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<br />O:J3H3 <br /> <br />( <br />, <br /> <br />DROUGHT HISTORY A!\'D RECO'iSTRUCIlONS FROM TREE RINGS <br /> <br />Introduction <br /> <br />Periods of shan-term or prolonged deficiencies in precipitation, generally <br /> <br />known as droughts, are common occurrences in global climatic regimes and it <br /> <br />would be rare indeed to find a time when the eanh was drought free. Even so, it is <br /> <br />difficult to define drought in terms that would apply in all circumstances. For ex- <br /> <br />ample, Sami, Krishna and Rao (1982) reponed finding no less than 60 definitions <br /> <br />of drought in the literature, based on the nature of water requirements and the time <br /> <br />of need for plants and animals. <br />There are, then, both technical and practical definitions of drought. For the <br /> <br />former, meteorologists would consider drought in terms of the number of rainless <br /> <br />dJ.Ys or precipitation deficits over a specific period of time. When describing the <br /> <br />( <br />, <br /> <br />(i,ought of 1930, for example, Henry (1930) cons;.:iered drought to exiSt when the <br /> <br />rainfall for a period of 21 days or longer was but 30 percent of the average for a <br /> <br />p:uticular time and place. From a practical standpoint, "agricultural drought" might <br /> <br />er.ist when precipitation has been insufficient to maintain soil moisture levels and <br /> <br />crops suffer decreased production or failure. Along the same line, "hydrologic <br /> <br />drought" would be an appropriate definition when surface water or ground water <br /> <br />supplies decline to the point where human and animal populations are adversely af- <br /> <br /> <br />fected_ To summarize, droughts are perhaps most meaningfully defined in terms of <br /> <br />water need, either for direct use to supply human, municipal or indusaial require- <br /> <br />ments, or indirectly to support the needs of growing plants such as agricultural and <br /> <br /> <br />forage crops, forests, etc. At the same time Thomas (1962) points out that there is a <br /> <br /> <br />danger in defining drought saictly on the basis of water need because precipitation <br /> <br />( <br /> <br />alone is never satisfactory for man, animal or plants due to its variability in amount, <br /> <br />rime and space. <br />