DevelopmentsinEarthSurfaceProcesses ...

Climate and Geomorphologic-related Disasters in Latin America 25
U (i)
6
4
2
–2
–4
–6
Paraná 1918–2006
Test of Trend in Annual Mean Precipitation
(Mann´s Test)
0
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Years
Figure 1.11 Graph of U(i) vs.Years (Mann’sTest) for precipitation at Parana¤ station (center of
the NEA region) (adapted from Garcíaetal.,2007).
The sudden increases in the means that marked a change in the climate were not
always coincident with the trend changes, although most of them happened in the
1970s. This seems to have been a key date for climatic change because, starting in
that period, the amount of water in the region, of both pluvial and fluvial origin,
has increased. At the same time, the phreatic levels have risen dramatically. This
positive trend embraced the whole region of northeastern Argentina as well as the
south of Paraguay (Fig. 1.11).
In this region, one area, where the greatest positive trends have taken place,
stands out: the south of the Province of Corrientes, where the increases in the mean
annual values were more than 400 mm. As in the region of northeastern Argentina,
the isohyets are approximately meridional, and the increase in the bi-annual precipitation
over the whole region results in a displacement of the isohyets toward the
west. Because of this, and also because of the implementation of new technologies,
the agricultural frontier expanded toward the west. However, the negative side of
these changes was the continuous or repeated flooding.
This increase in precipitation, which was not simultaneous across the whole of
the Plata Basin, is probably linked to the greatest frequency and intensity of the El
Niño phenomenon and to the strengthening of the South American low-level jet.
These positive trends took place simultaneously with a remarkable heating of the
polar areas (Barros and Doyle 1996; Barros et al. 2000).
Another indication of behavior change was seen through the recording of the
greatest frequency of daily precipitations of more than 100 mm in almost all of the
rain-gauging stations in the region (Canziani 2003, García et al. 2007). This is
manifested by a greater decadal frequency of convective events, which were almost
certainly mesoscale convective systems (MCS), although this was not demonstrated
as starting from 1970 but from the first part of the twentieth century.