Grasslands of the World.pdf - Disasters and Conflicts - UNEP

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Grasslands of the world
Figure 4.2
A map of the provinces of Argentinian Patagonia (shaded area) and
neighbouring provinces (Digital Cartography by Ing. Ag. Liliana González).
stand between the moist air and the Patagonian steppes, creating an extensive
rain-shadow that controls climatic patterns (Paruelo et al., 1998). There is a
very steep gradient of mean annual precipitation (MAP), decreasing towards
the east, from 4 000 mm at the Andes eastern foothills (at about 42°S) down
to 150 mm in the central plateau 180 km east of the mountains (Soriano,
1983). Inter-annual variation in precipitation increases exponentially with
decreasing rainfall, reaching coefficients of variation greater than 45 percent
at the drier end of the gradient (Jobággy, Paruelo and León, 1995). The east
coast is influenced by moist air from the Atlantic, with somewhat higher
annual precipitation (200 to 220 mm) evenly distributed, as opposed to the
winter rainfall of most of Patagonia (Paruelo et al., 1998; Soriano, 1983).
The ratio of mean annual precipitation to potential evapotranspiration
(MAP/PET ratio) of the steppes fluctuates between 0.45 and 0.11, with
marked deficits in spring and summer (Paruelo et al., 1998). Water is the most
important factor regulating primary production. Some of the variation can
be associated with El Niño-La Niña cycles (Paruelo et al., 1998), but Cibils
and Coughenour (2001) reported a longer-term cycle for MAP in southern
Patagonia: a significant decrease in precipitation from 1930 to 1960 and a
reversal of this trend (significant increase) over the subsequent thirty years
in Río Gallegos.