Long-term tree-ring series of Cedrus atlantica have been utilized to reconstruct and characterize past drought occurrences in Morocco. Based on the close relationship between tree growth and rainfall variations, the best tree-ring indicator of drought has been found to be, x₀ = μ -.75σ, with μ and σ the mean and standard deviation of the tree-ring series. In Morocco, drought is a short event lasting 1.7 years on average with 84% of the events lasting less than 2 years. The average maximum duration exceeds 4 years. It is associated with a 40% deficit about normal moisture availability which can add up to more than 125% when the anomalies persist. The average empirical recurrence interval of drought is 8 years. Long-term persistence has been characterized by the Hurst coefficient. Over the domain of study, the mean Hurst coefficient is .765. The tree-ring series show a periodic behavior with two major peaks at 22 and 66 years. Drought characteristics have been extended from the tree-ring sites to other regions such as the agricultural plains using geostatistical methods. The drought prone areas are located in central Morocco, including the Sais, the Gharb and central Middle Atlas, which experience longer, more severe and more persistent droughts. The northern latitudes experience milder and shorter drought events. The cost for spatial extension of the results was greater kriging errors which make the estimates less reliable. However, cokriging significantly improved the reliability and the quality of the estimates. A regionalization, using cluster analysis, was found to follow the natural climatic and orographic zones of north central Morocco. The tree-ring data were used to reconstruct spatial patterns of moisture anomalies for the period 1845-1974. Three atmospheric factors appear to be important: (1) the relative strength and position of the Azores High, (2) the importance and location of the local cyclogenesis centers, (3) the importance of the northeastern perturbations coming from the Mediterranean sea. The periods 1860-1890, 1925-1950 and the 1970's are associated with dry anomalies while the periods 1900-1920 and 1950-1970 are associated with wet anomalies. They exhibit a rhythmic succession with a 20-year periodicity.

The Southwest United States (SWUS) is facing an ongoing drought which has led to water short- ages, in addition to forest mortality due to wildfire and bark beetle outbreaks associated with increased temperatures. This region has a population of 9.6 million people and is one of the fastest growing parts of the United States, and pressure on its resources can be expected to increase in the future. The SWUS is also projected to become more arid in the coming century under greenhouse gas induced climate change, which will impact its environmental, economic and social vitality. This thesis explores the climate dynamics which control water availability, streamflow, and vegetation green-up in the SWUS, in order to constrain our understanding of the mechanisms controlling the ecohydrology of the region, and to inform projections for the 21st century. Chapters 1 and 2 investigate the climate drivers responsible for producing the observed vari- ability in streamflow for the Gila River, a tributary of the Colorado, and the upper Rio Grande. The Gila is the southernmost snowfed river in the SWUS, and has a spring streamflow peak that responds to melting of the snowpack at its headwaters in New Mexico. The Gila is also sufficiently south so that it has a secondary streamflow peak in the summer which is fed by rains from the North American Monsoon (NAM). On interannual timescales, the Gila’s spring peak is primarily influenced by natural variability associated with Pacific sea surface temperature (SST), while the summer peak apparently does not respond to interannual variability. The upper Rio Grande is fur- ther north and east in the SWUS, and only has one streamflow peak occurring in spring-summer which is influenced by both tropical Pacific SST and Atlantic SST. Spring streamflow has also declined in each river post-1998, and this is due to a shift in the tropical Pacific leading to negative precipitation anomalies and drying in the SWUS. Chapter 2 assess a region of the SWUS that receives both winter storm track precipitation and NAM, and therefore has two periods of vegetation green-up annually with an intervening spring dry season. The first peak in vegetation occurs during the spring, and is influenced by the magnitude of winter precipitation and snowmelt, which gradually adds water to the soils. The second peak in vegetation follows the spring dry season when soil moisture recovers with the arrival of the NAM. A climatic shift in the tropical Pacific occurred in 1997/98 and produced a shift to an earlier and more severe spring dry season, and reduced vegetation green-up. An earlier extended dry period in the mid-century (1948 to 1966) also was influenced by a cool phase of the tropical Pacific, which led to a reduction in precipitation of a similar magnitude as the recent drought. However, the recent drought is more severe - and temperatures also have been greater during the recent period. Using a decomposition of the impact of precipitation and potential evapotranspiration (PET) on soil moisture, we found that PET contributed 39% to the negative soil drying anomalies in the recent post-1998 drought, compared to 8% during the earlier extended dry period. This indicates an increased role of temperature during the recent drying. In Chapter 4 we evaluated 18 CMIP5 models based on comparisons with observations of pre- cipitation, net ecosystem exchange, leaf area index and soil moisture from land surface model output. Following our evaluation, we selected three models which best simulated the bimodal region: CanEMS2, GFDL-ESM2G and GFDL-ESM2M. These models indicate that overall this region will be drier in the 21st century; runoff is projected to decrease, particularly in the spring, soil moisture is reduced, and snow fall declines. The variability in projected precipitation, how- ever, is large, and we find that for the most part does not exceed what can be expected from model natural climate variability. The multi-model ensemble from the rest of the CMIP5 models indicate an overall decline in annual precipitation by the end of the 21st century, particularly during the spring. The three models also project an increase in net primary productivity in both the spring and summer growing seasons due to the effects of CO2 fertilization. Enhanced vegetation growth is likely to further exacerbate drying of the soils as vegetation draws down moisture, and enhances water losses via evapotranspiration. The fertilization process is, however, still uncertain and fur- ther studies are needed on the representation of CO2 enhanced vegetation growth in the SWUS to constrain this result. The findings of this thesis have contributed enhanced our knowledge of how climate dynamics, natural variability, and recent warming have influenced the ecohydrology of the SWUS, and also inform future climate projections. Constraining our understanding of this region is of importance given the growing populations, mounting pressures on natural resources, and anthropogenically induced climate change which is expected to affect this region in the 21st century.

Thesis (M.S.)--University of Nevada, Reno, 2005. / "December 2005." Includes bibliographical references (leaves 49-52). Online version available on the World Wide Web.

Compiled and Edited by Patricia Paylore / For 23rd International Geographical Congress, Moscow, 1976, Pre-Conference Meeting of the IGU Working Group on Desertification, Desert Research Institute, Ashkabad, Turkmen SSR, July 20-26, 1976.

The International Geographical Union Working Group on Desertification In and Around Arid Lands

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Thesis (M. Eng.)--University of Adelaide, 1988. / Includes bibliographical references (leaves 284-288).

Thesis (M.S.)--University of Wyoming, 2007. / Title from PDF title page (viewed on Nov. 13, 2008). Includes bibliographical references (p. 210-213).

Thesis (M.S.)--Ohio University, March, 1989. / Title from PDF t.p.