DevelopmentsinEarthSurfaceProcesses ...

Climate and Geomorphologic-related Disasters in Latin America 23
Figure 1.10 Differences (mm) between extreme high and low phases of the 3.5-year
oscillation over the La Plata Basin.
map of opposite phases for the 3.5-year component (Fig. 1.10) shows a lower spatial
variability compared to the 6-year pattern, and the largest values in the Brazilian
Highlands are around 25% of those of the 6-year oscillation.
Larger values for increases in annual precipitation between 1950 and 1984 were
recorded over the Pantanal (Brazil), and extreme decreases in values were recorded
over the Pilcomayo and Bermejo River basins (two tributaries coming from the
Andes). This had a direct consequence on the large-scale behavior of the major
rivers of the basin, especially for the Paraguay River, where the largest contributions
to the total variance, for periods longer than a year, corresponded to an
upward trend.
Between 1956 and 1991, in the northeast of Argentina, the increases in the
mean annual precipitation were greater than 10% in wide zones and up to more
than 30% in other zones (Castañeda and Barros, 1994; García, 2000). The existence
of changes in the trends and sudden increases in the means was shown by García
and Vargas (1998).
There is no exact uniformity in the dates of changes in the tendencies of annual
precipitation values among the scarce and widely spread rain-gauging stations.
However, a notable similarity of behavior exists, as the stations, almost without
exception, showed tendencies for changes in the same, or almost the same, years,
around 1917–1918, 1943–1944 and 1970–1971 (see Table 1.3). The trends in
annual precipitation have been positive throughout the entire set of stations, without
exception, starting from the year 1970–1971.