Droughts and floods are water-related natural disasters which affect a wide range of environmental factors and activities related to agriculture, vegetation, human and wild life and local economies. Drought is the single most important weather-related natural disaster often aggravated by human action, since it affects very large areas for months and years and thus has a serious impact on regional food production, life expectancy for entire populations and economic performance of large regions or several countries. During 1967-1991, droughts

Satellite Remote Sensing and GIS Applications in Agricultural Meteorology pp. 291-313

have affected 50 per cent of the 2.8 billion people who suffered from all natural disasters and killed 35 per cent of the 3.5 million people who lost their lives. In the recent years large-scale intensive droughts have been observed in all continents leading to huge economic losses, destruction of ecological resources, food shortages and starvation of millions people. Floods are among the most devastating natural hazards in the world, claiming more lives and causing more property damage than any other natural phenomena.

Several users such as top level policy makers at the national and international organisations, researchers, middle level policy makers at the state, province and local levels consultants, relief agencies and local producers including farmers, suppliers, traders and water managers are interested in reliable and accurate drought and flood information for effective management. The disaster management activities can be grouped into three major phases: The Preparedness phase where activities such as prediction and risk zone identification are taken up long before the event occurs; the Prevention phase where activities such as Early warning/Forecasting, monitoring and preparation of contingency plans are taken up just before or during the event; and the Response/Mitigation phase where activities are undertaken just after the event which include damage assessment and relief management.

Remote sensing techniques make it possible to obtain and distribute information rapidly over large areas by means of sensors operating in several spectral bands, mounted on aircraft or satellites. A satellite, which orbits the Earth, is able to explore the whole surface in a few days and repeat the survey of the same area at regular intervals, whilst an aircraft can give a more detailed analysis of a smaller area, if a specific need occurs. The spectral bands used by these sensors cover the whole range between visible and microwaves. Rapid developments in computer technology and the Geographical Information Systems (GIS) help to process Remote Sensing (RS) observations from satellites in a spatial format of maps - both individually and along with tabular data and "crunch" them together to provide a new perception - the spatial visualisation of information of natural resources. The integration of information derived from RS techniques with other datasets - both in spatial and non-spatial formats provides tremendous potential for identification, monitoring and assessment of droughts and floods.

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