COMPLEX CLIMATOLOGY IN COMPLEX TERRAIN: SLOPE PROCESSES AND THE SEASONAL CYCLE

Runoff and hillslope processes are strongly modulated by the climatological variability of precipitation. In semiarid regions the frequency and intensity of precipitation vary dramatically as a function of season, with the cold season characterized by lengthy low-intensity precipitation episodes, and the warm season characterized by brief but intense downbursts. The seasonal cycle of precipitation in regions of complex topography can exhibit sharp spatial gradients, both horizontally (across pronounced mountain ranges) and vertically (between valley bottoms and ridgetops). We describe the spatial variability of the seasonal cycle in precipitation across western North America, emphasizing monsoon dynamics and the difference between summer-dominated and winter-dominated precipitation regimes across the Southwest.

The geomorphological significance of these climatological precipitation gradients is illustrated through comparative analysis of watersheds in two mountain ranges in Arizona, one of which receives most of its precipitation in winter, and the other in the summer monsoon domain. The monsoon-dominated landscape is more variable in local relief and exhibits more exposed bedrock and higher drainage density. The seasonal cycle of stream discharge in the monsoonal regime is a function of basin size, with smaller basins responding more quickly and sensitively to summer thunderstorms. This analysis leads to interpretive comments on the role of the seasonal cycle of precipitation in priming hillslopes for sediment delivery, and speculation about the effects of projected future climate change on landscape evolution in the Southwest.