Land Surface Remote Sensing

Environment and Risks

Mehrez Zribi, Nicolas Baghdadi

• Publisher: ISTE Press - Elsevier
• Year: 2016
• Print ISBN: 9781785481055
• eBook ISBN: 9780081012659
• Topic: Technology & Engineering
• Subtopic: Environmental Science
• Index: Technology & Engineering

Land Surface Remote Sensing: Environment and Risks explores the use of remote sensing in applications concerning the environment, including desertification and monitoring deforestation and forest fires. The first chapter covers the characterization of aerosols and gases by passive remote sensing. The next chapter presents the correlation of optical images for quantifying the deformation of the Earth's surface and geomorphological processes. The third chapter is examines remote sensing applications in the mining environment. The fourth chapter depicts the strong potential of radar imagery for volcanology and urban and mining subsidence studies. The next two chapters deal respectively with the use of remote sensing in locust control and the contribution of remote sensing to the epidemiology of infectious diseases.In the last ten years, spatial observation of the Earth—particularly continental surfaces—has expanded considerably with the launch of increasing numbers of satellites covering various applications (hydrology, biosphere, flow of surface, snow, ice, landslide, floods). This has paved the way for an explosion in the use of remote sensing data.This book offers essential coverage of space-based observation techniques for continental surfaces. The authors explore major applications and provide a corresponding detailed chapter for the physical principles, physics of measurement, and data processing requirements for each technique, bringing you up-to-date descriptions of techniques used by leading scientists in the field of remote sensing and Earth observation.- Provides clear and concise descriptions of modern remote sensing methods- Explores the most current remote sensing techniques with physical aspects of the measurement (theory) and their applications- Provides chapters on physical principles, measurement, and data processing for each technique described- Describes optical remote sensing technology, including a description of acquisition systems and measurement corrections to be made

1

Drylands and Desertification

Richard Escadafal

Abstract

Regions with dry climates cover a large part of the world’s landmass (more than 40%). Collectively known by the English term “drylands”, they encompass a wide range of ecosystems, including the American “deserts” of California, Nevada and Mexico; the vast steppes of Central Asia and North Africa; the Sahelian savannah and real deserts such as the Sahara in Africa or the Gobi in China.

Keywords

Degradation; Desert aerosols; Desertification; Drylands; Ligneous plants; Sandstorms; Soils; Surface materials; Surfaces; Vegetation cover

1.1 Drylands

Regions with dry climates cover a large part of the world’s landmass (more than 40% [ADE 05], Figure 1.1). Collectively known by the English term “drylands”, they encompass a wide range of ecosystems, including the American “deserts” of California, Nevada and Mexico; the vast steppes of Central Asia and North Africa; the Sahelian savannah and real deserts such as the Sahara in Africa or the Gobi in China.

Figure 1.1 World map of the distribution of drylands; source: Millennium Ecosystem Assessment (2005)

1.1.1 Common features

The primary defining feature of drylands is a climate with average annual precipitation levels (P) that are substantially less than the evaporative capacity of the air (ETP, evapotranspiration). When using the aridity index P/ETP, drylands are defined as regions where this ratio is less than two-thirds [ADE 15]. In addition, these are further subdivided into classes using this ratio (for example hyper-arid regions or “real deserts” correspond to an aridity index of less than 0.05). There can be over-variability both in space and in time, even if these climates also experience seasons. Therefore, to roughly summarize their differences, the drylands in North Africa experience rain, especially in the winter, and the natural vegetation is the steppe type, dominated by low woody shrubs, while south of the Sahara there is savannah, whose entire landscape quickly develops and grows vegetation in the summer monsoon season, which largely disappears during the long dry season, leaving a landscape that is almost bare.

Although each of the world’s drylands has its own particular characteristics, the irregularity of precipitation and consequently the patchy aspect of vegetation cover over time and/or space is by far the most common feature. The world’s large regions with dry climates also have a very recognizable physiognomy: large open spaces with local dune formations, places subject to the action of the wind and associated phenomena. These are also territories that have mostly been used as grazing lands by pastoral societies, some of whom adopted a nomadic lifestyle, since the Neolithic. Drylands are therefore mostly used for pastoral activity on at least two-thirds of their surface area [ADE 15], while less than 10% is used for rain-fed crops and irrigated areas represent barely 3% [LHO 07].

The characteristics that distinguish drylands have implications for the environmental problems that affect them as well as for the use of remote sensing to study them.

1.1.2 Desertification: a form of degradation specific to drylands

In drylands, the soil is more directly exposed to changes in the climate, and particular forms of land degradation arise, collectively referred to as “desertification”. Although this term does not adequately describe the phenomena involved and often leads to an erroneous interpretation in which desertification is equated with invasion by the desert, it is nevertheless now commonly used. The United Nations convention in charge of combating this phenomenon provides the following definition: “desertification is land degradation in arid, semi-arid and dry sub humid zones resulting from diverse factors including climatic variations and human activity”¹.

This definition emphasizes both the climatic context and the land management aspect. As with the majority of land, arid or not, degradation generally occurs following maladaptive and unsustainable practices that reduce the environment’s capacity to provide ecosystem services, of which the most important is the production of food.

Numerous studies have been done to understand the mechanisms of desertification and to counteract it [REY 11, DOD 13, ESC 15]². For this, a diagnosis of the state of various environments and the methods for monitoring their changes are crucial to programs for action. In this chapter, we will explore cases where data and remote sensing methods have been implemented.

1.2 Dryland vegetation observed by satellite

With regard to remote sensing, drylands have a decisive advantage: minimal cloud cover. This makes the use of optical imaging much easier, especially for observing and monitoring vegetation cover. In addition, the use of satellite images to study these vast areas, which are often difficult to observe directly on the ground, has appeared to be promising since the beginning of civilian remote sensing. If optical data are dominant in the image providers’ catalogs, the most recent development of other spectral ranges, especially infrared and microwaves, allows for the diversification of the information obtained, and in particular the development of a multisensor approach.

1.2.1 Vegetation cover observed by satellite

Whether the sensor used to observe drylands is passive or active, the radiation that it receives is reflected or emitted by land surfaces primarily made up of soil and vegetation, with the latter generally being the less dominant element. The term “soil” is used here according to the broad definition used in French soil science, where rocky outcroppings and accumulations of sand (dunes, for example) are raw mineral soils (while in the North American classification system, these are “non-soils”). Here, the soil is a very important component, even if for remote sensing used for ecological monitoring the emphasis is on monitoring green vegetation and is done mostly with the NDVI (normalized difference vegetation index). In comparison with Figure 1.1, we can see the low NDVI index values for the drylands in Figure 1.2.

Figure 1.2 NDVI vegetation index values, varying from gray to dark green (National Oceanic and Atmospheric Administration – Advanced Very High Resolution Radiometer (NOAA – AVHRR) satellite data, 2012 summary, copyright NOAA). For a color version of this figure, see www.iste.co.uk/baghdadi/6.zip

To correctly characterize the vegetation of these regions, it should be noted that the vegetation is discontinuous over time and/or space. This is an important point, especially in choosing dates for observation. In cases where herbaceous vegetation covers the landscape after it rains, the NDVI is welladapted to evaluating the cover or the biomass of that vegetative cover and characterizing its fluctuations, especially in the long term.

Therefore, NOAA-AVHRR satellite images have been used by numerous teams to measure the changes in vegetation cover at the global level and to make diagnoses of the state of the environment. One of the difficulties present is the ability to link satellite measurements with the reality on the ground: a measuring point of the satellite sensors used here covers 9 km × 9 km, and it is often difficult to obtain vegetation measurements in the field of the same area at the same time when the satellite is passing over. In addition, the atmosphere’s transparency is affected by clouds and dust, which complicates the comparison of two types of measurement: that done on the ground and that done by satellite. The most recent large-field instruments, such as SPOT Vegetation (SPOT-VGT), MODIS and MERIS, have a finer spatial resolution, allowing for a more detailed monitoring, but over a shorter amount of time since they have been observing the planet for less time.

Among the recent work done, a French team was able to compare the level of vegetation coverage on the ground with the vegetation index measured by satellite on two reference sites: one in the Gourma region of Mali and one in the Fakara region of Niger [DAR 14a]. They were able to make this comparison for the years 1981 to 2011 using field measurements that have been carried out at these sites for almost that entire period (Figure 1.3).

Figure 1.3 Amount of vegetation cover on a site with sandy soil in Sahelian Africa: a) dry season; b) wet season (photos: P. Hiernaux)

Among the results of this study, Figure 1.4 shows the change in the NDVI during the same period for the entire Sahelian zone. It clearly shows the “re-vegetation” recorded in this region, since a large proportion of green is visible in the image, corresponding to an increase in the vegetation index during the 30 years being studied. There...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Foreword
• Acronyms
• Introduction
• 1: Drylands and Desertification
• 2: Remote Sensing and Measuring Deforestation
• 3: Remote Sensing of Wildfires
• 4: Characterization of Industrial Plumes
• 5: Monitoring of Earth Surface Motion and Geomorphologic Processes by Optical Image Correlation
• 6: Application of Optical Remote Sensing for Monitoring Environmental Impacts of Mining: From Exploitation to Postmining
• 7: The Contribution of SAR Data to Volcanology and Subsidence Studies
• 8: Applications of Remote Sensing to Locust Management
• 9: Applications of Remote Sensing to the Epidemiology of Infectious Diseases: Some Examples
• Glossary
• List of Authors
• Index
• Scientific Committee