196,303 research outputs found
Analysis of the land surface heterogeneity and its impact on atmospheric variables and the aerodynamic and thermodynamic roughness lengths
No full textThe land surface heterogeneity has a very significant impact on atmospheric variables (air temperature T-a, wind speed u, and humidity q), the aerodynamic roughness length z(0m), thermodynamic roughness length z(0h), and the excess resistance to heat transfer kB(-1). First, in this study the land surface heterogeneity has been documented through the comparison of surface reflectance r(0), surface temperature T-0, net radiation flux R-n, and sensible heat flux H partitioning over the different land cover types in the experimental areas of the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment on the Tibetan Plateau (GAME/Tibet), the Coordinated Enhanced Observing Period (CEOP) Asia-Australia Monsoon Project on the Tibetan Plateau (CAMP/Tibet), the Heihe Basin Field Experiment (HEIFE), the Arid Environment Comprehensive Monitoring Plan, 95 (AECMP' 95), and the Dun Huang Experiment (DHEX). The results show that the surface heterogeneity was very significant in the areas of the HEIFE, the AECMP' 95, and the DHEX and that it was less significant in the areas of CAMP/Tibet and GAME/Tibet. Second, the vertical profiles of T-a, u, and q in the near-surface layer and above the blending height z(b) have been analyzed using the atmospheric boundary layer (ABL) tower data, radiosonde data, and tethered balloon data observed during the HEIFE, the DHEX, and the CAMP/Tibet. The results show that the land surface heterogeneity leads in the near-surface layer to different vertical profiles of u, T-a, and q overlying the surfaces of the Gobi and the oasis in the areas of the HEIFE and DHEX. The values of u, T-a, and q become well mixed above a height of about 300 m at the HEIFE and 150 m at the DHEX. z(0m), z(0h), and kB(-1) over the different land surfaces have also been determined in this study. The results show that the land surface heterogeneity leads to different aerodynamic and thermodynamic parameters over the areas of the HEIFE, the AECMP' 95, and the GAME/Tibe
Physical aspects and determination of evaporation in deserts applying remote sensing techniques
No full textIn the deserts of the world groundwater losses by evaporation are shown to be a very important and almost unknown quantity in the water balance of aquifers present.After establishing a new combination formula to calculate actual evaporation from within the soil, the evaporational soil physical aspects are discussed in detail. It is shown that under specified conditions vapour flow can take place against the direction of heat flow. A new, fundamental, definition of evaporation sites is given and its bearing on a model of heat and moisture flow is discussed. A theory of heat exchange at the soil - air interface is presented in terms of the apparent soil thermal admittance. Equations to relate the latter soil property to soil layering and to the frequency of the surface temperature wave are given. A theory of evaporation in terms of multi-dimensional geometry is proposed and it is shown how to derive approximate formulae to calculate actual evaporation by making use of satellite data only. For more accurate results point ground reference measurements have been used to calibrate the satellite-based calculations.After describing a particular desert area in Libya, the accuracy of a number of satellite data of that area is discussed on the basis of ground reference measurements and numerical correction procedures.Finally, the theory is combined with satellite and point ground reference data to calculate the different terms of the surface energy balance and the actual evaporation for some 36,000 km 2of the Libyan desert
Performance indicators for the statistical evaluation of digital image classifications
No full textA statistical procedure is proposed to evaluate the algorithms for the numerical classification of images. The approach is based on the derivation of performance indicators from measurements of signature separability and thresholding analysis. Although these measurements are not new in image processing techniques, they are used in this study in an original way for the comparison of outputs resulting from different classification criteria. The theoretical description of the method suggested is followed by its practical application to a case-study for mapping crop coefficients in an irrigation district
Retrieval of small-relief marsh morphology from Terrestrial Laser Scanner, optimal spatial filtering, and laser return intensity
No full textMarshes are ubiquitous landforms in estuaries and lagoons, where important hydrological, morphological and ecological processes take place. These areas attenuate sea action on the coast and act as sediment trapping zones. Due to their ecosystem functions and effects on coastal stabilization, marshes are crucial structures in tidal environments, both biologically and geomorphologically, and are fundamental elements in wetland restoration and coastal realignment schemes. The spatially-distributed study of the geomorphology of intertidal areas using remotely-sensed digital terrain models remains problematic, owing to their small relief, often of the order of a few tens of centimetres, and to the presence of short and dense vegetation, which strongly reduces the number of resolvable ground returns. Here, we use high-resolution Terrestrial Laser Scanning (∼ 200 returns/m2) to retrieve a high-resolution and high-accuracy Digital Terrain Model within a tidal marsh in the Venice lagoon. To this aim we apply a new filtering scheme to Terrestrial Laser Scanner data which selects the lowest values within moving windows, whose optimal size is determined with the aid of a limited number of ancillary Differential GPS data in order to maximize resolution while ensuring the identification of true ground returns. The accuracy of the filtered data is further refined using classifications of the intensity of the returns to extract additional information on the surface (ground or canopy) originating the returning laser beam. Validations against about 200 reference Differential GPS ground elevation observations indicates that the best separation of canopy and ground signals is obtained using a low-pass filter with window size of the order of 1 m and the maximum likelihood classifier to further refine the detection of ground returns. In this case the average estimation error is about 1 cm (slight overestimation of ground elevation), while its standard deviation is about 3 cm. Our approach allows the separation of laser returns coming from the low marsh vegetation from those coming from the marsh surface. The overall result is a new observation technique producing Digital Terrain and Digital Surface Models in areas with very small relief, which is shown to provide unprecedented high-resolution and high-accuracy characterizations of marsh morphology
Thermal infrared observations of heterogeneous soil-vegetation systems
No full textRemote SensingAerospace Engineerin
Spectral indicators of photosynthesis. The impact of anisotropy of reflectance and heterogeneity of vegetation
No full textRemote sensing and simulation modelling for on-demand irrigation systems management
No full textThis paper describes a procedure for monitoring and improving the performance of on-demand irrigation networks, based on the integration of remote sensing techniques and simulation modelling of water flow in each component of the system. In order to adequately reproduce the actual operation of an on-demand irrigation system, the physical characteristics of the irrigation systems (crop, soil and distribution network) are linked to the farmers' irrigation criteria and preferences. The development of this procedure, which is currently being implemented in a district in South Italy, is of great support to improving the management and monitoring of irrigation systems
Adapting numerical models and remote sensing applications to the needs of irrigation agencies and farmers
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