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ABSTRACT: The purpose of this investigation was to develop a method for building simplified geometrical representations (GM) of cotton plants that could be used to simulate plant light interception at low planting densities. Individual plants were observed infield trials throughout their growing period and their 3D architecture (AM) was reproduced at all growth stages. The crown envelope of the GM was represented by an ellipsoid. The ellipsoid content consisted of foliage elements randomly dispersed in a planophile distribution. The radiative transfers of both plant representations were simulated in the PAR range using the ARCHIMED simulation platform. Different ellipsoid envelope and foliage element sizes were investigated. With an appropriate set of sizes, light interception calculated using GMs tallied well with that obtained using AMs. It can be concluded that a simple turbid medium model is sufficient for modelling GM content without needing to introduce a clumping factor.
Plant Growth Modeling and Applications, 2006. PMA '06. Second International Symposium on; 12/2006
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ABSTRACT: Existing experimental methods based on the measurement of crop temperature to estimate water stress have been applied for 20 years. However, the application of such techniques is limited because they are not able to totally overcome either soil interference on the measured signal or directional effects involved in temperature measurements according to sun/sensor angles configuration and crop structure. An energy balance model, based on the 3D description of plants at leaf level, is used to simulate directional cotton crop temperature variability according to crop structure and water status. The model is implemented with a bare soil compartment so that soil temperature, water balance as well heat exchanges with the crop can be computed. Once validated, this approach provides an accurate interpretation of thermal infrared information considering the directional effects involved in surface temperature measurements. This offers the opportunity of analyzing the limits of using temperature-based crop water status indices when dealing with partially covering crops. This study underlines the knowledge and tools to be further investigated in order to improve or perform such experimental techniques.
Precision Agriculture 08/2003; 4(3):297-309. · 1.55 Impact Factor
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ABSTRACT: A pertinent interpretation of thermal infrared (TIR) information to characterize crop water status requires at least to consider the fraction of crop cover. Even if the crop cover is known, such an interpretation remains difficult and the current issues to be overcome in the field of TIR remote sensing applications stands on bare soil effects. An experiment was conducted during summer 1999 in Montpellier (France) on a row-cotton crop in order to acquire a data set relating thermal and optical multidirectional measurements to crop structure and water status. The crop was monitored all along its development. Three plots were delimited: a reference plot with no water limitation and two plots without water supply respectively at flowering and cutout stage. On three dates, directional TIR and optical images were acquired both on the reference plot and on the one with limited water supply. Directional averaged temperatures (Ts) and Normalized Difference Vegetation Index (NDVI) values showed a strong dependence on canopy gap fraction. Ts appeared particularly influenced by directional sunlit soil fraction variability, depending on both sun/sensor angle configuration, crop structure and water status. Leaves at different levels in the canopy (with different ages and spectral properties) could be observed by the sensor, but the impact of the sunlit/shaded leaves ratio on directional temperature measurements was weak in comparison to soil effects.The different directional influence of sunlit soil fractions on Ts and NDVI values explains in a large part the limits encountered by water stress indices approaches, aiming at relating linearly such variables, when applied to partially covering crops. Such results provide an exhaustive experiment-based biophysical analysis of very high resolution multidirectional TIR signal. They point out further ways of investigations to be explored in the field of water stress indices improvement or performing. This comes as a preamble of an experiment-based analysis of the limits and opportunities of water stress indices methods, complemented with a 3D model-based analysis that allows confirmation and extrapolation of the results to larger ranges of crop characteristics and directional configurations.
Remote Sensing of Environment.
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ABSTRACT: Multidirectional remotely sensed optical and thermal images acquired within a row cotton crop in Montpellier (France) were used to test the opportunities and limitations of an existing water stress index, the Water Deficit Index (WDI, based on the trapezoid approach). The WDI was applied with multidirectional crop surface temperatures (Ts) and reflectance data acquired on a row-cotton crop with different water and cover conditions from 11 different view angles in the east/west plane. This data set allowed a biophysical analysis of this index both inside and outside its validity domain, initially limited in terms of Ts measurements in a [−20°, +20°] view angles interval around nadir. Results showed that the WDI was a robust approach, since its calculation is based on the relationship between crop cover and Ts. However, it yielded some directional errors in the case of sparse crops even in its validity domain where the relative variation of WDI between oblique angles and nadir could reach 14% (and more than 40% for larger view angles). The same degree of variability was observed between WDI values estimated on a same plot at two different times in a given day from a nadir observation. In a large range of crop heterogeneity, hourly sunlit soil fraction presented a stronger influence on Ts than the total soil fraction. However, by adapting the view angle to daytime measurements and crop structure, it seemed possible to overcome sunlit soil effects.These experimental results were tested and extrapolated using a 3D crop energy balance model (Thermo). It allowed simulations of directional Ts measurements according to various sun/sensor angular configurations, crop structure, and water status characteristics. This confirmed the limitations of the trapezoid method both within and outside its validity domain. Moreover, Thermo allowed the computation of a “directional” WDI accounting for angular and hourly sunlit soil effects variability on Ts. The interest of adapting the view angle to daytime measurements and crop structure was confirmed by comparing this “directional” WDI with the “theoretical” one (based on the original trapezoid approach). These results should encourage further development of water stress indices based on bidirectional thermal infrared and optical measurements to quantify and thus overcome sunlit soil fraction effects.
Remote Sensing of Environment.
