Evapotranspiration Estimation Based on MODIS Products and Surface Energy Balance Algorithms for Land(SEBAL) Model in Sanjiang Plain,Northeast China

In this study,the Surface Energy Balance Algorithms for Land(SEBAL) model and Moderate Resolution Imaging Spectroradiometer(MODIS) products from Terra satellite were combined with meteorological data to estimate evapotranspiration(ET) over the Sanjiang Plain,Northeast China.Land cover/land use was classified by using a recursive partitioning and regression tree with MODIS Normalized Difference Vegetation Index(NDVI) time series data,which were reconstructed based on the Savitzky-Golay filtering approach.The MODIS product Quality Assessment Science Data Sets(QA-SDS) was analyzed and all scenes with valid data covering more than 75% of the Sanjiang Plain were selected for the SEBAL modeling.This provided 12 overpasses during 184-day growing season from May 1st to October 31st,2006.Daily ET estimated by the SEBAL model was misestimaed at the range of-11.29% to 27.57% compared with that measured by Eddy Covariance system(10.52% on average).The validation results show that seasonal ET from the SEBAL model is comparable to that from ground observation within 8.86% of deviation.Our results reveal that the time series daily ET of different land cover/use increases from vegetation on-going until June or July and then decreases as vegetation senesced.Seasonal ET is lower in dry farmland(average(Ave):491 mm) and paddy field(Ave:522 mm) and increases in wetlands to more than 586 mm.As expected,higher seasonal ET values are observed for the Xingkai Lake in the southeastern part of the Sanjiang Plain(Ave:823 mm),broadleaf forest(Ave:666 mm) and mixed wood(Ave:622 mm) in the southern/western Sanjiang Plain.The ET estimation with SEBAL using MODIS products can provide decision support for operational water management issues.

Estimation of Regional Evapotranspiration in Alpine Area and Its Response to Land Use Change:A Case Study in Three-River Headwaters Region of Qinghai-Tibet Plateau,China

Three-River Headwaters （TRH） region involved in this paper refers to the source region of the Changjiang （Yangtze） River, the Huanghe （Yellow） River and the Lancang River in China. Taking the TRH region of the Qing- hai-Tibet Plateau as a case, the annual evapotranspiration （ET） model developed by Zhang et al. （2001） was applied to evaluate mean annual ET in the alpine area, and the response of annual ET to land use change was analyzed. The plant-available water coefficient （w） of Zhang＇s model was revised by using vegetation-temperature condition index （VTCI） before annual ET was calculated in alpine area. The future land use scenario, an input of ET model, was spa- tially simulated by using the conversion of land use and its effects at small regional extent （CLUE-S） to study the re- sponse of ET to land use change. Results show that the relative errors between the simulated ET and that calculated by using water balance equation were 3.81% and the index of agreement was 0.69. This indicates that Zhang＇s ET model based on revised plant-available water coefficient is a scientific and practical tool to estimate the annual ET in the al- pine area. The annual ET in 2000 in the study area was 221.2 ram, 11.6 mm more than that in 1980. Average annual ET decreased from southeast to northwest, but the change of annual ET between 1980 and 2000 increased from southeast to northwest. As a vast and sparsely populated area, the population in the TRH region was extremely unbalanced and land use change was concentrated in very small regions. Thus, land use change had little effect on total annual ET in the study area but a great impact on its spatial distribution, and the effect of land use change on ET decreased with in- creasing precipitation. ET was most sensitive to the interconversion between forest and unused land, and was least sen- sitive to the interconversion between cropland and low-covered grassland.

Evaluating Spatial Heterogeneity of Land Surface Hydrothermal Conditions in the Heihe River Basin

Land surface hydrothermal conditions(LSHCs) reflect land surface moisture and heat conditions, and play an important role in energy and water cycles in soil-plant-atmosphere continuum. Based on comparison of four evaluation methods(namely, the classic statistical method, geostatistical method, information theory method, and fractal method), this study proposed a new scheme for evaluating the spatial heterogeneity of LSHCs. This scheme incorporates diverse remotely sensed surface parameters, e.g., leaf area index-LAI, the normalized difference vegetation index-NDVI, net radiation-Rn, and land surface temperature-LST. The LSHCs can be classified into three categories, namely homogeneous, moderately heterogeneous and highly heterogeneous based on the remotely sensed LAI data with a 30 m spatial resolution and the combination of normalized information entropy(S’) and coefficient of variation(CV). Based on the evaluation scheme, the spatial heterogeneity of land surface hydrothermal conditions at six typical flux observation stations in the Heihe River Basin during the vegetation growing season were evaluated. The evaluation results were consistent with the land surface type characteristics exhibited by Google Earth imagery and spatial heterogeneity assessed by high resolution remote sensing evapotranspiration data. Impact factors such as precipitation and irrigation events, spatial resolutions of remote sensing data, heterogeneity in the vertical direction, topography and sparse vegetation could also affect the evaluation results. For instance, short-term changes(precipitation and irrigation events) in the spatial heterogeneity of LSHCs can be diagnosed by energy factors, while long-term changes can be indicated by vegetation factors. The spatial heterogeneity of LSHCs decreases when decreasing the spatial resolution of remote sensing data. The proposed evaluation scheme would be useful for the quantification of spatial heterogeneity of LSHCs over flux observation stations toward the global scale, and also contribute to the improvement of the accuracy of estimation and validation for remotely sensed(or model simulated) evapotranspiration.

How Does the Partitioning of Evapotranspiration and Runoff between Different Processes Affect the Variability and Predictability of Soil Moisture and Precipitation?

Water stored as part of the land surface is lost to evapotranspiration and runoff on different time scales, and the partitioning between these time scales is important for modeling soil water in a climate model. Different time scales are imposed on evapotranspiration primarily because it is derived from different reservoirs with different storage capacities, from the very rapid evaporation of canopy stores to the slow removal by transpiration of rooting zone soil moisture. Runoff likewise ranges in time scale from rapid surface terms to the slower base-flow. The longest time scale losses of water determine the slow variation of soil moisture and hence the longer time scale effects of soil moisture on precipitation. This paper shows with a simple analysis how shifting the partitioning of evapotranspiration between the different reservoirs affects the variability of soil moisture and precipitation. In particular, it is concluded that a shift to shorter time scale reservoirs shifts the variance of precipitation from that which is potentially predictable to unpredictable.

Estimating Evapotranspiration Using Improved Fractional Vegetation Cover and Land Surface Temperature Space

Vegetation index-land surface temperature （VI-T s ） space has been widely used to estimate evapotranspiration and soil moisture. The limitation of this method is the uncertainty of the observed dry edge, which is usually fitted by scatter plots. Here, a method was used to locate true dry and wet edges based on energy balance formulation, and a simple method to estimate surface energy flux is proposed based on the improved Fractional vegetation cover-Land surface temperature （F v -T s ） space. Seventeen days of MODIS products were selected to estimate evapotranspiration and the estimated sensible heat flux （H） is compared with Large Aperture Scintillometer （LAS） data at a site in Zhengzhou, resulting in a RMSE of 44.06 W m^-2 , bias of 36.99 W m^-2 and R^2 of 0.71. The H scatter plots of estimation versus observation show clearly that most points are around the 1：1 line. Overall, the located true and wet edges are more accurate than the observed true edge. Our results can also be applied to improve the estimation of soil moisture.

Satellite detection of increases in global land surface evapotranspiration during 1984-2007

As a key component of digital earth,remotely sensed data provides the compelling evidence that the amount of water vapour transferred from the entire global surface to the atmosphere increased from 1984 to 2007.The validation results from the earlier evapotranspiration(ET)estimation algorithm based on net radiation(Rn),Normalised Difference Vegetation Index(NDVI),air temperature and diurnal air temperature range(DTaR)showed good agreement between estimated monthly ET and ground-measured ET from 20 flux towers.Our analysis indicates that the estimated actual ET has increased on average over the entire global land surface except for Antarctica during 1984
2007.However,this increasing trend disappears after 2000 and the reason may be that the decline in net radiation and NDVI during this period depleted surface soil moisture.Moreover,the good correspondence between the precipitation trend and the change in ET in arid and semi-arid regions indicated that surface moisture linked to precipitation affects ET.The input parameters Rn,Tair,NDVI and DTaR show substantial spatio-temporal variability that is almost consistent with that of actual ET from 1984 to 2007 and contribute most significantly to the variation in actual ET.

Geospatial Analysis of Urban Heat Island Effects and Tree Equity

In recent decades, Urban Heat Island Effects have become more pronounced and more widely examined. Despite great technological advances, our current societies still experience great spatial disparity in urban forest access. Urban Heat Island Effects are measurable phenomenon that are being experienced by the world’s most urbanized areas, including increased summer high temperatures and lower evapotranspiration from having impervious surfaces instead of vegetation and trees. Tree canopy cover is our natural mitigation tool that absorbs sunlight for photosynthesis, protects humans from incoming radiation, and releases cooling moisture into the air. Unfortunately, urban areas typically have low levels of vegetation. Vulnerable urban communities are lower-income areas of inner cities with less access to heat protection like air conditioners. This study uses mean evapotranspiration levels to assess the variability of urban heat island effects across the state of Tennessee. Results show that increased developed land surface cover in Tennessee creates measurable changes in atmospheric evapotranspiration. As a result, the mean evapotranspiration levels in areas with less tree vegetation are significantly lower than the surrounding forested areas. Central areas of urban cities in Tennessee had lower mean evapotranspiration recordings than surrounding areas with less development. This work demonstrates the need for increased tree canopy coverage.

Estimating surface evapotranspiration using combined MODIS and CBERS-02 data

Spatial scale error is one of the most serious problems in the estimates of land surface heat fluxes of sensible and latent from satellite-borne data such as MODIS 1km resolution reflectance and emissive data. One of the feasible and economic ways to decrease the spatial scale error is to use high resolution land use class data together with the MODIS data. CBERS-02 data were used to produce land use class of Baiyangdian area, Hebei Province, China in the autumn of 2004. The area ratio of each class in MODIS pixel was calculated, and used to derive the heat fluxes of the mixed pixel. The results showed that the estimated heat fluxes of soil, sensible and latent have been changed remarkably after using the high resolution land class data. It could be concluded from the comparison between simulated and ground-measured fluxes as well as the theoretical analysis that high resolution land class data are useful to diminishing the scale error of heat fluxes estimated from low resolution satellite data.

Impacts of land use/cover change on water balance by using the SWAT model in a typical loess hilly watershed of China

Land use/cover change(LUCC)plays a key role in altering surface hydrology and water balance,finally affect-ing the security and availability of water resources.However,mechanisms underlying LUCC determination of water-balance processes at the basin scale remain unclear.In this study,the Soil and Water Assessment Tool(SWAT)model and partial least squares regression were used to detect the effects of LUCC on hydrology and water components in the Zuli River Basin(ZRB),a typical watershed of the Yellow River Basin.In general,three recommended coefficients(R^(2)and E ns greater than 0.5,and P bias less than 20%)indicated that the output results of the SWAT model were reliable and that the model was effective for the ZRB.Then,several key findings were obtained.First,LUCC in the ZRB was characterized by a significant increase in forest(21.61%)and settlement(23.52%)and a slight reduction in cropland(-1.35%),resulting in a 4.93%increase in evapotranspiration and a clear decline in surface runoffand water yield by 15.68%and 2.95%at the whole basin scale,respectively.Second,at the sub-basin scale,surface runoffand water yield increased by 14.26%-36.15%and 5.13%-15.55%,respectively,mainly due to settlement increases.Last,partial least squares regression indicated that urbanization was the most significant contributor to runoffchange,and evapotranspiration change was mainly driven by forest expansion.These conclusions are significant for understanding the relationship between LUCC and water balance,which can provide meaningful information for managing water resources and the long-term sustainability of such watersheds.

Regional Features and Seasonality of Land–Atmosphere Coupling over Eastern China

Land-atmosphere coupling is a key process of the climate system, and various coupling mechanisms have been proposed before based on observational and numerical analyses. The impact of soil moisture（SM） on evapotranspiration（ET） and further surface temperature（ST） is an important aspect of such coupling. Using ERA-Interim data and CLM4.0 offline simulation results, this study further explores the relationships between SM/ST and ET to better understand the complex nature of the land-atmosphere coupling（i.e., spatial and seasonal variations） in eastern China, a typical monsoon area. It is found that two diagnostics of land-atmosphere coupling（i.e., SM-ET correlation and ST-ET correlation） are highly dependent on the climatology of SM and ST. By combining the SM-ET and ST-ET relationships, two ＂hot spots＂ of land-atmosphere coupling over eastern China are identified： Southwest China and North China. In Southwest China, ST is relatively high throughout the year, but SM is lowest in spring, resulting in a strong coupling in spring. However, in North China, SM is relatively low throughout the year, but ST is highest in summer, which leads to the strongest coupling in summer. Our results emphasize the dependence of land-atmosphere coupling on the seasonal evolution of climatic conditions and have implications for future studies related to land surface feedbacks.

Aridity/humidity status of land surface in China during the last three decades

To clarify aridity/humidity status of land surface is helpful for studying environmental background and regional differences, seeking causes of environmental change, and providing a scientific basis for researches on climate change in the future. In this paper, the authors calcu- lated potential evapotranspiration of China using data from 616 meteorological stations during the period of 1971―2000 with the Penman-Monteith model recommanded by FAO in 1998. Vy- sothkii’s model was used to calculate aridity/humidity index. Then the calculated results of sta- tions were interpolated to land surface using ArcGIS. Results show that the annual average po- tential evapotranspiration is 400―1500 mm in the whole country, 600―800 mm in most parts of it; and 350―1400 mm in growing season (April―Octobor), which is nearly 200 mm less than the annual average. According to the aridity/humidity indexes of 1.0, 1.5 and 4.0, the aridity/humidity status is categorized to four types, namely, humid, subhumid, semiarid and arid. A majority of stations (76%) are more humid in growing season than the annual average. Results of com- prisons between the distribution map of aridity/humidity index with that of precipitation and vegetation indicate a good consistence of aridity/humidity status with natural environment. Therefore potential evapotranspiration calculated with modified FAO’s Penman-Monteith model in combination with aridity/humidity index that considers water balance can more reasonably explain the actual land surface aridity/humidity status of China.

Sub-pixel analysis to enhance the accuracy of evapotranspiration determined using MODIS images

A study was carried out to estimate the actual evapotranspiration(ET)over a 1074 km2 of the humid area of Perak State(Malaysia),where water and evaporation cycle deeply influences the climate,natural resources and human living aspects.Images from both Terra and Aqua platforms of the Moderate Resolution Imaging Spectroradiometer(MODIS)sensor were used for ET estimation by employing the Surface Energy Balance Algorithm for Land(SEBAL)model.As a part of the accuracy assessment process,in-situ measurements on soil temperature and reference ET(ET0)were recorded at the time of satellite overpass.In order to enhance the accuracy of the generated ET maps,MODIS images were subjected to sub-pixel analysis by assigning weights for different land surface cover(urban,agriculture and multi-surface areas)reflections.The weighting process was achieved by integrating ET from pure pixels with the respective site-specific ET0 of each land cover.The enhanced SEBAL model estimated ET exhibited a good correlation with the in-situ measured Penman-Montieth ET0,with R2 values for the Aqua and the Terra platforms of 0.67 and 0.73,respectively.However,the correlation of the non-enhanced ET maps resulted in R2 values of 0.61 and 0.68 for the Aqua and the Terra platforms,respectively.Hence,the results of this study revealed the feasibility of employing the sub-pixel analysis method for an accurate estimation of ET over large areas.

基于GF-5B VIMI数据和SEBS模型的区域蒸散发遥感估算研究

蒸散发是全球水循环的关键部分,对水循环和能量平衡有显著影响。然而现有的蒸散发数据显示出其在时空分辨率上的局限性。为了解决这一问题,文章采用了结合“高分五号”B卫星全谱段光谱成像仪(GF-5B VIMI)高分辨率数据与SEBS模型的新方法。选择重庆市大渡口区作为研究区域,利用GF-5B VIMI影像数据和ERA5-land气候再分析数据集,对该区域的日蒸散发量进行估算,并分析其时空分布特征及不同下垫面的蒸散量。同时,探讨了蒸散发量与归一化植被指数NDVI之间的相关性。实验结果表明,SEBS模型估算结果与涡度相关仪观测数据具有良好的一致性,表现在决定系数R^(2)为0.764,均方根误差RMSE为0.348 mm/d。不同土地利用类型的日蒸散量中,水体蒸散量最大,裸地最小。日蒸散量与归一化植被指数之间的拟合相关系数R为0.908,决定系数R^(2)为0.824,这进一步验证了模型的有效性和适用性。该方法有效提高蒸散发的时空分辨率并为水资源的高效利用提供重要科学依据。

地表蒸散定量遥感的研究进展

简要介绍了遥感监测地表蒸散和能量平衡研究的科学意义和应用价值 ,回顾了国内外的研究历史和现状 ,分析了目前该领域存在的一些问题和难点 。

基于遥感的蒸散发及地表温度对LUCC响应的研究

探讨土地利用/覆盖类型变化(LUCC)对地表温度(LST)和计算蒸散发(ET)的影响,进而了解LUCC对水资源消耗的影响。利用Landsat TM/ETM+遥感数据,解译获取山东省垦利县1987年和2000年的LUCC信息,并利用单窗算法和SEBAL模型分别反演LST,ET。研究发现:由于经济的快速发展,导致1987到2000年13年间山东省垦利县的土地利用/覆盖变化极为显著,变化面积达到总面积的36.12%;土地利用/覆盖的特点基本控制了研究区域的LST和ET的区域分布特点,滨海滩涂、沼泽地、水体的LST值低,而ET值高;建设用地和盐碱地的LST值高,ET值低。不同土地利用/覆盖类型下的各LST和ET基本都表现为单峰型变化。1987年和2000年LST的频率分布特征大体一致,但2000年的各地类LST的分布范围比1987年的LST分布范围变宽。ET的分布变化没有呈现出这个特征,2000年各类型的日ET量都大于1987年的日ET量,两个时期都表现为水体的ET最大,农田次之,建设用地的ET最小。且不同土地利用/覆盖类型下ET和LST之间都存在明显的反相关关系。

基于遥感技术的区域地表蒸散研究进展

遥感技术的发展为大面积的区域地表蒸散(evapotranspiration,ET)反演和估算提供了一种新的手段,国内外学者围绕该领域在理论和技术方法上开展了大量研究。能量平衡原理是遥感估算地表ET的理论基础,由此发展出多种遥感ET模型与算法。在分析几种常见ET模型算法的基本原理及优缺点的基础上,阐述了不同方法的适应性及相关研究进展情况,提出了目前遥感ET研究所存在的问题,展望了遥感ET的发展趋势。

作物需水量研究进展的回顾与展望

国内外有关作物需水研究的进展情况和相关估算方法,包括研究内容、定量估算方法和数学预测方法,通过其分析论述了各方法的主要原理、特点及局限性,并指出了目前作物需水研究领域存在的主要问题是在非均一下垫面条件下,植被与环境以及植被间的相互作用要比均一下垫面复杂得多。目前的计算模式存在着许多假设,普适性不够,难以推广应用,今后需进一步研究的课题与采取的研究思路主要是改进蒸散计算方法。最后简要介绍了一种利用遥感蒸散模型率定通用陆面模式CoLM来估算区域陆面蒸散量以及陆地耗水量的思路。

基于MODIS数据的黄河三角洲区域蒸散发量时空分布特征

充分利用遥感手段的快速、准确、大尺度及可视化等优势,对地球表层水量转化过程中最难估算的分量-蒸散发量进行了估算。针对黄河三角洲地理位置特殊、石油生产基地的经济重要性、生态系统水资源压力日益增大等特点,采用理论基础较坚实、区域应用限制小、反演陆面蒸散发量较为合理准确的SEBS(Surface Energy Balance System)模型, 基于MODIS数据估算了黄河三角洲区域陆面蒸散发量,分析了陆面蒸散发量的时空分布特征。研究结果表明:研究区年内陆面蒸散发量呈单峰型分布,季节性变化特征显著;陆面蒸散发量的空间分布在一定程度上受人类活动驱动的土地覆盖影响呈现出南高北低的趋势。水分盈亏分析表明:春季、秋季蒸散发相对较强烈,天然水分供给不足,灌溉需求较高,水资源丰枯状况对农业耕作、生态环境影响较大。

中国东部水分收支的初步分析

利用中国160站降水资料、中国气象局提供的探空资料、NCEP／NCAR提供的再分析资料（简称NCEP资料）和ECMWF提供的再分析资料（简称ERA40资料），根据水汽平衡方程，估算了1990-1999年中国东部的陆表水分收支，分析了华北、长江流域和华南三个典型区域的陆表水分收支，同时对NCEP、ERA40资料在东亚地区的陆表水分收支进行评估。结果表明，在中国东部区域，年平均和夏季是水汽汇区，冬季降水与蒸发基本平衡；华北在年平均、夏季以及冬季均为水汽源区；长江流域在年平均、夏季及冬季均为水汽汇区；华南在年平均和冬季为弱水汽汇区，夏季为水汽源区。两套再分析资料基本揭示出了上述特征。就区域平均的蒸发和降水的年际变化而言，两套再分析资料的结果与观测都存在显著相关，但估算的蒸发NCEP好于ERA40；相对于气候态的定量比较而言，由两套再分析资料得到的陆表水分收支距平（即降水减去蒸发的距平）的年际变化基本与观测一致。

遥感蒸散发模型研究进展

蒸散发是水圈、大气圈和生物圈水分和能量交换的主要过程,也是水循环中最重要的分量之一。遥感技术的应用使得区域尺度的蒸散发估算成为可能,由此涌现出许多旨在精确反演不同时空尺度蒸散发及地表通量的模型,以更好地服务于相关领域的研究和应用。根据模型建构思想和方法的不同,从基于能量平衡的单层和多层模型、基于能量平衡的彭曼类模型以及遥感经验模型3个方面入手,系统回顾国内外遥感蒸散发模型的算法,详细评述了目前应用较为广泛的SEBAL,SEBS,TSEB模型的原理和优劣,分析了蒸散发反演存在的时间拓展及不确定性、时空分辨率及尺度效应、模型选择及适用性判别、平流影响和精度检验5个问题。指出今后应加强陆面过程和地-气系统作用机制、模型输入参数精度、精度检验和尺度效应及误差传递规律等方面的研究。