Hydroclimatology of the North American Monsoon region in northwest Mexico
Introduction
An increasing body of literature is documenting salient features of the North American warm-season circulation and precipitation regime over the region of northwestern Mexico that is the core of the North American Monsoon System (NAMS). The region is generally semi-arid, with an annual precipitation regime dominated by warm-season convection that strongly interacts with the regional topography and surrounding bodies of seawater (For a complete discussion of the NAMS, please refer to the North American Monsoon Experiment (NAME) Science Plan (NAME Science Working Group, 2004) or Higgins et al., 2003). The circulation features responsible for this warm-season precipitation regime have been well documented (Higgins et al., 1997, Higgins et al., 1998, Higgins et al., 1999, Higgins and Shi, 2000, Carleton et al., 1990, Douglas et al., 1993, Schmitz and Mullen, 1996, Castro et al., 2001, Hu and Feng, 2002). Such studies consistently document a transition in the regional climate from an arid subtropical regime dominated by westerly flow at middle and upper levels, to a regime with substantially higher relative humidity, easterly flow at mid and upper levels, and strong diurnal convection. This transition occurs during June and early July and is designated as the ‘onset’ of the summer monsoon. The monsoon circulation, its onset, precipitation character, and the hydrological response to it, exhibit considerable spatial and temporal variability. This variability complicates diagnostic and predictive efforts and limits responsive management of regional water resources. This work explores the complex relationship between precipitation and streamflow in the NAM region by: (a) reviewing recent works that have examined precipitation and streamflow variability and (b) constructing a regional hydroclimatology from selected headwater catchments in northwest Mexico.
Section snippets
Overview of the North American Monsoon precipitation regime
The internal structure of NAM precipitation is complex and subject to considerable spatial and temporal variability. The centroid of NAM precipitation, which is often defined as the total rainfall in July, August, and September (JAS), is located along the western slope of the Sierra Madre Occidental (SMO) in northwestern Mexico (Douglas et al., 1993, Higgins et al., 1999, Gochis et al., 2004). In this region, the coefficient of variation of precipitation is high in the global context (Dettinger
Data
Selected basins, pertinent basin characteristics and streamgauge locations are shown in Fig. 1 and listed in Table 1. Monthly streamflow data were obtained from the BANDAS (Banco Nacional de Datos de Aguas Superficiales) data archive, BANDAS (1998). This data set is jointly developed by the Comision Nacional del Agua (CNA) and the Instituto Mexicano de Tecnologia del Agua (IMTA) of Mexico, and is continually updated. Streamflow periods of record vary for individual basins, but all basins
Streamflow climatology
The annual cycle of standardized monthly average flow volumes, monthly coefficients of variation and standardized monthly maximum flow volumes are shown in Fig. 2. Depending on the basin, 50–85% of the annual streamflow volume occurs during the months of July, August and September. Monthly streamflow volumes begin increasing in July and peak in either August or September. Notably, basins on the eastern slope of the SMO (shown in green) exhibit higher monthly percentages of annual streamflow
Summary and conclusions
This study presents a regional hydroclimatology of 15 headwater catchments that drain the Sierra Madre Occidental mountains in northwest Mexico. The basins range in size from 1000 to 10,000 km2 and are unregulated to the best of the authors' knowledge. Hence, this analysis is aimed at elucidating the natural streamflow response of headwater catchments from the annual cycle of precipitation in northwest Mexico, which is generally dominated by a warm-season monsoon. This study fills a niche in
Acknowledgements
The authors wish to thank Subrhendu Gangopadhyay for his insightful advice on interpreting the principal components analysis, the Comision Nacional del Agua of Mexico for collecting and providing the streamflow data used in this work, to Dr Tom Warner for his helpful review of this article and to two anonymous reviewers whose generous feedback greatly improved the quality and clarity of this work. Support for this work is provided by the National Center for Atmospheric Research, NOAA Office of
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