Alain Vidal

Evaluation of a temporal fire risk index in mediterranean forests from NOAA thermal IR

Abstract Mediterranean forests are regularly subjected to a large number of fires; 537,000 ha were burned during the severe European drought of 1990. The French Ministries of Environment, Interior, and Agriculture are trying to implement efficient methods to prevent forest fires and to reduce their incidence. Fire risk is composed of human, ecological, and climatic factors that are already accounted for in prevention methods. However, the importance of biophysical factors, especially the water status of forest trees, bushes, grasses, and litter should also be considered. In a first step, thermal infrared data from NOAA-AVHRR daily images covering an 18-month period were used to estimate temporal variations of forest evapotranspiration through an energy budget-based relationship. Results were related to statistics on fire starts, in order to derive a fire risk index that can be used for a real-time regional alarm.

Evaluating forest fire hazard with a Landsat TM derived water stress index

Abstract Mediterranean forests are regularly subjected to a large number of fires; 537 000 hectares were burned during the severe European drought of 1990. The French Ministries of Environment, Interior and Agriculture are trying to implement efficient methods to prevent forest fires and to reduce their incidence. Fire risk is composed of human, ecological and climatic factors that are already accounted for in prevention methods. However, the importance of biophysical factors, especially the water status of forest trees, bushes, grasses and litter should also be considered. The Water Deficit Index (WDI) is based on the combination of a vegetation index and the surface minus air temperature difference obtained from Landsat TM images and meteorological data. It aims at better estimating the water stress of partial canopies. The WDI has been shown to be easily estimated on forested surfaces, and a better indicator of water stress than the widely used CWSI (Crop Water Stress Index). Therefore it can be used for locating the areas where fire risk is high, even if for the studied cases, the prediction ability of fire starts was equivalent with both indexes. It has been shown that areas where WDI ≥ 0.6 concern 100% of fire events of more than 1 ha occurring after Landsat overpass but only 19% of the forested area: this index can thus be used as a tool for reducing survey operations dedicated to fire prevention.

Using multidirectional thermography to characterize water status of cotton

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.

An overview of the workshop on thermal remote sensing held at La Londe les Maures, France, September 20-24, 1993

Abstract This article reviews the major topics discussed at the Thermal Remote Sensing Workshop held at La Londe les Maures, France in 1993 and concludes with a list of new items and concepts suggested during the meeting

Combination of digital elevation models with SPOT-1 HRV multispectral imagery for reflectance factor mapping

Abstract Digital numbers in SPOT-1 HRV multispectral imagery were transformed to reflectance factor values. This approach is based on a simplified-scene radiance equation that is developed in this paper. The values of some parameters needed in the equation such as path radiance, total normal optical thickness, and diffuse spectral irradiance were estimated by using multiple linear regression in a small test site of homogeneous ground cover, located within the study area. The calculated values of reflectance factors were presented by dimensionless digital numbers ranging from 0 to 255, which corresponds respectively to the values of reflectance factors from 0 to 1. The feasibility of this approach has been tested against the SPOT-1 HRV multispectral imagery above a canopy of Picaussel Forest of the Quillan region in the eastern pyrenean mountain in France.

3D Simulation of directional temperature variability within a row-cotton crop : Toward an improvement of experimental crop water status monitoring using thermal infrared

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.