Renhua Zhang

On the separate retrieval of soil and vegetation temperatures from ATSR data

The Along-Track Scanning Radiometer (ATSR) onboard the European Remote Sensing satellite (ERS) is presently the only one available to provide quasi-simultaneous thermal infrared measurements at two view angles. Such data represent an opportunity to explore the potential information on the directional observations in the thermal infrared region, in view of the preparation of a new generation of multi-angle satellite sensors. Based on the analysis of one ATSR image, the results of this work indicate that the magnitude of the directional effect on the brightness temperature (ground anisotropic radiance), although quite sensitive to errors in atmospheric conditions, may still be retrieved with acceptable uncertainty. In order to retrieve both vegetation and soil temperatures from directional brightness temperatures, it is shown that an appropriate description of the nature and content of the pixel is needed, otherwise this retrieval will be quite uncertain.

A physically based algorithm for land surface emissivity retrieval from combined mid-infrared and thermal infrared data

On the basis of the concept of the two-channel Temperature-Independent Spectral Indices (TISI), a physically based method is developed to extract the directional emissivities in mid-infrared and thermal infrared channels from day-night space measurements. A phenomenological model with three parameters is also proposed in this paper to describe the angular variations of the reflectivity (or emissivity). Having applied the proposed method to AVHRR data on an area covering the lberian Peninsula (rather vegetated) and on a region centered on Tunisia (arid area), one can see from the results that the terrestrial surfaces do not behave as Lambertian reflector and angular variations of bidirectional reflectivity for bare soils appear to be azimuth-independent whereas those for vegetation present a pronounced backscattering effect. As for directional emissivities, values of vegetated areas are found to be higher and remain rather constant whatever the view angle is. On the contrary, on arid areas, values are found to be rather low and present significant angular variations.

SAR and Multispectral Image Fusion Using Generalized IHS Transform Based on à Trous Wavelet and EMD Decompositions

In the intensity hue saturation (IHS) based image fusion, the enhancement of the spatial information often leads to the distortion of the information in the spectral domain. In this paper, a spectral preserve fusion method is developed by introducing a trous wavelet transform (AWT) and empirical mode decomposition (EMD) into one generalized IHS transform, which extends panchromatic sharpening of low-resolution multispectral images (LRMIs) through detail injection to the fusion of LRMIs and synthetic aperture radar imagery (SARI). The original LRMIs are first transformed into one low-resolution intensity component (LRIC) and the details from the SARI are abstracted using AWT. The LRIC and the approximation image of the SARI are fused through an intrinsic mode functions based model. Subsequently, high RIC produced through an inverse AWT is then substituted for the old one. High RMIs are obtained through an inverse IHS transform. Experiments reveal that this approach performs better than the AWT and IHS-AWT-based methods in preserving the spectral information of the LRMIs. The performance of the image fusion is examined both visually and quantitatively.

A remote sensing model of CO2 flux for wheat and studying of regional distribution

A model for calculating CO2 flux in the wheat field and an algorithm for estimating CO2 flux in the mejonal scale were presented using the remote sensing data and supplementary micpo-met~orological data. First of all a-longertenn measurement wae carried out during winter wheat growing period in Yucheng Experimental Station udng the spectmradiometer system, the thermal infrared radiometer system, the Bowen-ratio device as well as the eddy-correlation device. Two kinds of issues concerning remote sensing and CO2 flux can be obtained. Based on the obeervations a remote sensing model was estabilished. Then when the NOAA-AVHRR passed over the experimental area simultaneous measurements were carried out with the satellites. A regional distribution image for CO2 flux over wheat canopy in North China (500×500 km2) was made using the supplementary ground data and NOAA-AVHRR remote sensing data which was calibrated by the synchronous observation. The sources and sinks for CO2 fluxes in the region can be seen obviously.

Thermal model for discrete vegetation and its solution on pixel scale using computer graphics

In this paper, we discuss how the multi-reflection of thermal emission affects the calculation of radiation balance. With the help of computer graphics, the four components of discrete vegetation are analyzed in detail and the curves of BRDF for the discrete vegetation can be obtained as well. A new model is put forward to inverse the temperatures of four components. The solution obtained by using computer graphics is consistent with observations in the field experiment in Yucheng Remote Sensing Comprehensive Site of CAS. Furthermore, the method can be used to retrieve land surface temperature based on multi-angle thermal infrared remotely sensed data.

A new measuring technique of soil thermal inertia

Thermal inertia is the function of substance density, heat capacity and heat diffusivity, and is an important parameter for researching the process of surface heat balance using remote sensing technique. In this paper, using soil heat plates, infrared thermometer, data logger and other instruments, by man-controlled changing the solar radiation status on the soil samples, the authors gave a new method to measure soil thermal inertia. Using the continuously surveying data of soil heat flux and infrared radiation temperature, thermal inertia can be calculated easily. Based on the thermal inertia calculation of three soil samples with different water content, good results were abtained by the authors, so this method is feasible. Meanwhile, this measuring technique is also a new attempt.

Estimation of regional evapotranspiration based on remote sensing: case study in the Heihe River Basin

Abstract. Accurate estimation of evapotranspiration (ET) has long been an important issue in hydrology. Many experimental observations indicate advection has a great impact on ET in arid and semiarid areas. However, most of the remote sensing models only focus on the vertical energy balance and deviate from reality. A revised Penman equation has been derived to estimate actual ET under normal conditions in order to account for advection. The parameter of the water availability for ET is introduced and the T s / f (surface temperature and vegetation fraction) space is used to characterize this parameter in the revised formula. The estimates were validated using the observations measured with eddy covariance systems at the Yingke station. In 22 of all 24 days, the difference between the observations and the estimates was smaller than 70     W / m 2 . The correlation coefficient is 0.91 and the RMSE is 48.38     W / m 2 . This finding reveals that this approach is capable of providing reliable results. In addition, when considering advection, the potential ET can be 83.23     W / m 2 larger than the available energy. This finding indicates that the advection effect needs to be considered in remote sensing models in order to derive more reliable regional ET.

Estimation of soil heat flux by apparent thermal inertia

By analyzing the relationship between midday soil heat flux (Gm) and apparent thermal inertia (ATI) by time series observations from YuCheng agroecological station, CAS, a method of estimating midday G by ATI was presented in the paper. ATI method is shown to have the close agreement with the observations. In situ observations from 2003 to 2005 were used to determine the coefficients of the formulation of Gm and ATI. Data in 2006 and 2008 were used to evaluate the method. In addition, effects of soil moisture on soil heat flux were specially investigated by field experiment. Soil water content is the dominant factor influencing soil heat flux for saturated or near-saturated soil (45%). As soil dries, soil temperature plays a more significant role and on the initial drying stage (from 45% to 30%), relatively higher soil temperature combined with high soil thermal conductivity make soil heat flux of wet soil much higher than that of dry soil.

A simplified method to separate latent and sensible heat fluxes using remotely sensed data

In this paper, the relationships between NDVI (normalized difference vegetation index) and pseudo thermal inertia (land surface temperature difference between day and night AVHRR data) are discussed. A simplified method was established to separate the latent and sensible heat fluxes on vegetation covered land surface. Analogous correlations between surface reflectance and LST (Land Surface Temperature) had been illustrated by several scientists who are concerned about regional estimation of soil evaporation in and or semi-arid areas. It proves work well in retrieving of soil evaporation or even aerodynamic resistance of beat transfer. The scattergram of pseudo thermal inertia versus NDVI demonstrates that higher frequency points clearly form a triangle shape, that is to say, the higher frequency points are within the triangle. The upper side of the triangle can be written as: DT/sub u/(NDVI)=a/sub u/+b/sub u/*NDVI. Similarly, the lower side of it can be denoted as: DT/sub l/(NDVI)=a/sub l/+b/sub l/*NDVI. Then, Bowen ratio B/sub i/ can be expressed as following: B/sub i/=(DT/sub u/(NDVI/sub i/)-DT/sub i/)/(DT/sub u/(NDVI/sub i/)-DT/sub l/(NDVI/sub i/))-1, where DT/sub i/ is the land surface temperature difference between day and night of the ith pixel, NDVI/sub i/ is as well. The soil heat flux G is determined empirically. After separating Rn (net radiation) into Rn/sub s/ and Rn/sub v/ (net radiation for soil and vegetation respectively) based on the vegetation fraction, the G can be obtained by: G=0.35* Rn/sub s/+0.05* Rn/sub v/, where the constant parameters may vary according to different land cover and land use.

A new algorithm to automatically determine the boundary of the scatter plot in the triangle method for evapotranspiration retrieval

Three different algorithms were developed in this paper to automatically determine the boundary of the triangle shape in the scatter plots. Experiment shows that the Algorithm III has the best performance among them based on the evaluation criteria of their robustness and computation time. The new algorithm proved to be effective to maintain the self-consistency and avoid the subjectivity in the traditional triangle method.