18
ERDC TR-04-1
2.2
HYDROLOGY
The multiple physiographic provinces in the western U.S. (Fig. 2) interact
with spatially and seasonally variable moisture sources to produce a highly
variable rainfall distribution pattern that is markedly different from the eastern
U.S. (Fig. 3). Differing sources of moisture result in sharp seasonal differences in
rainfall between climatic regions in the Southwest, while local differences in
physiography and geology sometimes lead to dramatic differences in precipita-
tion within a given climatic zone. A number of distinct ecological subregions, or
ecosystem provinces, are present in the southwestern U.S., each resulting from a
distinct assemblage of climate, physiography, and vegetation (Bailey 1983, 1995)
(Fig. 4, Table 2).
Figure 2. Physiographic provinces of the western
United States, showing the major provinces found in
the Southwest. (Modified from De Blij and Muller 1992,
Figure 52-1, p. 518.)
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