WSP05759 - Laserfiche WebLink

Home Browse Search

M3H5 3 ( and wet the soil surface. Such stonns, however would reduce cranspira[ion and ai- fect me water balance in mat manner. I[ is apparent. [hen, that drought impacts based strictly on me usual precip- iL1tion records have inherent shortcomings. Available soil moisrure is the ultima[e measure as it integrates precipitation, evaporation and transpiration, the latter two being temperarure related. The amount and rate of evaporation is an important factor in determining drought characteristics and it, in rum, is influenced by wind movement. For exam- cp ~ pie, average wind movement was almost doubled when a pan evaporation station at Roosevelt Dam in Arizona was moved some 900 feet. As a consequeence, evapo- ration increased almost 20 percent (Green and Sellers, 1964). Altered wind move- men[ and accompanying changes in evaporation were also noted when stations were moved a[ Yuma. Mesa and Tucson. It seems feasible then that changes in general atmospheric circulation could increase wind movement and thus haste:1 the onset of drought or cause existing dry conditions to become more severe. Almough droughts are related to changes in large-scale aanospheric circu- lation, me exact combination of circwnslances that result in extended periods of dry weather are not clearly understood. Until L'lese parameters are more clearly defined, the most logical approach is [0 examine past records to see if there is any pattern in the recurrence of drought. UnfortUnately the period of instrumented records is not long enough to verify the cyclic aspects of drought. Proxy data sources must then be employed that might reflect past climatic conditions. l The most commonly used sources of proxy data are stratified sediments in screams, lakes and swamps; p~llen profiles; layered ice cores; and tree rings. Al- though climatic reconstructions based on tree rings do not extend as far back in the past as the other methods. they can be accurately dated as to the year of ring forma-