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This paper investigates large-scale summertime moisture divergence associated with the southwestern branch of the North American monsoon. Diagnostic hydrologic estimates taken from thirty-three years of seasonal mean reanalysis data indicate that over most of the southwestern United States and northwestern Mexico there is positive vertically-integrated columnar (i.e. three dimensional) moisture divergence above approximately 700 mbar, implicitly suggesting that mid-level moisture convergence related to advection from remote source regions cannot support seasonal mean precipitation over the southwestern US. Furthermore, estimates of the horizontal component of this moisture divergence indicate that the southwestern US is actually one of the largest upper-air (<700 mbar) sources of moisture for the North American continent.