85 research outputs found
Retrieving Soil and Vegetation Temperatures From Dual-Angle and Multipixel Satellite Observations
Land surface component temperatures (LSCTs), i.e., the temperatures of soil and vegetation, are important parameters in many applications, such as estimating evapotranspiration and monitoring droughts. However, the multiangle algorithm is affected due to different spatial resolution between nadir and oblique views. Therefore, we propose a combined retrieval algorithm that uses dual-angle and multipixel observations together. The sea and land surface temperature radiometer onboard ESA\u27s Sentinel-3 satellite allows for quasi-synchronous dual-angle observations, from which LSCTs can be retrieved using dual-angle and multipixel algorithms. The better performance of the combined algorithm is demonstrated using a sensitivity analysis based on a synthetic dataset. The spatial errors in the oblique view due to different spatial resolution can reach 4.5 K and have a large effect on the multiangle algorithm. The introduction of multipixel information in a window can reduce the effect of such spatial errors, and the retrieval results of LSCTs can be further improved by using multiangle information for a pixel. In the validation, the proposed combined algorithm performed better, with LSCT root mean squared errors of 3.09 K and 1.91 K for soil and vegetation at a grass site, respectively, and corresponding values of 3.71 K and 3.42 K at a sparse forest site, respectively. Considering that the temperature differences between components can reach 20 K, the results confirm that, in addition to a pixel-average LST, the combined retrieval algorithm can provide information on LSCTs. This article demonstrates the potential of utilizing additional information sources for better LSCT results, which makes the presented combined strategy a promising option for deriving large-scale LSCT products
Retrieval of canopy component temperatures through Bayesian inversion of directional thermal measurements
Evapotranspiration is usually estimated in remote sensing from single temperature value representing both soil and vegetation. This surface temperature is an aggregate over multiple canopy components. The temperature of the individual components can differ significantly, introducing errors in the evapotranspiration estimations. The temperature aggregate has a high level of directionality. An inversion method is presented in this paper to retrieve four canopy component temperatures from directional brightness temperatures. The Bayesian method uses both a priori information and sensor characteristics to solve the ill-posed inversion problem. The method is tested using two case studies: 1) a sensitivity analysis, using a large forward simulated dataset, and 2) in a reality study, using two datasets of two field campaigns. The results of the sensitivity analysis show that the Bayesian approach is able to retrieve the four component temperatures from directional brightness temperatures with good success rates using multi-directional sensors (SrspectraË0.3, SrgonioË0.3, and SrAATSRË0.5), and no improvement using mono-angular sensors (SrË1). The results of the experimental study show that the approach gives good results for high LAI values (RMSEgrass=0.50 K, RMSEwheat=0.29 K, RMSEsugar beet=0.75 K, RMSEbarley=0.67 K); but for low LAI values the results were unsatisfactory (RMSEyoung maize=2.85 K). This discrepancy was found to originate from the presence of the metallic construction of the setup. As these disturbances, were only present for two crops and were not present in the sensitivity analysis, which had a low LAI, it is concluded that using masked thermal images will eliminate this discrepanc
Using middle-infrared reflectance for burned area detection
Tese de doutoramento, CiĂȘncias GeofĂsicas e da Geoinformação (Meteorologia), Universidade de Lisboa, Faculdade de CiĂȘncias, 2011A strategy is presented that allows deriving a new index for burned area
discrimination over the Amazon and Cerrado regions of Brazil. The index is based on
information from the near-infrared (NIR) and middle-infrared (MIR) channels of the Moderate
Resolution Imaging Spectroradiometer (MODIS). A thorough review is undertaken of existing
methods for retrieving MIR reflectance and an assessment is performed, using simulated and
real data, about the added value obtained when using the radiative transfer equation (RTE)
instead of the simplified algorithm (KR94) developed by Kaufman and Remer (1994), the
most used in the context of burned area studies. It is shown that use of KR94 in tropical
environments to retrieve vegetation reflectance may lead to errors that are at least of the
same order of magnitude of the reflectance to be retrieved and considerably higher for large
values of land surface temperature (LST) and solar zenith angle (SZA). Use of the RTE
approach leads to better estimates in virtually all cases, with the exception of high values of
LST and SZA, where results from KR94 are also not usable. A transformation is finally
defined on the MIR/NIR reflectance space aiming to enhance the spectral information such
that vegetated and burned surfaces may be effectively discriminated. The transformation is
based on the difference between MIR and NIR in conjunction with the distance from a
convergence point in the MIR/NIR space, representative of a totally burnt surface. The transformation allows defining a system of coordinates, one coordinate having a small scatter
for pixels associated to vegetation, burned surfaces and soils containing organic matter and
the other coordinate covering a wide range of values, from green and dry/stressed vegetation
to burned surfaces. The new set of coordinates opens interesting perspectives to
applications like drought monitoring and burned area discrimination using remote-sensed
information.O coberto vegetal da superfĂcie da Terra tem vindo a sofrer mudanças, por vezes
drĂĄsticas, que conduzem a alteraçÔes tanto na rugosidade da superfĂcie terrestre como no
seu albedo, afectando directamente as trocas de calor sensĂvel e latente e de diĂłxido de
carbono entre a superfĂcie terrestre e a atmosfera (Sellers et al., 1996). Neste contexto, as
queimadas assumem um papel de extremo relevo (Nobre et al., 1991; OâBrien, 1996; Xue,
1996) na medida em que constituem uma das mais importantes fontes de alteração do
coberto vegetal, resultando na destruição de florestas e de recursos naturais, libertando
carbono da superfĂcie continental para a atmosfera (Sellers et al., 1995) e perturbando as
interacçÔes biosfera-atmosfera (Levine et al., 1995; Scholes, 1995) através de mudanças na
rugosidade do solo, na ĂĄrea foliar e noutros parĂąmetros biofĂsicos associados ao coberto
vegetal. Ora, neste particular, a AmazĂłnia Brasileira constitui um exemplo notĂĄvel de
mudanças no uso da terra e do coberto vegetal nas Ășltimas dĂ©cadas, como resultado da
desflorestação induzida pelo homem bem como por causas naturais (Gedney e Valdes,
2000; Houghton, 2000; Houghton et al., 2000; Lucas et al., 2000), estimando-se que as regiÔes tropicais sejam responsåveis por cerca de 32% da emissão global de carbono para
a atmosfera (Andreae, 1991). Neste contexto, a disponibilidade de informaçÔes
pormenorizadas e actualizadas sobre as distribuiçÔes espacial e temporal de queimadas e
de åreas ardidas em regiÔes tropicais afigura-se crucial, não só para uma melhor gestão dos
recursos naturais, mas tambĂ©m para estudos da quĂmica da atmosfera e de mudanças
climĂĄticas (Zhan et al., 2002).
A detecção remota constitui, neste ùmbito, uma ferramenta indispensåvel na medida
em que permite uma monitorização em tempo quase real, a qual se revela especialmente
Ăștil em ĂĄreas extensas e/ou de difĂcil acesso afectadas pelo fogo (Pereira et al., 1997).
Diversos instrumentos, tais como o Land Remote Sensing Satellite/Thematic Mapper
(LANDSAT/TM) e o National Oceanic and Atmospheric Administration/Advanced Very High
Resolution Radiometer (NOAA/AVHRR) tĂȘm vindo a ser extensivamente utilizados na
gestĂŁo dos fogos florestais, em particular aos nĂveis da detecção de focos de incĂȘndio e da
monitorização de åreas queimadas. Mais recentemente, o instrumento VEGETATION a
bordo do Satellite Pour l'Observation de la Terre (SPOT) tem vindo a ser utilizado com
sucesso na monitorização de fogos. Finalmente, são de referir os sensores da série Along
Track Scanning Radiometer (ATSR) para os quais tĂȘm vindo a ser desenvolvidos algoritmos
de identificação de focos de incĂȘndio, e ainda o sensor Moderate Resolution Imaging
Spectroradiometer (MODIS) que tem vindo a demonstrar capacidades Ăłptimas no que
respeita à observação global de fogos, plumas e åreas queimadas.
Neste contexto, os métodos actuais de detecção de åreas ardidas através da
detecção remota tĂȘm vindo a dar prioridade Ă utilização das regiĂ”es do vermelho (0.64 ÎŒm)
e infravermelho-prĂłximo (0.84 ÎŒm) do espectro eletromagnĂ©tico. No entanto, tanto a regiĂŁo
do vermelho quanto a do infravermelho-prĂłximo apresentam a desvantagem de serem
sensĂveis Ă presença de aerossĂłis na atmosfera (Fraser e Kaufman, 1985; Holben et. al.,
1986). Desta forma, em regiÔes tropicais como a Amazónia, onde existem grandes camadas
de fumo devido à queima de biomassa, a utlização destas duas regiÔes do espectro eletromagnético torna-se insatisfatória para a detecção de åreas ardidas. Por outro lado, a
regiĂŁo do infravermelho mĂ©dio (3.7 â 3.9 ÎŒm) tem a vantagem de nĂŁo ser sensĂvel Ă
presença da maior parte dos aerossóis, exceptuando a poeira (Kaufman e Remer, 1994)
mostrando-se, ao mesmo tempo, sensĂvel a mudanças na vegetação devido Ă absorção de
ĂĄgua lĂquida.
Com efeito, estudos acerca dos efeitos do vapor de ågua na atenuação do espectro
eletromagnĂ©tico demonstraram que a regiĂŁo do infravermelho mĂ©dio Ă© uma das Ășnicas
regiÔes com relativamente pouca atenuação (Kerber e Schut, 1986). Acresce que a região
do infravermelho médio apresenta uma baixa variação da irradiùncia solar (Lean, 1991),
tendo-se ainda que a influĂȘncia das incertezas da emissividade na estimativa da
temperatura da superfĂcie Ă© pequena quando comparada com outras regiĂ”es tĂ©rmicas tais
como as de 10.5 e 11.5 ÎŒm (Salysbury e DâAria, 1994).
A utilização da radiùncia medida através de satélites na região do infravermelho
médio é, no entanto, dificultada pelo facto de esta ser afectada tanto pelo fluxo térmico
quanto pelo fluxo solar, contendo, desta forma, duas componentes, uma emitida e outra
reflectida, tendo-se que a componente reflectiva contém os fluxos térmico e solar reflectidos
pela atmosfera e pela superfĂcie enquanto que as emissĂ”es tĂ©rmicas sĂŁo oriundas da
atmosfera e da superfĂcie. Ora, a componente solar reflectida Ă© de especial interesse para a
detecção de åreas ardidas pelo que se torna necessårio isolå-la do sinal total medido pelo
sensor. Devido à ambiguidade deste sinal, a distinção dos efeitos da reflectùncia e da
temperatura torna-se uma tarefa muito complexa, verificando-se que os métodos em que se
não assume nenhuma simplificação, levando-se, portanto, em consideração todos os
constituintes do sinal do infravermelho mĂ©dio se tornam complexos e difĂceis de serem
aplicados na pråtica, na medida em que requerem dados auxiliares (e.g. perfis atmosféricos)
e ferramentas computacionais (e.g. modelos de tranferĂȘncia radiativa). Kaufman e Remer
(1994) desenvolveram um método simples para estimar a reflectùncia do infravermelho
médio o qual assenta em diversas hipóteses simplificadoras. Apesar do objectivo primårio que levou ao desenvolvimento do método ser a identificação de åreas cobertas por
vegetação densa e escura em regiÔes temperadas, este método tem sido lagarmente
utilizado nos estudos acerca da discriminação de åreas queimadas, algumas das vezes em
regiÔes tropicais (Roy et al., 1999; Barbosa et al., 1999; Pereira, 1999). Na literatura não
existe, no entanto, nenhum estudo acerca da exactidão e precisão deste método quando
aplicado com o objectivo de detectar åreas ardidas, em especial em regiÔes tropicais. Neste
sentido, no presente trabalho procedeu-se a um estudo de viabilidade do método proposto
por Kaufman e Remer (1994) em simultĂąneo com a anĂĄlise da equação de tranferĂȘncia
radiativa na região do infravermelho médio, tendo sido realizados testes de sensibilidade
dos algoritmos em relação aos erros nos perfis atmosfĂ©ricos, ruĂdo do sensor e erros nas
estimativas da temperatura da superfĂcie. Para tal recorreu-se ao modelo de transferĂȘncia
radiativa Moderate Spectral Resolution Atmospheric Transmittance and Radiance Code
(MODTRAN), dando-se especial atenção ao caso do sensor MODIS. Os resultados
demonstraram que a utilização do método proposto por Kaufman e Remer (1994) em
regiÔes tropicais para a estimativa da reflectùncia no infravermelho médio, leva a erros que
sĂŁo pelo menos da mesma ordem de magnitude do parĂąmetro estimado e, em alguns casos,
muito maior, quando ocorre a combinação de altas temperaturas da superfĂcie terrestre com
baixos Ăąngulos zenitais solares. A utilização da equação de transferĂȘncia radiativa mostrouse
uma boa alternativa, desde que estejam disponĂveis dados acerca da temperatura da
superfĂce terrestre assim como dos perfis atmosfĂ©ricos. Entretanto, nas regiĂ”es onde
ocorrem altos valores de temperatura da superfĂcie terrestre e baixos Ăąngulos zenitais
solares, quaisquer dos dois métodos se mostra pouco utilizåvel, jå que nesta região a
estimativa da reflectĂąncia constitui um problema mal-posto.
Em paralelo, utilizaram-se informaçÔes sobre aerossóis de queimada para efectuar
simulaçÔes do MODTRAN que permitiram avaliar a reposta do canal do infravermelho-médio
à este tipo de perturbação do sinal, muito comum na Amazónia Brasileira. A fim de tornar o
estudo o mais realĂstico possĂvel, procedeu-se Ă coleta de material resultante de queimadas na regiĂŁo AmazĂłnica, mais especificamente em Alta Floresta, Mato Grosso, Brasil. Estes
resultado foram então integrados nos estudos em questão, possibilitando a caracterização
espectral das ĂĄreas ardidas.
Com base nos resultados obtido definiu-se uma tranformação no espaço do
infravermelho próximo e médio com o objetivo de maximizar a informação espectral de
forma a que as superfĂcies vegetadas pudessem ser efectivamente discriminadas e as ĂĄreas
ardidas identificadas. A tranformação baseia-se na diferença entre a reflectùncia nos
infravermelhos prĂłximo e mĂ©dio, em conjunto com a distĂąncia a um ponto de convergĂȘncia
no espaço espectral dos infravermelhos próximo e médio, ponto esse representativo de uma
årea completamente ardida. A tranformação permitiu a definição de um novo sistema de
coordenadas, o qual provou ser bastante Ăștil no que diz respeito ĂĄ identificação de ĂĄreas
ardidas. Este novo espaço de coordenadas constitui uma inovação na årea dos estudos de
queimadas, jĂĄ que permite ao mesmo tempo definir dois tipos de Ăndices, o primeiro dos
quais identifica superfĂcies que contĂ©m ou nĂŁo biomassa e o segundo identifica, de entre as
superfĂcies que contĂȘm biomassa, a quantidade de ĂĄgua presente, podendo variar de
vegetação verde (abundĂąncia de ĂĄgua) atĂ© ĂĄreas ardidas (ausĂȘncia de ĂĄgua). AlĂ©m de
distiguir ĂĄreas ardidas, os Ăndices desenvolvidos podem ainda ser aplicados em outros
casos como, por exemplo, estudos de estresse hĂdrico e secas.DSA/INPE; Portuguese Foundation of Science and Technology (Fundação para a CiĂȘncia e Tecnologia / FCT)(SFRH/BD/21650/2005
Recommended from our members
Spaceborne monitoring of high temperature volcanic thermal features: studies using the ERS Along Track Scanning Radiometer
Satellite-based instruments have long been suggested as suitable for monitoring thermal phenomena occurring at the surface of active volcanoes. Past studies using data from high spatial resolution instruments indicated the effectiveness of this technique, but such data are expensive, time-consuming to obtain, and offer a poor temporal resolution. This thesis uses data from the European Remote Sensing satellitesâ Along Track Scanning Radiometer (ATSR) to analyse infrared thermal emittance from a variety of volcanic thermal features at low spatial resolution (1 km2) but high temporal resolution (~ 3 days), with data from vegetation fires also being investigated. I calibrate the (previously uncalibrated) 1.6 ÎŒm channel of ATSR-1, and go onto show how nighttime data in this waveband can be used to characterise emittance from high temperature surfaces, even if these are significantly smaller than the ATSR pixel size.
Procedures are developed to detect hotspots in ATSR data, filter out cloud contaminated observations, and quantitatively analyse the measurements of infrared thermal flux. ATSR time-series datasets are then used to study thermal emittance from active lava domes at Lascar Volcano (Chile) and Unzen Volcano (Japan), with volcanological interpretations being made from the observed variations in radiance. At both volcanoes the dominant source of nighttime shortwave infrared thermal flux is found to be high temperature surfaces heated by fumarolic degassing. During the monitoring period, decreases in shortwave infrared flux indicate an increased hazard at Lascar, such a change indicating blockage of the degassing system and an increased likelihood of a major pressure-driven explosive event. The reverse is found to be true at Unzen, where increases in shortwave infrared flux are found to be generally related to increases in magma supply (both being positively correlated with the flux rate of magmatic gas) and so to an increased frequency of hazardous pyroclastic flow from the growing dome.
ATSR time-series studies of active lava flows at Fernandina Volcano (GalĂĄpagos Islands) and Mount Etna (Sicily) indicate that such data can also be used to document the thermal evolution of a developing lava flow field. Though necessitating assumptions regarding the flow-field thermal structure, ATSR-based estimates of the area of emplaced lava compare favourably with those obtained using higher spatial resolution imagery. For the 1991 - 1993 Etna flow, the estimates of flow surface temperature and area are used to investigate the importance of the various heat loss mechanisms. Results indicate that radiative losses dominate, but that basal conduction is also highly significant.
The Moderate Resolution Imaging Spectrometer (MODIS) of NASAâs Earth Observing System (EOS) will soon provide a new source of multi-waveband, high temporal resolution data, available to the general volcanological and remote sensing community via the EOSDIS data network. I recommend that consideration be given to nighttime operation of the MODIS shortwave infrared channels, since studies using ATSR suggest that these data have considerable potential for the thermal monitoring of active volcanoes
A Comprehensive Review on Water Quality Parameters Estimation Using Remote Sensing Techniques
Remotely sensed data can reinforce the abilities of water resources researchers and decision makers to monitor waterbodies more effectively. Remote sensing techniques have been widely used to measure the qualitative parameters of waterbodies (i.e., suspended sediments, colored dissolved organic matter (CDOM), chlorophyll-a, and pollutants). A large number of different sensors on board various satellites and other platforms, such as airplanes, are currently used to measure the amount of radiation at different wavelengths reflected from the waterâs surface. In this review paper, various properties (spectral, spatial and temporal, etc.) of the more commonly employed spaceborne and airborne sensors are tabulated to be used as a sensor selection guide. Furthermore, this paper investigates the commonly used approaches and sensors employed in evaluating and quantifying the eleven water quality parameters. The parameters include: chlorophyll-a (chl-a), colored dissolved organic matters (CDOM), Secchi disk depth (SDD), turbidity, total suspended sediments (TSS), water temperature (WT), total phosphorus (TP), sea surface salinity (SSS), dissolved oxygen (DO), biochemical oxygen demand (BOD) and chemical oxygen demand (COD)
Half a century of satellite remote sensing of sea-surface temperature
Sea-surface temperature (SST) was one of the first ocean variables to be studied from earth observation satellites. Pioneering images from infrared scanning radiometers revealed the complexity of the surface temperature fields, but these were derived from radiance measurements at orbital heights and included the effects of the intervening atmosphere. Corrections for the effects of the atmosphere to make quantitative estimates of the SST became possible when radiometers with multiple infrared channels were deployed in 1979. At the same time, imaging microwave radiometers with SST capabilities were also flown. Since then, SST has been derived from infrared and microwave radiometers on polar orbiting satellites and from infrared radiometers on geostationary spacecraft. As the performances of satellite radiometers and SST retrieval algorithms improved, accurate, global, high resolution, frequently sampled SST fields became fundamental to many research and operational activities. Here we provide an overview of the physics of the derivation of SST and the history of the development of satellite instruments over half a century. As demonstrated accuracies increased, they stimulated scientific research into the oceans, the coupled ocean-atmosphere system and the climate. We provide brief overviews of the development of some applications, including the feasibility of generating Climate Data Records. We summarize the important role of the Group for High Resolution SST (GHRSST) in providing a forum for scientists and operational practitioners to discuss problems and results, and to help coordinate activities world-wide, including alignment of data formatting and protocols and research. The challenges of burgeoning data volumes, data distribution and analysis have benefited from simultaneous progress in computing power, high capacity storage, and communications over the Internet, so we summarize the development and current capabilities of data archives. We conclude with an outlook of developments anticipated in the next decade or so
The application of the surface energy balance system model to estimate evapotranspiration in South Africa
Includes abstract.Includes bibliographical references.In a water scarce country like South Africa with a number of large consumers of water, it is important to estimate evapotranspiration (ET) with a high degree of accuracy. This is especially important in the semi-arid regions where there is an increasing demand for water and a scarce supply thereof. ET varies regionally and seasonally, so knowledge about ET is fundamental to save and secure water for different uses, and to guarantee that water is distributed to water consumers in a sustainable manner. Models to estimate ET have been developed using a combination of meteorological and remote sensing data inputs. In this study, the pre-packaged Surface Energy Balance System (SEBS) model was used for the first time in the South African environment alongside MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data and validated with eddy covariance data measured in a large apple orchard (11 ha), in the Piketberg area of the Western Cape. Due to the relative infancy of research in this field in South Africa, SEBS is an attractive model choice as it is available as open-source freeware. The model was found to underestimate the sensible heat flux through setting it at the wet limit. Daily ET measured by the eddy covariance system represented 55 to 96% of the SEBS estimate, an overestimation of daily ET. The consistent underestimation of the sensible heat flux was ascribed to sensitivities to the land surface air temperature gradient, the choice of fractional vegetation cover formula as well as the height of the vegetation canopy (3.2 m) relative to weather station reference height (2 m). The methodology was adapted based on the above findings and was applied to a second study area (quaternary catchment P10A, near Grahamstown, Eastern Cape) where two different approaches for deriving surface roughness are applied. It was again demonstrated that the sensible heat flux is sensitive to surface roughness in combination with land surface air temperature gradient and again, the overestimation of daily ET persisted (actual ET being greater than reference ET). It was concluded that in complex environments, at coarse resolution, it is not possible to adequately describe the remote sensing derived input parameters at the correct level of accuracy and at the spatial resolution required for the accurate estimation of the sensible heat flux
Remote sensing of energy and water fluxes over Volta Savannah catchments in West Africa
The deterioration of the West African savannah in the last three decades is believed to be closely linked with about 0.5 C rise in temperature leading to evaporation losses and declining levels of the Volta Lake in Ghana. Although hydrological models can be used to predict climate change impacts on the regional hydrology, spatially-observed ground data needed for this purpose are largely unavailable. This thesis seeks to address this problem by developing improved methods for estimating energy and water fluxes (e.g. latent heat [ET]) from remotely sensed data and to demonstrate how these may be used to parameterize hydrological models. The first part of the thesis examines the potential of the Penman-Monteith method to estimate local-scale ET using groundbased hydrometeorological observations, vegetation coefficients and environmental data. The model results were compared with pan observations, scintillometer (eddy correlation) measurements and the Thomthwaite empirical method. The Penman- Monteith model produced better evaporation estimates (~3.90 mm day(^-1) for the Tamale district) than its counterpart methods. The Thomthwaite, for example, overestimated predictions by 5.0-11.0 mm day(^-1). Up-scaling on a monthly time scale and parameterization of the Grindley soil moisture balance model with the Thomthwaite and Penman-Monteith data, however, produced similar estimates of actual evaporation and soil moisture, which correlated strongly (R(^2) = 0.95) with water balance estimates. To improve ET estimation at the regional-scale, the second part of the thesis develops spatial models through energy balance modelling and data up-scaling methods, driven by radiometric measurements from recent satellite sensors such as the Landsat ETM+, MODIS and ENVISAT-AATSR. The results were validated using estimates from the Penman-Monteith method, field observations, detailed satellite measurements and published data. It was realised that the MODIS sensor is a more useful source of energy and water balance parameters than AA TSR. For example, stronger correlations were found between MODIS estimates of ET and other energy balance variables such as NDVI, surface temperature and net radiation (R(^2) = 0.67-0.73) compared with AATSR estimates (R(^2) = 0.31-0.40). There was also a good spatial correlation between MODIS and Landsat ETM+ results (R(^2) = 0.71), but poor correlations were found between AATSR and Landsat data (R(^2) = 0.0-0.13), which may be explained by differences in instrument calibration. The results further showed that ET may be underestimated with deviations of ~2.0 mm day 1 when MODIS/AATSR measurements are validated against point observations because of spatial mismatch. The final part of the thesis demonstrates the application of the ET model for predicting runoff (Q) using a simplified version of the regional water balance equation. This is followed byanalysis of flow sensitivity to declining scenarios of biomass volume. The results showed the absence of Q for >90% of the study area during the dry season due largely to crude model approximation and lack of rainfall data, which makes model testing during the wet season important. Runoff prediction may be improved if spatial estimates of rainfall, ET and geographical data (e.g. land-use/cover maps, soil & geology maps and DEM) could be routinely derived from satellite imagery
A Review of Current Methodologies for Regional Evapotranspiration Estimation from Remotely Sensed Data
An overview of the commonly applied evapotranspiration (ET) models using remotely sensed data is given to provide insight into the estimation of ET on a regional scale from satellite data. Generally, these models vary greatly in inputs, main assumptions and accuracy of results, etc. Besides the generally used remotely sensed multi-spectral data from visible to thermal infrared bands, most remotely sensed ET models, from simplified equations models to the more complex physically based two-source energy balance models, must rely to a certain degree on ground-based auxiliary measurements in order to derive the turbulent heat fluxes on a regional scale. We discuss the main inputs, assumptions, theories, advantages and drawbacks of each model. Moreover, approaches to the extrapolation of instantaneous ET to the daily values are also briefly presented. In the final part, both associated problems and future trends regarding these remotely sensed ET models were analyzed to objectively show the limitations and promising aspects of the estimation of regional ET based on remotely sensed data and ground-based measurements
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