Water is a critically limiting resource throughout much of Southwest Asia. Precipitation, which averages more than 1 m per year in the mountains of Turkey to the north, grades down to 100 mm or less on the southern Euphrates Plain and the Arabian Peninsula. Extreme scarcity and variability in available water is a concern for present day and future resource management and is also a point of interest in historical studies of the Near East. There is considerable speculation that humans in the past have exacerbated the natural aridity of the region through land use modification and degradation of native vegetation, and, by similar reasoning, some predict that the massive impoundment and irrigation schemes developed in recent decades will have a humidifying effect on the region. Such proposed land-atmosphere interactions, however, operate in a complex mesoscale context. Aridity in Southwest Asia is influenced by the Hadley circulation, topographically-induced subsidence, the location of surrounding water bodies, and seasonally variable wind patterns. An understanding of these drivers of climate is prerequisite to any claim of anthropogenic feedbacks and can aid our predictions regarding the regional impact of global climate change. Using the PSU/NCAR Mesoscale Model 5 (MM5) we investigate the relative roles of topography, local water bodies, and land surface modification in shaping the severity and spatial pattern of aridity in Southwest Asia. In these simulations large-scale irrigation had only a modest influence on regional humidity; in contrast, a heat-driven plateau circulation involving the Zagros Mountains of Iran had a significant impact on water vapor transport and the stability of the lower atmosphere.