Variation of Hydrometeorological Conditions Along a Topographic Transect in Northwestern Mexico During the North American Monsoon

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Relatively little is currently known about the spatiotemporal variability of land surface conditions duringthe North American monsoon, in particular for regions of complex topography. As a result, the role playedby land–atmosphere interactions in generating convective rainfall over steep terrain and sustaining monsoonconditions is still poorly understood. In this study, the variation of hydrometeorological conditionsalong a large-scale topographic transect in northwestern Mexico is described. The transect field experimentconsisted of daily sampling at 30 sites selected to represent variations in elevation and ecosystem distribution.Simultaneous soil and atmospheric variables were measured during a 2-week period in early August 2004. Transect observations were supplemented by a network of continuous sampling sites used to analyzethe regional hydrometeorological conditions prior to and during the field experiment. Results reveal thestrong control exerted by topography on the spatial and temporal variability in soil moisture, with distinctlandscape regions experiencing different hydrologic regimes. Reduced variations at the plot and transectscale during a drydown period indicate that homogenization of hydrologic conditions occurred over thelandscape. Furthermore, atmospheric variables are clearly linked to surface conditions, indicating thatheating and moistening of the boundary layer closely follow spatial and temporal changes in hydrologicproperties. Land–atmosphere interactions at the basin scale (100 km2), obtained via a technique accountingfor topographic variability, further reveal the role played by the land surface in sustaining high atmosphericmoisture conditions, with implications toward rainfall generation during the North American monsoon.

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