Laserfiche WebLink
<br />OOj~53 <br /> <br />10 <br /> <br />( <br /> <br />(, <br /> <br />Drought was prevalent through the southwestern United Slates, from Texas <br />to California and through the Great Basin, from 1942 through \ 956. A detailed <br />analysis of this period has been provided by Thomas (1962). Droughl was <br />considered 10 exist when precipitation was 85% less than the long-term average. <br />Figure 2 shows areas of such precipitation deficiencies on a year-to-year basis from <br />1942 to 1956. Although areas with precipitation deficiencies varied. drought <br />extended into at least four states at least every year, and in some years all seven <br />states were involved. Based on his analysis of available records, Thomas (1962) <br />suggested that droughts of longer dw-ation are most apt to occur where precipitation <br />is from a single source such as the Pacific Border Zone. In other zones, including <br />the Great Basin. Colorado Plateau and Sonoran Border. :lIlIlual precipitation is from <br />more than one source (e.g. Pacific Ocean and Gulf of Mexico) and droughts are <br />less regular in duration, recurrence and magnitude. <br />Precipitation evems resulting from eastern north Pacific tropical cyclon~s <br />(ENPTC) are not uncommon in the southwestern United States and may be an im- <br />portant factor in providi ng relief from normal dry weather or sustained periods of <br />drought. In many instances these storms produce very large amounts of rainfall. <br />often with disasITous results. Smith (Effects....1986) described the results of 84 <br />ENPTC from 1904-1986. During this period 11 of the storms produced rainfall <br />amounts in excess of 8 inches at specific locations. For example. II.4D inches were <br />recorded in 24 hours at Workman Creek, Arizona. in connection with Tropical <br />Storm Norma in September. 1970. Precipitation from Hurricaine Joanne <br />(September 29-October 6. 1972) measured 17.50 inches at the same location. <br />Thomas (l962) suggestS that most of the southwest can. endure several <br />months of precipitation deficiencies and that drought becomes critical when surface <br />and groundwater supplies are adversely affected. Conditions. however, have <br />changed considerably during the past 30 years as the overall population explosion <br />