Thomas J Jackson

Mitigating the effects of drought can be improved through better information on the current status, the prediction of occurrence, and the extent of drought. Soil moisture can now be measured using a new generation of microwave remote sensing satellites. These measurements can be used to monitor drought conditions on a daily basis over the entire earth. The quality of these products will continue to improve over time as new sensors are launched. These satellite products, combined with existing in situ observations and models, should be exploited in drought monitoring, assessment, and prediction.

Measuring soil moisture on a routine basis has the potential to significantly improve our understanding of climatic processes and strengthen our ability to model and forecast these processes. Leese et al. (2001) concluded that the optimal approach to monitoring soil moisture would be a combination of model-derived estimates using in situ and remotely sensed measurements. In this regard, each method produces soil moisture values that are both unique and complementary. This concept is essentially the process of data assimilation described by Houser et al. (1998). In situ measurements of soil moisture have been made in a few countries over the past 70 years (Robock et al., 2000). However, due to cost and sensor limitations, there are few soil moisture sensor systems available today, especially for automated measurements. A lack of routine observations of soil moisture has led to the use of surrogate measurements (i.e., antecedent precipitation index) and modeled estimates, which limits the possibility of physically based model validation and acceptance.

Current tools to predict drought, such as drought indices and Global Climate Models (GCMs), do not include any direct observations of the soil condition, which is critical for agriculture. Passive microwave remote sensing instruments respond to the amount of moisture in the soil. Several methods have the potential to provide both soil moisture and drought information. In the past, the options have been limited by the availability of satellite systems. Even with these limitations, investigators have explored the potential of these data in soil moisture studies with some success. Within the next few years, a wide range of new and significantly improved satellites will be launched that will offer new opportunities. In this chapter, the basis of passive microwave remote sensing is presented with a description of alternative techniques for retrieving soil moisture. A review of current and future satellite systems is presented, along with examples of soil moisture studies that illustrate how this information can be used for drought monitoring and assessment.

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