Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Climatic controls on aeolian activity in the Mojave and Colorado Deserts, California

Auteur : Bach, Andrew James

Université de soutenance : Arizona State University

Grade : Doctor of Philosophy (PhD) 1995

Résumé
This research assesses the response of aeolian sand to climatic conditions during the period 1973-1994. Sand dune (aeolian sand) mobility is in part controlled by sediment supply, wind energy, and surface conditions (i.e. vegetation cover and character, surface crusting, soil development etc.). In the Mojave and Colorado Deserts of California a dune mobility index based on climatic variables (wind energy, precipitation and potential evapotranspiration) predicts that dunes should be mostly active. In point of fact, the dunes are mostly inactive. A combination of complex topography, which leads to mesoclimatic variations, and the seasonality of precipitation gives rise to a pattern of aeolian activity that reflects local influences as much as, or more than regional climatic influences. In order to test the utility of the mobility index, an independent measure of dune mobility is developed. The field index of sand mobility (FISM) uses field observations to objectively measure the mobility of a sand dune landscape. Observations of vegetation type and cover, surface stabilizing features, and aeolian landforms and features are assigned relative numerical values. Dunes with low index values have high degrees of mobility. Higher values of FISM are found in more relict dune areas. The FISM performs well in evaluating the actual mobility state of a location. Active dune fields like Kelso and Algodones have FISM less than 100. The less active flanks of these dunes have higher FISM values. Many dunes in the eastern Mojave Desert are episodically active and have FISM values between 100-300. Dormant dunes have FISM values over 300, as illustrated by the dune field in Antelope Valley. Principal component analysis of the mobility index distinguishes four distinct regions of mobility : the moderately active to dormant central Mojave Desert ; the very active Colorado Desert ; the Lancaster and Palm Springs areas which are dormant, but prone to reactivation ; and the southeastern Mojave Desert where sand transport pathways exist. The field index of sand mobility provides an independent measure of dune activity. Regression analysis shows that the field index of sand mobility does a good job of estimating the Lancaster mobility index (57.4% of variance explained). The combination of both methods provides a powerful tool in evaluating the mobility state of desert dunes. Reactivation of stabilized dunes in the Mojave and Colorado Deserts may occur if the climate during the next century becomes warmer and drier as suggested by global climate models. Eight consecutive years of below normal precipitation and particularly the severe drought of 1989-1990 led to an overall reduction in natural vegetation cover by October 1990, when approximately 20,000 ha had been been wind eroded. Unfortunately it is not possible to disentangle the effects of 8-years of drought (1984-1990)) from 2-years of severe drought (1989-1990) from one year of very severe drought (1990) and determine which was/were responsible for the increase in wind erosion during 1990. Considerable paleoclimatic research has been conducted in the Mojave Desert, but there is not a satisfactory contemporary data base for comparison. A better understanding of the physical processes linking contemporary climate and the contemporary aeolian system will allow process-based interpretations of paleoclimatic indicators, such as relict dune features. This dissertation bridges that gap by increasing understanding of the dynamics of the contemporary aeolian system.

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