, with the aim to analyze the spatial-temporal variations of G0 over the heterogeneous area(with alpine grassland, farmland, and forest). The soil ice content was estimated by the difference in liquid soil water content before and after the melting of the frozen soil and its impact on the calculation of G0 was further analyzed. The results show that:(1) the diurnal variation of G0 is obvious under different underlying surfaces in the Heihe River Basin, and the time when the daily maximum value of G0 occurs is a few minutes to several hours earlier than that of the net radiation flux, which is related to the soil texture, soil moisture, soil thermal properties, and the vegetation coverage;(2) the net radiation flux varies with season and reaches the maximum in summer and the minimum in winter, whereas G0 reaches the maximum in spring rather than in summer, because more vegetation in summer hinders energy transfer into the soil;(3) the proportions of G0 to the net radiation flux are different with seasons and surface types, and the mean values in January are 25.6% at the Arou site, 22.9% at the Yingke site and 4.3% at the Guantan site, whereas the values in July are 2.3%, 1.6% and 0.3%, respectively; and(4) G0 increases when the soil ice content is included in thermal diffusion equation, which improves the surface energy balance closure by 4.3%.

A temperature prediction-correction method for estimating surface soil heat flux from soil temperature and moisture data

Surface soil heat flux is a component of surface energy budget and its estimation is needed in land-atmosphere interaction studies. This paper develops a new simple method to estimate soil heat flux from soil temperature and moisture observations. It give

Numerical Study of Impacts of Soil Moisture on the Diurnal and Seasonal Cycles of Sensible/Latent Heat Fluxes over Semi-arid Region

The semi-arid regions, as climatic and ecosystem transitional zones, are the most vulnerable to global environmental change. Earlier researches indicate that the semi-arid regions are characterized by strong landatmosphere coupling in which soil moisture is the crucial variable in land surface processes. In this paper, we investigate the sensitivity of the sensible/latent heat fluxes to soil moisture during the growing season based on the enhanced observations at Tongyu in the Jilin province of China, a reference site of international Coordinated Energy and Water Cycle Observations Project （CEOP） in the semi-arid regions, by using a sophisticated land surface model （NCAR_CLM3.0）. Comparisons between the observed and simulated sensible/latent heat fluxes indicate that the soil moisture has obvious effects on the sensible/latent heat fluxes in terms of diurnal cycle and seasonal evolution. Better representation of the soil moisture could improve the model performance to a large degree. Therefore, for the purpose of simulating the land-atmosphere interaction and predicting the climate and water resource changes in semi-arid regions, it is necessary to enhance the description of the soil moisture distribution both in the way of observation and its treatment in land surface models.

The analysis of heat and water fluxes in frozen silty soil

In this paper, based on the basic equations of water flow and heat transfer, the hydrothermal coupling model is established.The numerical model was realized in COMSOL Multiphysics software, and simulation results are compared with the experimental results of Watanabe and Wake(2008) to verify the effectiveness of the model. Through the calculation, we can obtain the dynamic changes of heat and water fluxes, thermal and hydrological properties, matric potential and temperature gradient in unsaturated freezing soil; and these variables are unmeasurable in practice.

Estimation of ground heat flux and its impact on the surface energy budget for a semi-arid grassland

Three approaches, i.e., the harmonic analysis （HA） technique, the thermal diffusion equation and correction （TDEC） method, and the calorimetric method used to estimate ground heat flux, are evaluated by using observations from the Semi-Arid Climate and Environment Observatory of Lanzhou University （SACOL） in July, 2008. The calorimetric method, which involves soil heat flux measurement with an HFP01SC self-calibrating heat flux plate buried at a depth of 5 cm and heat storage in the soil between the plate and the surface, is here called the ITHP approach. The results show good linear relationships between the soil heat fluxes measured with the HFP01SC heat flux plate and those calculated with the HA technique and the TDEC method, respectively, at a depth of 5 cm. The soil heat fluxes calculated with the latter two methods well follow the phase measured with the HFP01SC heat flux plate. The magnitudes of the soil heat flux calculated with the HA technique and the TDEC method are close to each other, and they are about 2 percent and 6 percent larger than the measured soil heat flux, respectively, which mainly occur during the nighttime. Moreover, the ground heat fluxes calculated with the TDEC method and the HA technique are highly correlated with each other （R2= 0.97）, and their difference is only about 1 percent. The TDEC-calculated ground heat flux also has a good linear relationship with the ITttP-calculated ground heat flux （R2 = 0.99）, but their difference is larger （about 9 percent）. Furthermore, compared to the HFP01SC direct measurements at a depth of 5 cm, the ground heat flux calculated with the HA technique, the TDEC method, and the ITHP approach can improve the surface energy budget closure by about 6 percent, 7 percent, and 6 percent at SACOL site, respectively. Therefore, the contribution of ground heat flux to the surface energy budget is very important for the semi-arid grassland over the Loess Plateau in China. Using turbulent heat fluxes with common corrections, soil heat storage between the surface and the heat flux plate can improve the surface energy budget closure by about 6 to 7 percent, resulting in a closure of 82 to 83 percent at the SACOL site.

Application of the Reciprocal Analysis for Sensible and Latent Heat Fluxes with Evapotranspiration at a Humid Region

Evapotranspiration acts an important role in hydrologic cycle and water resources planning. But the estimation issue still remains until nowadays. This research attempts to make clear this problem by the following way. In a humid region, by applying the Bowen ratio concept and optimum procedure on the soil surface, sensible and latent heat fluxes are estimated using net radiation (Rn) and heat flux into the ground (G). The method uses air temperature and humidity at a single height by reciprocally determining the soil surface temperature (Ts) and the relative humidity (rehs). This feature can be remarkably extended to the utilization. The validity of the method is confirmed by comparing of observed and estimated latent (lE) and sensible heat flux (H) using the eddy covariance method. The hourly change of the lE, H, Ts and rehs on the soil surface, yearly change of lE and H and relationship of estimated lE and H versus observed are clarified. Furthermore, monthly evapotranspiration is estimated from the lE. The research was conducted using hourly data of FLUXNET at a site of Japan, three sites of the United States and two sites of Europe in humid regions having over 1000 mm of annual precipitation.

川中丘陵区农田生态系统太阳辐射及土壤热通量数据集(2005-2021年)

太阳辐射是地表生态系统的主要能量来源,是生态系统维持本身正常运转和发展的原始动力;地表层土壤热通量是地表能量平衡的重要参数之一。四川盐亭农田生态系统国家野外科学观测研究站(简称盐亭站)是国家布局在亚热带四川盆地农业生态区的唯一农田生态系统监测野外站。该区域具有优越的光、热、水资源,非地带性土壤紫色土广泛分布,形成亚热带湿润季风气候与非地带性紫色土的最佳农业组合,农耕活跃,是四川盆地农业的主体区域。本研究基于盐亭站综合气象观测场Vaisala(芬兰)自动气象观测系统,经过数据采集、处理、数据质量控制与评估,集成2005-2021年紫色土典型农田生态系统太阳辐射和土壤热通量数据集,太阳辐射监测包括总辐射、反射辐射、紫外辐射、净辐射和光合有效辐射。数据集包含5个EXCEL文件,有各类辐射的时值、日值和月值观测及统计数据,数据量32.7 MB。本数据集可为生态系统生产能力评估、应对气候变化等方面的研究提供数据支持。

青藏高原土壤热通量估算及时空分布特征

土壤热通量(Soil heat flux,G)是影响青藏高原地表能量平衡的关键因素,对其进行估算以及时空分布特征分析,可为该区地表能量平衡研究提供参考依据。本文基于2003~2018年中分辨率成像光谱仪(Moderate-Resolution Imaging Spectroradiometer,MODIS)数据、中国区域地面气象要素驱动数据集以及中国西部1 km全天候地表温度数据集,利用土地表面能量平衡算法(Surface Energy Balance Algorithm for Land,SEBAL)模型结合青藏高原原位观测数据G0对模型的适用性和计算精度进行评估,发现该模型对青藏高原的土壤热通量G模拟精度较高。在此基础上利用遥感数据重构了该地区2003~2018年的土壤热通量数据,并分析了G值的时空分布特征。结果表明:(1)多年G均值整体呈波动下降趋势,最大谷值出现在2011年,最大峰值出现在2016年;各季节中,除冬季外,其余季节G均值呈波动下降趋势,且G均值值域高低依次呈现:夏季>春季>秋季>冬季,G均值波动变化大小顺序与之相一致。(2)G均值分布特征具有明显的空间异质性,总体呈现出北部柴达木盆地及其周边地区最高,西南阿里等地区较高,其余大部分地区普遍较低的空间分布特征;各季节G均值的空间分布特征基本与总体空间分布特征一致。(3)中部及东南地区G均值主要呈增加趋势,北部、西部和西南地区G均值主要呈减少趋势;各季节中,G均值有增加趋势的地区面积占比冬季最多,夏季最少,有减少趋势的地区面积占比夏季最多,冬季最少。本研究结果证明了SEBAL模型对反演青藏高原土壤热通量G的适用性,且丰富了青藏高原地表能量平衡的研究内容。

青海湖流域不同下垫面类型对地表温度的生物物理影响

本研究选取青海湖流域亚高山灌丛和温性草原两个不同土地覆盖类型的站点,利用湍流通量数据和自动气象站数据对比生长季和非生长季两个站点的微气象要素和地表能量平衡收支,评估土地利用/土地覆盖变化(Land Use/Land Cover Changes,LULCC)对地表温度的生物物理影响。亚高山灌丛相比温性草原具有更低的地表温度、气温和土壤温度,在生长季两个站点的地表温度、气温和土壤温度的差异更为明显,而非生长季相对湿度的差异更为明显。根据直接分解温度理论(Direct Decomposed Temperature Metric,DTM),分析不同下垫面对地表温度的生物物理影响。结果表明:白天灌丛相比草原的冷却作用主要贡献因素是短波辐射、地表土壤热通量和感热通量项,其中短波辐射在灌丛的冷却中起到正反馈作用,而后两者起到负反馈作用。夜间灌丛的冷却作用主要贡献因素是地表土壤热通量项。在相同气候和天气背景下,不同下垫面确实会对地表温度有明显的生物物理反馈作用。

Thermal conductivity of reinforced soils: A literature review

This paper aims a review of the literature related to soil reinforcements to achieve lower soil thermal conductivity （2）. The use of various natural and synthetic fibers, polymers, geosynthetics, agricultural waste/materials, and nanoclays is discussed and existing prediction models that have been thought to affect low thermal conductivity are presented.

地铁车站围岩土体蓄放热特性实测研究

针对地铁车站围岩土体蓄放热量问题,以运行5年的上海地铁12号线某车站为研究对象,对该车站室外空气温度、车站内空气温度及站厅壁面热流密度进行了为期1年的实测,分析了地铁车站围岩土体的蓄放热特性及在维持车站空气温度稳定性方面的积极作用。测试结果表明:地铁车站围岩土体在过渡季与夏季以吸热为主,冬季以放热为主;全年蓄放热量净值为-5743 MJ,吸热量为36069 MJ,放热量为30326 MJ;围岩土体通过蓄放热有效抑制了室外空气温度波动对车站内空气温度的影响。

Biophysical warming patterns of an open-top chamber and its short-term influence on a Phragmites wetland ecosystem in China

Passive-warming, open-top chambers(OTCs) are widely applied for studying the effects of future climate warming on coastal wetlands. In this study, a set of six OTCs were established at a Phragmites wetland located in the Yellow River Delta of Dongying City, China. With data collected through online transmission and in-situ sensors, the attributes and patterns of realized OTCs warming are demonstrated.The authors also quantified the preliminary influence of experimental chamber warming on plant traits.OTCs produced an elevated average air temperature of 0.8°C(relative to controls) during the growing season(June to October) of 2018, and soil temperatures actually decreased by 0.54°C at a depth of 5 cm and 0.46°C at a depth of 30 cm in the OTCs. Variations in diel patterns of warming depend greatly on the heat sources of incoming radiation in the daytime versus soil heat flux at night. Warming effects were often larger during instantaneous analyses and influenced OTCs air temperatures from-2.5°C to 8.3°C dependent on various meteorological conditions at any given time, ranging from cooling influences from vertical heat exchange and vegetation to radiation-associated warming. Night-time temperature depressions in the OTCs were due to the low turbulence inside OTCs and changes in surface soilatmosphere heat transfer. Plant shoot density, basal diameter, and biomass of Phragmites decreased by23.2%, 6.3%, and 34.0%, respectively, under experimental warming versus controls, and plant height increased by 4.3%, reflecting less carbon allocation to stem structures as plants in the OTCs experienced simultaneous wind buffering. While these passive-warming OTCs created the desired warming effects both to the atmosphere and soils, pest damages on the plant leaves and lodging within the OTCs were extensive and serious, creating the need to consider control options for these chambers and the replicated OTCs studies underway in other Chinese Phragmites marshes(Panjin and Yancheng).

松嫩平原北部季节冻土冻融过程及热量传递规律

为深入了解松嫩平原北部季节冻土冻融过程及热量传递规律,同时为东北寒区工程及寒区农业的土体环境的高效利用提供科学依据。基于松嫩平原北部季节冻土原位监测,开展季节冻土温度变化特性及分层热通量变化规律研究。结果表明:深度小于50 cm土体温度日变化明显,土体温度季节差异随着土体深度增大而减小。2017年3月3日达到最大冻深(164 cm),4月22日为最终融化日期,最终融化深度为130 cm。不同深度土体温度对地表温度响应呈滞后效应,随着土体深度的增加,滞后时间延长;季节冻土在冻融期内浅层土体受到净辐射的影响,热量交换极其频繁;随着土层深度的增加,净辐射的作用减小,热量在土体中传递的损耗增加,热量交换程度减弱,在冻结期,土体损失的热量大于吸收的热量。在整个冻融期内保持负值,冻深线以下土体中的热量持续向上传输,表明160 cm深度以下土体持续对冻土层传递热量。

地下近邻平行精密恒温隧道间传热的模拟研究

针对某大科学工程地下3条近邻隧道运行时的隧道间传热进行了研究,分析了单条运行、2条运行及3条同时运行模式下隧道围护结构热流密度和周围土壤温度场的差异。结果表明:各运行模式下,初始第1个月内热流密度下降最快,可达90%以上;不同运行模式下6个月后隧道间传热开始出现明显差异,最大值是最小值的115%,而不同模式运行10年后,最大值是最小值的268%;当隧道运行数量和运行顺序不同时,热流密度衰减不同,隧道运行数量越多、距离越近,相互影响就越大,且围护结构热流密度越低。

黄河源区玛曲土壤冻融过程中地表水热交换特征分析

土壤冻融过程显著影响地表含水量和能量收支变化。利用玛曲2017年8月至2018年7月的土壤温度/湿度、涡动观测资料以及公用陆面模式(Community Land Model,CLM)最新版本CLM5.0的模拟资料,其中冻结过程阶段的辐射和能量通量使用模式模拟的数据,通过分析土壤冻融过程中土壤温湿度、地表能量平衡各分量的时间演变特征,探讨冻融过程中地表水热交换的特征。数据分析表明:(1)土壤冻融过程包括冻结过程、完全冻结、消融过程及完全消融四个阶段,各阶段中的土壤温度/湿度、辐射和能量通量存在明显的日变化,在冻结过程和消融过程阶段,土壤湿度随土壤温度变化显示出明显的日冻融循环。(2)冻融过程通过影响表层土壤水分影响地表辐射收支和能量分配。冻融过程中土壤中的水相变为冰,改变下垫面性质影响地表辐射收支。土壤中的液态水通过相变影响地表潜热通量,完全消融(冻结)阶段,地气之间能量交换以潜热(感热)通量为主。相比于以潜热通量为主的冻结过程阶段,消融过程阶段净辐射通量逐渐增大,地气之间能量交换主要受感热通量影响。土壤中水分的昼融夜冻导致频繁的潜热通量释放影响地表热通量。土壤热通量在冻结过程(G_(0)=-9.1 W·m^(-2))和消融过程阶段(G_(0)=3.4 W·m^(-2))绝对值大于完全消融阶段(G_(0)=1.2 W·m^(-2)),土壤日冻融循环加强地表热通量交换。(3)能量闭合率为感热、潜热通量之和与净辐射通量、土壤热通量之差的比值。冻结过程、完全冻结、消融过程和完全消融阶段平均能量闭合率为1.44、1.56、0.99和0.81,消融过程和完全消融过程能量闭合率更趋近于1。土壤中存在日冻融循环时,冻结过程阶段土壤中的水冻结释放热量,高估土壤热通量从而高估能量闭合率,消融过程阶段土壤中的冰融化吸收热量,低估土壤热通量从而低估能量闭合率,影响地表能量收支平衡。

A Variational Method for Estimating Near-Surface Soil Moisture and Surface Heat Fluxes

A variational data assimilation method is proposed to estimate the near-surface soil moisture and surface sensible and latent heat fluxes. The method merges the five parts into a cost function, i.e., the differences of wind, potential temperature, and specific humidity gradient between observations and those computed by the profile method, the difference of latent heat fluxes calculated using the ECMWF land surface evaporation scheme and the profile method, and a weak constraint for surface energy balance. By using an optimal algorithm, the best solutions are found. The method is tested with the data collected at Feixi Station （31.41^oN, 117.08^oE） supported by the China Heavy Rain Experiment and Study （HeRES） during 7-30 June 2001. The results show that estimated near-surface soil moistures can quickly respond to rainfall, and their temporal variation is consistent with that of measurements of average soil moisture over 15-cm top depth with a maximum error less than 0.03 m^3 m^-3. The surface heat fluxes calculated by this method are consistent with those by the Bowen ratio method, but at the same time it can overcome the instability problem occurring in the Bowen ratio method when the latter is about -1. Meanwhile, the variational method is more accurate than the profile method in terms of satisfying the surface energy balance. The sensitivity tests also show that the variational method is the most stable one among the three methods.

半干旱草地地表土壤热通量的计算及其对能量平衡的影响

利用2008年7月兰州大学半干旱气候与环境观测(SACOL)站的观测资料,对比分析了地表土壤热通量的三种计算方法,即谐波法、温度预报校正法(TDEC法)以及结合自校正热通量板(HFP01SC)测量的温度积分法(ITHP法);进而分析了三种不同方法的计算结果对地表能量平衡的影响。比较5cm深度处谐波法和TDEC法的计算结果与HFP01SC的实测结果,三者的相位基本一致,相互之间均具有很好的线性关系;谐波法与TDEC法的计算值较为接近,但分别比HFP01SC的实测值偏大了2%和6%(主要发生在夜间)。对于地表的土壤热通量(G0),谐波法与TDEC法两者的计算结果仅偏差约1%;TDEC法与ITHP法的计算结果之间也具有很好的线性关系(R2=0.99),但偏差达到9%左右。相对于HFP01SC的实测结果,由谐波法和TDEC法计算的G0可将SACOL站的地表能量闭合率分别提高6%和7%左右;利用温度积分法将HFP01SC的实测结果校正到地表后,地表能量闭合率也提高了约6%。因此,在对涡动相关通量做了常规订正的情况下,当充分考虑了土壤热存储后,SACOL站的地表能量闭合率可提高6%～7%,达到82%～83%左右。

不同土壤类型的热通量变化特征

利用2004—2007年中国科学院中国生态系统研究网络(CERN)生态站实测土壤热通量、辐射等资料,分析了不同土壤类型表层热通量的日变化和季节变化,以及不同土壤类型的热通量与总辐射、净辐射的关系。结果表明,由于导热率越大,热量传输就越快;热容量越小,热量传输也越快,造成土壤热通量的日较差和年较差较大,所以黄绵土和紫色土的表层热通量日较差最大(220～280 W.m-2),高寒草甸土和水稻土最小(55W.m-2);季节变化中土壤表层热通量的年较差变化范围在12～28W.m-2之间,灰漠土最大,为28W.m-2,热通量年较差从大到小依次为灰漠土、黄绵土、盐碱潮土、红壤土、紫色土、沼泽土、水稻土和高寒潮土,高寒潮土最小,为12W.m-2。不同土壤类型的热通量与总辐射、净辐射呈正相关关系,但不同土壤类型的土壤热通量在12:00(地方时)所占净辐射的比例各不相同,高寒草甸土最小,约为8%;黄绵土最大,为38%,多数土壤的热通量占净辐射的比例在15%～20%之间,这充分表明不同土壤类型表层热通量的传输存在很大差异。