Remote sensing can provide timely and economical monitoring of large areas. It provides the ability to generate information on a variety of spatial and temporal scales. Generally, remote sensing is divided into passive and active depending on the sensor system. The majority of remote-sensing studies concerned with drought monitoring have involved visible-infrared sensor systems, which are passive and depend on the sun's illumination. Radar (radio detection and ranging) is an active sensor system that transmits energy in the microwave region of the electromagnetic spectrum and measures the energy reflected back from the landscape target. The energy reflected back is called backscatter. The attraction of radar over visible-infrared remote sensing (chapters 5 and 6) is its independence from the sun, enabling day/night operations, as well as its ability to penetrate cloud and obtain data under most weather conditions. Thus, unlike visible-infrared sensors, radar offers the opportunity to acquire uninterrupted information relevant to drought such as soil moisture and vegetation stress.
Drought conditions manifest in multiple and complex ways. Accordingly, a large number of drought indices have been defined to signal abnormally dry conditions and their effects on crop growth, river flow, ground-water, and so on (Tate and Gustard, 2000).
In the field of radar remote sensing, much work has been devoted to developing algorithms to retrieve geophysical parameters such as soil moisture, crop biomass, and vegetation water content. In principle, these parameters would be highly relevant for monitoring agricultural drought. However, despite the existence of a number of radar satellite systems, progress in the use of radar in environmental monitoring, particularly in respect to agriculture, has been slower than anticipated. This may be attributed to the complex nature of radar interactions with agricultural targets and the suboptimal configuration of the satellite sensors available in the 1990s
(Ulaby, 1998; Bouman et al., 1999). Because most attention is still devoted to the problem of deriving high-quality soil moisture and vegetation products, there have been few investigations on how to combine such radar products with other data and models to obtain value-added agricultural drought products. This chapter provides a brief overview of radar sensor systems and the principles involved in the interaction of microwave energy with agricultural targets.
The two main radar systems with potential for agricultural monitoring are synthetic aperture radars (SARs) and scatterometers. While SARs offer high ground resolution suitable for providing information on a farm level, scatterometers allow frequent sampling (daily to weekly) at a regional scale. Scatterometers have similar spatial and temporal sampling characteristics as spaceborne radiometers, which are discussed in chapter 7. Progress with the use of these two radar systems is discussed, with emphasis on how the information could be used to monitor agricultural droughts.
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