Using a thermal-based two source energy balance model with time-differencing to estimate surface energy fluxes with day-night MODIS observations

The Dual Temperature Difference (DTD) model, introduced by Norman et al. (2000), uses a two source energy balance modelling scheme driven by remotely sensed observations of diurnal changes in land surface temperature (LST) to estimate surface energy fluxes. By using a time-differential temperature measurement as input, the approach reduces model sensitivity to errors in absolute temperature retrieval. The original formulation of the DTD required an early morning LST observation (approximately 1 h after sunrise) when surface fluxes are minimal, limiting application to data provided by geostationary satellites at sub-hourly temporal resolution. The DTD model has been applied primarily during the active growth phase of agricultural crops and rangeland vegetation grasses, and has not been rigorously evaluated during senescence or in forested ecosystems. In this paper we present modifications to the DTD model that enable applications using thermal observations from polar orbiting satellites, such as Terra and Aqua, with day and night overpass times over the area of interest. This allows the application of the DTD model in high latitude regions where large viewing angles preclude the use of geostationary satellites, and also exploits the higher spatial resolution provided by polar orbiting satellites. A method for estimating nocturnal surface fluxes and a scheme for estimating the fraction of green vegetation are developed and evaluated. Modification for green vegetation fraction leads to significantly improved estimation of the heat fluxes from the vegetation canopy during senescence and in forests. When the modified DTD model is run with LST measurements acquired with the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites, generally satisfactory agreement with field measurements is obtained for a number of ecosystems in Denmark and the United States. Finally, regional maps of energy fluxes are produced for the Danish Hydrological ObsErvatory (HOBE) in western Denmark, indicating realistic patterns based on land use

Carbon Dioxide Dynamics During a Growing Season in Midwestern Cropping Systems

Daily and seasonal CO2-exchange dynamics between the boundary layer and biosphere is important to understanding Net Ecosystem Exchange of terrestrial ecosystems. Spatial and temporal variations of CO2 fluxes across midwestern cropping systems have not been well documented. This study was designed to monitor and evaluate spatial and temporal dynamics of CO2 exchange across a watershed region for typical production fields of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] in the Midwest by quantifying the above-canopy, within-canopy, and soil components of C balance for this cropping system. An energy-balance approach using eddy covariance was utilized across different fields making year-around measurements in both corn and soybean fields to quantify the exchange of CO2 and H2O between the crop canopy and the atmospheric boundary layer. Within-canopy concentrations of CO2 and H2O vapor were measured with an eight-port CO2/H2O infrared analyzer. Soil respiration was quantified using soil chambers at various landscape positions throughout the growing season. Fluxes of CO2 and H2O vapor throughout the day were dependent on net radiation and the stage of canopy development. Diurnal variations in CO2 and H2O vapor fluxes revealed that the magnitude of the fluxes is large and the variation of the fluxes among fields was consistent throughout the season. Integration of the daily fluxes into seasonal totals showed large differences among crops and fields. Flux differences were the result of the effect of varying soil types on water-holding capacity. Seasonal integrated values were lower than estimates derived from biomass samples collected within the fields and the measurement of the C content of the biomass. Within-canopy recycling of soil CO2 may provide insight to this discrepancy. The techniques are available to quantify the CO2 and H2O vapor fluxes across different management systems and landscapes to help refine our understanding of the magnitude of the CO2 and H2O dynamics in cropping systems

Evaporation monitoring over land surface using microwave radiometry

National audienc

Radiometry at infrared wavelengths for agricultural applications

Measurements of thermal radiation at infrared wavelengths (7-14 μm) yield much information about the land surface. The primary use of these observations is for surface temperature determination as the emissivity is usually close to one. For this purpose it is fortuitous that the peak in the thermal emission occurs in an atmospheric transmission window. In additions there are variations in the emissivity of minerals and soils in the 7-14-μm region which can be interpreted for identification purposes. The emissivity for vegetative canopies has been found to be close to one with little spectral variation. Applications of the derived surface temperature to study the surface energy balance and to estimate the energy fluxes from the land surface are discussed. The basic concepts of the energy balance at the land surface are presented along with an example of how remotely sensed surface brightness temperatures can be used to estimate the sensible heat and to estimate plant water use. The example is from the Monsoon 90 experiment conducted over an arid watershed in the state of Arizona in the United States. In this case, surface temperatures derived from an aircraft thermal infrared sensor and vegetation and land use characteristics derived from a Landsat TM image were used in a two-source model to predict the surface heat fluxes. The agreement with ground measurements is reasonably good for the 3 days of observations. (© Inra/Elsevier, Paris.)Applications agricoles de la radiométrie infrarouge. Les mesures de radiation thermique aux longueurs d'ondes infrarouges (7 à 14 μm) fournissent une riche information concernant la surface des terres. Ces observations sont utilisées en premier lieu pour déterminer la température de surface, puisque l'émissivité en est généralement proche. Dans ce cas il arrive par hasard que le pic dans l'émission thermique se trouve dans la fenêtre de transmission atmosphérique. En outre il y a des variations de l'émissivité des minéraux et des sols dans la bande 7 à 14 μm, qui peuvent être interprétées dans des buts d'identification. On a trouvé que l'émissivité des canopées de la végétation était proche de la valeur un, avec une petite variation spectrale. On discute des applications de la température de surface dérivée à l'étude de l'équilibre énergétique de surface et à l'estimation des flux d'énergie envoyé par la surface des terres. Les concepts de base de l'équilibre énergétique à la surface des terres sont présentés avec un exemple qui montre comment les températures de brillance de surface télédétectées peuvent être utilisées pour estimer la chaleur sensible et l'utilisation de l'eau par la plante. L'exemple est tiré de l'expérience Mousson 90 qui a été faite au-dessus d'une ligne de partage des eaux en région aride, dans l'état d'Arizona. Dans ce cas, les températures de surface provenant du détecteur infrarouge thermique de l'avion et les caractéristiques de la végétation et de l'utilisation des terres provenant de l'image Landsat TM ont été utilisées dans un modèle à deux sources pour prédire les flux de chaleur de surface. Les résultats concordent assez bien avec les mesures de terrain faites durant les trois jours d'observation. (© Inra/Elsevier, Paris.